Fault ride-through coordinated control method, system and equipment for flexible direct current power transmission system with energy margin of fusion sub-module

Abstract

The present application relates to the technical field of power system protection and flexible DC power transmission protection, and specifically discloses a kind of flexible DC power transmission system fault ride-through cooperative control method, system and equipment fusing submodule energy margin.The scheme is on one hand with three-dimensional fault safety domain accurately quantifying the accommodation potential of unbalanced power and system control boundary, avoids the risk of DC voltage out-of-limit due to fuzzy control boundary, guarantees the safety and stability of DC system during fault;On the other hand, by adaptively switching control strategy, while ensuring the safety of DC voltage, the collaborative optimization of maximizing reactive power support and minimizing load shedding is realized, reducing equipment loss and energy waste, and making full use of the transient energy of submodule capacitor, realizing the collaborative accommodation of unbalanced energy, improving the fault ride-through capability under complex conditions, which can well ensure the safe operation of flexible DC power transmission system, and significantly improve the efficiency and economy of system executing fault ride-through control.

Description

Technical Field

[0001] This invention relates to the fields of power system protection and flexible DC transmission protection technology, specifically to a collaborative control method, system, and equipment for fault ride-through of flexible DC transmission systems that integrates submodule energy margins. Background Technology

[0002] Flexible DC transmission, with its significant advantages in long-distance, large-capacity power transmission and large-scale renewable energy consumption, has become a key technology for power system interconnection. However, when a short-circuit fault occurs in the receiving-end AC grid, the power transmission capacity of the receiving-end converter station to the AC side drops sharply, resulting in a severe transient power imbalance between the sending and receiving ends. This triggers a rapid rise in DC voltage, threatening the safety of power electronic devices and even causing system shutdown and triggering a chain reaction.

[0003] To address the aforementioned issues, existing fault ride-through strategies primarily include: adjusting the active power reference value at the sending end via communication links, coordinating voltage reduction and load shedding of wind turbines at the sending end, and activating DC unloading devices. However, sending-end coordinated control is highly dependent on the reliability and real-time performance of communication and expands the scope of fault impact; while DC unloading offers a fast response, centralized unloading is prone to triggering impact thermal stress, and distributed unloading still faces severe heat dissipation pressure and significant energy loss. Therefore, unloading should be minimized during fault ride-through.

[0004] On the other hand, adjusting the active and reactive power distribution at the receiving-end converter station can mitigate some of the unbalanced power. However, due to AC current limitations, simply increasing active power weakens the reactive voltage support for the AC grid, while simply increasing reactive power exacerbates DC voltage increases. In practical engineering, the submodule capacitors of modular multilevel converters typically have a certain voltage margin, which can be used as short-term energy storage to absorb and temporarily store unbalanced DC power. However, most existing studies only focus on the single utilization of the submodule energy margin, lacking a method for global quantification and dynamic coordination with the power control and load shedding of the receiving-end converter station. If the submodule charging power is too high, the energy may be exhausted prematurely before the fault is cleared, forcing it to be put into load shedding; if the charging power is too low, the DC voltage may exceed the limit due to insufficient active power at the receiving-end converter station. Therefore, existing methods are still insufficient to maximize reactive power support and minimize load shedding while ensuring DC voltage safety.

[0005] Therefore, in flexible DC transmission systems, when the receiving-end AC grid experiences a short-circuit fault, how to coordinate the power of the converter station and the energy of the submodules to avoid DC overvoltage while ensuring DC voltage safety, maximize reactive power support and minimize unloading input, and improve fault ride-through control efficiency has become an important research direction. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a collaborative control method for fault ride-through in flexible DC transmission systems that integrates the energy margin of submodules. The method aims to better coordinate the power of the converter station and the energy of the submodules, thereby maximizing reactive power support and minimizing load shedding input while ensuring system safety, thus improving fault ride-through control efficiency.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a collaborative control method for fault ride-through of a flexible DC transmission system that integrates the energy margin of submodules, comprising the following steps:

[0009] S1. When a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, fault ride-through control is initiated to obtain the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time.

[0010] S2. Calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. And the critical active power that enables the fault-safe domain at the receiving end, taking into account the energy margin of the submodule, to exist. ;

[0011] S3. Based on the maximum active power that the receiving-end converter station can output. With the critical active power The size relationship is used to determine whether the received-end fault safety domain that takes into account the energy margin of the submodule exists under the current fault condition; if the fault safety domain does not exist, execute S4; if the fault safety domain exists, execute S5.

[0012] S4. Calculate the time it takes for the submodule's capacitor to deplete its energy. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At that moment, jump to execute S7; , These are the d-axis current and q-axis current that enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit; The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0013] S5. Set the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Provide reactive power to the grid as much as possible while avoiding DC voltage exceeding limits, and then execute S6; , These are the d-axis and q-axis currents, respectively, that enable the receiving-end converter station to achieve its maximum reactive power output in the receiving-end fault-safe domain, taking into account the energy margin of the submodule. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0014] S6. At the expected activation time of the main protection, if the AC bus voltage of the receiving-end converter station is detected to have recovered to its pre-fault value, the d-axis and q-axis current reference values ​​are set to their pre-fault values, and the submodule charging power reference value is set to 0, thus restoring the flexible DC transmission system to normal operation. If the AC bus voltage of the receiving-end converter station does not recover, the d-axis and q-axis current reference values ​​of the receiving-end converter station are set to... , Then jump to execute S7;

[0015] S7. Set the submodule charging power reference value to 0, and input power equal to... The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

[0016] As a preferred option, in step S2, the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit is determined. Calculate as follows:

[0017] ;

[0018] in, and The d-axis and q-axis currents, respectively, are the currents that maximize the active power of the receiving-end converter station under the maximum permissible AC current limit, and are calculated as follows:

[0019] ;

[0020] ;

[0021] Among them, abbreviated parameter symbols , , They are represented as follows:

[0022] ;

[0023] ;

[0024] ;

[0025] in, The maximum allowable current coefficient is determined by the converter station parameters; This is the rated current of the receiving-end converter station; This represents the equivalent potential amplitude of the AC power grid at the receiving end; and These are the equivalent resistance and equivalent reactance of the virtual short-circuit transition impedance of the receiving-end AC grid, respectively, calculated from the AC bus voltage and AC current measured after the fault at the receiving-end converter station. It represents the phase angle difference between the equivalent electromotive force of the AC power grid and the AC bus voltage of the receiving-end converter station.

[0026] As a preferred embodiment, in step S2, the critical active power of the receiving-end fault safety domain, taking into account the energy margin of the submodule, is determined. Calculate as follows:

[0027] ;

[0028] Among them, abbreviated parameter symbols , They are represented as follows:

[0029] ;

[0030] ;

[0031] in, This refers to the active power input from the sending-end converter station to the DC side; The DC equivalent capacitance; This is the DC overvoltage coefficient; This is the rated DC voltage. This is the DC voltage during normal operation. To communicate the timing of the main protective action; The transient energy margin of the capacitor in the receiving-end converter station submodule is calculated as follows:

[0032] ;

[0033] in, The rated capacitor energy of the capacitor in the receiving-end converter station submodule:

[0034] ;

[0035] in, This represents the maximum allowable value for the capacitor voltage of the submodule. This is the rated voltage of the submodule capacitor; The number of modular multilevel converters in the receiving-end converter station; This is the rated capacitance of the submodule capacitor.

[0036] As a preferred embodiment, in step S3, the receiving-end fault safety domain considering the submodule energy margin under the current fault conditions is a three-dimensional feasible space for the receiving-end converter station's d-axis and q-axis currents and submodule charging power to avoid exceeding the limits of DC voltage and AC current. The existence of the receiving-end fault safety domain considering the submodule energy margin is determined in the following manner:

[0037] like If so, it is determined that the received-end fault safety domain, which takes into account the energy margin of the submodule, exists; if If so, it is determined that the received-end fault safety domain that takes into account the energy margin of the submodule does not exist.

[0038] As a preferred embodiment, in step S4, the d-axis and q-axis currents of the receiving-end converter station are... , The submodule capacitor charging power that enables power balance between the sending and receiving ends under certain conditions. Calculate as follows:

[0039] ;

[0040] Time it takes for the submodule capacitor to run out of energy Calculate as follows:

[0041] ;

[0042] in, This refers to the transient energy margin of the capacitor in the receiving-end converter station submodule.

[0043] As a preferred option, in step S5, the d-axis current that enables the reactive power output of the receiving-end converter station to reach its maximum value is determined within the receiving-end fault-safe domain, taking into account the energy margin of the submodule. and q-axis current This is determined by solving the following system of equations:

[0044] ;

[0045] in, To ensure the critical active power exists in the fault-safe domain of the receiving end, taking into account the energy margin of the submodule.

[0046] As a preferred embodiment, in step S5, the d-axis and q-axis currents of the receiving-end converter station are... , The submodule capacitor charging power that enables power balance between the sending and receiving ends under certain conditions. Calculate as follows:

[0047] ;

[0048] in, This refers to the active power input from the sending-end converter station to the DC side.

[0049] As a preferred embodiment, the submodule charging power control is achieved through a submodule capacitor energy control loop. The submodule capacitor energy control loop takes the submodule charging power reference value as input and adjusts the DC modulation ratio of the hybrid MMC converter station to change the ratio of half-bridge submodules to full-bridge submodules, thereby achieving dynamic absorption and regulation of transient unbalanced energy.

[0050] Secondly, the present invention also provides a flexible DC transmission system fault ride-through cooperative control system that integrates submodule energy margins, comprising:

[0051] The acquisition module is activated to initiate fault ride-through control when a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, and to acquire the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time.

[0052] The first calculation module is used to calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. , and the critical active power that makes the receiving-end fault safety domain, which takes into account the energy margin of the submodule, exist;

[0053] The judgment module is used to determine the maximum active power that the receiving-end converter station can output. With the critical active power The size relationship is used to determine whether the received-end fault safety domain that takes into account the energy margin of the submodule exists under the current fault condition; if the fault safety domain does not exist, the first control module is triggered; if the fault safety domain exists, the second control module is triggered.

[0054] The first control module is used to calculate the time when the capacitor energy of the submodule is depleted. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At any given moment, the unloading and input module is triggered; , These are the d-axis current and q-axis current that enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit; The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0055] The second control module is used to set the d-axis and q-axis current reference values ​​of the receiving-end converter station to the specified values ​​at the time of a fault. , Set the submodule charging power reference value to To provide reactive power to the grid as much as possible while avoiding DC voltage over-limit, and then trigger the backup protection control module; , These are the d-axis and q-axis currents, respectively, that enable the receiving-end converter station to achieve its maximum reactive power output in the receiving-end fault-safe domain, taking into account the energy margin of the submodule. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0056] The backup protection control module, at the expected activation time of the main protection, if it detects that the AC bus voltage of the receiving-end converter station has recovered to its pre-fault value, sets the d-axis and q-axis current reference values ​​to their pre-fault values ​​and sets the submodule charging power reference value to 0, thus restoring the flexible DC transmission system to normal operation. If the AC bus voltage of the receiving-end converter station has not recovered, it sets the d-axis and q-axis current reference values ​​of the receiving-end converter station to their pre-fault values. , And trigger the unloading and input module;

[0057] The unloading and activation module is used to set the submodule's charging power reference value to 0 and activate the power equal to... The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

[0058] Thirdly, the present invention also provides a flexible DC transmission system fault ride-through collaborative control device with integrated submodule energy margin, characterized in that it includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor executes the above-mentioned flexible DC transmission system fault ride-through collaborative control method with integrated submodule energy margin when executing the computer program.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] 1. The present invention utilizes the control degrees of freedom of the receiving-end converter station and the overvoltage margin of the sub-module capacitors within the hybrid MMC to construct a three-dimensional fault safety domain that integrates the transient energy margin of the sub-module capacitors, the AC current limit of the converter station, and the DC voltage safety threshold. This accurately quantifies the potential for unbalanced power absorption and the system control boundary. Therefore, by determining whether the safety domain exists, the collaborative control strategy of adaptively switching the current reference value of the receiving-end converter station and the charging power of the sub-modules is adopted. This avoids the DC voltage exceeding the limit of the flexible DC transmission system during three-phase short-circuit faults in the receiving-end AC grid and ensures DC voltage safety. At the same time, it minimizes unloading input and provides reactive power support to the receiving-end AC grid, maximizing reactive power support and minimizing unloading input. This achieves the collaborative absorption of unbalanced energy and improves fault ride-through control efficiency.

[0061] 2. The present invention utilizes a constructed fault-safe domain to quantify the absorption potential of unbalanced power and the system control boundary. When the fault-safe domain exists, coordinated control enables the receiving-end converter station to operate at the maximum voltage support point, achieving maximum reactive power support for the AC grid without engaging any unloading devices, thus avoiding energy waste and thermal stress damage and heat dissipation pressure on the unloading resistors. When the fault-safe domain does not exist, the receiving-end converter station is controlled to output maximum active power, and the charging power of the sub-modules is dynamically matched, achieving maximum internal absorption of DC unbalanced energy. This not only delays the engagement time of the unloading device but also effectively reduces the peak value of the required unloading power, reduces thermal stress damage and heat dissipation pressure on the unloading resistors, and significantly improves the safety and economy of the system.

[0062] 3. The present invention, on the one hand, accurately quantifies the potential for unbalanced power absorption and the system control boundary using a three-dimensional fault-safe domain, avoiding the risk of DC voltage exceeding limits due to fuzzy control boundaries, and ensuring the safety and stability of the DC system during faults; on the other hand, through an adaptive switching control strategy, while ensuring DC voltage safety, it achieves synergistic optimization of maximizing reactive power support and minimizing unloading input, reducing equipment losses and energy waste, and making full use of the transient energy of submodule capacitors to achieve synergistic absorption of unbalanced energy. This improves the fault ride-through capability of the flexible DC transmission system under complex operating conditions when the receiving-end AC grid experiences a short-circuit fault, effectively ensuring the safe operation of the flexible DC transmission system, and significantly improving the efficiency and economy of the system in executing fault ride-through control, thus having excellent technical application prospects. Attached Figure Description

[0063] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0064] Figure 1This is a flowchart of the fault ride-through collaborative control method for flexible DC transmission systems that integrates sub-module energy margins according to the present invention;

[0065] Figure 2 This is a schematic diagram of the architecture of the flexible DC transmission system fault ride-through collaborative control system with integrated sub-module energy margin of the present invention;

[0066] Figure 3 This is a simulation system topology diagram used in an embodiment of the present invention;

[0067] Figure 4 This is a comparison diagram of the simulated electrical quantities of the receiving-end converter station when the safety domain exists in an embodiment of the present invention;

[0068] Figure 5 This is a comparison diagram of the simulated electrical quantities of the receiving-end converter station when the safety domain does not exist in the embodiments of the present invention. Detailed Implementation

[0069] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but only to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0070] Firstly, addressing the shortcomings of existing technologies, this invention provides a collaborative control method for fault ride-through in flexible DC transmission systems that integrates submodule energy margins. The core idea is to utilize the control degrees of freedom of the receiving-end converter station and the overvoltage margin of the submodule capacitors within the hybrid MMC to construct a three-dimensional fault safety domain that integrates the transient energy margin of the submodule capacitors, the AC current limit of the converter station, and the DC voltage safety threshold. By determining whether the safety domain exists, the collaborative control strategy of the receiving-end converter station current reference value and the submodule charging power is adaptively switched. Through the collaborative control of the receiving-end converter station power and the submodule capacitor charging power, while avoiding DC voltage over-limit during three-phase short-circuit faults in the receiving-end AC grid, the method minimizes unloading input and provides reactive power support to the receiving-end AC grid, thereby achieving collaborative absorption of unbalanced energy.

[0071] Based on the above technical design concept, the present invention provides a collaborative control method for fault ride-through of flexible DC transmission systems that integrates submodule energy margins. The specific process is as follows: Figure 1As shown, it includes the following steps:

[0072] S1. When a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, fault ride-through control is initiated to obtain the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time.

[0073] The status data after a fault occurs in the receiving-end AC power grid mainly includes the AC voltage, AC current, DC voltage, submodule capacitance and voltage of the receiving-end converter station, and the power of the sending-end converter station. Using Thevenin's equivalent, a three-phase short-circuit fault at any location in the power grid is equivalent to a virtual short-circuit fault occurring at the AC bus, which is used to calculate the equivalent transition resistance and reactance in real time.

[0074] S2. Calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. And the critical active power that enables the fault-safe domain at the receiving end, taking into account the energy margin of the submodule, to exist. .

[0075] In practical implementation, under the current fault conditions and the maximum allowable AC current limit, the maximum active power that the receiving-end converter station can output is... Calculate as follows:

[0076] ;

[0077] in, and The d-axis and q-axis currents, respectively, are the currents that maximize the active power of the receiving-end converter station under the maximum permissible AC current limit, and are calculated as follows:

[0078] ;

[0079] ;

[0080] in, , , The abbreviated parameter symbols are represented as follows:

[0081] ;

[0082] ;

[0083] ;

[0084] in, The maximum allowable current coefficient is determined by the converter station parameters; This is the rated current of the receiving-end converter station; This represents the equivalent potential amplitude of the AC power grid at the receiving end; and These are the equivalent resistance and equivalent reactance of the virtual short-circuit transition impedance of the receiving-end AC grid, respectively, calculated from the AC bus voltage and AC current measured after the fault at the receiving-end converter station. It represents the phase angle difference between the equivalent electromotive force of the AC power grid and the AC bus voltage of the receiving-end converter station.

[0085] The critical active power that enables the fault-safe domain at the receiving end, taking into account the energy margin of the submodule, to exist is... Calculate as follows:

[0086] ;

[0087] Among them, abbreviated parameter symbols , They are represented as follows:

[0088] ;

[0089] ;

[0090] in, This refers to the active power input from the sending-end converter station to the DC side; The DC equivalent capacitance; This is the DC overvoltage coefficient; This is the rated DC voltage. This is the DC voltage during normal operation. To communicate the timing of the main protective action; The transient energy margin of the capacitor in the receiving-end converter station submodule is calculated as follows:

[0091] ;

[0092] in, The rated capacitor energy of the capacitor in the receiving-end converter station submodule:

[0093] ;

[0094] in, This represents the maximum allowable value for the capacitor voltage of the submodule. This is the rated voltage of the submodule capacitor; The number of modular multilevel converters (MMCs) in the receiving-end converter station; This is the rated capacitance of the submodule capacitor.

[0095] S3. Based on the maximum active power that the receiving-end converter station can output. With the critical active power The magnitude relationship is used to determine whether the received-end fault safety domain, which takes into account the energy margin of the submodule, exists under the current fault conditions.

[0096] The receiving-end fault safety domain, taking into account the submodule energy margin under the current fault conditions, is a three-dimensional feasible space for the receiving-end converter station's d-axis and q-axis currents and submodule charging power to avoid exceeding DC voltage and AC current limits. In specific implementation, the existence of the receiving-end fault safety domain, taking into account the submodule energy margin, is determined as follows:

[0097] like If the fault safety domain of the receiving end, which takes into account the energy margin of the submodule, exists, it means that there is at least one combination of the d-axis and q-axis currents of the receiving end converter station and the charging power of the submodule, which can prevent the DC voltage and AC current of the receiving end converter station from exceeding the limits; at this time, jump to execute S5.

[0098] like If the fault safety domain of the receiving end, which takes into account the energy margin of the submodule, does not exist, it means that there is no combination of the d-axis and q-axis currents of the receiving end converter station and the charging power of the submodule, which can simultaneously prevent the DC voltage and AC current of the receiving end converter station from exceeding the limit; at this time, execute S4.

[0099] S4. Control strategy when the fault-safe domain does not exist.

[0100] When the fault-safe domain does not exist, relying solely on the energy regulation of the receiving-end converter station and submodules is insufficient to withstand the main protection operation time. In this case, the time for the submodule capacitor energy to deplete should be calculated first. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At that moment, jump to execute S7.

[0101] in, , These are the d-axis current and q-axis current, respectively, which enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit. Their calculation method has been introduced in step S2 above. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power that enables power balance between the sending and receiving ends can be calculated using the power fed into the safe sending-end converter station and the maximum active power of the receiving-end converter station as follows:

[0102] ;

[0103] Time it takes for the submodule capacitor to run out of energy In charging power The time required for the submodule capacitor voltage to reach its maximum allowable value is calculated as follows:

[0104] ;

[0105] exist At a certain point, the submodule's energy margin is exhausted. At this time, it is necessary to jump to S7 to trigger unloading and implement fault ride-through control. This strategy maximizes internal energy absorption, delays the unloading activation time, and reduces the unloading peak.

[0106] S5. Control strategy when a fault-safe domain exists.

[0107] When a fault-safe domain exists, the system has sufficient regulation potential. In this case, the d-axis and q-axis current reference values ​​of the receiving-end converter station are set to... , Set the submodule charging power reference value to Provide reactive power to the grid as much as possible while avoiding DC voltage exceeding limits, and then execute S6.

[0108] in, , These are the d-axis and q-axis currents, respectively, that enable the maximum reactive power output of the receiving-end converter station in the fault-safe domain, taking into account the energy margin of the submodule. They are determined by solving the following set of equations:

[0109] ;

[0110] The d-axis and q-axis currents of the receiving-end converter station are , The submodule capacitor charging power that enables power balance between the sending and receiving ends under certain conditions is calculated as follows:

[0111] .

[0112] Under this strategy, the system provides reactive power to the grid as much as possible while avoiding DC voltage exceeding limits, without needing to unload the load. Then, S6 is executed for backup protection judgment and control.

[0113] S6. Backup protection control at the expected moment of main protection operation.

[0114] If the main protection operates correctly at the expected time of operation, detecting that the AC bus voltage of the receiving-end converter station has recovered to its pre-fault value, the d-axis and q-axis current reference values ​​will be set to their pre-fault values, and the submodule charging power reference value will be set to 0, thus restoring the flexible DC transmission system to normal operation. If the main protection fails to operate and the AC bus voltage of the receiving-end converter station does not recover, then to ensure DC safety, the d-axis and q-axis current reference values ​​of the receiving-end converter station will be forcibly set to their pre-fault values. , Then, jump to execute S7, trigger unloading and implement fault crossing control until the fault is cleared by backup protection.

[0115] This strategy involves implementing backup coordinated control to smoothly restore the system to its pre-fault state or forcibly engage unloading based on the recovery status of the AC bus voltage.

[0116] S7, Unloading and Putting into Operation.

[0117] Set the submodule charging power reference value to 0, and input power equal to The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

[0118] In the above processing flow, the submodule charging power control is achieved through the submodule capacitor energy control loop. The submodule capacitor energy control loop takes the submodule charging power reference value as input, and changes the ratio of half-bridge submodules to full-bridge submodules by adjusting the DC modulation ratio of the hybrid MMC converter station. This dynamically transfers unbalanced energy to the full-bridge submodule capacitor, thereby achieving dynamic absorption and precise control of transient unbalanced energy.

[0119] As can be seen from the above process, the fault ride-through collaborative control method for flexible DC transmission systems integrating submodule energy margins of this invention utilizes the control degrees of freedom of the receiving-end converter station and the overvoltage margin of the submodule capacitors within the hybrid MMC to construct a three-dimensional fault safety domain that integrates the transient energy margin of the submodule capacitors, the AC current limit of the converter station, and the DC voltage safety threshold. This accurately quantifies the potential for unbalanced power absorption and the system control boundary. Therefore, by determining whether the safety domain exists, the collaborative control strategy of adaptively switching the current reference value of the receiving-end converter station and the charging power of the submodules is used. This avoids the DC voltage exceeding the limit of the flexible DC transmission system during three-phase short-circuit faults in the receiving-end AC grid and ensures DC voltage safety, while minimizing unloading input and providing reactive power support to the receiving-end AC grid. This maximizes reactive power support and minimizes unloading input, achieving collaborative absorption of unbalanced energy and improving fault ride-through control efficiency.

[0120] Specifically, in this invention, the fault-safe domain is used to quantify the absorption potential of unbalanced power and the system control boundary. When the fault-safe domain exists, the receiving-end converter station is operated at the maximum voltage support point through coordinated control. Without engaging any unloading device, maximum reactive power support for the AC grid is achieved, avoiding energy waste and thermal stress damage and heat dissipation pressure on the unloading resistor. When the fault-safe domain does not exist, the receiving-end converter station is controlled to output maximum active power, and the charging power of the sub-modules is dynamically matched. This achieves maximum internal absorption of DC unbalanced energy, which not only delays the engagement time of the unloading device but also effectively reduces the peak value of the unloading power required, reduces thermal stress damage and heat dissipation pressure on the unloading resistor, and significantly improves the safety and economy of the system.

[0121] In summary, the present invention improves the fault ride-through capability of the receiving-end AC grid in flexible DC transmission systems under complex operating conditions when a short-circuit fault occurs. It can effectively ensure the safe operation of the flexible DC transmission system and significantly improve the efficiency and economy of the system in executing fault ride-through control, thus having a promising future for technical applications.

[0122] Secondly, this invention also proposes a flexible DC transmission system fault ride-through cooperative control system that integrates submodule energy margins. The system architecture is as follows: Figure 2 As shown, it includes:

[0123] The acquisition module is activated to initiate fault ride-through control when a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, and to acquire the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time.

[0124] The first calculation module is used to calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. , and the critical active power that makes the receiving-end fault safety domain, which takes into account the energy margin of the submodule, exist;

[0125] The judgment module is used to determine the maximum active power that the receiving-end converter station can output. With the critical active power The size relationship is used to determine whether the received-end fault safety domain that takes into account the energy margin of the submodule exists under the current fault condition; if the fault safety domain does not exist, the first control module is triggered; if the fault safety domain exists, the second control module is triggered.

[0126] The first control module is used to calculate the time when the capacitor energy of the submodule is depleted. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At any given moment, the unloading and input module is triggered; , These are the d-axis current and q-axis current that enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit; The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0127] The second control module is used to set the d-axis and q-axis current reference values ​​of the receiving-end converter station to the specified values ​​at the time of a fault. , Set the submodule charging power reference value to To provide reactive power to the grid as much as possible while avoiding DC voltage over-limit, and then trigger the backup protection control module; , These are the d-axis and q-axis currents, respectively, that enable the receiving-end converter station to achieve its maximum reactive power output in the receiving-end fault-safe domain, taking into account the energy margin of the submodule. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends;

[0128] The backup protection control module, at the expected activation time of the main protection, if it detects that the AC bus voltage of the receiving-end converter station has recovered to its pre-fault value, sets the d-axis and q-axis current reference values ​​to their pre-fault values ​​and sets the submodule charging power reference value to 0, thus restoring the flexible DC transmission system to normal operation. If the AC bus voltage of the receiving-end converter station has not recovered, it sets the d-axis and q-axis current reference values ​​of the receiving-end converter station to their pre-fault values. , And trigger the unloading and input module;

[0129] The unloading and activation module is used to set the submodule's charging power reference value to 0 and activate the power equal to... The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

[0130] This invention relates to a flexible DC transmission system fault ride-through collaborative control system integrating submodule energy margins. The first calculation module calculates the maximum active power output capacity of the receiving-end converter station based on the current fault conditions and AC current limitations, determining the critical active power required to meet the submodule energy margin requirements. This constructs a three-dimensional fault safety domain integrating the submodule capacitor transient energy margin, converter station AC current limiting, and DC voltage safety threshold, thereby accurately quantifying the potential for unbalanced power absorption and the system control boundary. The judgment module determines the existence of the fault safety domain based on the relationship between the maximum active power output of the receiving-end converter station and the critical active power, and then adaptively triggers different control... Control Strategy: If the fault-safe domain does not exist, the first control module sets the current reference value and submodule charging power with the maximum allowable active power of the receiving-end converter station as the target. It maintains power balance until the submodule capacitor energy is depleted, and then triggers unloading and connection. If the fault-safe domain exists, the second control module prioritizes setting the current reference value with the target of maximizing reactive power support, while ensuring submodule power balance, and then triggers the backup protection control module to dynamically respond to voltage recovery. The backup protection control module flexibly switches to normal operation or unloading and connection mode according to whether the AC bus voltage has recovered. Finally, the unloading and connection module completes energy absorption and system stability control during the fault period.

[0131] As can be seen, the flexible DC transmission system fault ride-through collaborative control system with integrated submodule energy margin is designed to realize the aforementioned flexible DC transmission system fault ride-through collaborative control method with integrated submodule energy margin, and has the corresponding technical advantages of the aforementioned method of this invention.

[0132] Thirdly, the present invention also provides a fault ride-through collaborative control device for a flexible DC transmission system with integrated submodule energy margin, comprising a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it executes the aforementioned fault ride-through collaborative control method for a flexible DC transmission system with integrated submodule energy margin.

[0133] The memory can be any combination of one or more computer-readable media. The computer-readable medium can be a computer-readable signal medium or a computer-readable storage device. The computer-readable storage device can include: an electrical connection having one or more wires, a portable computer disk, a hard disk, random access memory, read-only memory, erasable programmable read-only memory, optical fiber, portable compact disk read-only memory, optical storage device, magnetic storage device, or any suitable combination thereof. In this invention, the computer-readable storage device can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

[0134] The code for a computer program that performs the operations of this invention can be written in one or more programming languages ​​or a combination thereof. Programming languages ​​include object-oriented programming languages ​​such as Java, Smalltalk, and C++, as well as conventional procedural programming languages ​​such as C or similar languages. The program code can be executed entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer can be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computer.

[0135] Example:

[0136] To verify the effectiveness of the method of the present invention, the present invention solution is analyzed and verified through the following embodiments.

[0137] This embodiment uses Figure 3 The simulation system shown is used as an example. In this system, the flexible DC transmission adopts a pseudo-bipolar structure, with a rated voltage of ±100kV, a transmission capacity of 200MVA, a maximum allowable AC current coefficient of 1.2, a maximum allowable voltage coefficient of 1.5 for submodules, and an AC main protection action time of 0.1s. This invention (Method 1), the traditional receiving-end converter station power collaborative control method (Method 2), and the reactive power support priority method (Method 3) are compared.

[0138] When the safety domain exists, a three-phase short circuit occurs in the receiving-end AC network at 0.5s. The voltage drops to 0.6 pu at the moment of the fault, calculated as follows... =0.73 pu, critical power =0.65pu, the safety domain exists. Using method 1 of this invention, the current reference value of the receiving-end converter station is set to the maximum reactive power point (0.92, 0.71 pu), and the submodule charging power is 0.18 pu. From Figure 4 It can be seen that Method 1, without any load shedding, stabilizes the DC voltage within the safe threshold (1.1 pu) before the main protection trips, and the receiving-end converter station outputs 0.71 pu of reactive power, raising the AC bus voltage to 0.73 pu. In contrast, Method 2, due to not utilizing the submodule energy, is forced to shed 0.35 pu of load at 0.57s; Method 3, prioritizing reactive power, results in excessively low active power output, and even with the utilization of submodule energy, it still requires shed 0.20 pu of load at 0.57s.

[0139] When the safety domain does not exist, a critical short circuit occurs at 0.5s. The voltage drops to 0.5pu, and calculations show... = 0.61 pu, critical power = 0.65 pu, the safety domain does not exist. Using method 1 of this invention, the current of the receiving-end converter station is set to the maximum active point (1.05, 0.58 pu), and the submodule charging power is set to 0.32 pu. From Figure 5 As can be seen, this invention prioritizes energy absorption by the submodule capacitor, and only after the energy is depleted in 0.55s does it engage a 0.32pu unloading, successfully supporting the DC voltage. In contrast, method 2 triggers unloading in 0.53s with a power as high as 0.40pu; method 3, due to its extremely low active power absorption, requires a high unloading of 0.56pu after the energy is depleted in 0.53s. The above embodiments fully demonstrate that this invention, by quantifying the safety domain boundary and coordinating the energy of the submodule, can maximize system safety, improve reactive power support, and reduce hardware power consumption under any fault severity.

[0140] Overview:

[0141] This invention discloses a fault ride-through collaborative control method, system, and equipment for flexible DC transmission systems that integrates submodule energy margins. Based on current fault conditions and AC current limitations, it calculates the maximum active power output capacity of the receiving-end converter station, determines the critical active power required to meet the submodule energy margin requirements, and constructs a three-dimensional fault safety domain that integrates the submodule capacitor transient energy margin, converter station AC current limit, and DC voltage safety threshold. This allows for precise quantification of the unbalanced power absorption potential and system control boundaries. Furthermore, based on the relationship between the maximum active power output of the receiving-end converter station and the critical active power, it determines whether the fault safety domain exists. Adaptive triggering of different control strategies: If the fault-safe domain does not exist, the current reference value and submodule charging power are set with the maximum allowable active power of the receiving-end converter station as the target. Power balance is maintained until the submodule capacitor energy is exhausted, and then unloading is triggered. If the fault-safe domain exists, the current reference value is set with maximizing reactive power support as the target, while ensuring submodule power balance. Then, the backup protection control module is triggered to dynamically respond to voltage recovery. Finally, depending on whether the AC bus voltage has recovered, the system flexibly switches to normal operation or unloading mode. Ultimately, the energy absorption and system stability control during the fault period are completed through the unloading module.

[0142] In summary, the present invention, on the one hand, accurately quantifies the potential for unbalanced power absorption and the system control boundary using a three-dimensional fault-safe domain, avoiding the risk of DC voltage exceeding limits due to fuzzy control boundaries and ensuring the safety and stability of the DC system during faults; on the other hand, through an adaptive switching control strategy, it achieves synergistic optimization of maximizing reactive power support and minimizing unloading input while ensuring DC voltage safety, reducing equipment losses and energy waste, and making full use of the transient energy of submodule capacitors to achieve synergistic absorption of unbalanced energy. This improves the fault ride-through capability of the flexible DC transmission system under complex operating conditions during short-circuit faults in the receiving-end AC grid, effectively ensuring the safe operation of the flexible DC transmission system and significantly improving the efficiency and economy of the system in executing fault ride-through control, demonstrating excellent technical application prospects.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A collaborative control method for fault ride-through in flexible DC transmission systems with integrated submodule energy margins, characterized in that, Includes the following steps: S1. When a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, fault ride-through control is initiated to obtain the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time. S2. Calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. And the critical active power that enables the fault-safe domain at the receiving end, taking into account the energy margin of the submodule, to exist. ; S3. Based on the maximum active power that the receiving-end converter station can output. With the critical active power Based on the size relationship, determine whether the received-end fault safety domain considering the energy margin of the submodule exists under the current fault condition; If the fault-safe domain does not exist, proceed to step S4; if the fault-safe domain exists, proceed to step S5. S4. Calculate the time it takes for the submodule's capacitor to deplete its energy. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At that moment, jump to execute S7; , These are the d-axis current and q-axis current that enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit; The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends; S5. Set the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Provide reactive power to the grid as much as possible while avoiding DC voltage exceeding limits, and then execute S6; , These are the d-axis and q-axis currents, respectively, that enable the receiving-end converter station to achieve its maximum reactive power output in the receiving-end fault-safe domain, taking into account the energy margin of the submodule. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends; S6. At the expected activation time of the main protection, if the AC bus voltage of the receiving-end converter station is detected to have recovered to its pre-fault value, the d-axis and q-axis current reference values ​​are set to their pre-fault values, and the submodule charging power reference value is set to 0, thus restoring the flexible DC transmission system to normal operation. If the AC bus voltage of the receiving-end converter station does not recover, the d-axis and q-axis current reference values ​​of the receiving-end converter station are set to... , Then jump to execute S7; S7. Set the submodule charging power reference value to 0, and input power equal to... The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

2. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin as described in claim 1, characterized in that, In step S2, the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit is determined. Calculate as follows: ； in, and The d-axis and q-axis currents, respectively, are the currents that maximize the active power of the receiving-end converter station under the maximum permissible AC current limit, and are calculated as follows: ； ； Among them, abbreviated parameter symbols , , They are represented as follows: ； ； ； in, The maximum allowable current coefficient is determined by the converter station parameters; This is the rated current of the receiving-end converter station; The equivalent potential amplitude of the AC power grid at the receiving end; and These are the equivalent resistance and equivalent reactance of the virtual short-circuit transition impedance of the receiving-end AC grid, respectively, calculated from the AC bus voltage and AC current measured after the fault at the receiving-end converter station. It represents the phase angle difference between the equivalent electromotive force of the AC power grid and the AC bus voltage of the receiving-end converter station.

3. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin as described in claim 2, characterized in that, In step S2, the critical active power that enables the fault-safe domain at the receiving end, taking into account the energy margin of the submodule, is determined. Calculate as follows: ； Among them, abbreviated parameter symbols , They are represented as follows: ； ； in, This refers to the active power input from the sending-end converter station to the DC side; The equivalent capacitance is the DC capacitance. This is the DC overvoltage coefficient; This is the rated DC voltage. This is the DC voltage during normal operation. To communicate the timing of the main protective action; The transient energy margin of the capacitor in the receiving-end converter station submodule is calculated as follows: ； in, The rated capacitor energy of the capacitor in the receiving-end converter station submodule: ； in, This refers to the maximum allowable value of the capacitor voltage in the submodule. This is the rated voltage of the submodule capacitor; The number of modular multilevel converters in the receiving-end converter station; This is the rated capacitance of the submodule capacitor.

4. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin according to claim 1, characterized in that, In step S3, the receiving-end fault safety domain considering the submodule energy margin under the current fault conditions is a three-dimensional feasible space for the receiving-end converter station's d-axis and q-axis currents and submodule charging power to avoid exceeding DC voltage and AC current limits. The existence of the receiving-end fault safety domain considering the submodule energy margin is determined in the following manner: like If so, it is determined that the received-end fault safety domain, which takes into account the energy margin of the submodule, exists; like If so, it is determined that the received-end fault safety domain that takes into account the energy margin of the submodule does not exist.

5. The fault ride-through collaborative control method for flexible DC transmission systems with integrated submodule energy margin as described in claim 3, characterized in that, In step S4, the d-axis and q-axis currents of the receiving-end converter station are: , The submodule capacitor charging power that enables power balance between the sending and receiving ends under certain conditions. Calculate as follows: ； Time it takes for the submodule capacitor to run out of energy Calculate as follows: ； in, This refers to the transient energy margin of the capacitor in the receiving-end converter station submodule.

6. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin according to claim 2, characterized in that, In step S5, the d-axis current in the receiving-end fault-safe domain, which takes into account the energy margin of the submodule, is the current that maximizes the reactive power output of the receiving-end converter station. and q-axis current This is determined by solving the following system of equations: ； in, To ensure the critical active power exists in the fault-safe domain of the receiving end, taking into account the energy margin of the submodule.

7. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin according to claim 6, characterized in that, In step S5, the d-axis and q-axis currents at the receiving-end converter station are... , The submodule capacitor charging power that enables power balance between the sending and receiving ends under certain conditions. Calculate as follows: ； in, This refers to the active power input from the sending-end converter station to the DC side.

8. The fault ride-through cooperative control method for flexible DC transmission systems with integrated submodule energy margin according to claim 1, characterized in that, The submodule charging power control is achieved through the submodule capacitor energy control loop. The submodule capacitor energy control loop takes the submodule charging power reference value as input and adjusts the DC modulation ratio of the hybrid MMC converter station to change the ratio of half-bridge submodules to full-bridge submodules, thereby achieving dynamic absorption and regulation of transient unbalanced energy.

9. A flexible DC transmission system fault ride-through collaborative control system with integrated submodule energy margin, characterized in that, include: The acquisition module is activated to initiate fault ride-through control when a fault occurs in the AC grid at the receiving end of the flexible DC transmission system, and to acquire the operating parameters of the flexible DC transmission system and the status data of the AC grid at the receiving end after the fault occurs in real time. The first calculation module is used to calculate the maximum active power that the receiving-end converter station can output under the current fault conditions and the maximum allowable AC current limit. , and the critical active power that makes the receiving-end fault safety domain, which takes into account the energy margin of the submodule, exist; The judgment module is used to determine the maximum active power that the receiving-end converter station can output. With the critical active power The size relationship is used to determine whether the received-end fault safety domain that takes into account the energy margin of the submodule exists under the current fault condition; if the fault safety domain does not exist, the first control module is triggered; if the fault safety domain exists, the second control module is triggered. The first control module is used to calculate the time when the capacitor energy of the submodule is depleted. And start timing, setting the d-axis and q-axis current reference values ​​of the receiving-end converter station to... , Set the submodule charging power reference value to Until the countdown ends At any given moment, the unloading and input module is triggered; , These are the d-axis current and q-axis current that enable the active power of the receiving-end converter station to reach its maximum value under the maximum allowable AC current limit; The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends; The second control module is used to set the d-axis and q-axis current reference values ​​of the receiving-end converter station to the specified values ​​at the time of a fault. , Set the submodule charging power reference value to To provide reactive power to the grid as much as possible while avoiding DC voltage over-limit, and then trigger the backup protection control module; , These are the d-axis and q-axis currents, respectively, that enable the receiving-end converter station to achieve its maximum reactive power output in the receiving-end fault-safe domain, taking into account the energy margin of the submodule. The d-axis and q-axis currents of the receiving-end converter station are , Under certain conditions, the submodule capacitor charging power can achieve power balance between the sending and receiving ends; The backup protection control module, at the expected activation time of the main protection, if it detects that the AC bus voltage of the receiving-end converter station has recovered to its pre-fault value, sets the d-axis and q-axis current reference values ​​to their pre-fault values ​​and sets the submodule charging power reference value to 0, thus restoring the flexible DC transmission system to normal operation. If the AC bus voltage of the receiving-end converter station has not recovered, it sets the d-axis and q-axis current reference values ​​of the receiving-end converter station to their pre-fault values. , And trigger the unloading and input module; The unloading and activation module is used to set the submodule's charging power reference value to 0 and activate the power equal to... The load is unloaded until the fault is cleared, thereby implementing fault ride-through control.

10. A fault ride-through collaborative control device for flexible DC transmission systems with integrated submodule energy margins, characterized in that, The system includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein, when executing the computer program, the processor performs a collaborative control method for fault ride-through of a flexible DC transmission system with fused submodule energy margin as described in any one of claims 1 to 8.

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