Coupling inductor series compensation circuit

Abstract

The invention discloses a coupling inductor series compensation circuit, which is characterized in that the direct current side of a flexible direct current power transmission system is connected in series through a coupling coil, the compensation circuit is composed of double H-bridge modules, and the compensation circuit has the functions of zero sequence voltage suppression and rapid direct current circuit breaker, and can suppress the fluctuation of the direct current side voltage when a single-phase fault occurs. And the transmission of the zero-sequence component at the alternating current side is blocked, and the fault current is quickly inhibited to be zero under the direct current side fault. The compensation circuit can effectively restrain direct current side voltage fluctuation caused by alternating current side faults, avoids transmission of zero-sequence components under the alternating current side faults, has a rapid direct current circuit breaking function under the direct current side faults, and prevents the faults from being further expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current power transmission systems, and in particular to a coupled inductor series compensation circuit. Background Art

[0002] Urban power grid is one of the infrastructures for urban modernization and the main load center of the power system. It has the characteristics of large power consumption, high load density, high reliability and power supply quality requirements. The flexible interconnection system can realize the closed-loop operation of power supply systems in different partitions without increasing the short-circuit capacity. It has the advantages of flexible and controllable flow, fast fault isolation, no phase change failure, and independent decoupling control of active and reactive power. It maintains the stability of AC system voltage and frequency and provides support for rapid load recovery after faults. This effectively realizes the collection, transmission and grid connection of large-scale renewable energy. Therefore, the research on urban medium-voltage flexible DC transmission system is of great significance.

[0003] The city's flexible DC transmission system needs to consider reliability, economy, land occupation and fault characteristics. The flexible DC transmission system directly connected to the 10kV AC power grid without a connecting transformer will bring two problems: 1) When a single-phase grounding fault occurs on the AC side, the transmission of the zero-sequence component cannot be blocked, which will cause voltage fluctuations on the DC side and further cause the fault to be transmitted to the non-fault side; 2) When a fault occurs on the DC side, the fault current will continue to flow through the power diode, and the converter and the AC side will form a three-phase uncontrolled rectifier bridge. The AC phase current lacks a zero-crossing point and cannot be disconnected. The common solution in the prior art is to use a full-bridge sub-module converter or a hybrid converter and configure a DC circuit breaker. These two methods are more complex to control, and the full-bridge sub-module has large losses and high costs, making it difficult to maintain economy. Summary of the invention

[0004] The purpose of the present invention is to provide a coupled inductor series compensation circuit, which can effectively suppress the DC side voltage fluctuation caused by AC measurement fault, avoid the transmission of zero-sequence components under AC measurement fault, and has a fast DC circuit breaker function under DC side fault to prevent the fault from further expanding.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] A coupled inductor series compensation circuit is connected in series with a coupled coil on the DC side of a flexible DC power transmission system. The compensation circuit is composed of a double H-bridge module, a filter inductor L4, a filter capacitor C3, controllable diodes VT1-VT4, and a primary winding of a coupled inductor, wherein the double H-bridge module includes energy storage capacitors C1 and C2 and a fully controlled power device Q.

[0007] The compensation circuit has the functions of zero-sequence voltage suppression and fast DC circuit breaker, which can suppress the fluctuation of DC side voltage during single-phase fault, block the transmission of zero-sequence component on AC side, and quickly suppress the fault current to zero under DC side fault, wherein:

[0008] The converter stations at both ends of the compensation circuit use half-bridge submodules, the AC grid power supply on both sides is 10kV, the MMC converters are connected in a back-to-back mode, and the neutral point is grounded through the arc suppression coil;

[0009] U Sj1 and U Sj2 (j=a, b, c phase) are the phase voltages of the AC sides of MMC1 and MMC2 respectively;

[0010] U dk1 and U dk2 (k=p,n pole) are respectively the positive and negative pole to ground voltages at the DC side outlets of MMC1 and MMC2;

[0011] i1 is the current value flowing through the primary side of the coupling coil in the compensation circuit; i2 is the DC bus current;

[0012] U L1 is the output voltage value of the compensation circuit through the coupling coil primary side, U L2 It is the voltage value injected into the DC bus by the compensation circuit through the secondary side of the coupling coil.

[0013] It can be seen from the technical solution provided by the present invention that the compensation circuit can effectively suppress the DC side voltage fluctuation caused by the AC fault, avoid the transmission of the zero-sequence component under the AC fault, and has a fast DC circuit breaker function under the DC side fault to prevent the fault from further expanding. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0015] Figure 1 A schematic diagram of the structure of a coupled inductor series compensation circuit provided by an embodiment of the present invention;

[0016] Figure 2 A schematic diagram of current and voltage waveforms at each stage of the fast DC circuit breaker according to an embodiment of the present invention;

[0017] Figure 3 This is a schematic diagram of the simulation waveform of the response characteristics before and after a single-phase grounding fault in the example of the present invention;

[0018] Figure 4 It is a schematic diagram of simulation of waveforms at various stages of a DC circuit breaker according to an example of the present invention. DETAILED DESCRIPTION

[0019] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments, which does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] like Figure 1 The figure shows a schematic diagram of the structure of a coupled inductor series compensation circuit provided by an embodiment of the present invention. A coupled coil series compensation circuit is connected on the DC side of the flexible DC power transmission system. The compensation circuit is composed of a double H-bridge module, a filter inductor L4, a filter capacitor C3, controllable diodes VT1-VT4, and a coupled inductor primary winding, wherein the double H-bridge module includes energy storage capacitors C1, C2 and a fully controlled power device Q;

[0021] The compensation circuit has the functions of zero-sequence voltage suppression and fast DC circuit breaker, which can suppress the fluctuation of DC side voltage during single-phase fault, block the transmission of zero-sequence component on AC side, and quickly suppress the fault current to zero under DC side fault, wherein:

[0022] The converter stations at both ends of the compensation circuit use half-bridge submodules, the AC grid power supply on both sides is 10kV, the MMC converters are connected in a back-to-back mode, and the neutral point is grounded through the arc suppression coil;

[0023] U Sj1 and U Sj2 (j=a, b, c phase) are the phase voltages of the AC sides of MMC1 and MMC2 respectively;

[0024] U dk1 and U dk2 (k=p,n pole) are respectively the positive and negative pole to ground voltages at the DC side outlets of MMC1 and MMC2;

[0025] i1 is the current value flowing through the primary side of the coupling coil in the compensation circuit; i2 is the DC bus current;

[0026] U L1 is the output voltage value of the compensation circuit through the coupling coil primary side, U L2 It is the voltage value injected into the DC bus by the compensation circuit through the secondary side of the coupling coil.

[0027] By utilizing the structure of the compensation circuit, the connection transformer is eliminated, a DC circuit breaker is not required, a half-bridge converter is used, the floor space is reduced, the loss is reduced, and the economy is high.

[0028] In the specific implementation, Figure 1 Taking the C-phase single-phase grounding fault of the left converter of the flexible DC transmission network as an example, the process of zero-sequence voltage suppression by the compensation circuit is as follows:

[0029] In the case of a single-phase grounding fault of phase C in the left converter of the flexible DC transmission network, the entire flexible system at both ends is divided into three parts: the fault AC system, the DC system and the non-fault AC system. The three-phase voltage measured before the fault is for:

[0030]

[0031] Among them, U m is the phase voltage peak value; ω is the angular frequency; is the initial phase angle;

[0032] When a single-phase grounding fault occurs in a neutral point low current grounding system, the voltage of the fault phase is 0, and the voltage of the non-fault phase increases to the original value. times, as shown in formula (2):

[0033]

[0034] in, They are the three-phase voltages after the fault;

[0035] The zero-negative sequence decomposition of the three-phase voltage before and after the fault is performed to obtain the following formula:

[0036]

[0037] Where: They are the positive, negative and zero sequence voltage components before the fault respectively; They are the positive, negative and zero sequence voltage components after the fault respectively; where a = e -j120° ;

[0038] Based on the classic circuit analysis and MMC positive and negative sequence vector control strategy, the reference wave formulas of the upper and lower bridge arm voltages of the MMC before and after the fault are as follows:

[0039]

[0040] Where: u mp and u mn are the equivalent bridge arm voltages of the upper and lower bridge arms in steady state; u dc，ref is the DC side reference voltage; Us (s=a, b, c) is the S-phase voltage amplitude at the grid-connected side of the AC system;

[0041] Through the analysis of formulas (3) and (4), it can be seen that there is only a change on the 0 axis before and after the fault. The positive sequence component remains unchanged before and after the fault, and the negative sequence component is always 0; there is no zero sequence component before the fault, but there is a zero sequence component after the fault, and the zero sequence component is in reverse phase with the normal phase voltage of the fault phase;

[0042] According to Kirchhoff's voltage law (KVL), after a single-phase grounding fault occurs, ignoring the voltage drop of the bridge arm reactance, the DC side positive voltage is equal to the bridge arm voltage of the fault phase, and the DC side negative voltage is opposite to the bridge arm voltage of the fault phase. Combining formula (5), the DC side positive and negative voltages are obtained as follows:

[0043]

[0044] According to formula (6), when a single-phase grounding fault occurs, the positive and negative bus voltages on the DC side are composed of the DC voltage itself and the zero-sequence component. The essence of the DC side fluctuation is the presence of a fault zero-sequence component on the DC bus. If there is no fault, the three-phase voltage of the non-fault station is Then the voltage reference wave of the upper and lower bridge arms of the non-fault side MMC is:

[0045]

[0046] According to Kirchhoff's voltage law, the DC side positive and negative voltage minus the upper and lower bridge arm voltages equals the non-fault side AC phase voltage. The non-fault side AC phase voltage is obtained from formula (7):

[0047]

[0048] In the formula, is the AC phase voltage on the non-fault side, j = a, b, c;

[0049] It can be seen from formula (8) that the AC phase voltage on the non-fault side has zero-sequence components. The zero-sequence components on the fault side will be transmitted to the non-fault side through the DC bus, causing non-fault voltage distortion and the expansion of the accident. The DC bus is essentially a transmission channel for fault components. The compensation circuit suppresses the zero-sequence components on the DC side to keep the DC bus voltage stable, thereby blocking the transmission of zero-sequence components. In the zero-sequence suppression stage, the upper half of the double H-bridge module of the compensation circuit is put into operation, the lower half is locked, VT1 and VT2 are disconnected, and VT3 or VT4 is turned on. The voltage U output by the upper half of the double H-bridge module is C1 After passing through the filter capacitor and filter inductor, the AC voltage U is output on the primary side of the coupled inductor. L1 and injects AC voltage U into the DC bus through the coupling inductor L2 .L2 The two voltages are superimposed on each other, which ensures the stability of the DC bus voltage and blocks the transmission of the zero-sequence component.

[0050] In a specific implementation, the process of the compensation circuit realizing the function of a fast DC circuit breaker is as follows:

[0051] like Figure 2 The figure shows the current and voltage waveforms of each stage of the fast DC circuit breaker according to the embodiment of the present invention. The short-circuit current is suppressed by magnetic coupling voltage injection. When a DC side short-circuit fault occurs, a transient reverse voltage is injected into the DC bus. When the transient voltage is greater than the DC bus voltage, the sum of the DC bus voltage is negative, forcing the fault short-circuit current to drop rapidly. When the fault current drops to 0, a square wave is injected into the DC bus to modulate the fault current into a high-frequency AC ripple, so that the mechanical switch can have multiple operation windows, allowing the mechanical switch to be turned off without arcing. The entire compensation suppression process is divided into five stages, including:

[0052] 1) Normal operation stage: When the system is operating normally and no short-circuit fault occurs, the mechanical switch MS is closed, the DC bus transmits the load current normally, and the compensation circuit is locked. When a DC side short circuit occurs, the DC bus current i2 rises rapidly, and the current slope is expressed as:

[0053]

[0054] Where L2 is the secondary winding of the coupled inductor; L line is the line inductance; R f represents the transient resistance at the time of fault; K1 represents a constant related to system parameters; i1 is the current value flowing through the primary side of the coupling coil in the compensation circuit; t is the time after the DC short circuit fault occurs;

[0055] 2) Rapid suppression stage: When the DC bus current i2 surges to the threshold current i th , the compensation circuit starts to be activated, the upper half of the double H-bridge module is put into operation, the lower half of the double H-bridge module is in a locked state, and the injected voltage U L2 =nU L1 =nU C1 , where n is the transformation ratio of the coupled inductor; the primary winding and secondary winding of the coupled inductor satisfy the following formula:

[0056]

[0057] L m is the leakage inductance of the coupled inductor; M is the mutual inductance coefficient; the voltage-current relationship of the coupled inductor should satisfy the following equation:

[0058]

[0059] According to Kirchhoff's voltage law, the voltages of each group of DC bus are analyzed and the formula is as follows:

[0060]

[0061] The expression of the slope di2 / dt of the DC bus current i2 is further obtained as:

[0062]

[0063] Because U L2 The current slope di2 / dt is higher than the bus voltage and opposite to the bus voltage, which forces the DC bus current i2 to drop sharply, thus achieving rapid suppression of the fault current. The fault interruption time is T F It is expressed by the following formula:

[0064]

[0065] 3) Zero current modulation stage: When the DC bus current i2, that is, the fault current, drops to 0, the lower half module of the double H-bridge of the compensation circuit starts to work and continuously outputs ±U C2 , the injected voltage U L2 =nU L1 =n(U C1 ±U C2 ), so during the entire current modulation stage, the voltage injected by the compensation circuit satisfies formula (15):

[0066]

[0067] When the lower half of the double H-bridge outputs -U C2 When the current slope di2 / dt>0, the fault current increases. In this state, the current increase slope is:

[0068]

[0069] Where K2 is a constant related to system parameters; through high-frequency switching of the energy storage capacitor C2, the fault current i2 is actively modulated into a high-frequency ripple current near zero, providing multiple near-zero current shutdown operation windows for the mechanical switch MS; avoiding the arc phenomenon caused by the inaccurate clock control of the traditional circuit breaker, creating conditions for arc-free shutdown of the mechanical switch MS;

[0070] 4) Arc extinguishing stage: The mechanical switch MS is turned on under near-zero current conditions, which can safely interrupt the AC ripple current and enter the arc extinguishing stage. At this time, the voltage U MS for:

[0071] U MS =U dc -n(U C1 +UC2 ) (17)

[0072] It can be seen from formula (17) that at this time, the voltage across the mechanical switch MS is negative; this has advantages in preventing arc reignition and improving the voltage blocking capability of the mechanical switch;

[0073] 5) Energy recovery stage: When the mechanical switch MS is completely disconnected, there is a large amount of energy on the primary winding of the coupled inductor, which is reversely charged through the energy storage capacitor to recover the line energy; the energy storage capacitors C1 and C2 are both reversely conducted and are in the charging stage. The charging time of this stage is:

[0074]

[0075] Where R loop_E is the equivalent resistance of the entire compensation circuit; U′ C2 is the pre-charge voltage of the energy storage capacitor C2; U C2_4 is the voltage of the energy storage capacitor C2 at the end of the arc extinction stage;

[0076] When the energy storage capacitor C2 with a smaller capacitance value returns to its initial voltage, the lower half module of the double H-bridge is locked and continues to charge the energy storage capacitor C1. The energy of the winding on one side of the coupled inductor is completely absorbed by the energy storage capacitor, energy recovery is completed, the entire DC fault is completely cleared and isolated, and the DC circuit breaker is disconnected.

[0077] It is worth noting that the contents not described in detail in the embodiments of the present invention belong to the prior art known to professional and technical personnel in the field.

[0078] The following is a simulation verification of the results of the compensation circuit described in the embodiment of the present invention using a specific example:

[0079] 1) Zero-sequence suppression simulation verification

[0080] In order to verify the zero-sequence suppression function of the compensation circuit, assuming t = 0.2s, a C-phase grounding fault occurs on the AC side of MMC1. The compensation circuit is connected for zero-sequence suppression in 0.3s, and the fault is cleared in 0.4s.

[0081] like Figure 3 The figure shows the simulation waveform of the response characteristics before and after the single-phase grounding fault in the example of the present invention. It can be seen from the simulation waveform that at 0.2s, a c-phase grounding fault occurs on the AC side of MMC1, the c-phase voltage becomes 0, and the a-phase and b-phase voltages rise to the line voltage amplitude. times, the fault zero-sequence voltage is the same as the normal phase voltage amplitude on the AC side, and the voltage fluctuation amplitude and phase on the DC side are consistent with the fault zero-sequence component. At this time, the system can still work normally for a short time, but the fault zero-sequence voltage component will be transmitted to the AC side of MMC2, causing voltage fluctuation on the MMC2 side. After the DC side is connected through the coupled inductor series compensation circuit, the DC voltage fluctuation is quickly suppressed, and the voltage on the AC side of MMC2 quickly recovers symmetry and stability, indicating that the compensation circuit has a good suppression effect on the fault zero-sequence voltage component, preventing the fault range from further expanding.

[0082] 2) Simulation verification of fast DC circuit breaker function

[0083] In order to verify the fast DC circuit breaker function of the compensation circuit, the rated current is 200A and the fault current threshold is 600A. At 0.2s, an inter-pole short circuit fault occurs. When the fault current reaches the threshold, the compensation circuit starts to work. Since the simulation results of the positive and negative poles of the system are the same, the DC side positive pole voltage and current are taken as an example for analysis. Figure 4 Shown is a schematic diagram of simulation of waveforms at various stages of a DC circuit breaker according to an example of the present invention.

[0084] It can be seen from the waveform that the fault current is quickly suppressed to 0 after reaching the threshold, and then the fault current is modulated into a high-frequency AC ripple current of several hundred μs near zero, with multiple zero crossing points. Entering the arc extinction stage, the mechanical switch begins to disconnect at a negative voltage of -13000V. After the mechanical switch is completely disconnected, it enters the energy recovery stage, and the voltage of the energy storage capacitor begins to rise; when the primary side current is 0, the DC fault is completely cleared and isolated, and the circuit breaker opening process is completed.

[0085] In summary, the compensation circuit described in the embodiment of the present invention has the following advantages:

[0086] 1. The compensation circuit suppresses zero-sequence components and fault currents on the DC side through dual H-bridge modules. There is no need to configure isolation transformers and DC circuit breakers. The converters can all use half-bridge sub-modules, which reduces converter losses, reduces system losses, and has high overall economic efficiency. It has a strong advantage over traditional fault suppression solutions.

[0087] 2. By injecting reverse voltage, the short-circuit current on the side can be quickly forced to 0, and the fault current can be modulated into a high-frequency ripple near zero, providing multiple zero-crossing points for shutdown operations without the need for precise electronic / mechanical timing control, providing microsecond DC fault protection;

[0088] 3. The compensation circuit is composed of a dual H-bridge module, which has bidirectional fault suppression and residual energy recovery capabilities, is easy to integrate multiple modules and can be applied to high-voltage systems.

[0089] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims. The information disclosed in the background technology section of this article 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 in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A coupled inductor series compensation circuit, characterized in that: A compensation circuit is connected in series through a coupling coil on the DC side of the flexible DC transmission system, wherein the compensation circuit is composed of a double H-bridge module, a filter inductor L4, a filter capacitor C3, controllable diodes VT1-VT4, and a primary winding of a coupled inductor, wherein the double H-bridge module includes energy storage capacitors C1 and C2 and a fully controlled power device Q; The compensation circuit has the functions of zero-sequence voltage suppression and fast DC circuit breaker, which can suppress the fluctuation of DC side voltage during single-phase fault, block the transmission of zero-sequence component on AC side, and quickly suppress the fault current to zero under DC side fault, wherein: The converter stations at both ends of the compensation circuit use half-bridge submodules, the AC grid power supply on both sides is 10kV, the MMC converters are connected in a back-to-back mode, and the neutral point is grounded through the arc suppression coil; U Sj1 and U Sj2 (j=a, b, c phase) are the phase voltages of the AC sides of MMC1 and MMC2 respectively; U dk1 and U dk2 (k=p,n pole) are respectively the positive and negative pole to ground voltages at the DC side outlets of MMC1 and MMC2; i1 is the current value flowing through the primary side of the coupling coil in the compensation circuit; i2 is the DC bus current; U L1 is the output voltage value of the compensation circuit through the coupling coil primary side, U L2 It is the voltage value injected into the DC bus by the compensation circuit through the secondary side of the coupling coil.

2. The coupled inductor series compensation circuit according to claim 1, characterized in that: The process of the compensation circuit performing zero-sequence voltage suppression is specifically as follows: In the case of a single-phase grounding fault of phase C in the left converter of the flexible DC transmission network, the entire flexible system at both ends is divided into three parts: the fault AC system, the DC system and the non-fault AC system. The three-phase voltage measured before the fault is for: Among them, U m is the phase voltage peak value; ω is the angular frequency; is the initial phase angle; When a single-phase grounding fault occurs in a neutral point low current grounding system, the voltage of the fault phase is 0, and the voltage of the non-fault phase increases to the original value. times, as shown in formula (2): in, They are the three-phase voltages after the fault; The zero-negative sequence decomposition of the three-phase voltage before and after the fault is performed to obtain the following formula: Where: They are the positive, negative and zero sequence voltage components before the fault respectively; They are the positive, negative and zero sequence voltage components after the fault respectively; in a=e -j120° ; Based on the classic circuit analysis and MMC positive and negative sequence vector control strategy, the reference wave formulas of the upper and lower bridge arm voltages of the MMC before and after the fault are as follows: Where: u mp and u mn are the equivalent bridge arm voltages of the upper and lower bridge arms in steady state; u dc，ref is the DC side reference voltage; U s (s=a, b, c) is the S-phase voltage amplitude at the grid-connected side of the AC system; Through the analysis of formulas (3) and (4), it can be seen that there is only a change on the 0 axis before and after the fault. The positive sequence component remains unchanged before and after the fault, and the negative sequence component is always 0; there is no zero sequence component before the fault, but there is a zero sequence component after the fault, and the zero sequence component is in reverse phase with the normal phase voltage of the fault phase; According to Kirchhoff's voltage law, after a single-phase grounding fault occurs, ignoring the voltage drop of the bridge arm reactance, the DC side positive voltage is equal to the bridge arm voltage of the fault phase, and the DC side negative voltage is opposite to the bridge arm voltage of the fault phase. Combining formula (5), the DC side positive and negative voltages are as follows: According to formula (6), when a single-phase grounding fault occurs, the positive and negative bus voltages on the DC side are composed of the DC voltage itself and the zero-sequence component. The essence of the DC side fluctuation is the presence of a fault zero-sequence component on the DC bus. If there is no fault, the three-phase voltage of the non-fault station is Then the voltage reference wave of the upper and lower bridge arms of the non-fault side MMC is: According to Kirchhoff's voltage law, the DC side positive and negative voltage minus the upper and lower bridge arm voltages equals the non-fault side AC phase voltage. The non-fault side AC phase voltage is obtained from formula (7): In the formula, is the AC phase voltage on the non-fault side, j = a, b, c; The voltage U output by the upper half module of the double H bridge through the compensation circuit C1 After passing through the filter capacitor and filter inductor, the AC voltage U is output on the primary side of the coupled inductor. L1 and injects AC voltage U into the DC bus through the coupling inductor L2 , thereby suppressing the zero-sequence component on the DC side, keeping the DC bus voltage stable, and thus blocking the transmission of the zero-sequence component.

3. The coupled inductor series compensation circuit according to claim 1, characterized in that: The specific process of the compensation circuit realizing the function of fast DC circuit breaker is as follows: The short-circuit current is suppressed by magnetic coupling voltage injection. When a short-circuit fault occurs on the DC side, a transient reverse voltage is injected into the DC bus. When the transient voltage is greater than the DC bus voltage, the sum of the DC bus voltage is negative, forcing the fault short-circuit current to drop rapidly. When the fault current drops to 0, a square wave is injected into the DC bus to modulate the fault current into a high-frequency AC ripple, so that the mechanical switch can have multiple operating windows, allowing the mechanical switch to be turned off without arcing. The entire compensation suppression process is divided into five stages, including: 1) Normal operation stage: When the system operates normally and no short-circuit fault occurs, the mechanical switch MS is closed, the main branch transmits the load current normally, and the compensation circuit is locked. When a DC side short circuit occurs, the DC bus current i2 rises rapidly, and the current slope is expressed as: Where L2 is the secondary winding of the coupled inductor; L line is the line inductance; R f represents the transient resistance at the time of fault; K1 represents a constant related to system parameters; i1 is the current value flowing through the primary side of the coupling coil in the compensation circuit; t is the time after the DC short circuit fault occurs; 2) Rapid suppression stage: When the DC bus current i2 surges to the threshold current i th , the compensation circuit starts to be activated, the upper half of the double H-bridge module is put into operation, the lower half of the double H-bridge module is in a locked state, and the injected voltage U L2 =nU L1 -nU C1 , where n is the transformation ratio of the coupled inductor; the primary winding and secondary winding of the coupled inductor satisfy the following formula: L m is the leakage inductance of the coupled inductor; M is the mutual inductance coefficient; the voltage-current relationship of the coupled inductor should satisfy the following equation: According to Kirchhoff's voltage law, the voltages of each group of DC bus are analyzed and the formula is as follows: The expression of the slope di2 / dt of the DC bus current i2 is further obtained as: Because U L2 Higher than the bus voltage and opposite in polarity, the current slope d i2 / dt<0, forcing the DC bus current i2 to drop sharply, achieving rapid suppression of fault current, and the fault interruption time T F It is expressed by the following formula: 3) Zero current modulation stage: When the DC bus current i2, that is, the fault current, drops to 0, the lower half module of the double H-bridge of the compensation circuit starts to work and continuously outputs ±U C2 , the injected voltage U L2 =nU L1 =n(U C1 ±U C2 ), so during the entire current modulation stage, the voltage injected by the compensation circuit satisfies formula (15): When the lower half of the double H-bridge outputs -U C2 When the current slope di2 / dt>0, the fault current increases. In this state, the current increase slope is: Where K2 is a constant related to system parameters; through high-frequency switching of the energy storage capacitor C2, the fault current i2 is actively modulated into a high-frequency ripple current near zero, providing multiple near-zero current shutdown operation windows for the mechanical switch MS; 4) Arc extinguishing stage: The mechanical switch MS is turned on under near-zero current conditions, which can safely interrupt the AC ripple current and enter the arc extinguishing stage. At this time, the voltage U MS for: U MS =U dc -n(U C1 +U C2 ) (17) It can be seen from formula (17) that at this time, the voltage across the mechanical switch MS is negative; 5) Energy recovery stage: When the mechanical switch MS is completely disconnected, there is a large amount of energy on the primary winding of the coupled inductor, which is reversely charged through the energy storage capacitor to recover the line energy; the energy storage capacitors C1 and C2 are both reversely conducted and are in the charging stage. The charging time of this stage is: Where R loop_E is the equivalent resistance of the entire compensation circuit; U′ C2 is the pre-charge voltage of the energy storage capacitor C2; U C2_4 is the voltage of the energy storage capacitor C2 at the end of the arc extinction stage; When the energy storage capacitor C2 with a smaller capacitance value returns to its initial voltage, the lower half module of the double H-bridge is locked and continues to charge the energy storage capacitor C1. The energy of the winding on one side of the coupled inductor is completely absorbed by the energy storage capacitor, energy recovery is completed, the entire DC fault is completely cleared and isolated, and the DC circuit breaker is disconnected.

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