A relay protection method and device for a multi-terminal direct current system based on MMC active current limiting

Abstract

The application discloses a kind of based on the multi-terminal direct current system relay protection method and device of MMC active current limiting, belong to power grid protection field;By limiting the direct current fault current of MMC at k1 times rated current, thereby effectively eliminating short-time direct current short-circuit fault, control is relatively simple, and reliability is higher.On this basis, a kind of MMC hardware and software combined end direct current system relay protection method is presented, reduces the demand to direct current side current limiting reactor, can realize the accurate control of fault current.In addition, a kind of strategy for coping with permanent fault is also proposed, to ensure that multi-terminal direct current system MMC has excellent fault response capability whether in short-time fault or permanent fault.Finally, the MMC dominant direct current short-circuit fault selective protection strategy is presented, using the injected amplitude droop type ac current signal realizes the three-stage selective protection of short-circuit fault, reduces the cost and design requirement of direct current line protection, and is suitable for the multi-terminal direct current system containing complex branch structure.

Description

Technical Field

[0001] This invention belongs to the field of power grid protection, and more specifically, relates to a method and device for relay protection of multi-terminal DC systems based on MMC active current limiting. Background Technology

[0002] Against the backdrop of "dual carbon" (carbon and carbon emissions), the energy structure is undergoing profound changes, with electricity substitution and clean energy substitution becoming the mainstream. With the growth of new energy installed capacity, power supply is shifting from centralized to distributed, and DC load is increasing significantly. Important and sensitive loads are also placing increasingly higher demands on power supply reliability. Traditional AC distribution networks are finding it increasingly difficult to meet the diversified electricity demands of this growing market. In response to these new characteristics of the distribution structure, hybrid AC / DC distribution networks are emerging.

[0003] A hybrid AC / DC distribution network is established on the basis of an existing AC grid by adding key flexible interconnection switches to create a multi-terminal DC distribution system among multiple AC grids. This distribution network architecture facilitates power exchange and AC / DC voltage complementarity between different grids. Simultaneously, constructing a multi-voltage-level, multi-layered ring network distribution system with flexible interconnection between sources, grids, loads, and storage helps promote the effective utilization of new energy sources and multi-energy complementarity, improves power conversion efficiency, and enhances grid reliability and responsiveness to diverse demands.

[0004] Multi-terminal DC distribution systems are similar in architecture to traditional AC distribution networks. They provide DC power at the required voltage level to electricity users and are also multi-level interconnected networks. Based on voltage level, they can be divided into high-voltage DC distribution systems, medium-voltage DC distribution systems, and low-voltage DC distribution systems. While the functions of each level of the distribution network are not entirely the same, the users they serve and the scope of their access differ. These different levels of the network cooperate with each other. The DC distribution network and the existing AC distribution network can be interconnected through power electronic devices such as flexible interconnection converters and two-terminal soft switches (SOPs) to meet the power needs of different scenarios.

[0005] Existing relay protection methods mostly employ overcurrent protection, longitudinal differential protection, and boundary protection based on traveling wave theory. Overcurrent protection is simple in principle and fast in operation, but it suffers from complex protection threshold settings and timing coordination issues in multi-terminal and ring-shaped DC systems. Longitudinal differential protection uses the amplitude of the current at both ends of the faulty line for protection determination, and is less affected by the fault type and transition resistance, but this method heavily relies on data synchronization via communication. Boundary protection based on traveling wave theory uses the transient characteristics of the boundary elements of the DC line for protection determination, offering fast protection speed and eliminating the need for communication, making it the most widely used engineering method in high-voltage DC systems. However, this method is susceptible to fault impedance, noise interference, and boundary strength, and usually requires the configuration of large-inductance current-limiting reactors at both ends of the line to ensure high selectivity and reliability. Due to the numerous power sources in multi-terminal DC systems, including a large number of main power sources, wind power, and photovoltaic power, and the large number and short length of lines at each level of the system, the system operation is complex, making the application and coordination of the above-mentioned existing relay protection methods in multi-terminal DC systems difficult and severely limiting their reliability. Summary of the Invention

[0006] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a multi-terminal DC system relay protection method based on MMC active current limiting, so as to solve the technical problem that the existing technology cannot achieve multi-terminal DC system relay protection with high reliability.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a relay protection method for a multi-terminal DC system based on active current limiting of MMC, comprising: performing the following operations on each MMC in the multi-terminal DC system respectively:

[0008] S1. When the software fault detection identifies a DC short-circuit fault in the MMC, the reference value of the positive DC current of the MMC will be adjusted. Switch to positive DC fault current limit value Then, feedback i with DC positive current dc_P The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the positive DC current control feedforward term u. u_ff Add them together to obtain the corresponding positive voltage signal;

[0009] Use the negative DC current reference value of MMC Switch to negative DC fault current limit value Then, feedback i with DC negative current dc_N The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the negative DC current control feedforward term u. l_ff Add them together to obtain the corresponding negative voltage signal;

[0010] The obtained positive and negative voltage signals are added to the voltage signal output by the AC side current control loop to form corresponding modulation wave signals, which are then input to the PWM loop to control the MMC, thereby limiting the DC fault current of the MMC to k1 times the rated current; k1 is a positive integer greater than 1.

[0011] S2. Repeat step S1 until the software fault detection fails to detect a DC short circuit fault in the MMC, or the duration reaches t. p time;

[0012] Among them, when the DC short circuit fault is a pole-to-pole fault, The value is k1 times the rated current; The value is -k1 times the rated current; when the DC short circuit fault is a positive-to-ground fault, The value is k1 times the rated current. The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. The value is -k1 times the rated current.

[0013] More preferably, the method for software fault detection to identify whether a DC short-circuit fault has occurred in the MMC includes:

[0014] Calculate the DC fault impedance of the MMC based on the voltage and current output from the DC side of the MMC.

[0015] If the DC fault impedance of the MMC is less than 1 / k1 times the rated impedance, the MMC is determined to have a DC short circuit fault; otherwise, the MMC is determined not to have a DC short circuit fault.

[0016] More preferably, when the DC short-circuit fault is a pole-to-pole fault, the DC fault impedance Z of the MMC is... dcf =u dc / i dc ;

[0017] When the DC short-circuit fault is a positive-to-ground fault, the DC fault impedance Z of the MMC is... dcf =u dc_u / i dc_P ;

[0018] When the DC short-circuit fault is a negative-to-ground fault, the DC fault impedance Z of the MMC is... dcf =u dc_l / i dc_N ;

[0019] Among them, u dc The DC voltage output from the DC side of the MMC; i dc This refers to the DC current output from the DC side of the MMC; udc_u i is the DC voltage output from the upper arm of the MMC bridge; dc_P This is the DC current output from the positive terminal of the MMC; u dc_l i is the DC voltage output by the lower bridge arm; dc_N This is the DC current output from the negative terminal of the MMC.

[0020] More preferably, the above-mentioned MMC active current limiting method for short-circuit faults in multi-terminal DC systems further includes step S0, which is performed before step S1;

[0021] Step S0 includes: when a DC short-circuit fault occurs, hardware current limiting is applied to the DC fault current of the MMC until the software fault detection can identify that the MMC has a DC short-circuit fault.

[0022] More preferably, the method for hardware current limiting of the DC fault current of the MMC includes:

[0023] S01. The control signal of the MMC is blocked by a hysteresis current limiting circuit. When the DC fault current drops to i th1 Afterwards, the hysteresis current limiting circuit stops operating;

[0024] S02. Repeat step S01 at a fixed frequency to limit the DC fault current of the MMC to k2 times the rated current.

[0025] Among them, i th1 The rated current is greater than k1 times and less than k2 times; k2 > k1;

[0026] The aforementioned hysteresis current limiting circuit includes: a hysteresis comparator, a diode, a resistor, a capacitor, and an AND operation structure;

[0027] The hysteresis comparator is used to compare the sampled signal of the DC fault current with a threshold signal. When the sampled signal is greater than the upper threshold, the hysteresis comparator outputs a low level; when the sampled signal drops to less than the lower threshold, the hysteresis comparator outputs a high level. The upper threshold is k2 times the rated current, and the lower threshold is greater than k1 times the rated current.

[0028] The cathode of the diode is connected to the output of the hysteresis comparator, and the anode is connected to the first terminal of the operational circuit and grounded through a capacitor; the resistor is connected in parallel with the diode.

[0029] The operational structure obtains the PWM signal through the second terminal, which is used to perform an AND operation on the signals input from the first and second terminals to output the drive signal for driving the IGBT in the MMC, thereby locking the control signal of the MMC.

[0030] More preferably, the above-mentioned multi-terminal DC system relay protection method further includes: steps S3 and S4 executed after step S2, including:

[0031] S3, when the duration reaches t p If the software fault detection still identifies a DC short circuit fault in the MMC, then a corresponding frequency AC signal is continuously fed into the corresponding fault point of the MMC so that the amplitude of the fault current of the MMC and the impedance from the MMC to the corresponding fault point exhibit a drooping characteristic.

[0032] S4. In a single-source system dominated by MMC:

[0033] The following processing is performed on each relay participating in the first stage of protection: If the first setting value of the relay participating in the first stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then the relay is controlled to quickly trigger the corresponding circuit breaker to clear the fault.

[0034] The following processing is performed on each relay participating in the second stage of protection: If the second setting value of the relay participating in the second stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then after a delay of Δt, the relay is controlled to trigger the corresponding circuit breaker to clear the fault.

[0035] For each relay participating in the third stage of protection, the following processing is performed: If the third setting value of the relay participating in the third stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then after a delay of Δt′, the relay is controlled to trigger the corresponding circuit breaker to clear the fault.

[0036] In the multi-terminal DC system, the frequencies of the AC signals corresponding to different MMCs are all different; the multi-terminal DC system is divided into single-source systems dominated by each MMC; the dominant frequency of each single-source system dominated by each MMC is the frequency of the corresponding AC signal.

[0037] In a single-source system dominated by MMC, all relays on all lines participate in the first stage of protection;

[0038] In the single-source system led by MMC, relays on all lines except the terminal line participate in the second stage of protection.

[0039] In the single-source system dominated by MMC, relays on all lines except the terminal and secondary terminal lines participate in the third stage of protection.

[0040] The amplitudes of the positive and negative AC signals at the dominant frequency are obtained by frequency analysis of the positive and negative currents of the DC line, respectively.

[0041] In a single-source system dominated by MMC, the first setting value of each relay is determined based on the fault point and the droop curve of the fault current. The second setting value of each relay is the first setting value of the next segment of the line in which it is located. The third setting value of each relay is the first setting value of the end line of the single-source system.

[0042] Both Δt and Δt′ are greater than the sum of the frequency analysis time and the time from when the relay issues the action command to when the circuit breaker fully disconnects, and Δt < Δt′.

[0043] More preferably, the method of feeding a corresponding frequency AC signal to the corresponding fault point of the MMC includes:

[0044] The reference value of the positive DC current of MMC Switch to positive frequency AC signal Then, feedback i with DC positive current dc_P The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the positive DC current control feedforward term u. u_ff Add them together to obtain the corresponding positive voltage signal;

[0045] Use the negative DC current reference value of MMC Switch to negative frequency AC signal Then, feedback i with DC negative current dc_N The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the negative DC current control feedforward term u. l_ff Add them together to obtain the corresponding negative voltage signal;

[0046] The obtained positive and negative voltage signals are added to the voltage signal output by the AC side current control loop to form corresponding modulation wave signals, which are then input into the PWM loop.

[0047] Among them, when the DC short circuit fault is a pole-to-pole fault, Values Values When the DC short circuit fault is a positive-to-ground fault... Values The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. Values The preset maximum value of the AC injection signal; d is the droop coefficient; Z dcf ω is the DC fault impedance of the MMC. fThe frequency of the signal injected into the AC circuit; t represents time.

[0048] More preferably, the sag coefficient is:

[0049]

[0050] Among them, the minimum value of the AC injection signal δ is the ratio of the total line impedance to the rated impedance; k rel The preset reliability coefficient; The line impedance of the entire line in a single-source system dominated by MMC.

[0051] Secondly, the present invention provides a multi-terminal DC system relay protection device based on MMC active current limiting, comprising: a memory and a processor, wherein the memory stores a computer program, and the processor executes the multi-terminal DC system relay protection method provided in the first aspect of the present invention when executing the computer program.

[0052] Thirdly, the present invention also provides a computer-readable storage medium comprising a stored computer program, wherein the computer program, when executed by a processor, controls the device containing the storage medium to perform the multi-terminal DC system relay protection method provided in the first aspect.

[0053] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0054] 1. This invention provides a relay protection method for multi-terminal DC systems based on MMC active current limiting. By limiting the DC fault current of the MMC to k1 times the rated current, it can effectively ensure the reliable and safe operation of the MMC during DC short-circuit faults without the risk of overcurrent in the system. On the other hand, the fault current limiting of the MMC can allow it to avoid short-term short-circuit faults or use its own output current to break down and clear debris that causes the fault, thereby effectively enabling the system to achieve the purpose of short-term fault self-healing. By utilizing the current output capability of the MMC itself, this invention can effectively cope with short-term short-circuit faults in multi-terminal DC systems caused by short-term contact with wires caused by strong winds, debris such as tree branches or bird nests, and flashover of insulator surfaces caused by lightning, improving the reliability of continuous system operation and short-circuit fault response, and the control is relatively simple and the cost is low.

[0055] 2. The multi-terminal DC system relay protection method provided by this invention, after a DC short-circuit fault occurs, considering the control delay introduced by sampling, fault identification, PWM control and dynamic response in digital control, relying solely on software current limiting based on digital control may cause the peak fault current to rise to a very high level. Therefore, before the software fault detection can identify the DC short-circuit fault in the MMC, this invention first performs hardware current limiting on the DC fault current of the MMC, limiting the DC fault current of the MMC to k2 (k2>k1) times the rated current. The active current limiting method for DC short-circuit faults through the combination of MMC hardware and software further improves the reliability of relay protection, reduces the demand for DC-side current limiting reactors, and can achieve precise control of fault current under different current limiting purposes.

[0056] 3. In the multi-terminal DC system relay protection method provided by the present invention, when performing hardware current limiting, the hysteresis current limiting circuit is introduced into the control system of MMC. The hysteresis current limiting circuit is triggered based on the detection signal of DC side current, and its delay only includes the sampling delay of the current transformer. Compared with software current limiting based on digital control, it has a faster response speed and better speed performance.

[0057] 4. The multi-terminal DC system relay protection method provided by this invention, when exceeding t p When a short-circuit fault has not been effectively cleared, the entire multi-terminal DC system is decoupled at different frequencies, making it multiple radial single-source systems dominated by each MMC. Each line protection device only cooperates with the MMC on the same side of the line, and uses the injected amplitude drooping AC current signal to achieve three-stage selective protection of short-circuit faults. This reduces the cost and design requirements of DC line protection and is suitable for multi-terminal DC systems with complex branch structures. The injected AC current signal has a natural zero-crossing point, so the fault can be effectively cleared by using a microprocessor AC circuit breaker. There is no need to configure a high-voltage DC circuit breaker with a complex arc suppression design. Low-cost relay protection devices can be used to achieve selective protection of short-circuit faults in complex multi-terminal DC systems. This ensures that the MMCs of the multi-terminal DC system have excellent fault response capabilities for both short-term and permanent faults, and works in conjunction with the line relay protection devices to form an integrated control and protection system. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the basic structure of the multi-terminal DC system provided by the present invention.

[0059] Figure 2 This is a schematic diagram of the active rate limiting strategy system configuration provided by the present invention;

[0060] Figure 3 This is a block diagram of the software current limiting control for DC short-circuit faults provided by the present invention;

[0061] Figure 4 This is a schematic diagram of capacitor voltage balancing and circulating current control provided by the present invention;

[0062] Figure 5 The present invention provides a four-stage control flowchart for MMC.

[0063] Figure 6 This is a schematic diagram of the protection configuration for a multi-terminal DC system provided by the present invention;

[0064] Figure 7 This is a schematic diagram of the fault protection principle of a multi-terminal DC system provided by the present invention;

[0065] Figure 8 The amplitude droop characteristic curve provided by this invention;

[0066] Figure 9 A schematic diagram illustrating the construction principle of the AC injection signal provided by this invention;

[0067] Figure 10 This is a schematic diagram of the protection action value setting provided by the present invention;

[0068] Figure 11 The experimental results provided by this invention are for an occurrence of a pole-to-pole metallic short-circuit fault.

[0069] Figure 12 The experimental results provided by this invention are for a positive-to-ground metallic short-circuit fault. Detailed Implementation

[0070] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0071] To achieve the above objectives, in a first aspect, the present invention provides a relay protection method for a multi-terminal DC system based on active current limiting of MMC, comprising: performing the following operations on each MMC in the multi-terminal DC system respectively:

[0072] S1. When the software fault detection identifies a DC short-circuit fault in the MMC, the reference value of the positive DC current of the MMC will be adjusted. Switch to positive DC fault current limit value Then, feedback i with DC positive current dc_P The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the positive DC current control feedforward term u. u_ffAdd them together to obtain the corresponding positive voltage signal;

[0073] Use the negative DC current reference value of MMC Switch to negative DC fault current limit value Then, feedback i with DC negative current dc_N The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the negative DC current control feedforward term u. l_ff Add them together to obtain the corresponding negative voltage signal;

[0074] The obtained positive and negative voltage signals are added to the voltage signal output by the AC side current control loop to form corresponding modulation wave signals, which are then input to the PWM loop to control the MMC, thereby limiting the DC fault current of the MMC to k1 times the rated current; k1 is a positive integer greater than 1.

[0075] S2. Repeat step S1 until the software fault detection fails to detect a DC short circuit fault in the MMC, or the duration reaches t. p Time; specifically, t p This is an empirical value, and can be set to 500ms.

[0076] Among them, when the DC short circuit fault is a pole-to-pole fault, The value is k1 times the rated current; The value is -k1 times the rated current; when the DC short circuit fault is a positive-to-ground fault, The value is k1 times the rated current. The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. The value is -k1 times the rated current.

[0077] Preferably, the method for software fault detection to identify whether a DC short-circuit fault has occurred in the MMC includes:

[0078] Calculate the DC fault impedance of the MMC based on the voltage and current output from the DC side of the MMC.

[0079] If the DC fault impedance of the MMC is less than 1 / k1 times the rated impedance, the MMC is determined to have a DC short-circuit fault; otherwise, the MMC is determined not to have a DC short-circuit fault. In one optional embodiment, k1 is taken as 1.2.

[0080] Among them, when the DC short-circuit fault is a pole-to-pole fault, the DC fault impedance Z of the MMC is... dcf =u dc / i dc ;

[0081] When the DC short-circuit fault is a positive-to-ground fault, the DC fault impedance Z of the MMC is... dcf =u dc_u / i dc_P ;

[0082] When the DC short-circuit fault is a negative-to-ground fault, the DC fault impedance Z of the MMC is... dcf =u dc_l / i dc_N ;

[0083] Among them, u dc The DC voltage output from the DC side of the MMC; i dc This refers to the DC current output from the DC side of the MMC; u dc_u i is the DC voltage output from the upper arm of the MMC bridge; dc_P This is the DC current output from the positive terminal of the MMC; u dc_l i is the DC voltage output by the lower bridge arm; dc_N This is the DC current output from the negative terminal of the MMC.

[0084] Furthermore, in an optional implementation, the above-described MMC active current limiting method for short-circuit faults in multi-terminal DC systems further includes step S0, which is performed before step S1;

[0085] Step S0 includes: when a DC short-circuit fault occurs, hardware current limiting is applied to the DC fault current of the MMC (limiting the DC fault current of the MMC to k2 times the rated current) until the software fault detection can identify that the MMC has a DC short-circuit fault.

[0086] It should be noted that there are multiple methods for hardware current limiting of the DC fault current of the MMC. In one optional implementation, hardware current limiting of the MMC's DC fault current is achieved by increasing the inductance of the DC reactor (to suppress the rate of change of the DC fault current during short-circuit fault transients). In another optional implementation, hardware current limiting of the MMC's DC fault current is achieved by adding a DC circuit breaker (to quickly clear the DC fault current). In yet another optional implementation, the method for hardware current limiting of the MMC's DC fault current includes:

[0087] S01. The control signal of the MMC is blocked by a hysteresis current limiting circuit. When the DC fault current drops to i th1 Afterwards, the hysteresis current limiting circuit stops operating;

[0088] S02. Repeat step S01 at a fixed frequency to limit the DC fault current of the MMC to k2 times the rated current.

[0089] Among them, i th1The rated current is greater than k1 times and less than k2 times; k2 > k1; in this embodiment, k2 is 1.5.

[0090] Specifically, the aforementioned hysteresis current limiting circuit includes: a hysteresis comparator, a diode, a resistor, a capacitor, and an AND operation structure;

[0091] The hysteresis comparator is used to compare the sampled signal of the DC fault current with a threshold signal. When the sampled signal is greater than the upper threshold, the hysteresis comparator outputs a low level; when the sampled signal drops to less than the lower threshold, the hysteresis comparator outputs a high level. The upper threshold is k2 times the rated current, and the lower threshold is greater than k1 times the rated current.

[0092] The cathode of the diode is connected to the output of the hysteresis comparator, and the anode is connected to the first terminal of the operational circuit and grounded through a capacitor; the resistor is connected in parallel with the diode.

[0093] The operational structure obtains the PWM signal through the second terminal, which is used to perform an AND operation on the signals input from the first and second terminals to output the drive signal for driving the IGBT in the MMC, thereby locking the control signal of the MMC.

[0094] Furthermore, in an optional implementation, the above-described MMC active current limiting method for short-circuit faults in multi-terminal DC systems further includes steps S3 and S4, executed after step S2, specifically including:

[0095] S3, when the duration reaches t p If the software fault detection still identifies a DC short circuit fault in the MMC, then a corresponding frequency AC signal is continuously fed into the corresponding fault point of the MMC so that the amplitude of the fault current of the MMC and the impedance from the MMC to the corresponding fault point exhibit a drooping characteristic.

[0096] S4. In a single-source system dominated by MMC:

[0097] The following processing is performed on each relay participating in the first stage of protection: If the first setting value of the relay participating in the first stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then the relay is controlled to quickly trigger the corresponding circuit breaker to clear the fault.

[0098] The following processing is performed on each relay participating in the second stage of protection: If the second setting value of the relay participating in the second stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then after a delay of Δt, the relay is controlled to trigger the corresponding circuit breaker to clear the fault.

[0099] For each relay participating in the third stage of protection, the following processing is performed: If the third setting value of the relay participating in the third stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then after a delay of Δt′, the relay is controlled to trigger the corresponding circuit breaker to clear the fault.

[0100] In a multi-terminal DC system, the frequencies of the AC signals corresponding to different MMCs are all different. The multi-terminal DC system is divided into single-source systems dominated by each MMC through frequency analysis. The dominant frequency of each single-source system dominated by each MMC is the frequency of the corresponding AC signal.

[0101] In a single-source system dominated by MMC, all relays on all lines participate in the first stage of protection;

[0102] In the single-source system led by MMC, relays on all lines except the terminal line participate in the second stage of protection.

[0103] In the single-source system dominated by MMC, relays on all lines except the terminal and secondary terminal lines participate in the third stage of protection.

[0104] The amplitudes of the positive and negative AC signals at the dominant frequency are obtained by frequency analysis of the positive and negative currents of the DC line, respectively.

[0105] In a single-source system dominated by MMC, the first setting value of each relay is determined based on the fault point and the droop curve of the fault current. The second setting value of each relay is the first setting value of the next segment of the line in which it is located. The third setting value of each relay is the first setting value of the end line of the single-source system.

[0106] Both Δt and Δt′ are greater than the sum of the frequency analysis time and the time from when the relay issues the action command to when the circuit breaker fully disconnects, and Δt < Δt′.

[0107] Preferably, the method of feeding a corresponding frequency AC signal to the corresponding fault point of the MMC includes:

[0108] The reference value of the positive DC current of MMC Switch to positive frequency AC signal Then, feedback i with DC positive current dc_P The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the positive DC current control feedforward term u. u_ff Add them together to obtain the corresponding positive voltage signal;

[0109] Use the negative DC current reference value of MMC Switch to negative frequency AC signal Then, feedback i with DC negative current dc_N The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is compared with the negative DC current control feedforward term u. l_ff Add them together to obtain the corresponding negative voltage signal;

[0110] The obtained positive and negative voltage signals are added to the voltage signal output by the AC side current control loop to form corresponding modulation wave signals, which are then input into the PWM loop.

[0111] Among them, when the DC short circuit fault is a pole-to-pole fault, Values Values When the DC short circuit fault is a positive-to-ground fault... Values The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. Values The preset maximum value of the AC injection signal; d is the droop coefficient; Z dcf ω is the DC fault impedance of the MMC. f The frequency of the signal injected into the AC circuit; t represents time.

[0112] Specifically, the droop coefficient is:

[0113]

[0114] Among them, the minimum value of the AC injection signal δ is the ratio of the total line impedance to the rated impedance; k rel The preset reliability coefficient; The line impedance of the entire line in a single-source system dominated by the MMC to be protected.

[0115] To further illustrate the technical solution provided by the present invention, a specific embodiment is described in detail below:

[0116] The basic structure of a multi-terminal DC system is as follows: Figure 1 As shown, the power distribution system consists of DC feeders flexibly interconnected by three MMCs. The converter enables bidirectional power flow, DC-side voltage support, and uninterrupted power supply to the load during AC-side feeder faults.

[0117] MMC system configuration as follows Figure 2 As shown, during normal operation, the three-phase AC voltage u detected by the current transformer is... abc AC side three-phase current i abcDC side voltage u dc DC side current i dc Submodule capacitor voltage u cap After the analog signal is converted into a digital signal by the digital-to-analog converter circuit (AD chip and peripheral circuit), it is sent to the digital controller (composed of DSP and FPGA). The PWM signal generated by the digital controller is sent to the local controller of each submodule as the drive signal for the IGBT in the submodule. After a DC short circuit fault occurs, considering the control delay introduced by sampling, fault identification, PWM control and dynamic response in the digital control, relying solely on software current limiting based on digital control may cause the peak fault current to rise to a very high level. Therefore, if Figure 2 As shown, a hysteresis current limiting circuit is introduced into the MMC control system. The hysteresis current limiting circuit is triggered based on the DC-side current detection signal, and its delay only includes the sampling delay of the current transformer, resulting in a faster response speed compared to software current limiting based on digital control. A latching circuit serves as a backup protection circuit for the hysteresis current limiting circuit. When the hardware hysteresis circuit cannot limit the fault current, the latching circuit activates, causing the MMC to shut down, thus achieving self-protection. The latching circuit's operating threshold is higher than the hysteresis current threshold (k² times the rated current).

[0118] The working principle of the hardware hysteresis current limiting circuit is as follows: During normal operation, the DC current i r For threshold i th2 The hysteresis current limiting circuit does not control the MMC. Once a DC short-circuit fault occurs, the DC current rises sharply and reaches i th2 The hysteresis current limiting circuit is activated and immediately blocks the control signal of the MMC. After a brief blockage by the MMC, the DC fault current drops to i. th1 Afterward, the hysteresis current limiting circuit exits operation, and the MMC is controlled again by the digital controller. The above process will repeat at a fixed frequency, and the DC fault current will be reliably limited within a certain range until the software current limiting control takes effect.

[0119] Software rate limiting control strategies such as Figure 3 As shown, it is divided into three parts: (a) MMC internal power control, (b) AC side power-current control, and (c) DC side power-current control. The reference value for MMC internal power control is the rated capacitor voltage. The feedback value is the DC component of the average capacitor voltage of all three-phase submodules, and the PI output is the reference value for the internal power of the three-phase bridge arm. The AC-side power-current control is a power outer loop + current inner loop control structure in the dq synchronous rotating coordinate system. Its output, after dq-abc coordinate transformation, becomes the reference value of the differential mode component of the three-phase bridge arm output voltage. The DC-side power-current control adopts a power outer loop + current inner loop control structure, and its output is the reference value of the zero-sequence common-mode component of the three-phase bridge arm output voltage. The controlled object in capacitor voltage balancing and circulating current control is the bridge arm circulating current i flowing between the three-phase bridge arms. cir Furthermore, it is controlled using a parallel repetitive PI control method, such as... Figure 4 As shown.

[0120] The control mode of the MMC is determined by the power control mode flag F. P With DC fault flag F f The relationship between the two is determined by (1). When F P When F = 0, the MMC control power flows from the DC side to the AC side, and the power consumed by the MMC itself is provided by the DC side; when F P When = 1, the MMC control power flows from the AC side to the DC side, and the power consumed by the MMC itself is provided by the AC side. When a DC fault occurs, the DC fault flag F... f =1 and F P =1, the power consumption of the MMC itself is provided by the AC side. During steady-state operation, the DC current is typically less than 1.2 times the rated value. Therefore, the DC fault impedance Z is calculated in real time. dcf This is compared with 0.83 times the rated impedance. Compare to determine the DC fault flag F f As shown in (2).

[0121]

[0122]

[0123] The DC-side current control adopts an independent control mode for the positive and negative DC currents, and the AC-side neutral point voltage u u_ff with u l_ff Introduce feedforward. Where i dc_P with i dc_N These are the positive and negative DC bus currents, respectively, with the reference direction pointing from the converter to the DC side. Upon detecting a DC short-circuit fault, the reference values ​​for the positive and negative DC currents are... and Switch to positive and negative DC fault current limiting values ​​respectively. and When a pole-to-pole fault occurs in a DC system, both the positive and negative poles of the DC system are faulty poles. When a pole-to-ground fault occurs in a DC system, the positive and negative poles can be controlled separately, therefore

[0124] A four-state MMC current-limiting protection scheme for DC short-circuit faults, such as Figure 5 As shown. Under normal operating conditions, the MMC operates in a steady-state state. When a short-circuit fault occurs in the DC system, the hardware hysteresis current limiting is activated first to limit the fault current within a preset range. After the fault is detected and identified, software current limiting control is initiated. The software current limiting control first limits the DC fault current to 1.2 times the rated current and calculates the DC fault impedance Z based on the DC side output voltage and current. dcf As shown in (3), the duration is t p This stage is used to clear short-term short-circuit faults in the DC system. If t p If the short-circuit fault is effectively cleared within a certain time, the MMC will return to steady-state operation and terminate the operation; otherwise, if the fault time t is greater than t p When the fault is cleared, the MMC enters the AC signal injection state and works with the line protection device to perform fault protection. After the protection device clears the fault, the MMC returns to the steady-state operation state.

[0125]

[0126] Multi-terminal DC system protection configuration such as Figure 6 As shown, the system consists of multi-terminal MMCs and complex branches, with protection devices (PDs) installed at both ends of the line for locating and isolating faults.

[0127] The basic idea of ​​the selective protection strategy based on MMC AC signal injection is as follows:

[0128] 1) During the fault control phase, each MMC operates in current-limiting control mode (current source mode) and independently feeds a specific frequency AC signal to the fault point. The signal amplitude and the impedance from the MMC to the fault point exhibit a drooping characteristic.

[0129] 2) The entire multi-terminal DC system is decoupled at different frequencies to make it into multiple radial single-source systems dominated by each MMC. Each line protection device only cooperates with the MMC located on the same side of the line.

[0130] 3) The digital relay of the protection device performs three-stage protection settings for different frequency signals according to its position in each single-source system. When making protection judgments, it will issue a protection action signal as long as any frequency signal meets the action criteria.

[0131] The fault protection principle of a multi-terminal DC system is illustrated as follows: Figure 7As shown. During the fault protection phase, MMC1, MMC2, and MMC3 inject AC signals with frequencies f1, f2, and f3 into the fault point, respectively. The multi-terminal DC system is decoupled into three single-source systems at different frequencies. The digital relays in each protection device perform three-stage protection settings for different frequency signals. Taking the protection devices in line 1 as an example, the left-side protection device only sets the f1 frequency signal, while the right-side protection device sets both f2 and f3 frequency signals simultaneously. Similarly, in line 2, the right-side protection device only sets the f2 frequency signal, while the left-side protection device sets both f1 and f3 frequency signals simultaneously. During protection action determination, a protection action signal is issued as long as any frequency signal meets the action criterion.

[0132] AC injection signal amplitude droop characteristic curve like Figure 8 As shown, when the DC short-circuit fault is a metallic short circuit, the amplitude variation range of the AC injected signal varies depending on the magnitude of the fault impedance. A wider judgment range ensures the selectivity and sensitivity of fault protection. For high-impedance short-circuit faults, the amplitude variation range of the AC injection signal will be correspondingly shortened, inevitably affecting the selectivity and sensitivity of fault protection. However, based on the minimum value... The configuration (as shown in (4)) still ensures the coordination between upstream and downstream protection.

[0133]

[0134] Where δ is the ratio of the total line impedance to the rated impedance, its maximum value is usually no more than 5% to 10%; the reliability coefficient k rel The value range is generally 0.8 to 0.9.

[0135] By actively injecting AC signals of different frequencies into each MMC, the multi-terminal DC system is decomposed into multiple single-source systems, each dominated by one of the MMCs. AC injection signal i f_ac The structure is as follows Figure 9 As shown, its expression is

[0136]

[0137] Where, ω f The frequency of the AC injection signal can be taken as odd harmonics such as 3, 5, and 7, depending on the number of MMCs. The amplitude of the AC injected signal is related to the calculated DC fault impedance Z. dcf It features a "drooping" design. The expression is

[0138]

[0139] in, The maximum value of the AC injection signal can be up to 2 p.u., but to avoid the upper limit of the hardware hysteresis current limit (1.5 p.u.), this invention uses... d is the droop coefficient, defined as follows:

[0140]

[0141] in, The line impedance of the entire line to be protected.

[0142] For the control of the injected AC signal, the MMC uses a PIR controller in the DC side current control stage, and its transfer function is shown in (8).

[0143]

[0144] Where, k p k i and k r These are the proportional parameter, integral parameter, and resonant parameter, respectively; ω0 is the resonant frequency, set as the frequency ω of the AC injection signal. f ;ω c The bandwidth used to adjust the resonant frequency. During the permanent fault handling phase, the MMC switches the DC fault current limit value to an AC injection signal. For DC pole-to-pole short-circuit faults, it sets... For a DC positive terminal to ground short circuit fault, let For a DC negative terminal to ground short circuit fault, let

[0145] The digital relays in the line protection device use recursive discrete Fourier transform (RDFT) to extract each AC signal. For a signal x(t) with period T0, when the sampling period T... s When = T0 / N, the discrete Fourier expression of x(t) is:

[0146]

[0147] Where ω is the fundamental angular frequency; A k and B k These are the real and imaginary parts of the k-th frequency component, respectively, calculated as follows:

[0148]

[0149] Therefore, the amplitude of the k-th frequency component is

[0150]

[0151] The RDFT algorithm can extract the amplitudes of multiple frequency components at once. The extracted amplitude of the AC signal at a specific frequency is then compared with the set protection action value. Based on the amplitude droop characteristic curve of the AC injected signal, taking instantaneous overcurrent protection (Stage I), time-limited overcurrent protection (Stage II), and definite-time overcurrent protection (Stage III) in traditional three-stage current protection as examples, the schematic diagram of the protection action value setting of the line protection device is as follows: Figure 10 As shown.

[0152] Stage I protection is activated when a digital relay detects a specific frequency signal amplitude flowing through the line. When the setting value is exceeded, the circuit breaker is quickly triggered to clear the fault. The setting of the operating value of the I-stage protection is based on avoiding the injection signal amplitude when a short-circuit fault occurs at the beginning of the next line segment. To ensure that a short-circuit fault at the beginning of the next line segment will not cause the protection of this line to malfunction, the injection signal amplitude when a short-circuit fault occurs at the end of the current line needs to be multiplied by a selection coefficient k. sel Used as the protection action value. Selection coefficient k sel The value range is generally 1.1 to 1.2. For the last section of the line, in order to ensure the reliability of the protection of section I of the line, it is necessary to multiply the amplitude of the injected signal when a short-circuit fault occurs at the end of the line by a reliability coefficient k. rel This is used as the protection action value. Reliability coefficient k rel The value range is generally 0.8 to 0.9. It should be noted that although Stage I protection can quickly execute protective actions after detecting a fault, except for the last section of the line where Stage I protection can protect the entire length, the Stage I protection of other lines has a selection coefficient k. sel These methods can only protect 80% to 90% of the line. Therefore, it is necessary to introduce a second-stage protection system based on the first-stage protection system.

[0153] Stage II protection occurs when a digital relay detects a specific frequency signal amplitude flowing through the line. When the set value is exceeded, the circuit breaker is not triggered immediately, but rather after a delay Δt before a decision is made on whether to operate. If the detected signal still exceeds the set value after the delay Δt, the circuit breaker is triggered to clear the fault. Stage II protection can protect not only the entire length of the current line but also extend its protection range to the next line. Therefore, the operating value of Stage II protection can be set to the operating value of Stage I protection for the next line segment. The delay time Δt must be greater than the inherent operating time t of the protection device. act Its definition

[0154] t act =t det +t CB (10)

[0155] Among them, t detThe time for the microprocessor to detect the amplitude of the AC injection signal (i.e., the frequency analysis time); t CB The time from when the relay issues an operation command to when the circuit breaker fully disconnects is Δt. In this embodiment, Δt = 50 ms.

[0156] The principle of Stage III protection is similar to that of Stage II protection. Its protection range extends to the entire length of the line. It also determines whether to operate after a delay Δt′. If the detected signal still exceeds the set value after the delay Δt′, the circuit breaker is triggered to clear the fault. The operating value of Stage III protection can be set to the same as that of Stage I protection for the last section of the line. The delay time Δt′ must be greater than the inherent operating time t of the protection device. a ′ ct Similar to (10), it will not be elaborated further here; it should be noted that Δt′>Δt; in this embodiment, Δt′=60ms. The three-stage protection setting values ​​of the present invention example are shown in Table 1, wherein, This is the first setting value for relay DR1; This is the first setting value for relay DR2; This is the first setting value for relay DR3; This is the second setting value for relay DR1; This is the second setting value for relay DR2; This is the third setting value for relay DR1; I fault1 I represents the amplitude of the AC signal injected by the MMC when a short-circuit fault occurs at the end of line 1. fault2 The amplitude of the AC signal injected by the MMC when a short-circuit fault occurs at the end of line 2; I fault3 The amplitude of the AC signal injected by MMC when a short-circuit fault occurs at the end of line 3.

[0157] Table 1. Protection Action Values ​​Setting of Line Protection Devices

[0158]

[0159] Figure 11 The experimental waveform for a pole-to-pole short circuit in a DC system is shown; where u dc For DC voltage, i dc For direct current, i l1 Let i be the DC current of line 1. l2 U is the DC current of line 2. ab with u bc For AC line voltage, i a with i b For alternating current, u cap i represents the capacitor voltage of the MMC submodule. armThis refers to the MMC arm current. According to the proposed four-stage MMC control process, after a fault occurs, hardware hysteresis current limiting takes effect first, limiting the DC-side fault current within a predetermined range (8-10A); subsequently, software current limiting control is initiated, controlling the DC fault current to... Entering software rate limiting control t p =After 500ms, the MMC switches to fault protection mode and actively injects an AC signal into the fault point. Fault point 1 is located at the midpoint of line 1, therefore the DC fault impedance Z dcf The impedance is 0.5Ω. According to the amplitude droop characteristic curve, the amplitude of the third-harmonic AC signal injected by the MMC is 6.225A. The amplitude of the injected AC signal is greater than the first-stage protection setting value of the digital relay in the protection device PD1, which quickly triggers the circuit breaker of the protection device PD1 to clear the fault. After the fault is cleared, the system smoothly returns to normal operation.

[0160] Figure 12 The experimental waveform for a DC system with a pole-to-ground short circuit is given; where u dc For DC voltage, i dcp with i dcn These are the positive and negative DC currents, i l1p U is the DC positive current of line 1. ab with u bc For AC line voltage, i a with i b For alternating current, u cap i represents the capacitor voltage of the MMC submodule. arm This refers to the MMC bridge arm current. During steady-state operation, the positive current i on the DC side of the MMC is... dcp DC side negative current i dcn Zero-sequence fault current The current ratings are 4A, -4A, and 0A, respectively. According to the proposed four-stage MMC control flow, after a fault occurs, the hardware hysteresis current limiting takes effect first, limiting the zero-sequence fault current within a predetermined range (8-10A); subsequently, software current limiting control is initiated, controlling the zero-sequence fault current to... Entering software rate limiting control t p After 500ms, the MMC switches to fault protection mode and actively injects an AC signal into the fault point. The amplitude of the triple-harmonic AC signal injected by the MMC is 6.225A. After the fault is cleared, the system smoothly returns to normal operation.

[0161] In summary, in this embodiment, when a DC short-circuit fault occurs, the hardware hysteresis circuit limits the DC fault current to within 1.5 times the rated current during the system control delay. After the control delay ends, the software fault detection identifies the short-circuit fault and enters software current limiting control. The software current limiting control strategy operates at time t. pThe internal current limit for DC faults is set at 1.2 times the rated current to handle short-term DC short-circuit faults. If the DC short-circuit fault occurs within t... p If the fault is not cleared, the MMC enters a current-limiting mode for permanent short-circuit faults. It employs an AC signal injection current-limiting control strategy based on amplitude droop, working in conjunction with the relay protection devices in the line to disconnect the faulty branch in the multi-terminal DC system. Once the DC fault is cleared, the MMC returns to normal operating mode, enabling the system to operate normally.

[0162] The beneficial effects of this invention are as follows: First, this invention proposes an active current-limiting strategy for DC short-circuit faults that combines MMC hardware and software, reducing the need for DC-side current-limiting reactors and enabling precise control of fault current under different current-limiting purposes. Second, it proposes a four-state MMC current-limiting protection scheme for DC short-circuit faults, ensuring that the MMC in multi-terminal DC systems has excellent fault response capabilities for both short-term and permanent faults, and forms an integrated control and protection system in conjunction with line relay protection devices. Finally, combining the flexible control capabilities of MMC and the mature three-stage protection method, a selective protection strategy for DC short-circuit faults dominated by MMC is proposed. The injected amplitude-drooping AC current signal achieves three-stage selective protection of the fault, reducing the cost and design requirements of DC line protection and making it suitable for multi-terminal DC systems with complex branch structures. The injected AC current signal has a natural zero-crossing point, so a microprocessor-based AC circuit breaker can effectively clear the fault, eliminating the need for high-voltage DC circuit breakers with complex arc-suppression designs. This allows for the use of low-cost relay protection devices to achieve selective protection of short-circuit faults in complex multi-terminal DC systems.

[0163] Secondly, the present invention provides a multi-terminal DC system relay protection device based on MMC active current limiting, comprising: a memory and a processor, wherein the memory stores a computer program, and the processor executes the multi-terminal DC system relay protection method provided in the first aspect of the present invention when executing the computer program.

[0164] The related technical solutions are the same as those provided in the first aspect of this invention for the relay protection method of multi-terminal DC systems, and will not be described in detail here.

[0165] Thirdly, the present invention also provides a computer-readable storage medium comprising a stored computer program, wherein the computer program, when executed by a processor, controls the device containing the storage medium to perform the multi-terminal DC system relay protection method provided in the first aspect.

[0166] The related technical solutions are the same as those provided in the first aspect of this invention for the relay protection method of multi-terminal DC systems, and will not be described in detail here.

[0167] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A relay protection method for multi-terminal DC systems based on MMC active current limiting, characterized in that, include: Perform the following operations on each MMC in the multi-terminal DC system respectively: S1. When the software fault detection identifies a DC short-circuit fault in the MMC, the reference value of the positive DC current of the MMC will be adjusted. Switch to positive DC fault current limit value Then, feedback with DC positive current. The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is then compared with the positive DC current control feedforward term. Add them together to obtain the corresponding positive voltage signal; Use the negative DC current reference value of MMC Switch to negative DC fault current limit value Then, feedback with DC negative current. The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is then compared with the negative DC current control feedforward term. Add them together to obtain the corresponding negative voltage signal; The obtained positive and negative voltage signals are added to the voltage signal output from the AC side current control circuit to form corresponding modulation wave signals, which are then input to the PWM circuit to control the MMC, thereby limiting the DC fault current of the MMC to a certain level. At times the rated current; It is a positive integer greater than 1; S2. Repeat step S1 until the software fault detection fails to identify a DC short circuit fault in the MMC, or the duration reaches [a certain value]. time; Among them, when the DC short circuit fault is a pole-to-pole fault, Values Times the rated current; Values The rated current is times that of the DC short circuit fault; when the DC short circuit fault is a positive-to-ground fault... Values times the rated current, The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. Values Times the rated current; The multi-terminal DC system relay protection method further includes step S0, which is performed before step S1. Step S0 includes: when a DC short circuit fault occurs, hardware current limiting is applied to the DC fault current of the MMC until the software fault detection can identify that the MMC has a DC short circuit fault. The method for hardware current limiting of DC fault current in MMC includes: S01. The control signal of the MMC is blocked by a hysteresis current limiting circuit. When the DC fault current drops to Afterwards, the hysteresis current limiting circuit stops operating; S02. Repeat step S01 at a fixed frequency to limit the DC fault current of the MMC to... At times the rated current; in, Greater than times the rated current, and less than Times the rated current; ; The hysteresis current limiting circuit includes: a hysteresis comparator, a diode, a resistor, a capacitor, and an AND operation structure; The hysteresis comparator is used to compare the sampled signal of the DC fault current with a threshold signal; when the sampled signal is greater than the upper threshold, the hysteresis comparator outputs a low level; when the sampled signal drops to less than the lower threshold, the hysteresis comparator outputs a high level; the upper threshold is... The lower limit threshold is greater than times the rated current. Times the rated current; The negative terminal of the diode is connected to the output terminal of the hysteresis comparator, and the positive terminal is connected to the first terminal of the operational structure and grounded through the capacitor; the resistor is connected in parallel with the diode. The operational structure obtains the PWM signal through the second terminal, and uses it to perform an AND operation on the signals input from the first and second terminals to output the drive signal for driving the IGBT in the MMC, thereby locking the control signal of the MMC.

2. The multi-terminal DC system relay protection method according to claim 1, characterized in that, Methods for software fault detection to identify whether a DC short-circuit fault has occurred in the MMC include: Calculate the DC fault impedance of the MMC based on the voltage and current output from the DC side of the MMC. If the DC fault impedance of the MMC is less than If the impedance is twice the rated impedance, the MMC is determined to have a DC short circuit fault; otherwise, the MMC is determined not to have a DC short circuit fault.

3. The multi-terminal DC system relay protection method according to claim 1, characterized in that, When the DC short-circuit fault is a pole-to-pole fault, the DC fault impedance of the MMC is... ; When the DC short-circuit fault is a positive-to-ground fault, the DC fault impedance of the MMC is... ; When the DC short-circuit fault is a negative-to-ground fault, the DC fault impedance of the MMC is... ; in, This refers to the DC voltage output from the DC side of the MMC. This refers to the DC current output from the DC side of the MMC. This refers to the DC voltage output from the upper arm of the MMC bridge. This is the DC current output from the positive terminal of the MMC; This is the DC voltage output by the lower bridge arm; This is the DC current output from the negative terminal of the MMC.

4. The multi-terminal DC system relay protection method according to any one of claims 1-3, characterized in that, Also includes: Steps S3 and S4, performed after step S2, specifically include: S3, when the duration reaches If the software fault detection still identifies a DC short circuit fault in the MMC, then a corresponding frequency AC signal is continuously fed into the corresponding fault point of the MMC so that the amplitude of the fault current of the MMC and the impedance from the MMC to the corresponding fault point exhibit a drooping characteristic. S4. In a single-source system dominated by MMC: The following processing is performed on each relay participating in the first stage of protection: If the first setting value of the relay participating in the first stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then the relay is controlled to quickly trigger the corresponding circuit breaker to clear the fault. For each relay participating in the second stage of protection, the following processing is performed: If the second setting value of the relay participating in the second stage of protection is less than the amplitude of the positive AC signal or the amplitude of the negative AC signal at the corresponding dominant frequency, then a delay is applied. Then, the relay is controlled to trigger the corresponding circuit breaker to disconnect the fault; For each relay participating in the third stage of protection, the following processing is performed: If the third setting value of the relay participating in the third stage of protection is less than the amplitude of the positive or negative AC signal at the corresponding dominant frequency, then a delay is applied. Then, the relay is controlled to trigger the corresponding circuit breaker to disconnect the fault; In the multi-terminal DC system, the frequencies of the AC signals corresponding to different MMCs are all different; the multi-terminal DC system is divided into single-source systems dominated by each MMC; the dominant frequency of each single-source system dominated by an MMC is the frequency of the corresponding AC signal. In a single-source system dominated by MMC, all relays on all lines participate in the first stage of protection; In the single-source system led by MMC, relays on all lines except the terminal line participate in the second stage of protection. In the single-source system dominated by MMC, relays on all lines except the terminal and secondary terminal lines participate in the third stage of protection. The amplitudes of the positive and negative AC signals at the dominant frequency are obtained by frequency analysis of the positive and negative currents of the DC line, respectively. In a single-source system dominated by MMC, the first setting value of each relay is determined based on the fault point and the droop curve of the fault current. The second setting value of each relay is the first setting value of the next segment of the line in which it is located. The third setting value of each relay is the first setting value of the end line of the single-source system. and Both are greater than the sum of the frequency analysis time and the time from when the relay issues an action command to when the circuit breaker fully disconnects, and .

5. The multi-terminal DC system relay protection method according to claim 4, characterized in that, Methods for feeding corresponding frequency AC signals to the corresponding fault point of the MMC include: The reference value of the positive DC current of MMC Switch to positive frequency AC signal Then, feedback with DC positive current. The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is then compared with the positive DC current control feedforward term. Add them together to obtain the corresponding positive voltage signal; Use the negative DC current reference value of MMC Switch to negative frequency AC signal Then, feedback with DC negative current. The results are compared, and the resulting error value is input into the PIR controller; the output of the PIR controller is then compared with the negative DC current control feedforward term. Add them together to obtain the corresponding negative voltage signal; The obtained positive and negative voltage signals are added to the voltage signal output by the AC side current control loop to form corresponding modulation wave signals, which are then input into the PWM loop. Among them, when the DC short circuit fault is a pole-to-pole fault, Values ; Values When the DC short circuit fault is a positive-to-ground fault, Values , The value is 0A; when the DC short circuit fault is a negative-to-ground fault, The value is 0A. Values ; ; Set the preset maximum value of the AC injection signal; This is the droop coefficient; The DC fault impedance of the MMC; The frequency of the signal injected into the AC circuit; t Indicates time.

6. The multi-terminal DC system relay protection method according to claim 5, characterized in that, The droop coefficient is: Among them, the minimum value of the AC injection signal ; This is the ratio of the total line impedance to the rated impedance. The preset reliability coefficient; The line impedance of the entire line in a single-source system dominated by MMC.

7. A multi-terminal DC system relay protection device based on MMC active current limiting, characterized in that, include: A memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to perform the multi-terminal DC system relay protection method according to any one of claims 1-6.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored computer program, wherein when the computer program is executed by a processor, it controls the device where the storage medium is located to perform the multi-terminal DC system relay protection method according to any one of claims 1-6.

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