Overvoltage control and protection method for submodules in symmetrical bipolar flexible DC transmission system

Abstract

The present invention discloses a method for controlling overvoltage of submodules in a symmetrical bipolar flexible direct current transmission system. The method comprises the following steps: step 1: replacing the valve top and neutral line arresters with controllable arresters; step 2: improving the overvoltage bypass of the submodule; in the unlocked state of the converter, the overvoltage bypass of the submodule adopts the set value u set1 ; In the inverter locked state, the submodule overvoltage bypass setting value is u set2 ; Step 3: The controllable lightning arrester and the submodule overvoltage bypass cooperate to control overvoltage. The present invention overcomes the problem of overvoltage generated by the bridge arm submodule after the valve side single-phase grounding fault is locked in the prior art, and cannot effectively suppress and reduce the overvoltage of the MMC submodule; it has the advantages of being able to effectively suppress and reduce the overvoltage of the MMC submodule after locking, and the submodule bypass switch does not need to be closed, which can reduce the maintenance workload of the submodule and accelerate the recovery of the DC voltage after the fault.

Description

Technical Field

[0001] The present invention relates to the field of direct current transmission technology, and more specifically, it is a method for controlling overvoltage of a submodule of a symmetrical bipolar flexible direct current transmission system. More specifically, it is a method for controlling overvoltage of an MMC submodule after a converter station of a symmetrical bipolar flexible direct current transmission system is locked. Background Art

[0002] In recent years, with the development of power electronics technology, modular multilevel converter (MMC) has become the preferred topology for high-voltage and high-power transmission due to its low power consumption and low output harmonics. As a new type of voltage source converter (VSC), MMC-HVDC (modular multilevel converter type high-voltage direct current transmission) system has no commutation failure problem, which is one of the most important advantages compared with the traditional thyristor-based line commutated converter (LCC). However, with the development of economy and technology, people have higher and higher requirements for the transmission capacity and transmission reliability of flexible direct current transmission system, so some scholars have proposed a bipolar flexible direct current transmission system. The transmission capacity of the bipolar flexible direct current transmission system is twice that of the unipolar transmission capacity, and the operation mode is more flexible. When a unipolar fault occurs in the bipolar flexible direct current transmission system, the non-fault pole can still operate normally and transmit half of the rated power, thereby greatly improving the transmission reliability.

[0003] As a station fault, the single-phase grounding fault on the valve side is less likely to occur, but it will cause a large overvoltage in the entire system. In a bipolar flexible DC transmission system that uses a metal return line as a return line, after a single-phase grounding occurs on the valve side, the fault phase voltage is 0, and the non-fault phase voltage rises to the line voltage. In steady-state operation, there is a DC bias on the AC side voltage. Therefore, after a fault, because the AC side voltage can be negative, the bridge arm will be subjected to a maximum voltage of the difference between the DC voltage and the AC voltage, and the bridge arm submodule capacitor will be quickly charged and boosted. After the bridge arm is locked, due to the unidirectional conductivity of the diode, the submodule capacitor can only be charged and boosted. Therefore, the single-phase grounding fault on the valve side will cause the bridge arm submodule capacitor to generate a maximum overvoltage of about 1.866 times, which is a huge threat to the submodule capacitor.

[0004] Tang Guangfu et al. proposed a submodule overvoltage protection control method in their invention patent "A method for overvoltage control protection of MMC flexible DC submodules". The three-level protection for submodule overvoltage faults is achieved through valve base controller, submodule controller and breakdown diode protection. The method controls the bypass switch to cut off the submodule from the circuit to protect the submodule, but lacks measures to limit overvoltage, and the fault current flows through the bypass switch, which has high requirements on the starting capacity of the bypass switch, thus limiting its application.

[0005] Zhou Yang et al. studied the phenomenon that AC circuit breaker cannot be disconnected due to AC current bias and sub-module of faulty station is continuously charged, resulting in sub-module overvoltage in the “In-station Ground Fault Protection Strategy for Bipolar Flexible DC Transmission System” in the Journal of Electrical Engineering. The method locked the converter and triggered the sub-module bypass thyristor, and triggered the lower arm sub-module of the non-fault phase to the cut-off state to isolate the AC side fault and reduce the sub-module overvoltage. However, this method did not fundamentally eliminate the DC side fault energy. After using this method, the sub-module still had an overvoltage about 1.5 times higher.

[0006] Mian Wang et al. studied the voltage recovery and balancing strategy of the positive and negative poles of the bipolar system in "Pole Rebalancing Methods for Pole-to-Ground Faults in Symmetrical Monopolar HVDC Grids" in "IEEE transactions on power delivery". The DC voltage recovery was achieved by discharging the fault energy after the fault was cleared through the configuration of a reasonable DBS device. However, this method has no effect on the transient characteristics during the fault period and only promotes the recovery phase of the system after the fault. Therefore, this method is not suitable for the protection and control of the overvoltage of the submodule after the lockout.

[0007] In general, there is currently a lack of methods that can effectively suppress and reduce the overvoltage of the MMC submodule in the flexible DC transmission system after the valve-side single-phase grounding fault occurs. Therefore, it is urgent to develop a method to protect the submodule during the entire fault period. Summary of the invention

[0008] The purpose of the present invention is to provide a symmetrical bipolar flexible direct current transmission system submodule overvoltage control protection method, which is used to suppress the overvoltage of the submodule after the MMC is locked when a single-phase grounding fault occurs on the valve side of the converter station, so that the submodule avoids the risk of overvoltage after the converter station is locked; the method of the present invention has a simple principle, a wide range of application scenarios, and has great use value in engineering.

[0009] In order to achieve the above-mentioned object, the technical scheme of the present invention is: a symmetrical bipolar flexible direct current transmission system submodule overvoltage control protection method, characterized in that: the grounding point of the bipolar MMC-HVDC transmission system is set on the DC side; the bipolar MMC-HVDC transmission system includes a power transmission end and a power receiving end; wherein the power transmission end and the power receiving end both include a positive electrode and a negative electrode; the positive and negative electrodes are connected through a metal return line and a grounding resistor, and the grid-side winding of the transformer is connected to a circuit breaker;

[0010] The post-lockout submodule overvoltage control protection method specifically includes the following steps:

[0011] Step 1: Replace the valve top and neutral line arresters with controllable arresters;

[0012] The controllable lightning arrester comprises a controllable part and a fixed part; wherein the controllable part is connected in parallel with the switch;

[0013] Step 2: Improve the overvoltage bypass of the submodule;

[0014] In the inverter unlocked state, the submodule overvoltage bypass adopts the set value u set1 ;

[0015] In the inverter blocking state, the submodule overvoltage bypass setting value is u set2 ;

[0016] Among them, u set1 ≤u set2 ;

[0017] Step 3: The controllable lightning arrester and the submodule overvoltage bypass cooperate to control overvoltage;

[0018] 1) The valve base controller monitors the MMC system in real time. When a fault in the MMC system is detected, the valve base overpressure protection controller is turned off. When the MMC system is not faulty and the valve base controller detects an overpressure fault in a single submodule, the submodule overvoltage bypasses the corresponding faulty submodule.

[0019] 2) When a single-phase grounding fault occurs on the valve side, the MMC controller quickly locks the converter station and closes the controllable circuit breaker switch;

[0020] 3) When it is detected that the submodule voltage is higher than the submodule controller overvoltage control protection setting, the submodule overvoltage bypasses the overvoltage submodule.

[0021] In the above technical solution, in step 1, the controllable part and the fixed part of the controllable lightning arrester use the same valve plate, and their normalized volt-ampere characteristic curves are the same.

[0022] In the above technical solution, in step 1, the rated voltage of the fixed part is determined as follows:

[0023] k ref U N ≤u s (1)

[0024]

[0025] Where: k ref U is the ratio of the arrester's initial operating voltage to the rated voltage. N is the rated voltage of the arrester, u s is the peak value of the AC system line voltage, t blc is the system locking time, t AC is the AC circuit breaker disconnection moment, u arr (t) is the voltage across the arrester at time t, i arr (t) is the current flowing through the arrester at time t, E lim It is the limit of energy absorbed by the arrester.

[0026] In the above technical solution, in step 1, when the bipolar MMC-HVDC transmission system is operating normally, the controllable lightning arrester maintains the overall input state unchanged;

[0027] When a single-phase grounding fault occurs on the valve side, the converter station sends a blocking command after detecting the fault, and simultaneously closes the switch of the controllable part of the controllable lightning arrester and connects the fixed part to the circuit;

[0028] When the AC side circuit breaker is disconnected, the switch of the controllable lightning arrester is disconnected to accelerate the attenuation of the fault current.

[0029] In the above technical solution, in step 2, u set1 Take the original stator module bypass switch overvoltage controller control protection setting value;

[0030] u set2 The determination method is as follows: after selecting the rated voltage of the fixed part of the controllable lightning arrester, it is adjusted according to the maximum overvoltage that may occur in the submodule after the actual MMC is locked. The overvoltage can be determined through simulation, and a certain margin is given on this basis.

[0031] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0032] (1) The present invention replaces the valve protection arrester with a controllable arrester under the existing arrester configuration of the power transmission system, provides a discharge path for the fault energy on the DC side, and avoids the problem of charging and boosting the submodule capacitor through the AC side on the DC side after the MMC is locked;

[0033] (2) The improvement measure proposed in the present invention of raising the threshold for the submodule to perform overvoltage bypass protection in the locked state can avoid the submodule from bypassing by mistake to the greatest extent, prevent the large fault current on the DC side from flowing through the bypass switch to increase the pressure on the switch device, and at the same time reduce the maintenance workload after the converter station is locked and shut down;

[0034] (3) The improved configuration scheme with controllable lightning arrester proposed in the present invention provides a discharge channel for the metal return line and the DC pole line, which helps to quickly restore the DC voltage after the fault is cleared.

[0035] The present invention transforms the valve top and neutral line arrester into a controllable arrester, and avoids the submodule from generating overvoltage after the MMC is locked by coordinating the submodule bypass control strategy and the control strategy of the controllable arrester. At the same time, the controllable arrester provides a fault energy release path after the fault is cleared, which is helpful for the recovery of the DC voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Flowchart of the control and protection method of the present invention.

[0037] Figure 2 It is a schematic diagram of the structure of the bipolar flexible direct current transmission system of the present invention.

[0038] Figure 3 This is a schematic diagram of the structure of the controllable lightning arrester of the present invention.

[0039] Figure 4 This is the volt-ampere characteristic curve of the controllable lightning arrester of the present invention.

[0040] Figure 5 It is a schematic diagram of the switching strategy of the controllable lightning arrester of the present invention.

[0041] Figure 6 It is a schematic diagram of the coordination of the submodule overvoltage bypass strategy and the controllable lightning arrester switching strategy of the present invention.

[0042] Figure 7 This is a schematic diagram of the DC cable structure of the present invention.

[0043] Figure 8 This is a schematic diagram of the waveform of the sum of the capacitor voltages of the upper and lower bridge arm submodules of phase A before and after the controllable lightning arrester used in the present invention.

[0044] Fig. 9 This is a schematic diagram of the waveform of the sum of the capacitor voltages of the upper and lower bridge arm submodules of the B phase before and after the controllable lightning arrester used in the present invention.

[0045] Fig.10 This is a schematic diagram of the waveform of the sum of the capacitor voltages of the upper and lower bridge arm submodules of the C phase before and after the controllable lightning arrester used in the present invention.

[0046] Fig.11This is a schematic diagram of the DC side healthy pole voltage waveform before and after the controllable lightning arrester is used in the present invention.

[0047] Figure 2 In the figure, A represents AC circuit breaker; B represents transformer; C represents valve side; D represents positive pole; E represents negative pole; F represents metal return line; and F represents metal return line grounding resistance.

[0048] Figure 3 In the figure, MOA1 represents the fixed part of the controllable lightning arrester; MOA2 represents the controllable part of the controllable lightning arrester; K represents the power electronic switch.

[0049] Figure 7 In the figure, A represents conductor; B represents insulation I; C represents sheath; and D represents insulation II. DETAILED DESCRIPTION

[0050] The following is a detailed description of the implementation of the present invention in conjunction with the accompanying drawings, but they do not constitute a limitation of the present invention and are only given as examples. At the same time, the advantages of the present invention are made clearer and easier to understand through the description.

[0051] The present invention is applied to a symmetrical bipolar flexible direct current transmission system using a metal return line as a return line; the present invention transforms the valve top arrester and the neutral line arrester on the protection valve side into controllable arresters, and appropriately increases the overvoltage control protection setting value of the submodule after locking. When a single-phase grounding fault on the valve side is locked, the submodule will not have overvoltage, and the bypass switch of the submodule does not need to be closed, which can reduce the maintenance workload of the submodule and accelerate the recovery of the DC voltage after the fault.

[0052] Referring to the attached drawings, it can be seen that: a symmetrical bipolar flexible direct current transmission system submodule overvoltage control protection method, the grounding point of the bipolar MMC-HVDC transmission system is set on the DC side, the DC side grounding point only provides potential clamping, and does not provide a DC current path; the bipolar MMC-HVDC transmission system includes a power transmission end and a power receiving end; the power transmission end completes AC-DC conversion through the MMC and transmits electric energy to the receiving end, and the receiving end completes the DC-AC conversion and outputs it through the MMC; wherein the power transmission end and the receiving end both include a positive pole and a negative pole; the positive and negative poles are connected by a metal return line and a grounding resistor, and the grid-side winding of the transformer is connected to a circuit breaker (such as Figure 2 As shown); the unbalanced current of positive and negative electrodes returns through the metal return line;

[0053] Since the mechanism of charging and boosting after the submodule is locked has nothing to do with the power direction before locking, that is, this phenomenon exists whether the power is at the sending end or the receiving end, and the structures of the power sending end and the power receiving end are symmetrical after locking, the present invention is only described for the power sending end, and the overvoltage control protection method of the submodule after locking at the power receiving end is the same as that of the power sending end;

[0054] Submodule overvoltage control protection method after locking (such as Figure 1 As shown), it specifically includes the following steps:

[0055] Step 1: Replace the valve top and neutral line arresters with controllable arresters;

[0056] Among them, the valve top arrester is installed between the bridge arm reactor and the converter valve to protect the converter station from the harm of the intrusion wave from the DC side; and the neutral line arrester is installed between the metal return line and the converter valve to limit the overvoltage intruding from the neutral line to the converter station;

[0057] The controllable arrester includes a controllable part and a fixed part; the controllable part is connected in parallel with the switch, and the switch is composed of power electronic devices; the structure of the controllable arrester is as follows Figure 3 As shown, the volt-ampere characteristic curve of the controllable lightning arrester is as follows Figure 4 As shown, by controlling the action of the switch device, the arrester is in the whole on state during normal operation, and only the fixed part remains connected to the circuit after the converter station is locked in the event of a fault;

[0058] The sum of the rated voltages of the controllable and fixed parts of the two types of arresters and their respective volt-ampere characteristic curves are the same as those of the original arresters;

[0059] The selection of the rated voltage of the fixed part of the controllable arrester needs to take into account the limitations of the AC side voltage and the energy absorbed by the arrester, so as to avoid the risk of overvoltage in the submodule and avoid the arrester absorbing too much energy and heating;

[0060] The difference between the controllable part and the fixed part of the controllable arrester lies in the different rated voltage settings, while the basic volt-ampere characteristic curve remains the same; the settings of the rated voltages of the controllable part and the fixed part of the controllable arrester meet two restrictions. One is that the setting of the sum of the rated voltages meets the requirements of system insulation coordination; the other is that the setting of the rated voltage of the fixed part can ensure that after the converter station is locked, the energy of the line can be released through the arrester of the fixed part, and does not exceed the upper limit of the energy absorbed by the arrester of the fixed part;

[0061] First, select the appropriate rated voltage of the fixed part and the variable part of the controllable arrester to ensure that the energy stored in the DC line after the fault is locked can be safely discharged through the fixed part of the arrester. Then, according to the maximum value of the submodule overvoltage in this case, the overvoltage control protection constant is set by retaining a certain margin.

[0062] Step 2: improving the overvoltage bypass of the submodule (i.e., improving the overvoltage protection strategy of the submodule bypass);

[0063] In the inverter unlocked state, the submodule overvoltage bypass adopts the set value u set1 ;

[0064] In the inverter blocking state, the submodule overvoltage bypass setting value is u set2 ;

[0065] Among them, u set1 ≤u set2 ; The present invention improves the compatibility of the controllable lightning arrester by improving the traditional submodule overvoltage bypass strategy; the improvement of the traditional submodule overvoltage bypass strategy is: appropriately increase the overvoltage control protection setting value of the submodule controller, so that when the valve-side single-phase grounding fault converter station is locked, the risk of overvoltage in the submodule capacitor is avoided by controlling the switching of the controllable lightning arrester, without triggering the submodule overvoltage protection control, thereby reducing the subsequent maintenance workload;

[0066] Step 3: The controllable lightning arrester and the submodule overvoltage bypass cooperate to perform overvoltage control (i.e., coordinate the disconnection of the controllable lightning arrester and the submodule overvoltage bypass protection strategy);

[0067] When the MMC detects a converter station fault and issues a locking command, the switch of the controllable arrester is controlled to close, and only the fixed part of the arrester is connected to the circuit; the submodule overvoltage bypass strategy needs to be selected according to the maximum value of the overvoltage that the submodule can generate under the rated voltage determined in step 2; usually the setting value of the submodule overvoltage bypass is based on the unlocked state, and the set value used by the submodule overvoltage bypass in the locked state is greater than the set value in the unlocked state, which is used to prevent the submodule from being bypassed by mistake after the fault, thereby increasing the maintenance workload;

[0068] 1) The valve base controller monitors the MMC system in real time. When a fault in the MMC system is detected, the valve base overpressure protection controller is turned off. When the MMC system is not faulty and the valve base controller detects an overpressure fault in a single submodule, the submodule overvoltage bypasses the corresponding faulty submodule.

[0069] 2) When a single-phase grounding fault occurs on the valve side, the MMC controller quickly locks the converter station and closes the controllable circuit breaker switch;

[0070] 3) When it is detected that the submodule voltage is higher than the submodule controller overvoltage control protection setting, the submodule overvoltage bypasses the overvoltage submodule.

[0071] Further, in step one, in step one, the controllable part and the fixed part of the controllable lightning arrester use the same valve plate, and their normalized volt-ampere characteristic curves are the same, and only the rated voltage is different.

[0072] Furthermore, in step 1, the rated voltage of the fixed part of the controllable lightning arrester is determined as follows:

[0073] k ref U N ≤u s (1)

[0074]

[0075] In the above formula (1) and formula (2): k ref U is the ratio of the arrester's initial operating voltage to the rated voltage. N is the rated voltage of the arrester, u s is the peak value of the AC system line voltage, t blc is the system locking time, t AC is the AC circuit breaker disconnection moment, u arr (t) is the voltage across the arrester at time t, i arr (t) is the current flowing through the arrester at time t, E lim It is the limit of energy absorbed by the arrester.

[0076] Formula (1) is the limit of the rated voltage of the arrester; Formula (1) ensures that after the MMC is locked, the action voltage of the arrester is smaller than the voltage on the AC side, so as to ensure that after the MMC is locked, the DC voltage discharges energy through the arrester, and does not pass through the AC side to charge the submodule capacitor. Formula (2) is the limit of the energy absorbed by the arrester; Formula (2) ensures that the energy absorbed by the arrester during the entire protection action does not exceed the rated value and is damaged; the smaller the rated voltage, the more energy is discharged through the arrester on the DC side, and the less likely the submodule is charged and boosted, but the more energy is absorbed by the arrester, so the two are a compatible and coordinated process; in the specific implementation process, the rated voltage is selected according to formula (1), and then the energy absorbed by the arrester can be verified by simulation.

[0077] Further, in step 1, when the bipolar MMC-HVDC transmission system is operating normally, the controllable lightning arrester maintains an overall input state unchanged;

[0078] When a single-phase grounding fault occurs on the valve side, the converter station sends a blocking command after detecting the fault, and simultaneously closes the switch of the controllable part of the controllable lightning arrester and connects the fixed part to the circuit;

[0079] When the AC side circuit breaker is disconnected, the switch of the controllable lightning arrester is disconnected to accelerate the attenuation of the fault current. Under normal circumstances, the AC circuit breaker action time is longer than the MMC locking time, so the MMC is generally locked first, and then the AC circuit breaker is disconnected. After a fault is detected, the AC circuit breaker will be disconnected.

[0080] The control strategy of controllable lightning arrester is as follows Figure 5 As shown, Figure 5It is expressed as follows: when the converter station is locked, the controllable part of the controllable lightning arrester is disconnected, but the fixed part is put into use at the same time. Under normal circumstances, the fixed part and the controllable part of the controllable lightning arrester are put into use at the same time; no matter whether the converter station is locked or not, the fixed part of the controllable lightning arrester is always in use; after fault detection and protection delay, the AC circuit breaker will be sent a disconnection instruction, but the disconnection time of the AC circuit breaker is usually tens of ms; the controllable circuit breaker is disconnected at the same time as the AC circuit breaker is disconnected, and the disconnection means disconnection of the usable part.

[0081] Furthermore, in step 2, u set1 Take the original stator module bypass switch overvoltage controller control protection setting value; u set1 A method for setting the action value of the overvoltage bypass switch of the existing submodule;

[0082] u set2 The determination method is as follows: after selecting the rated voltage of the fixed part of the controllable lightning arrester, it is adjusted through the simulation results of the actual system after locking, and a certain margin is given on the basis of the simulation results to determine the submodule overvoltage bypass setting value to prevent the submodule from being bypassed by mistake.

[0083] u set2 The determination method is as follows: after selecting the rated voltage of the fixed part of the controllable arrester, it is set according to the maximum overvoltage that may occur in the submodule after the actual MMC is locked, and a certain margin is given on this basis, u set2 The determination method is to set the maximum overvoltage that can be avoided in the submodule after the lockout. The occurrence of the maximum overvoltage is related to the system parameters and structure. There is no accurate analytical solution for the maximum overvoltage of the submodule, so it needs to be determined based on simulation (such as Figure 6 shown), where Figure 6 It is the overvoltage bypass protection control method of the submodule, and also explains the selection method of the overvoltage protection control setting value of the submodule after blocking; Figure 6 In the figure, the vertical axis is the voltage value.

[0084] u set2 The calculation formula is: set2 ≥1.2u max (3)

[0085] In formula (3), u max It is the maximum overvoltage that may be generated by the three-phase bridge arm submodule after blocking occurs.

[0086] Example

[0087] This embodiment takes a ±320kV symmetrical bipolar double-terminal DC grid as an example for simulation verification. Figure 2In the simulation model shown in FIG. 1 , MMC1 and MMC2 are constant power control stations, MMC3 and MMC4 are constant voltage control stations, and the main parameters of MMC (MMC includes MMC1, MMC2, MMC3 and MMC4) are shown in Table 1. Both the positive and negative poles and the metal return line use DC cables, and the main parameters of the cables are shown in Table 2. The structure of the cables is shown in FIG. Figure 7 shown.

[0088] Table 1 Main parameters of MMC

[0089]

[0090]

[0091] Table 2 Main parameters of the cable

[0092] Material Thickness / m Resistivity / (Ω·m) Relative dielectric constant conductor 0.0239m <![CDATA[1.75×10 -8 ]]> / Insulation I 0.0179m / 2.30 jacket 0.009m <![CDATA[2.2×10 -7 ]]> / Insulation II 0.005m / 2.25

[0093] When the system is in steady-state operation, the sending end transmits 1000MW of DC power to the receiving end. The system is in steady-state 1s ago. At time t=1s, a grounding fault of phase A on the valve side occurs at the sending end. At time t=1.003s, the MMC converter station is locked. At time 1.01s, the AC circuit breaker is disconnected. The entire simulation test on the AC side lasts for 1s. The corresponding simulation waveform is as follows: Figure 8 to Figure 11 shown.

[0094] From Figure 8 to Figure 11 It can be seen that: when a controllable lightning arrester is used, when a single-phase grounding fault occurs on the valve side and is locked, the submodule will not experience overvoltage, and after the AC circuit breaker is disconnected, the DC voltage quickly returns to a steady state and will not trigger the bypass control protection of the submodule.

[0095] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be within the scope of protection of the present invention.

[0096] Other parts not described belong to the prior art.

Claims

1. A method for controlling and protecting overvoltage of submodules in a symmetrical bipolar flexible direct current transmission system, characterized in that: The grounding point of the bipolar MMC-HVDC transmission system is set on the DC side; the bipolar MMC-HVDC transmission system includes a power transmission end and a power receiving end; wherein the power transmission end and the power receiving end both include a positive pole and a negative pole; the positive and negative poles are connected through a metal return line and a grounding resistor, and the grid-side winding of the transformer is connected to a circuit breaker; The post-lockout submodule overvoltage control protection method specifically includes the following steps: Step 1: Replace the valve top and neutral line arresters with controllable arresters; The top arrester is installed between the bridge arm reactor and the converter valve to protect the converter station from the harm of the intrusion wave from the DC side; while the neutral line arrester is installed between the metal return line and the converter valve to limit the overvoltage intruding from the neutral line into the converter station; The controllable lightning arrester comprises a controllable part and a fixed part; wherein the controllable part is connected in parallel with the switch; The controllable part and the fixed part of the controllable arrester use the same valve plate, and their normalized volt-ampere characteristic curves are the same; When the bipolar MMC-HVDC transmission system is operating normally, the controllable lightning arrester maintains the overall input state unchanged; When a single-phase grounding fault occurs on the valve side, the converter station sends a blocking command after detecting the fault, and simultaneously closes the switch of the controllable part of the controllable lightning arrester and connects the fixed part to the circuit; When the AC circuit breaker is disconnected, the switch of the controllable lightning arrester is disconnected to accelerate the decay of the fault current; after detecting a fault, the AC circuit breaker will be disconnected; When the converter station is locked, the controllable part of the controllable lightning arrester is disconnected, but the fixed part is put into use at the same time. Under normal circumstances, the fixed part and the controllable part of the controllable lightning arrester are put into use at the same time; whether the converter station is locked or not, the fixed part of the controllable lightning arrester is always in use; after fault detection and protection delay, the AC circuit breaker will be sent a disconnection instruction, but the disconnection time of the AC circuit breaker is tens of ms; The rated voltage of the fixed part is determined as follows: k ref IN N in s (1) Where: k ref U is the ratio of the arrester's initial operating voltage to the rated voltage. N is the rated voltage of the arrester, u s is the peak value of the AC system line voltage, t blc is the system locking time, t AC is the AC circuit breaker disconnection moment, u arr (t) is the voltage across the arrester at time t, i arr (t) is the current flowing through the arrester at time t, E lim The limit of energy absorbed by the arrester; Step 2: Improve the overvoltage bypass of the submodule; In the inverter unlocked state, the submodule overvoltage bypass adopts the set value u set1 ; In the inverter blocking state, the submodule overvoltage bypass setting value is u set2 ; Among them, u set1 ≤u set2 ; u set1 Take the original stator module bypass switch overvoltage controller control protection setting value; u set2 The determination method is as follows: after selecting the rated voltage of the fixed part of the controllable arrester, it is set according to the maximum overvoltage that may occur in the submodule after the actual MMC is locked. The overvoltage can be determined through simulation, and a certain margin is given on this basis; Step 3: The controllable lightning arrester and the submodule overvoltage bypass cooperate to control overvoltage; 1) The valve base controller monitors the MMC system in real time. When a fault in the MMC system is detected, the valve base overpressure protection controller is turned off. When the MMC system is not faulty and the valve base controller detects an overpressure fault in a single submodule, the submodule overvoltage bypasses the corresponding faulty submodule. 2) When a single-phase grounding fault occurs on the valve side, the MMC controller quickly locks the converter station and closes the controllable circuit breaker switch; 3) When it is detected that the submodule voltage is higher than the submodule controller overvoltage control protection setting value, the submodule overvoltage bypasses the overvoltage submodule; u set2 The determination method is as follows: after selecting the rated voltage of the fixed part of the controllable arrester, it is set according to the maximum overvoltage of the submodule after the actual MMC is locked, and a certain margin is given on this basis, u set2 The determination method is to set the maximum overvoltage that can be avoided in the submodule after the lockout. The occurrence of the maximum overvoltage is related to the system parameters and structure. There is no accurate analytical solution for the maximum overvoltage of the submodule, and it needs to be determined based on simulation. u set2 The calculation formula is: set2 ≥1.2u max u max It is the maximum overvoltage generated by the three-phase bridge arm submodule after blocking occurs.

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