Direct current transient overvoltage suppression method and system suitable for hybrid MMC extra-high voltage layered flexible direct current

Abstract

The invention discloses a DC transient overvoltage suppression method and system suitable for hybrid MMC extra-high voltage layered flexible DC, and belongs to the technical field of power systems. The method comprises the following steps: determining a topological structure of the hybrid MMC extra-high voltage layered flexible direct current, and determining equipment parameters based on the topological structure; determining steady-state control strategies of a sending end converter station, a receiving end high-end converter station and a receiving end low-end converter station of the hybrid MMC extra-high voltage layered flexible direct current converter; based on the equipment parameters and the steady-state control strategy, determining an alternating-current fault ride-through strategy of a sending-end converter station, a receiving-end high-end converter station and a receiving-end low-end converter station of the hybrid MMC extra-high-voltage layered flexible direct current; and executing the alternating current fault ride-through strategy to suppress the direct current transient overvoltage of the hybrid MMC extra-high voltage layered flexible direct current. According to the invention, the technical advantage of negative investment of the full-bridge sub-module in the hybrid MMC is utilized, no equipment is added, and the construction cost is not increased.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power systems, and more particularly, to a DC transient overvoltage suppression method and system suitable for a hybrid MMC ultra-high voltage layered flexible DC. BACKGROUND

[0002] The voltage source converter-based DC transmission technology (flexible DC, referred to as flexible DC) has good controllability, flexible structure and operation mode, strong adaptability, and low harmonic content, and exhibits unique technical advantages and economic benefits in cross-regional DC transmission, new energy grid connection, asynchronous networking, and urban center power supply. At present, flexible DC has rapidly developed from low-voltage and small-capacity demonstration projects to high-voltage and large-capacity, providing a new solution for the rational allocation and efficient use of new energy on a large scale, and rapidly becoming a key technology for building a new type of power system dominated by new energy.

[0003] With the operation and application of flexible DC projects, the topology of flexible DC is more flexible and complex, evolving from the original point-to-point two-end topology to other complex topologies such as multi-end parallel and high-low end series. Among them, the ultra-high voltage layered flexible DC based on hybrid MMC learns from the topology structure of ultra-high voltage layered conventional DC, which can not only meet the transmission demand of DC power dispersedly accessing load centers, but also rely on hybrid MMC to complete DC line fault ride-through, while having the economic advantage of relatively low construction cost, gradually becoming the first topology of ultra-high voltage flexible DC.

[0004] Compared with conventional DC, the flexible DC transmission technology based on fully controlled power electronic devices can avoid commutation failure and has reactive power control capability. However, during the AC fault period of the receiving end power grid, the decrease of AC voltage leads to the obstruction of flexible DC AC active power transmission, resulting in the impact of sending end DC power on the sub-module capacitor and the rapid rise of DC voltage (referred to as DC transient overvoltage). Due to the limited withstand voltage and current level of the current fully controlled power electronic devices, if the sub-module capacitor voltage is not effectively controlled or suppressed, the overvoltage protection of the power electronic devices will act, the flexible DC will be locked, and power transmission will be impossible, thereby affecting the safe and stable operation of the AC power grid.

[0005] To address the DC transient overvoltage problem in flexible DC transmission systems (DCS) during receiver-end faults, domestic and international scholars have proposed several DC transient overvoltage suppression strategies, including configuring AC energy dissipation devices, configuring DC energy dissipation devices, increasing submodule capacitance, reducing sending-end power, and transferring active power from other receiver-end converter stations. However, strategies involving configuring AC energy dissipation devices (Zhangbei DCS project), configuring DC energy dissipation devices (Jiansu and Rudong DCS projects), and increasing submodule capacitance increase the land area required for DCS projects, posing a risk of infeasibility and increasing construction costs. The strategy of reducing sending-end power is mainly applied to LCC-type sending-end converter stations capable of rapid phase shifting (Jiansu and Kunliulong), and this measure will prevent active power transmission at both the high and low ends of the layered DCS topology. The strategy of transferring active power from other receiver-end converter stations requires multiple receiver-end converter stations connected in parallel on the DC side. Therefore, none of these DC transient overvoltage suppression strategies are suitable for layered DCS topologies with series connection between the high and low ends at the receiver. Summary of the Invention

[0006] To address the above problems, this invention proposes a DC transient overvoltage suppression method suitable for hybrid MMC ultra-high voltage layered flexible DC systems, comprising:

[0007] The topology of the hybrid MMC UHV layered flexible DC transmission system was determined, and the equipment parameters were determined based on the topology.

[0008] Determine the steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC type.

[0009] Based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategy of the sending-end converter station, receiving-end high-end converter station and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC system is determined.

[0010] The AC fault ride-through strategy is implemented to suppress the DC transient overvoltage of the hybrid MMC UHV stratified flexible DC.

[0011] Optionally, the topology of the hybrid MMC UHV layered flexible DC transmission is determined, and the equipment parameters are determined based on the topology, including:

[0012] The topology of the hybrid MMC UHV stratified flexible DC transmission is determined, and based on the topology, the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are determined.

[0013] The equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, as well as the flow capacity of the converter valves in each converter station, are determined respectively.

[0014] The equipment parameters include: the rated capacity, rated voltage, and rated current of the converter valve.

[0015] Optionally, the steady-state control strategy for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system includes:

[0016] Determine the steady-state control strategy for the sending-end converter station, including:

[0017] During steady state, the sending-end converter station is set to operate in constant active power control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0018] Determine the steady-state control strategy for the receiving-end high-end converter station, including:

[0019] During steady state, the sending-end converter station is set to operate in constant high-side DC voltage control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0020] Determine the steady-state control strategy for the receiving-end low-end converter station, including:

[0021] During steady state, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

[0022] Optionally, based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategy for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC transmission system is determined, including:

[0023] Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages;

[0024] Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. Based on the magnitude of the surplus power, the AC fault flag is determined, and it is determined whether the AC fault ride-through strategy should be activated.

[0025] The control strategy for the sending-end converter station is determined based on the AC fault flag bit.

[0026] Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station;

[0027] Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station.

[0028] Optionally, the calculation formulas for determining the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages are as follows:

[0029]

[0030] in, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

[0031] Optionally, based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. The AC fault flag is determined based on the magnitude of the surplus power, and it is then determined whether to activate the AC fault ride-through strategy, including:

[0032] The steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station is calculated using the following formula:

[0033]

[0034] in, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. I represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc Indicates direct current;

[0035] Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bits, including:

[0036] like Greater than And continue for t in If the receiving end high-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigH of the receiving end high-end converter station will be switched from 0 to 1.

[0037] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station is switched from 1 to 0.

[0038] like Greater than And continue for t inIf the receiving end low-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigL of the receiving end low-end converter station will switch from 0 to 1.

[0039] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station is switched from 1 to 0.

[0040] Optionally, based on the AC fault flag bit, the control strategy of the sending-end converter station is determined, including:

[0041] If any of the signals in SigH and SigL is 1, it is determined that there is a risk of power surplus accumulation and DC transient overvoltage in the receiving-end converter station. The sending-end converter station signal SigS is switched from 0 to 1, and the control is switched from constant active power to constant DC current.

[0042] If both SigH and SigL are 0, it is determined that there is no power surplus accumulation or DC transient overvoltage risk at the receiving-end converter station. The sending-end converter station's SigS signal is switched from 1 to "0", and a steady-state control strategy with constant active power is adopted.

[0043] Optionally, based on the AC fault flag bit, the control strategy for the receiving-end high-end converter station is determined, including:

[0044] If SigH is 0, the receiving-end high-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is put into operation. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0045]

[0046] Internal potential of AC voltage at the receiving end satisfy:

[0047]

[0048] The receiving-end high-end converter station, through the positive input sub-module, ensures a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0049] Where, δ H(t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle;

[0050] in:

[0051]

[0052]

[0053] in: This represents the active power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end high-end converter station and the AC power grid;

[0054] If SigH is 1, the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The calculation method for the DC voltage command value is as follows:

[0055]

[0056] Among them, I dc0 This is the DC current value before the fault. The current effective value of the AC voltage is given by k, where k is a safety margin coefficient between 0 and 1.

[0057] The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0058]

[0059] Internal potential of AC voltage at the receiving end satisfy:

[0060]

[0061] By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0062] Optionally, based on the AC fault flag bit, the control strategy for the receiving-end low-end converter station is determined, including:

[0063] If SigL is 0, the receiving-end low-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is in operation at time t. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0064]

[0065] Internal potential of AC voltage at the receiving end satisfy:

[0066]

[0067] The receiving-end low-end converter station, through the positive input submodule, maintains a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. The calculation formula is as follows:

[0068]

[0069] Where, δ L (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle, This represents the active power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end low-end converter station and the AC power grid;

[0070] If SigL is 1, the receiving-end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1, and the calculation formula is as follows:

[0071]

[0072] The receiving-end low-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0073]

[0074] Internal potential of AC voltage at the receiving end satisfy:

[0075]

[0076] By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0077] Furthermore, this invention also proposes a DC transient overvoltage suppression system suitable for hybrid MMC ultra-high voltage layered flexible DC, comprising:

[0078] The parameter determination unit is used to determine the topology of the hybrid MMC UHV layered flexible DC transmission and to determine the equipment parameters based on the topology.

[0079] The steady-state strategy unit is used to determine the steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system.

[0080] The output unit is used to determine the AC fault ride-through strategy of the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission based on the equipment parameters and the steady-state control strategy.

[0081] An execution unit is used to execute the AC fault ride-through strategy to suppress the DC transient overvoltage of the hybrid MMC ultra-high voltage stratified flexible DC.

[0082] Optionally, the topology of the hybrid MMC UHV layered flexible DC transmission is determined, and the equipment parameters are determined based on the topology, including:

[0083] The topology of the hybrid MMC UHV stratified flexible DC transmission is determined, and based on the topology, the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are determined.

[0084] The equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, as well as the flow capacity of the converter valves in each converter station, are determined respectively.

[0085] The equipment parameters include: the rated capacity, rated voltage, and rated current of the converter valve.

[0086] Optionally, the steady-state control strategy for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system includes:

[0087] Determine the steady-state control strategy for the sending-end converter station, including:

[0088] During steady state, the sending-end converter station is set to operate in constant active power control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0089] Determine the steady-state control strategy for the receiving-end high-end converter station, including:

[0090] During steady state, the sending-end converter station is set to operate in constant high-side DC voltage control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0091] Determine the steady-state control strategy for the receiving-end low-end converter station, including:

[0092] During steady state, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

[0093] Optionally, based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategy for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC transmission system is determined, including:

[0094] Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages;

[0095] Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. Based on the magnitude of the surplus power, the AC fault flag is determined, and it is determined whether the AC fault ride-through strategy should be activated.

[0096] The control strategy for the sending-end converter station is determined based on the AC fault flag bit.

[0097] Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station;

[0098] Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station.

[0099] Optionally, the calculation formulas for determining the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages are as follows:

[0100]

[0101] in, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

[0102] Optionally, based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. The AC fault flag is determined based on the magnitude of the surplus power, and it is then determined whether to activate the AC fault ride-through strategy, including:

[0103] The steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station is calculated using the following formula:

[0104]

[0105] in, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. i represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc Indicates direct current;

[0106] Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bits, including:

[0107] like Greater than And continue for t in If the receiving end high-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigH of the receiving end high-end converter station will be switched from 0 to 1.

[0108] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station is switched from 1 to 0.

[0109] like Greater than And continue for t inIf the receiving end low-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigL of the receiving end low-end converter station will switch from 0 to 1.

[0110] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station is switched from 1 to 0.

[0111] Optionally, based on the AC fault flag bit, the control strategy of the sending-end converter station is determined, including:

[0112] If any of the signals in SigH and SigL is 1, it is determined that there is a risk of power surplus accumulation and DC transient overvoltage in the receiving-end converter station. The sending-end converter station signal SigS is switched from 0 to 1, and the control is switched from constant active power to constant DC current.

[0113] If both SigH and SigL are 0, it is determined that there is no power surplus accumulation or DC transient overvoltage risk at the receiving-end converter station. The sending-end converter station's SigS signal is switched from 1 to "0", and a steady-state control strategy with constant active power is adopted.

[0114] Optionally, based on the AC fault flag bit, the control strategy for the receiving-end high-end converter station is determined, including:

[0115] If SigH is 0, the receiving-end high-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is put into operation. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0116]

[0117] Internal potential of AC voltage at the receiving end satisfy:

[0118]

[0119] The receiving-end high-end converter station, through the positive input sub-module, ensures a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0120] Where, δ H(t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle;

[0121] in:

[0122]

[0123] in: This represents the active power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end high-end converter station and the AC power grid;

[0124] If SigH is 1, the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The calculation method for the DC voltage command value is as follows:

[0125]

[0126] Among them, i dc0 This is the DC current value before the fault. The current effective value of the AC voltage is given by k, where k is a safety margin coefficient between 0 and 1.

[0127] The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0128]

[0129] Internal potential of AC voltage at the receiving end satisfy:

[0130]

[0131] By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0132] Optionally, based on the AC fault flag bit, the control strategy for the receiving-end low-end converter station is determined, including:

[0133] If SigL is 0, the receiving-end low-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is in operation at time t. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0134]

[0135] Internal potential of AC voltage at the receiving end satisfy:

[0136]

[0137] The receiving-end low-end converter station, through the positive input submodule, maintains a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. The calculation formula is as follows:

[0138]

[0139] Where, δ L (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle, This represents the active power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end low-end converter station and the AC power grid;

[0140] If SigL is 1, the receiving-end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1, and the calculation formula is as follows:

[0141]

[0142] The receiving-end low-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0143]

[0144] Internal potential of AC voltage at the receiving end satisfy:

[0145]

[0146] By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0147] In another aspect, the present invention also provides a computing device, comprising: one or more processors;

[0148] A processor is used to execute one or more programs;

[0149] When the one or more programs are executed by the one or more processors, the method described above is implemented.

[0150] In another aspect, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed, implements the method described above.

[0151] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0152] This invention provides a method for suppressing DC transient overvoltages in hybrid MMC UHV tiered flexible DC systems, comprising: determining the topology of the hybrid MMC UHV tiered flexible DC system and determining equipment parameters based on the topology; determining steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC system; determining AC fault ride-through strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC system based on the equipment parameters and the steady-state control strategies; and executing the AC fault ride-through strategies to suppress DC transient overvoltages in the hybrid MMC UHV tiered flexible DC system. This invention utilizes the technical advantage of the full-bridge submodules in the hybrid MMC system, which can be negatively integrated, without increasing equipment or construction costs; within the overvoltage and overcurrent range of the flexible DC power electronic devices, it maximizes the active power transmission capacity of the flexible DC system before and after a fault, which is more conducive to the rapid restoration of stable operation of the receiving-end power grid. Attached Figure Description

[0153] Figure 1 This is a flowchart of the method of the present invention;

[0154] Figure 2 The flowchart of the DC transient overvoltage suppression strategy applicable to hybrid MMC ultra-high voltage layered flexible DC is provided by the present invention;

[0155] Figure 3 The topology diagram of the hybrid MMC provided by this invention;

[0156] Figure 4 The diagram of the UHV hierarchical flexible DC topology based on hybrid MMC provided by this invention;

[0157] Figure 5 The control structure diagram of the UHV stratified flexible direct transmission converter station based on hybrid MMC provided by the present invention;

[0158] Figure 6 The control structure diagram of the UHV stratified flexible DC receiving-end converter station based on hybrid MMC provided by the present invention;

[0159] Figure 7 This is a diagram illustrating the DC transient overvoltage suppression strategy applicable to ultra-high voltage layered hybrid MMC flexible DC.

[0160] Figure 8 (ae) is a comparison diagram of the response before and after the suppression strategy is adopted under a three-phase fault at the receiving end high-end converter station;

[0161] Figure 9 (ae) is a comparison diagram of the response before and after the suppression strategy is adopted under a single-phase fault at the receiving end low-end converter station;

[0162] in, Figure 8 (ae) are comparison charts of AC voltage / pu, DC voltage / pu, DC power / pu, AC power / pu, and average value of submodule capacitor voltage / kV.

[0163] in, Figure 9 (ae) are comparison charts of AC voltage / pu, DC voltage / pu, DC power / pu, AC power / pu, and average value of submodule capacitor voltage / kV. Detailed Implementation

[0164] Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0165] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0166] Example 1:

[0167] This invention proposes a DC transient overvoltage suppression method applicable to hybrid MMC ultra-high voltage layered flexible DC systems, such as... Figure 1 As shown, it includes:

[0168] Step (1) Determine the topology of the hybrid MMC UHV layered flexible DC transmission and determine the equipment parameters based on the topology;

[0169] Step (2) Determine the steady-state control strategy for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system.

[0170] Step (3) Based on the equipment parameters and the steady-state control strategy, determine the AC fault ride-through strategy for the sending-end converter station, receiving-end high-end converter station and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system.

[0171] Step (4) executes the AC fault ride-through strategy to suppress the DC transient overvoltage of the hybrid MMC UHV stratified flexible DC.

[0172] The following provides further explanation of steps (1) to (3):

[0173] (1) Determine the topology and equipment parameters of the UHV stratified flexible DC transmission system;

[0174] (2) Determine the steady-state control strategies for the UHV layered flexible DC converter stations at the sending end, receiving end, and receiving end.

[0175] (3) Based on the equipment parameters of UHV layered flexible DC, design AC fault ride-through strategies for the sending-end converter station, receiving-end high-end converter station and receiving-end low-end converter station of UHV layered flexible DC respectively.

[0176] Preferably, step (1) includes:

[0177] Step 101: Determine the topology of the UHV layered flexible DC transmission system, and clarify the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station;

[0178] Step 102: Determine the equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station respectively, including but not limited to the rated capacity, rated voltage, and rated current of the converter valves, and determine the overcurrent capacity of the converter valves in each converter station;

[0179] Preferably, step (2) includes:

[0180] Step 201: Determine the steady-state control strategy of the sending-end converter station. Considering that the full-bridge submodule in the hybrid MMC has a negative input function, during the steady-state period, the d-axis of the sending-end converter station is set to operate in constant active power control mode, the q-axis is set to operate in constant AC voltage or constant reactive power control mode, and the DC bias loop is set to operate in constant modulation ratio control mode.

[0181] Step 202: Determine the steady-state control strategy of the receiving-end high-end converter station. Considering that the full-bridge submodule in the hybrid MMC has a negative input function, during the steady-state period, the sending-end converter station is set to operate in a constant high-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

[0182] Step 203: Determine the steady-state control strategy for the receiving-end low-end converter station. Considering that the full-bridge submodule in the hybrid MMC has a negative input function, during the steady-state period, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

[0183] Preferably, step (3) includes:

[0184] Step 301: Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages, where, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

[0185]

[0186] Step 302: Calculate the steady-state DC power of the receiving-end converter station based on the steady-state DC current at the sending end; assess whether there is a risk of power surplus at the receiving-end converter station based on the active power transmission capacity and steady-state DC power of the receiving-end converter station; determine the AC fault flag bit based on the size of the surplus power; and determine whether to activate the AC fault ride-through strategy.

[0187] 1) Calculate the steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station:

[0188] The formulas for calculating the steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station are as follows: where, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. I represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc This represents direct current.

[0189]

[0190] 2) Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bit.

[0191] like Greater than And continue for t in If this is the case, it is determined that the receiving-end high-end converter station has the risk of surplus power accumulation and DC transient overvoltage, and the AC fault flag bit SigH of the receiving-end high-end converter station switches from "0" to "1"; if Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station switches from "1" to "0".

[0192] like Greater than And continue for t in If this is detected, it is determined that the receiving-end low-end converter station has the risk of surplus power accumulation and DC transient overvoltage, and the AC fault flag bit SigL of the receiving-end low-end converter station switches from "0" to "1". If Less than And continue for t out If the result is positive, it is determined that there is no surplus power accumulation or DC transient overvoltage risk at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station switches from "1" to "0".

[0193] Step 303: Determine the control strategy of the sending-end converter station based on the AC fault flag bit.

[0194] If either SigH or SigL is "1", it is determined that the receiving-end converter station has a risk of power surplus accumulation and DC transient overvoltage. The sending-end converter station's SigS signal switches from "0" to "1", and the control strategy changes from constant active power to constant DC current. If both SigH and SigL are "0", it is determined that the receiving-end converter station does not have a risk of power surplus accumulation and DC transient overvoltage. The sending-end converter station's SigS signal switches from "1" to "0", and a steady-state control strategy with constant active power is adopted.

[0195] Step 304: Determine the control strategy for the receiving-end high-end converter station based on the AC fault flag bit.

[0196] 1) If SigH is "0", the receiving-end high-end converter station does not enter the AC fault ride-through strategy and adopts the steady-state control strategy determined in step 202. Taking any time t as an example, under the action of the control system, the upper arm of the hybrid MMC is being put into operation. The sub-module and lower bridge arm are being put into operation. Each submodule has an average capacitor voltage of [number] submodules. High-side DC voltage at the receiving end satisfy:

[0197]

[0198] Internal potential of AC voltage at the receiving end satisfy:

[0199]

[0200] The receiving-end high-end converter station, through the positive input submodule, controls the AC power (active power) exchanged between the converter station and the AC grid by adjusting the AC internal potential while ensuring a constant DC voltage. reactive power ) is the set value. Where δ H (t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle.

[0201]

[0202] 2) If SigH is "1", the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The calculation method for the DC voltage command value is as follows: I dc0 This is the DC current value before the fault. is the effective value of the AC voltage at the current moment, and k is the safety margin coefficient between 0 and 1.

[0203]

[0204] The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Taking any time t as an example, under the action of the DC bias loop, the upper arm of the hybrid MMC at time t is positively engaged. Sub-module, lower bridge arm negative input There are 1 full-bridge submodules (submodules may be negatively engaged in both the upper and lower bridge arms; this analysis focuses only on the negatively engaged lower bridge arm submodule). The average capacitor voltage of the submodule is... High-side DC voltage at the receiving end satisfy:

[0205]

[0206] Internal potential of AC voltage at the receiving end satisfy:

[0207]

[0208] By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0209] Step 305: Determine the control strategy for the receiving-end low-end converter station based on the AC fault flag bit.

[0210] If SigL is "0", the receiving-end low-end converter station does not enter the AC fault ride-through strategy and adopts the steady-state control strategy determined in step 202. Taking any time t as an example, under the action of the control system, the upper arm of the hybrid MMC is in operation at time t. The sub-module and lower bridge arm are being put into operation. Each submodule has an average capacitor voltage of [number] submodules. DC voltage at the receiving end satisfy:

[0211]

[0212] Internal potential of AC voltage at the receiving end satisfy:

[0213]

[0214] The receiving-end low-end converter station, through the positive input submodule, controls the AC power (active power) exchanged between the converter station and the AC grid by adjusting the AC internal potential while ensuring a constant DC voltage. reactive power ) is the set value. Where δ H (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle.

[0215]

[0216] If SigL is "1", the receiving end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1.

[0217]

[0218] The receiving-end low-end converter station actively reduces the high-side DC voltage by negatively engaging the full-bridge submodule. Taking any time t as an example, under the action of the DC bias loop, the upper arm of the hybrid MMC at time t is positively engaged. Sub-module, lower bridge arm negative input There are several full-bridge submodules (submodules may be negatively connected in both the upper and lower bridge arms; here, we will analyze the negatively connected submodule in the lower bridge arm as an example). The average capacitor voltage of the submodule is... DC voltage at the receiving end satisfy:

[0219]

[0220] Internal potential of AC voltage at the receiving end satisfy:

[0221]

[0222] By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0223] The following is in conjunction with the appendix Figures 2-9 The present invention will be further described as follows:

[0224] like Figure 2 As shown, the present invention provides a DC transient overvoltage suppression method applicable to hybrid MMC ultra-high voltage layered flexible DC transmission, the process of which is as follows:

[0225] (1) Determine the topology and equipment parameters of the UHV stratified flexible DC transmission system;

[0226] 1) Determine the topology of the UHV stratified flexible DC type, and the topology of the hybrid MMC type, such as... Figure 3 As shown, it includes three phases and six arms. Each arm consists of multiple half-sub-modules (HSMs), full-sub-modules (FSBs), and cascaded arm reactors. The full-sub-modules are mainly used for DC line fault ride-through. The UHV hierarchical flexible DC topology is as follows: Figure 4 As shown, the locations and numbers of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are clearly defined;

[0227] 2) Determine the equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station respectively, including but not limited to the rated capacity, rated voltage, and rated current of the converter valves, and determine the overcurrent capacity of the converter valves in each converter station;

[0228] (2) Determine the steady-state control strategies for the UHV layered flexible DC converter stations at the sending end, receiving end, and receiving end.

[0229] 1) Determine the steady-state control strategy for the sending-end converter station, such as... Figure 5 As shown, considering the negative input function of the full-bridge submodule in the hybrid MMC, during steady-state operation, the d-axis of the sending-end converter station is set to operate in constant active power control mode, the q-axis in constant AC voltage or constant reactive power control mode, and the DC bias loop in constant modulation ratio control mode. (Q in the figure...) ref Q and P represent the commanded value and measured value of AC reactive power, respectively. ref P and V represent the commanded value and measured value of AC active power, respectively. acref V ac These represent the commanded value and the measured value of the AC voltage, respectively. dc I dcref M represents the commanded value and the measured value of the DC current, respectively. dc0 M is the modulation ratio rating. dc The modulation ratio is the actual value; PI represents the proportional-integral controller, I... sd I sq Let I represent the d-axis and q-axis components of the alternating current, respectively. sdref I sqref V represents the command values ​​for the d-axis and q-axis components of the alternating current, respectively. sd V sq X represents the d-axis and q-axis components of the AC voltage, respectively. t Represents equivalent reactance, m a m b m c T represents the modulation waves of phases a, b, and c respectively. -1 V represents the inverse Park transform. dc This indicates DC voltage.

[0230] 2) Determine the steady-state control strategy for the receiving-end high-end converter station, such as... Figure 6 As shown, considering the negative input function of the full-bridge submodule in the hybrid MMC, during steady-state operation, the d-axis of the sending-end converter station is set to operate in a constant high-side DC voltage control mode, the q-axis in a constant AC voltage or constant reactive power control mode, and the DC bias loop in a constant modulation ratio control mode. In the figure, U... cref U cThese represent the rated and actual values ​​of the average capacitor voltage of the submodule, respectively, V. dref (t) represents the DC voltage command value output by the fault ride-through module.

[0231] 3) Determine the steady-state control strategy for the receiving-end low-end converter station, such as... Figure 6 As shown, considering that the full-bridge submodule in the hybrid MMC has a negative input function, during the steady state, the d-axis of the sending-end converter station is set to operate in a constant low-end DC voltage control mode, the q-axis is set to operate in a constant AC voltage or constant reactive power control mode, and the DC bias loop is set to operate in a constant modulation ratio control mode.

[0232] (3) Based on the equipment parameters of the UHV tiered flexible DC transmission system, design AC fault ride-through strategies for the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, respectively. For example... Figure 6 As shown:

[0233] 1) Real-time monitoring of the effective value of the AC bus voltage at the converter station outlet, and calculation of the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station using the following formula:

[0234]

[0235] 2) Calculate the steady-state DC power of the receiving-end converter station based on the steady-state DC current at the sending end; assess whether there is a risk of power surplus at the receiving-end converter station based on the active power transmission capacity and steady-state DC power of the receiving-end converter station; determine the AC fault flag bit based on the size of the surplus power; and determine whether to activate the AC fault ride-through strategy.

[0236] Calculate the steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station.

[0237] The formulas for calculating the steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station are as follows:

[0238]

[0239] Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag.

[0240] like Greater than And continue for t in If this is the case, it is determined that the receiving-end high-end converter station has the risk of surplus power accumulation and DC transient overvoltage, and the AC fault flag bit SigH of the receiving-end high-end converter station switches from "0" to "1"; if Less than And continue for t outIf the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station switches from "1" to "0".

[0241] like Greater than And continue for t in If this is detected, it is determined that the receiving-end low-end converter station has the risk of surplus power accumulation and DC transient overvoltage, and the AC fault flag bit SigL of the receiving-end low-end converter station switches from "0" to "1". If Less than And continue for t out If the result is positive, it is determined that there is no surplus power accumulation or DC transient overvoltage risk at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station switches from "1" to "0".

[0242] 3) Determine the control strategy of the sending-end converter station based on the AC fault flag bit.

[0243] If either SigH or SigL is "1", it is determined that the receiving-end converter station has a risk of power surplus accumulation and DC transient overvoltage. The sending-end converter station's SigS signal switches from "0" to "1", and the control strategy changes from constant active power to constant DC current. If both SigH and SigL are "0", it is determined that the receiving-end converter station does not have a risk of power surplus accumulation and DC transient overvoltage. The sending-end converter station's SigS signal switches from "1" to "0", and a steady-state control strategy with constant active power is adopted.

[0244] 4) Determine the control strategy for the receiving-end high-end converter station based on the AC fault flag bit.

[0245] Case 1:

[0246] If SigH is "0", the receiving-end high-end converter station does not enter the AC fault ride-through strategy and adopts the steady-state control strategy determined in step 202. Taking any time t as an example, under the action of the control system, the upper arm of the hybrid MMC is being put into operation. The sub-module and lower bridge arm are being put into operation. Each submodule has an average capacitor voltage of [number] submodules. High-side DC voltage at the receiving end satisfy:

[0247]

[0248] Internal potential of AC voltage at the receiving end satisfy:

[0249]

[0250] The receiving-end high-end converter station, through the positive input submodule, controls the AC power (active power) exchanged between the converter station and the AC grid by adjusting the AC internal potential while ensuring a constant DC voltage. reactive power () is the set value.

[0251]

[0252]

[0253] Case 2:

[0254] If SigH is "1", the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The DC voltage command value is calculated as follows: I dc0 This is the DC current value before the fault. is the effective value of the AC voltage at the current moment, and k is the safety margin coefficient between 0 and 1.

[0255]

[0256] The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Taking any time t as an example, under the action of the DC bias loop, the upper arm of the hybrid MMC at time t is positively engaged. Sub-module, lower bridge arm negative input There are 1 full-bridge submodules (submodules may be negatively engaged in both the upper and lower bridge arms; this analysis focuses only on the negatively engaged lower bridge arm submodule). The average capacitor voltage of the submodule is... High-side DC voltage at the receiving end satisfy:

[0257]

[0258] Internal potential of AC voltage at the receiving end satisfy:

[0259]

[0260] By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0261] 5) Determine the control strategy for the receiving-end low-end converter station based on the AC fault flag bit.

[0262] Case 1:

[0263] If SigL is "0", the receiving-end low-end converter station does not enter the AC fault ride-through strategy and adopts the steady-state control strategy determined in step 202. Taking any time t as an example, under the action of the control system, the upper arm of the hybrid MMC is in operation at time t. The sub-module and lower bridge arm are being put into operation. Each submodule has an average capacitor voltage of [number] submodules. DC voltage at the receiving end satisfy:

[0264]

[0265] Internal potential of AC voltage at the receiving end satisfy:

[0266]

[0267] The receiving-end low-end converter station, through the positive input submodule, controls the AC power (active power) exchanged between the converter station and the AC grid by adjusting the AC internal potential while ensuring a constant DC voltage. reactive power ) is the set value. Where δ H (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle.

[0268]

[0269] Case 2:

[0270] If SigL is "1", the receiving end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1.

[0271]

[0272] The receiving-end low-end converter station actively reduces the high-side DC voltage by negatively engaging the full-bridge submodule. Taking any time t as an example, under the action of the DC bias loop, the upper arm of the hybrid MMC at time t is positively engaged. Sub-module, lower bridge arm negative input There are several full-bridge submodules (submodules may be negatively connected in both the upper and lower bridge arms; here, we will analyze the negatively connected submodule in the lower bridge arm as an example). The average capacitor voltage of the submodule is... DC voltage at the receiving end satisfy:

[0273]

[0274] Internal potential of AC voltage at the receiving end satisfy:

[0275]

[0276] By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0277] The following is an example:

[0278] In the PSD power system analysis software developed by the China Electric Power Research Institute, an electromechanical transient model of the receiving-end AC power grid was built, and the simulation scale is shown in Table 1. In the PSModel (Power System Model) electromagnetic transient simulation software, an ultra-high voltage layered flexible DC electromagnetic transient simulation model was built, and the parameters are shown in Table 2.

[0279] Table 1

[0280]

[0281] Table 2

[0282]

[0283] The proposed hybrid MMC UHV stratified flexible DC transient overvoltage suppression method was adopted, with control parameters shown in Appendix Table 3, to simulate a three-phase AC short-circuit fault in the near-field area of ​​the receiving-end high-end converter station and a single-phase AC short-circuit fault in the near-field area of ​​the receiving-end low-end converter station.

[0284] Table 3

[0285]

[0286] Figure 8 This is a comparison chart of the response before and after adopting a suppression strategy under a three-phase fault at the receiving-end high-end converter station. Figure 9This diagram compares the response of the receiving-end low-end converter station before and after implementing a suppression strategy under a single-phase fault. During the fault, the AC voltage drops, and the active power of the flexible DC power unit decreases. Without the DC transient overvoltage suppression strategy proposed in this invention, the DC power remains essentially unchanged during the fault, while the submodule capacitor voltage and DC voltage rise rapidly, posing an overvoltage lockout risk to the flexible DC power unit. However, with the proposed DC transient overvoltage suppression strategy, the receiving-end converter actively reduces the DC voltage based on the AC voltage during the fault, thereby reducing the DC power. The submodule capacitor voltage and DC voltage rise slowly and then quickly recover to their rated values, eliminating the overvoltage lockout risk for the flexible DC power unit. The proposed DC transient overvoltage suppression strategy effectively suppresses DC transient overvoltage phenomena during receiving-end grid faults, ensuring the safe and stable operation of the flexible DC power unit.

[0287] Example 2:

[0288] Furthermore, this invention also proposes a DC transient overvoltage suppression system 200 suitable for hybrid MMC ultra-high voltage layered flexible DC, comprising:

[0289] The parameter determination unit 201 is used to determine the topology of the hybrid MMC UHV layered flexible DC transmission and to determine the equipment parameters based on the topology.

[0290] The steady-state strategy unit 202 is used to determine the steady-state control strategies of the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system.

[0291] Output unit 203 is used to determine the AC fault ride-through strategy of the sending-end converter station, receiving-end high-end converter station and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission based on the equipment parameters and the steady-state control strategy.

[0292] The execution unit 204 is used to execute the AC fault ride-through strategy to suppress the DC transient overvoltage of the hybrid MMC ultra-high voltage stratified flexible DC.

[0293] The process includes determining the topology of the hybrid MMC UHV stratified flexible DC transmission system and determining equipment parameters based on the topology, including:

[0294] The topology of the hybrid MMC UHV stratified flexible DC transmission is determined, and based on the topology, the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are determined.

[0295] The equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, as well as the flow capacity of the converter valves in each converter station, are determined respectively.

[0296] The equipment parameters include: the rated capacity, rated voltage, and rated current of the converter valve.

[0297] The steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system include:

[0298] Determine the steady-state control strategy for the sending-end converter station, including:

[0299] During steady state, the sending-end converter station is set to operate in constant active power control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0300] Determine the steady-state control strategy for the receiving-end high-end converter station, including:

[0301] During steady state, the sending-end converter station is set to operate in constant high-side DC voltage control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop.

[0302] Determine the steady-state control strategy for the receiving-end low-end converter station, including:

[0303] During steady state, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

[0304] Specifically, based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC system are determined, including:

[0305] Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages;

[0306] Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. Based on the magnitude of the surplus power, the AC fault flag is determined, and it is determined whether the AC fault ride-through strategy should be activated.

[0307] The control strategy for the sending-end converter station is determined based on the AC fault flag bit.

[0308] Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station;

[0309] Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station.

[0310] The calculation formulas for determining the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages are as follows:

[0311]

[0312] in, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

[0313] Specifically, based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. The AC fault flag is determined based on the magnitude of the surplus power, and it is then determined whether to activate the AC fault ride-through strategy, including:

[0314] The steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station is calculated using the following formula:

[0315]

[0316] in, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. i represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc Indicates direct current;

[0317] Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bits, including:

[0318] like Greater than And continue for t in If the receiving end high-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigH of the receiving end high-end converter station will be switched from 0 to 1.

[0319] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station is switched from 1 to 0.

[0320] like Greater than And continue for t inIf the receiving end low-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigL of the receiving end low-end converter station will switch from 0 to 1.

[0321] like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station is switched from 1 to 0.

[0322] The control strategy for the sending-end converter station is determined based on the AC fault flag bit, including:

[0323] If any of the signals in SigH and SigL is 1, it is determined that there is a risk of power surplus accumulation and DC transient overvoltage in the receiving-end converter station. The sending-end converter station signal SigS is switched from 0 to 1, and the control is switched from constant active power to constant DC current.

[0324] If both SigH and SigL are 0, it is determined that there is no power surplus accumulation or DC transient overvoltage risk at the receiving-end converter station. The sending-end converter station's SigS signal is switched from 1 to "0", and a steady-state control strategy with constant active power is adopted.

[0325] The control strategy for the receiving-end high-end converter station is determined based on the AC fault flag bit, including:

[0326] If SigH is 0, the receiving-end high-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is put into operation. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0327]

[0328] Internal potential of AC voltage at the receiving end satisfy:

[0329]

[0330] The receiving-end high-end converter station, through the positive input sub-module, ensures a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0331] Where, δ H(t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle;

[0332] in:

[0333]

[0334] in: This represents the active power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end high-end converter station and the AC power grid;

[0335] If SigH is 1, the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The calculation method for the DC voltage command value is as follows:

[0336]

[0337] Among them, i dc0 This is the DC current value before the fault. The current effective value of the AC voltage is given by k, where k is a safety margin coefficient between 0 and 1.

[0338] The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy:

[0339]

[0340] Internal potential of AC voltage at the receiving end satisfy:

[0341]

[0342] By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0343] The control strategy for the receiving-end low-end converter station is determined based on the AC fault flag bit, including:

[0344] If SigL is 0, the receiving-end low-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is in operation at time t. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0345]

[0346] Internal potential of AC voltage at the receiving end satisfy:

[0347]

[0348] The receiving-end low-end converter station, through the positive input submodule, maintains a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. The calculation formula is as follows:

[0349]

[0350] Where, δ L (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle, This represents the active power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end low-end converter station and the AC power grid;

[0351] If SigL is 1, the receiving-end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1, and the calculation formula is as follows:

[0352]

[0353] The receiving-end low-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy:

[0354]

[0355] Internal potential of AC voltage at the receiving end satisfy:

[0356]

[0357] By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

[0358] This invention utilizes the technical advantage of the full-bridge submodule in the hybrid MMC that can be negatively invested, without increasing equipment or construction costs; within the overvoltage and overcurrent range of the flexible DC power electronic device, it maximizes the active power transmission capacity of the flexible DC before and after the fault, which is more conducive to the rapid restoration of stable operation of the receiving-end power grid.

[0359] Example 3:

[0360] Based on the same inventive concept, this invention also provides a computer device, which includes a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to implement corresponding method flows or corresponding functions, thereby implementing the steps of the methods in the above embodiments.

[0361] Example 4:

[0362] Based on the same inventive concept, this invention also provides a storage medium, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, this storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the steps of the method in the above embodiments.

[0363] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present invention can be implemented using various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0364] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0365] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0366] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0367] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0368] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for suppressing transient DC overvoltages in hybrid MMC ultra-high voltage stratified flexible DC systems, characterized in that, include: The topology of the hybrid MMC UHV layered flexible DC transmission system was determined, and the equipment parameters were determined based on the topology. Determine the steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC type. Based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategy of the sending-end converter station, receiving-end high-end converter station and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC system is determined. The AC fault ride-through strategy is implemented to suppress the DC transient overvoltage of the hybrid MMC UHV stratified flexible DC.

2. The DC transient overvoltage suppression method according to claim 1, characterized in that, Determine the topology of the hybrid MMC UHV layered flexible DC transmission system, and determine the equipment parameters based on the topology, including: The topology of the hybrid MMC UHV stratified flexible DC transmission is determined, and based on the topology, the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are determined. The equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, as well as the flow capacity of the converter valves in each converter station, are determined respectively. The equipment parameters include: the rated capacity, rated voltage, and rated current of the converter valve.

3. The DC transient overvoltage suppression method according to claim 1, characterized in that, The steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system are determined, including: Determine the steady-state control strategy for the sending-end converter station, including: During steady state, the sending-end converter station is set to operate in constant active power control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop. Determine the steady-state control strategy for the receiving-end high-end converter station, including: During steady state, the sending-end converter station is set to operate in constant high-side DC voltage control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop. Determine the steady-state control strategy for the receiving-end low-end converter station, including: During steady state, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

4. The DC transient overvoltage suppression method according to claim 1, characterized in that, Based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC system are determined, including: Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages; Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. Based on the magnitude of the surplus power, the AC fault flag is determined, and it is determined whether the AC fault ride-through strategy should be activated. The control strategy for the sending-end converter station is determined based on the AC fault flag bit. Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station; Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station.

5. The DC transient overvoltage suppression method according to claim 4, characterized in that, The calculation formulas for determining the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages are as follows: in, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

6. The DC transient overvoltage suppression method according to claim 4, characterized in that, Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. The AC fault flag is determined based on the magnitude of the surplus power, and it is then determined whether to activate the AC fault ride-through strategy, including: The steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station is calculated using the following formula: in, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. I represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc Indicates direct current; Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bits, including: like Greater than And continue for t in If the receiving end high-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigH of the receiving end high-end converter station will be switched from 0 to 1. like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station is switched from 1 to 0. like Greater than And continue for t in If the receiving end low-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigL of the receiving end low-end converter station will switch from 0 to 1. like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station is switched from 1 to 0.

7. The DC transient overvoltage suppression method according to claim 4, characterized in that, Based on the AC fault flag, determine the control strategy for the sending-end converter station, including: If any of the signals in SigH and SigL is 1, it is determined that there is a risk of power surplus accumulation and DC transient overvoltage in the receiving-end converter station. The sending-end converter station signal SigS is switched from 0 to 1, and the control is switched from constant active power to constant DC current. If both SigH and SigL are 0, it is determined that there is no power surplus accumulation or DC transient overvoltage risk at the receiving-end converter station. The sending-end converter station's SigS signal is switched from 1 to "0", and a steady-state control strategy with constant active power is adopted.

8. The DC transient overvoltage suppression method according to claim 4, characterized in that, Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station, including: If SigH is 0, the receiving-end high-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is put into operation. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: The receiving-end high-end converter station, through the positive input sub-module, ensures a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. Where, δ H (t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle; in: in: This represents the active power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end high-end converter station and the AC power grid; If SigH is 1, the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The calculation method for the DC voltage command value is as follows: Among them, i dc0 This is the DC current value before the fault. The current effective value of the AC voltage is given by k, where k is a safety margin coefficient between 0 and 1. The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

9. The DC transient overvoltage suppression method according to claim 4, characterized in that, Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station, including: If SigL is 0, the receiving-end low-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is in operation at time t. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: The receiving-end low-end converter station, through the positive input submodule, maintains a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. The calculation formula is as follows: Where, δ L (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle, This represents the active power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end low-end converter station and the AC power grid; If SigL is 1, the receiving-end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1, and the calculation formula is as follows: The receiving-end low-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

10. A DC transient overvoltage suppression system suitable for hybrid MMC ultra-high voltage layered flexible DC transmission, characterized in that, include: The parameter determination unit is used to determine the topology of the hybrid MMC UHV layered flexible DC transmission and to determine the equipment parameters based on the topology. The steady-state strategy unit is used to determine the steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system. The output unit is used to determine the AC fault ride-through strategy of the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission based on the equipment parameters and the steady-state control strategy. An execution unit is used to execute the AC fault ride-through strategy to suppress the DC transient overvoltage of the hybrid MMC ultra-high voltage stratified flexible DC.

11. The DC transient overvoltage suppression system according to claim 10, characterized in that, Determine the topology of the hybrid MMC UHV layered flexible DC transmission system, and determine the equipment parameters based on the topology, including: The topology of the hybrid MMC UHV stratified flexible DC transmission is determined, and based on the topology, the location and numbering of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station are determined. The equipment parameters of the sending-end converter station, the receiving-end high-end converter station, and the receiving-end low-end converter station, as well as the flow capacity of the converter valves in each converter station, are determined respectively. The equipment parameters include: the rated capacity, rated voltage, and rated current of the converter valve.

12. The DC transient overvoltage suppression system according to claim 10, characterized in that, The steady-state control strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV stratified flexible DC transmission system are determined, including: Determine the steady-state control strategy for the sending-end converter station, including: During steady state, the sending-end converter station is set to operate in constant active power control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop. Determine the steady-state control strategy for the receiving-end high-end converter station, including: During steady state, the sending-end converter station is set to operate in constant high-side DC voltage control mode for the d-axis, constant AC voltage or constant reactive power control mode for the q-axis, and constant modulation ratio control mode for the DC bias loop. Determine the steady-state control strategy for the receiving-end low-end converter station, including: During steady state, the sending-end converter station is set to operate in a constant low-end DC voltage control mode for the d-axis, in a constant AC voltage or constant reactive power control mode for the q-axis, and in a constant modulation ratio control mode for the DC bias loop.

13. The DC transient overvoltage suppression system according to claim 10, characterized in that, Based on the equipment parameters and the steady-state control strategy, the AC fault ride-through strategies for the sending-end converter station, receiving-end high-end converter station, and receiving-end low-end converter station of the hybrid MMC UHV tiered flexible DC system are determined, including: Determine the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages; Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. Based on the magnitude of the surplus power, the AC fault flag is determined, and it is determined whether the AC fault ride-through strategy should be activated. The control strategy for the sending-end converter station is determined based on the AC fault flag bit. Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station; Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station.

14. The DC transient overvoltage suppression system according to claim 13, characterized in that, The calculation formulas for determining the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station under different AC voltages are as follows: in, These represent the maximum effective values ​​of AC current operating at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the effective values ​​of AC voltage at the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. These represent the active power transmission capacity of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively.

15. The DC transient overvoltage suppression system according to claim 13, characterized in that, Based on the steady-state DC current at the sending end, the steady-state DC power of the receiving end converter station is calculated. Based on the active power transmission capacity and steady-state DC power of the receiving end converter station, the risk of power surplus at the flexible DC receiving end converter station is assessed. The AC fault flag is determined based on the magnitude of the surplus power, and it is then determined whether to activate the AC fault ride-through strategy, including: The steady-state DC power of the receiving-end high-end converter station and the receiving-end low-end converter station is calculated using the following formula: in, These represent the DC power of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. i represents the DC voltage of the receiving-end high-end converter station and the receiving-end low-end converter station, respectively. dc Indicates direct current; Determine whether there is a risk of DC transient overvoltage at each converter station at the receiving end, and determine the AC fault flag bits, including: like Greater than And continue for t in If the receiving end high-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigH of the receiving end high-end converter station will be switched from 0 to 1. like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end high-end converter station, and the AC fault flag bit SigH of the receiving end high-end converter station is switched from 1 to 0. like Greater than And continue for t in If the receiving end low-end converter station is found to have surplus power accumulation and DC transient overvoltage risk, the AC fault flag bit SigL of the receiving end low-end converter station will switch from 0 to 1. like Less than And continue for t out If the result is positive, it is determined that there is no risk of surplus power accumulation and DC transient overvoltage at the receiving end low-end converter station, and the AC fault flag bit SigL of the receiving end low-end converter station is switched from 1 to 0.

16. The DC transient overvoltage suppression system according to claim 13, characterized in that, Based on the AC fault flag, determine the control strategy for the sending-end converter station, including: If any of the signals in SigH and SigL is 1, it is determined that there is a risk of power surplus accumulation and DC transient overvoltage in the receiving-end converter station. The sending-end converter station signal SigS is switched from 0 to 1, and the control is switched from constant active power to constant DC current. If both SigH and SigL are 0, it is determined that there is no power surplus accumulation or DC transient overvoltage risk at the receiving-end converter station. The sending-end converter station's SigS signal is switched from 1 to "0", and a steady-state control strategy with constant active power is adopted.

17. The DC transient overvoltage suppression system according to claim 13, characterized in that, Based on the AC fault flag, determine the control strategy for the receiving-end high-end converter station, including: If SigH is 0, the receiving-end high-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is put into operation. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: The receiving-end high-end converter station, through the positive input sub-module, ensures a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. Where, δ H (t) represents the internal potential of the AC voltage at the receiving end high-end converter station. With system voltage The included angle; in: in: This represents the active power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end high-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end high-end converter station and the AC power grid; If SigH is 1, the receiving-end high-end converter station enters the AC fault ride-through strategy. The original d-axis power loop switches from steady-state control to stator module capacitor voltage control, and the DC bias loop switches from steady-state control to constant DC voltage control. The calculation method for the DC voltage command value is as follows: Among them, I dc0 This is the DC current value before the fault. The current effective value of the AC voltage is given by k, where k is a safety margin coefficient between 0 and 1. The receiving-end high-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is High-side DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: By negatively engaging the full-bridge submodule, the receiving-end high-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

18. The DC transient overvoltage suppression system according to claim 13, characterized in that, Based on the AC fault flag, determine the control strategy for the receiving-end low-end converter station, including: If SigL is 0, the receiving-end low-end converter station does not enter the AC fault ride-through strategy, but adopts the steady-state control strategy of the receiving-end high-end converter station. Under the action of the control system at any time t, the hybrid MMC upper arm is in operation at time t. Sub-modules, The lower bridge arm is being put into operation. Sub-modules, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: The receiving-end low-end converter station, through the positive input submodule, maintains a constant DC voltage while adjusting the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value. The calculation formula is as follows: Where, δ H (t) represents the internal potential of the AC voltage at the receiving end of the low-end converter station. With system voltage The included angle, This represents the active power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the reactive power exchanged between the receiving-end low-end converter station and the AC power grid. This represents the equivalent reactance between the receiving-end low-end converter station and the AC power grid; If SigL is 1, the receiving-end low-end converter station enters the AC fault ride-through strategy. The d-axis power loop switches from steady-state control strategy to stator module capacitor voltage control, and the DC bias loop switches from steady-state control strategy to constant DC voltage control strategy. The DC voltage command value is calculated as follows, where k is a safety margin coefficient between 0 and 1, and the calculation formula is as follows: The receiving-end low-end converter station actively reduces the high-end DC voltage by negatively engaging the full-bridge submodule. Under the action of the DC bias loop at any time t, the upper arm of the hybrid MMC is positively engaged at time t. Sub-modules, Lower bridge arm negative input A full-bridge sub-module, The average voltage of the submodule capacitor is DC voltage at the receiving end satisfy: Internal potential of AC voltage at the receiving end satisfy: By negatively engaging the full-bridge submodule, the receiving-end low-end converter station reduces the DC voltage and adjusts the AC internal potential to control the AC power exchanged between the converter station and the AC grid to a set value.

19. A computer device, characterized in that, include: One or more processors; A processor is used to execute one or more programs; When the one or more programs are executed by the one or more processors, the method described in any one of claims 1-9 is implemented.

20. A computer-readable storage medium, characterized in that, It contains a computer program, which, when executed, implements the method as described in any one of claims 1-9.

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