Direct-current fault ride-through method suitable for large-scale wind power transmission system

Abstract

The invention discloses a direct-current fault ride-through method suitable for a large-scale wind power transmission system, and relates to the technical field of high-voltage flexible direct-current power transmission. The method comprises the following steps: firstly, determining a structure of a bipolar MMC-SESS sending-out system based on large-scale wind power and an MMC-SESS topology; secondly, analyzing direct-current fault characteristics of a large-scale wind power output system through a bipolar MMC-SESS; then respectively providing control strategies for an MMC part and an SESS part in the MMC-SESS at the sending end; then providing system fault ride-through strategies under different DC fault types and designing energy storage system parameters; and finally, a simulation model is built in the PSCAD / EMTDC, simulation is carried out, and the provided control method is verified. According to the invention, the surplus power of the wind power plant can be consumed, and stable operation of a large-scale wind power output system converter and stable DC side voltage are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage flexible direct current (HVDC) transmission technology, and in particular to a DC fault ride-through method applicable to large-scale wind power transmission systems. More specifically, it relates to a DC fault ride-through solution for large-scale wind power transmission systems based on a modular multilevel converter with supercapacitor energy storage system (MMC-SESS). Background Art

[0002] As an important component of renewable energy power generation, wind power generation has gradually increased its penetration rate in the power grid and has become the most widely used form of renewable energy power generation. However, traditional wind power has obvious disadvantages in terms of reactive power compensation requirements and commutation failure when connected to the grid on a large scale through AC lines or conventional DC. One of the characteristics of wind power development in my country is that wind farms are usually far away from load centers, requiring long-distance power transmission. Compared with AC or conventional DC grid connection technology, modular multilevel converter based high voltage direct current (MMC-HVDC), as a form of flexible DC transmission, is particularly suitable for large-scale wind power transmission and is the current mainstream direction of multi-terminal flexible DC transmission development. As the technology continues to mature and its application scope expands, MMC technology will play an increasingly important role in renewable energy grid connection, grid stability improvement, and stable management of the entire power system.

[0003] As power system voltage levels and transmission distances increase, overhead lines have become the preferred option for long-distance, high-capacity DC transmission due to their economical efficiency. However, these lines also suffer from a high failure rate. In flexible DC transmission systems, DC faults can have a more immediate and severe impact on the grid. Currently, my country has operationalized several large-scale wind power transmission projects via flexible DC transmission, primarily using overhead lines for transmission. Therefore, improving the system's DC fault ride-through capability has become a research priority to ensure safe and stable grid operation.

[0004] In order to improve the energy utilization efficiency of the system and ensure the stable operation capability of the system during DC faults, the present invention proposes a DC fault ride-through method suitable for large-scale wind power transmission systems. Summary of the Invention

[0005] The purpose of the present invention is to propose a DC fault ride-through method suitable for large-scale wind power transmission systems to solve the problems raised in the background technology. Compared with the existing technology, the present invention can absorb the surplus power of the wind farm and ensure the stable operation of the converter and the DC side voltage stability of the large-scale wind power transmission system.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A DC fault ride-through method applicable to a large-scale wind power transmission system comprises the following steps:

[0008] S1. Establish the structure of the large-scale wind power transmission system via bipolar MMC-SESS and the MMC-SESS topology;

[0009] S2. After a fault occurs, analyze the DC fault characteristics of the large-scale wind power transmission system via the bipolar MMC-SESS;

[0010] S3, propose control strategies for MMC and SESS in the sending-end MMC-SESS respectively;

[0011] S4. Consider the requirements of DC fault ride-through strategies, propose fault ride-through strategies for different DC fault types and design energy storage system parameters;

[0012] S5. Based on the contents of S1 to S4, a simulation model was built in PSCAD / EMTDC to verify the proposed control strategy.

[0013] Preferably, the S1 specifically includes the following contents:

[0014] The structure of the bipolar MMC-SESS transmission system is as follows: the positive and negative MMCs are both half-bridge MMCs, the wind turbine generator set adopts a permanent magnet direct-drive wind turbine generator set, and a DC circuit breaker is installed on the DC outlet side of the double-end positive and negative MMCs;

[0015] The MMC-SESS topology is as follows: SESS is connected in parallel with the MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter. The equivalent capacitance C SESS Expressed as:

[0016] C SESS =N p C SC / N s

[0017] Where C SC is the capacitance value of a single supercapacitor; R SC is the equivalent series resistance; N p is the number of parallel branches in SESS; N sThe number of supercapacitors in series in each branch; by accurately configuring N s and N p value to meet the system's requirements for energy storage capacity and power output.

[0018] Preferably, the S2 specifically includes the following contents:

[0019] When a single-pole grounding fault occurs on the DC side, the DC voltage at the fault pole drops to zero, and the AC system and submodule capacitors inject current into the fault point, among which the short-circuit current injected by the submodule capacitor plays a dominant role. As the submodule capacitor voltage drops, the short-circuit current injected by the AC system begins to increase. At this time, the DC side fault is quickly isolated before the fault pole MMC is locked to maintain the stability of the grid connection point voltage.

[0020] Preferably, the S3 specifically includes the following contents:

[0021] (3.1) Sending-end MMC control strategy

[0022] During the steady-state operation of the system, the sending-end bipolar MMC adopts a master-slave control strategy, which enables the bipolar MMCs to collaboratively establish the wind farm AC voltage, achieve synchronous control, and evenly distribute the active and reactive power input from the wind farm to the bipolar MMCs.

[0023] After a single-pole ground fault occurs on the DC side of the system, the DCCB responds quickly and disconnects the DC-side overhead line of the faulty pole. The faulty pole MMC cannot transmit power. The control mode of the sending-end MMC during the fault is as follows:

[0024] 3.1.1) Before the fault type is determined, the faulty pole MMC adopts grid-forming control to maintain the stability of the wind farm's AC voltage and frequency; the non-faulty pole MMC adopts grid-following control to control active power and AC voltage;

[0025] 3.1.2) When the fault type is determined to be a permanent fault, the non-fault pole MMC adopts grid-forming control to maintain the stability of the wind farm's AC voltage and frequency; the fault pole MMC adopts grid-following control to control the stability of the DC and AC voltages;

[0026] 3.1.3) When the fault type is determined to be a non-permanent fault, the system's sending-end bipolar MMC returns to the master-slave control strategy to re-achieve balanced power distribution between the bipolar MMCs;

[0027] (3.2) Sending-end SESS control strategy

[0028] During the steady-state operation of the system, the SESS has sufficient throughput capacity to cope with the next fault ride-through. The actual SOC value of the SESS in steady state is calculated as follows:

[0029]

[0030] Where, SOC SESS is the actual SOC value of SESS in steady state;

[0031] Assuming the throughput capacity of SESS is consistent, the maximum energy that SESS can absorb is W SESSab Equal to its maximum releasable energy W SESSre Based on this, the U of SESS in steady-state operation is determined SESSref , W SESSab and W SESSre Calculated by the following formula:

[0032]

[0033] Where U SESSref W SESSab =W SESSre When , the voltage of SESS;

[0034] In the event of a system failure, if U dcw Exceeding the maximum allowable operating voltage U dcmax Or lower than the minimum allowable operating voltage U dcmin , SESS switches to DC voltage control mode to maintain the DC side voltage stability of the system; in other cases, SESS is in SOC balance control mode, adopting the SOC balance optimization control strategy based on discrete time domain predictive power model;

[0035] When the fault is cleared and the system returns to steady state, if U SESS ≠U SESSref , then the SESS switches to the SESS voltage control mode and exchanges energy with the DC side to adjust U SESS to U SESSref To ensure that sufficient energy storage capacity is reserved for the next fault ride-through; if U SESS =U SESSref , the SESS remains in the SOC balance control mode to maintain the SOC balance among the SESSs.

[0036] Preferably, the S4 specifically includes the following contents:

[0037] (4.1) Fault ride-through strategy: Permanent and non-permanent fault control strategies for DC single-pole grounding faults include:

[0038] 4.1.1) When the system detects a DC fault, the DCCB of the fault pole will act quickly to isolate the fault;

[0039] 4.1.2) Before the fault type is determined, the sending-end bipolar MMC adopts the control strategy described in 3.1.1), and the SESS adopts the fault control strategy described in (3.2), and the system begins fault ride-through;

[0040] 4.1.3) After determining the fault type, if it is a non-permanent fault, the DCCB recloses after the fault ends, and the sending-end MMC-SESS returns to the steady-state control strategy. If it is a permanent fault, the sending-end MMC adopts the control strategy described in 3.1.2), and the SESS continues to adopt the fault control strategy described in (3.2);

[0041] (4.2) Energy storage system parameter design:

[0042] 4.2.1) Filter inductor parameter design

[0043] When SESS is connected to the sending-end MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter, the SESS series filter inductor L SESS , which is designed as follows:

[0044]

[0045] Where U SM is the capacitor voltage of the MMC submodule at the sending end; ΔI SESS is the maximum value of the SESS current change; f is the carrier frequency of the PWM modulation of the non-isolated bidirectional Buck / Boost converter;

[0046] 4.2.2) SESS parameter design

[0047] SESS by N p Number N s The series branches formed by connecting supercapacitors in series are connected in parallel to determine the number of supercapacitors in series N. s and the number of parallel branches N p , the specific calculation method is as follows:

[0048] Number of series connections N s :

[0049]

[0050] Where, Indicates rounding up;

[0051] Number of series branches in parallel N p :

[0052] The power of SESS is equivalent to the rated power of wind farm P wN Capacity, N p satisfy:

[0053]

[0054] Where, P SCN is the rated power of the supercapacitor monomer;

[0055] SESS has an absorption system SESS The capacity of the surplus energy during the worst fault period is expressed as:

[0056] W SESSmax =P wN t SESS (6)

[0057] At this time, N p satisfy:

[0058]

[0059] Where W SCmax is the maximum capacity of a single supercapacitor, which is calculated using the following formula:

[0060]

[0061] In summary, in order to satisfy both power and capacity constraints, N p The values ​​are:

[0062]

[0063] Compared with the existing technology, the present invention provides a DC fault ride-through method applicable to large-scale wind power transmission systems, which has the following beneficial effects:

[0064] The present invention proposes a DC fault ride-through method suitable for large-scale wind power transmission systems. Compared with the existing technology, the present invention is simpler and easier to implement, and is convenient for solving the power shock problem in the high-voltage power grid; the present invention can absorb the surplus power of the wind farm, ensure the stable operation of the converter and the DC side voltage stability of the large-scale wind power transmission system; and can also realize the storage power to be re-transmitted back to the power grid after the fault ends, thereby improving the utilization efficiency of wind power and enhancing the overall economy of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a flow chart of a DC fault ride-through method applicable to a large-scale wind power transmission system mentioned in Example 1 of the present invention;

[0066] Figure 2 This is a schematic diagram of the large-scale wind power transmission system via a bipolar MMC-SESS mentioned in Example 1 of the present invention;

[0067] Figure 3 This is the MMC-SESS topology diagram mentioned in Example 1 of the present invention;

[0068] Figure 4 This is the submodule capacitor discharge mechanism during a DC side fault mentioned in Example 1 of the present invention;

[0069] Figure 5 The fault type mentioned in Example 1 of the present invention does not determine the MMC control strategy at the forwarding end;

[0070] Figure 6 This is the permanent fault sending-end MMC control strategy mentioned in Example 1 of the present invention;

[0071] Figure 7 This is the sending-end SESS control block diagram mentioned in Example 1 of the present invention;

[0072] Figure 8 This is the fault ride-through strategy for different fault types mentioned in Example 1 of the present invention;

[0073] Figure 9 This is the simulation result under the permanent fault condition mentioned in Example 1 of the present invention;

[0074] Figure 10 This is the simulation result under the non-permanent fault condition mentioned in Example 1 of the present invention. DETAILED DESCRIPTION

[0075] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0076] Example 1:

[0077] See also Figure 1 The present invention proposes a DC fault ride-through method applicable to large-scale wind power transmission systems, which specifically includes the following contents:

[0078] Step 1: Establish the system structure and MMC-SESS topology based on large-scale wind power transmission via bipolar MMC-SESS:

[0079] Based on the large-scale wind power transmission system structure via bipolar MMC-SESS Figure 2 The positive and negative MMCs are both half-bridge MMCs, and the wind turbine generator set uses a gearbox-free, simple-structured, and low-failure permanent magnet direct-drive wind turbine generator set. A direct current circuit breaker (DCCB) capable of rapid disconnection is installed on the DC output side of both the positive and negative MMC terminals.

[0080] In the energy storage MMC solution of the present invention, the SESS is connected in parallel with the MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter, such as Figure 3 In this configuration, U dcp and U dcn Refers to the positive and negative DC side voltages at the sending end, respectively. This topology gives the SESS a high degree of modularity, allowing the expansion of energy storage capacity by increasing the number of submodules and ensuring sufficient redundancy in the system. Given that the rated voltage of a single supercapacitor usually does not exceed 3V, the SESS in actual applications is composed of multiple supercapacitors connected in series and parallel. Assuming the capacitance of a single supercapacitor is C SC , the equivalent series resistance is R SC , the number of parallel branches in SESS is N p , the number of supercapacitors in series in each branch is N s , then the equivalent capacitance C of SESS SESS It can be expressed as C SESS =N p C SC / N s By precisely configuring N s and N p The value can meet the system's specific requirements for energy storage capacity and power output.

[0081] Step 2: Analyze the DC fault characteristics of the large-scale wind power transmission system via bipolar MMC-SESS:

[0082] against Figure 2 In the example shown, large-scale wind power is transmitted through a true bipolar MMC-SESS system. Once a single-pole ground fault occurs on the DC side, the DC voltage at the fault pole rapidly drops to zero. The AC system and submodule capacitors inject current into the fault point, with the short-circuit current injected by the submodule capacitors playing a dominant role. As the submodule capacitor voltage drops, the short-circuit current injected by the AC system begins to increase. From the perspective of the wind farm, this is equivalent to a three-phase short-circuit fault on the AC side, causing a severe drop in the grid connection point voltage, threatening the safe and stable operation of the wind farm. Therefore, it is necessary to quickly isolate the DC side fault before the fault pole MMC is locked to maintain grid connection point voltage stability and prevent wind turbines from disconnecting from the grid. In engineering practice, DCCBs can quickly isolate faults within 6ms, effectively curbing the sharp rise in fault current. Combined with switching the converter station control strategy, this can effectively control voltage stability and ensure continuous operation of wind turbines without disconnection.

[0083] However, after fault isolation, the wind farm's output power cannot be rapidly reduced, and the non-fault MMC's power transmission capacity is insufficient. Consequently, unbalanced power will accumulate in the fault-pole MMC at the system's sending end, causing overvoltage in its submodule capacitors and compromising the stability of the entire system. Therefore, a reasonable control strategy is needed to absorb the unbalanced power and maintain stable system operation. The following section examines fault ride-through control strategies for two different DC fault types.

[0084] Step 3: Propose control strategies for the MMC and SESS parts of the sending-end MMC-SESS respectively:

[0085] Since the probability of single-pole grounding fault on the DC side of the HVDC system is the highest, the present invention studies the permanent and non-permanent fault types under this fault condition.

[0086] In a true bipolar MMC transmission system, when a single-pole ground fault occurs on a DC line, the MMC on the faulted pole cannot transmit power, while the unfaulted pole continues to operate normally. In this situation, it is necessary to coordinate control strategies between the wind farm, MMC, and SESS to jointly absorb the wind farm's surplus power that the unfaulted pole MMC cannot transmit, thereby ensuring stable system operation during the DC fault.

[0087] against Figure 2 The DC side single-pole grounding fault of the system shown in the figure is shown in the figure. This step will introduce in detail the fault ride-through strategy based on MMC-SESS under different single-pole grounding fault types. The specific contents are as follows.

[0088] (1) DCCB reclosing strategy

[0089] Figure 2 In the system shown, a single-pole grounding fault occurs on the DC side and the DCCB operates, disconnecting the DC line at the fault pole. The fault type is determined based on the principle of refracted and reflected waves at the fault point. The refracted and reflected waves are related to the line parameters.

[0090] The DC system's automatic reclosing scheme draws on the same principles as the AC system. After the DCCB operates, it waits for a brief DC line de-energization period (approximately 200ms) before determining the fault type. If the fault is determined to be non-permanent, the double-ended DCCB recloses, gradually restoring system operation. If the fault is determined to be permanent, the DCCB remains open, ensuring rapid power restoration in the case of non-permanent faults and protecting the system in the case of permanent faults.

[0091] (2) Sending-end MMC-SESS control strategy

[0092] The present invention uses subscript p to represent the positive electrode variable, and subscript n to represent the negative electrode variable.

[0093] 1. Sending-end MMC control strategy

[0094] During the steady-state operation of the system, the sending-end bipolar MMC adopts a master-slave control strategy, which enables the bipolar MMCs to collaboratively establish the wind farm AC voltage, achieve synchronous control (i.e., zero-difference control), and evenly distribute the active and reactive power input from the wind farm to the bipolar MMCs.

[0095] After a single-pole ground fault occurs on the DC side of the system, the DCCB responds quickly, disconnecting the DC-side overhead line at the faulty pole. The faulty pole MMC is unable to transmit power. To address this issue, the control mode of the sending-end MMC during the fault is as follows:

[0096] (1) Before the fault type is determined, the sending end MMC adopts Figure 5 The control strategy shown is as follows: the fault pole MMC adopts grid-forming control to maintain the stability of the AC voltage and frequency of the wind farm; the non-fault pole MMC adopts grid-following control to control the active power and AC voltage. wd and u wq are the d-axis and q-axis components of the AC voltage of the wind farm respectively; u wdref and u wqref are the reference values ​​of the d-axis and q-axis components of the AC voltage of the wind farm respectively; P wmax is the maximum output power of the non-fault MMC; i wdref1 、i wqref1 、i wdref2 、i wqref2 are the reference values ​​of active and reactive currents of the fault pole and non-fault pole respectively.

[0097] (2) When the fault type is determined to be a permanent fault, the sending end MMC adopts Figure 6 The control strategy shown is as follows: the non-fault pole MMC adopts grid-forming control to maintain the stability of the wind farm AC voltage and frequency; the fault pole MMC adopts grid-following control to control the stability of DC voltage and AC voltage. dcw and U dcref They are the actual value and reference value of the DC side voltage of the MMC at the sending end respectively.

[0098] (3) When the fault type is determined to be a non-permanent fault, the system sending-end bipolar MMC returns to the master-slave control strategy to re-achieve balanced power distribution between the bipolar MMCs.

[0099] 2. Sending-end SESS control strategy

[0100] The control strategy of the sending-end SESS proposed in the present invention is as follows: Figure 7 As shown, there are three control modes, where N is the number of MMC submodules; P SESSref is the power reference value for SOC balance control, P SESS is the actual power of SESS; U SESSref and USESS are the voltage reference value and actual value of SESS respectively; I SESSref is the SESS control inner loop current reference value, I SESSj is the actual current value of the jth SESS.

[0101] In the event of a system failure, if U dcw Exceeding the maximum allowable operating voltage U dcmax (In this solution, the reference voltage U is set to 1.1 times dcref ) or lower than the minimum allowable operating voltage U dcmin (In this solution, the reference voltage U is set to 0.9 times dcref ), the SESS will switch to Mode 1; in other cases, the SESS is in Mode 2, adopting the SOC balance optimization control strategy based on the discrete time domain predictive power model.

[0102] In order to ensure that the SESS has sufficient throughput capacity to cope with the next fault ride-through when the system is running in steady state, the actual SOC value of the SESS in steady state must be determined, that is, SOC SESS .SOC SESS The calculation method of is given by formula (1). From formula (1), it can be seen that SOC SESS You can use U SESS To express:

[0103]

[0104] Where U SESS 、U SESSmax and U SESSmin They are the current voltage, maximum operating voltage and minimum operating voltage of SESS respectively.

[0105] This scheme sets the throughput capacity of SESS to be consistent, that is, the maximum energy that SESS can absorb W SESSab Should be equal to its maximum releasable energy W SESSre Based on this, the U of SESS in steady-state operation can be determined SESSref , W SESSab and W SESSre It can be calculated by formula (2):

[0106]

[0107] Where U SESSref W SESSab =W SESSre When , the voltage of SESS. Combining equations (1) and (2), we can get: U SESSref ≈0.59kV, so this scheme uses the voltage reference value U of SESS in steady state SESSrefSet to 0.59kV.

[0108] When the fault is cleared and the system returns to steady state, if U SESS ≠U SESSref , then the SESS switches to mode 3 and exchanges energy with the DC side to adjust U SESS to U SESSref , to ensure that sufficient energy storage capacity is reserved for the next fault ride-through. SESS =U SESSref , the SESS remains in Mode 2 to maintain SOC balance among the SESSs.

[0109] Step 4: Propose system fault ride-through strategies under different DC fault types and design energy storage system parameters:

[0110] (1) Fault ride-through strategy

[0111] After the above analysis, the permanent and non-permanent fault control strategies of the present invention for DC single-pole grounding faults are as follows: Figure 8 shown.

[0112] (1) When the system detects a DC fault, the fault pole DCCB will act quickly to isolate the fault.

[0113] (2) Before the fault type is determined, the sending end bipolar MMC adopts Figure 5 The control strategy shown in the figure is that SESS adopts Figure 7 Fault control strategy, the system starts fault ride-through.

[0114] (3) After determining the fault type, if it is a non-permanent fault, the DCCB recloses after the fault ends, and the sending-end MMC-SESS returns to the steady-state control strategy; if it is a permanent fault, the sending-end MMC adopts Figure 6 Control strategy, SESS continues to adopt Figure 7 Fault control strategy.

[0115] (2) Energy storage system parameter design

[0116] 1. Filter inductor parameter design

[0117] like Figure 3 As shown in the figure, when SESS is connected to the sending-end MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter, SESS needs to be connected in series with a filter inductor L SESS , its design is shown in formula (3).

[0118]

[0119] Where U SM is the capacitor voltage of the MMC submodule at the sending end, ΔI SESSis the maximum value of the SESS current change, and f is the carrier frequency of the PWM modulation of the non-isolated bidirectional Buck / Boost converter.

[0120] 2. SESS parameter design

[0121] like Figure 3 As shown, SESS needs to be composed of N p Number N s The series branches formed by connecting supercapacitors in series are connected in parallel, so the number of supercapacitors in series N needs to be determined. s and the number of parallel branches N p .

[0122] (1) Number of series connections N s

[0123] In order to avoid overcharge and overdischarge of supercapacitors, this solution sets the rated voltage of supercapacitor monomer U SCN Set to its maximum operating voltage U SCmax , 0.3U SCN Set to its minimum operating voltage U SCmin Considering the duty cycle limitation of non-isolated bidirectional Buck / Boost converter, the rated submodule capacitor voltage is set to 0.8U dcref / N is its maximum operating voltage U SESSmax , then 0.24U dcref / N is the minimum operating voltage U SESSmin .

[0124] To ensure U SESS In line with the above design requirements, N s It can be calculated by formula (4).

[0125]

[0126] Where, Indicates rounding up.

[0127] (2) Number of parallel series branches N p

[0128] As shown in formula (4), the number of supercapacitors connected in series is N s Depends on the SESS voltage limit, so the number of series branches connected in parallel is N p The power and capacity limitations of the system should be met.

[0129] In order to meet the requirement of system fault ride-through under the worst fault condition, the power of SESS should be able to receive the rated power P of wind farm. wN The ability to do this p Should meet

[0130]

[0131] Where, P SCN is the rated power of the supercapacitor cell.

[0132] In addition, the SESS should have an absorption system SESS The capacity of the surplus energy during the worst fault period is expressed as:

[0133] W SESSmax =P wN t SESS (6)

[0134] At this time, N p Should meet the following requirements:

[0135]

[0136] Where W SCmax is the maximum capacity of a single supercapacitor, which can be calculated by formula (8):

[0137]

[0138] In summary, in order to satisfy both power and capacity constraints, N p The value should be:

[0139]

[0140] Step 5: Build a simulation model in PSCAD / EMTDC and perform simulation to verify the proposed control strategy:

[0141] Build the following in PSCAD / EMTDC simulation software Figure 2 The large-scale wind power transmission system shown here is delivered via a double-terminal true bipolar MMC-SESS. Simulations verify the proposed permanent and non-permanent fault ride-through strategies. To align with the research scope of this proposal, the cables used at the Rudong wind farm are replaced with overhead lines. A frequency-dependent model is employed to better analyze the system's transient characteristics. The supercapacitors used are MaxWell's BCAP0100 P300 S17 models.

[0142] Among them, the receiving-end bipolar MMC adopts constant DC voltage and constant reactive power control to maintain DC side voltage stability, and the control mode of the receiving-end MMC remains unchanged during system DC faults.

[0143] (1) Permanent failure

[0144] In order to verify the effectiveness of the fault ride-through strategy under permanent fault, a permanent DC side positive pole single-pole grounding fault occurs when the system is running at t=3s. The DCCB is disconnected 6ms after the fault occurs to isolate the fault. The simulation results are shown in Figure 2. Figure 9 shown.

[0145] Figure 9 (a)-(c) to Figure 9 (e)-(g) show the AC voltage, power, DC voltage of the MMC-SESS at the sending end and the SESS voltage U after the system failure. SESS Waveform: From the beginning of the fault to the determination of the fault type, the sending end MMC adopts Figure 5 From the control strategy shown in the figure, it can be seen that the AC voltage at the sending end is stable, the power output of the wind farm fluctuates slightly with the AC voltage, the non-fault pole MMC maintains the maximum power output, the DC side voltage at the sending end is within the stable operating range, and the fault pole SESS at the sending end absorbs the wind farm power that the non-fault pole MMC cannot transmit; after the system determines that the fault type is a permanent fault, the wind farm cuts off the wind turbines corresponding to the non-fault pole that cannot transmit power, and the sending end MMC adopts Figure 6 The control strategy shown is that the MMC at the non-faulty sending end sets the AC side voltage and frequency and sends the wind farm power. The DC side voltage of the faulty MMC returns to the rated value. The SESS maintains the control strategy during the fault period. Since there is no surplus power after the system stabilizes, U SESS No longer changes.

[0146] Figure 9 (f) and Figure 9 Figure (g) shows the power and DC voltage of the receiving-end MMC, respectively: from the onset of the fault to the determination of the fault type, the receiving-end fault-pole MMC absorbs power from the receiving-end grid to maintain the DC side voltage back to the rated value; after the system determines that the fault type is a permanent fault, the DC voltages of both the receiving-end fault-pole and non-fault-pole MMCs return to the rated value, and at the same time, all the power sent to the sending-end is delivered by the receiving-end non-fault-pole MMC.

[0147] (2) Non-permanent faults

[0148] To verify the effectiveness of the fault ride-through strategy under non-permanent faults, a single-pole ground fault on the positive DC side of 0.2s occurs when the system is running at t=3s. The DCCB is disconnected 6ms after the fault to isolate the fault. The simulation results are shown in Figure 2. Figure 10 shown.

[0149] Figure 10 (a) to Figure 10 (e) shows the MMC-SESS AC voltage, power, DC voltage and SESS voltage U at the sending end after the system failure. SESSWaveform: From the beginning of the fault to the determination of the fault type, the system sending end MMC adopts Figure 5 From the control strategy shown, it can be seen that the AC voltage at the sending end is stable, the power output of the wind farm fluctuates slightly with the AC voltage, the non-fault pole MMC maintains the maximum power output, the DC side voltage at the sending end is within the stable operating range, and the fault pole SESS at the sending end absorbs the wind farm power that the non-fault pole MMC cannot transmit. After the system determines that the fault type is a non-permanent fault, the DCCB recloses, the sending end MMC returns to the master-slave pole control, the active power of the sending end bipolar MMC is evenly distributed, the DC side voltage returns to the rated value, and at the same time, the sending end SESS exchanges power with the MMC to make U SESS Regression reference value.

[0150] Figure 10 (f) and Figure 10 (g) shows the input power and DC voltage of the receiving-end MMC respectively: from the beginning of the fault to the determination of the fault type, the receiving-end fault-pole MMC absorbs power from the receiving-end grid to maintain the DC side voltage back to the rated value; after the system determines that the fault type is a non-permanent fault, the DCCB recloses, and the active power of the receiving-end bipolar MMC is reduced by the sending-end SESS voltage U SESS During the power exchange between the system and the reference value, the receiving MMC power is different. SESS After returning to the reference value, the active power is evenly distributed in the receiving-end bipolar MMC, and the DC voltage returns to the rated value.

[0151] From step 1 to step 5, the previous step is the basis for the execution of the next step. These five steps are closely linked to each other and executed sequentially, forming an organic and indivisible whole.

[0152] It should be noted that the contents not described in detail in the embodiments of the present invention belong to the prior art known to those skilled in the art.

[0153] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims. The information disclosed in the background technology section of the present invention is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that the information constitutes prior art already known to those skilled in the art.

Claims

1. A DC fault ride-through method applicable to large-scale wind power transmission systems, characterized in that: The steps include: S1. Establish the structure of the large-scale wind power transmission system via bipolar MMC-SESS and the MMC-SESS topology; S2. After a fault occurs, analyze the DC fault characteristics of the large-scale wind power transmission system via the bipolar MMC-SESS; S3, propose control strategies for MMC and SESS in the sending-end MMC-SESS respectively; S4. Consider the requirements of DC fault ride-through strategies, propose fault ride-through strategies for different DC fault types and design energy storage system parameters; S5. Based on the contents of S1 to S4, a simulation model was built in PSCAD / EMTDC to verify the proposed control strategy.

2. A DC fault ride-through method applicable to a large-scale wind power transmission system according to claim 1, characterized in that: The S1 specifically includes the following contents: The structure of the bipolar MMC-SESS transmission system is as follows: the positive and negative MMCs are both half-bridge MMCs, the wind turbine generator set adopts a permanent magnet direct-drive wind turbine generator set, and a DC circuit breaker is installed on the DC outlet side of the double-end positive and negative MMCs; The MMC-SESS topology is as follows: SESS is connected in parallel with the MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter. The equivalent capacitance C SESS Expressed as: C SESS =N p C SC / N s Where C SC is the capacitance value of a single supercapacitor; R SC is the equivalent series resistance; N p is the number of parallel branches in SESS; N s The number of supercapacitors in series in each branch; by accurately configuring N s and N p value to meet the system's requirements for energy storage capacity and power output.

3. A DC fault ride-through method applicable to a large-scale wind power transmission system according to claim 2, characterized in that: The S2 specifically includes the following contents: When a single-pole grounding fault occurs on the DC side, the DC voltage at the fault pole drops to zero, and the AC system and submodule capacitors inject current into the fault point, among which the short-circuit current injected by the submodule capacitor plays a dominant role. As the submodule capacitor voltage drops, the short-circuit current injected by the AC system begins to increase. At this time, the DC side fault is quickly isolated before the fault pole MMC is locked to maintain the stability of the grid connection point voltage.

4. A DC fault ride-through method applicable to a large-scale wind power transmission system according to claim 3, characterized in that: The S3 specifically includes the following contents: (3.1) Sending-end MMC control strategy During the steady-state operation of the system, the sending-end bipolar MMC adopts a master-slave control strategy, which enables the bipolar MMCs to collaboratively establish the wind farm AC voltage, achieve synchronous control, and evenly distribute the active and reactive power input from the wind farm to the bipolar MMCs. After a single-pole ground fault occurs on the DC side of the system, the DCCB responds quickly and disconnects the DC-side overhead line of the faulty pole. The faulty pole MMC cannot transmit power. The control mode of the sending-end MMC during the fault is as follows: 3.1.1) Before the fault type is determined, the faulty pole MMC adopts grid-forming control to maintain the stability of the wind farm's AC voltage and frequency; the non-faulty pole MMC adopts grid-following control to control active power and AC voltage; 3.1.2) When the fault type is determined to be a permanent fault, the non-fault pole MMC adopts grid-forming control to maintain the stability of the wind farm's AC voltage and frequency; the fault pole MMC adopts grid-following control to control the stability of the DC and AC voltages; 3.1.3) When the fault type is determined to be a non-permanent fault, the system's sending-end bipolar MMC returns to the master-slave control strategy to re-achieve balanced power distribution between the bipolar MMCs; (3.2) Sending-end SESS control strategy During the steady-state operation of the system, the SESS has sufficient throughput capacity to cope with the next fault ride-through. The actual SOC value of the SESS in steady state is calculated as follows: Where, SOC SESS is the actual SOC value of SESS in steady state; Assuming the throughput capacity of SESS is consistent, the maximum energy that SESS can absorb is W SESSab Equal to its maximum releasable energy W SESSre Based on this, the U of SESS in steady-state operation is determined SESSref , W SESSab and W SESSre Calculated by the following formula: Where U SESSref W SESSab =W SESSre When , the voltage of SESS; In the event of a system failure, if U dcw Exceeding the maximum allowable operating voltage U dcmax Or lower than the minimum allowable operating voltage U dcmin , SESS switches to DC voltage control mode to maintain the DC side voltage stability of the system; in other cases, SESS is in SOC balance control mode, adopting the SOC balance optimization control strategy based on discrete time domain predictive power model; When the fault is cleared and the system returns to steady state, if U SESS ≠U SESSref , then the SESS switches to the SESS voltage control mode and exchanges energy with the DC side to adjust U SESS to U SESSref To ensure that sufficient energy storage capacity is reserved for the next fault ride-through; If U SESS =U SESSref , the SESS remains in the SOC balance control mode to maintain the SOC balance among the SESSs.

5. A DC fault ride-through method applicable to a large-scale wind power transmission system according to claim 4, characterized in that: The S4 specifically includes the following contents: (4.1) Fault ride-through strategy: Permanent and non-permanent fault control strategies for DC single-pole grounding faults include: 4.1.1) When the system detects a DC fault, the DCCB of the fault pole will act quickly to isolate the fault; 4.1.2) Before the fault type is determined, the sending-end bipolar MMC adopts the control strategy described in 3.1.1), and the SESS adopts the fault control strategy described in (3.2), and the system begins fault ride-through; 4.1.3) After determining the fault type, if it is a non-permanent fault, the DCCB will reclose after the fault ends, and the sending-end MMC-SESS will return to the steady-state control strategy; If it is a permanent fault, the sending-end MMC adopts the control strategy in 3.1.2) and the SESS continues to adopt the fault control strategy described in (3.2); (4.2) Energy storage system parameter design: 4.2.1) Filter inductor parameter design When SESS is connected to the sending-end MMC submodule capacitor through a non-isolated bidirectional Buck / Boost converter, the SESS series filter inductor L SESS , which is designed as follows: Where U SM is the capacitor voltage of the MMC submodule at the sending end; ΔI SESS is the maximum value of SESS current change; f is the carrier frequency of the PWM modulation of the non-isolated bidirectional Buck / Boost converter; 4.2.2) SESS parameter design SESS by N p Number N s The series branches formed by connecting supercapacitors in series are connected in parallel to determine the number of supercapacitors in series N. s and the number of parallel branches N p , the specific calculation method is as follows: Number of series connections N s : Where, Indicates rounding up; Number of series branches in parallel N p : The power of SESS is equivalent to the rated power of wind farm P wN Capacity, N p satisfy: Where, P SCN is the rated power of the supercapacitor monomer; SESS has an absorption system SESS The capacity of the surplus energy during the worst fault period is expressed as: W SESSmax =P wN t SESS (6) At this time, N p satisfy: Where W SCmax is the maximum capacity of a single supercapacitor, which is calculated using the following formula: In summary, in order to satisfy both power and capacity constraints, N p The values ​​are:

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