A MMC type distribution network bipolar short circuit fault protection and distance measurement device and method

Abstract

The present invention provides a MMC type distribution network bipolar short circuit fault protection and distance measurement device and method, and relates to the technical field of flexible DC distribution network. The system includes a central monitoring unit, a bipolar short circuit energy release unit, and a capacitor distance measurement unit. The present invention uses an energy dissipation resistor in an energy release circuit to release the capacitor energy storage in a multi-level converter (MMC) after a DC bus bipolar short circuit. Afterwards, an additional capacitor is added between the positive and negative busbars by an injection method, and a closed loop is formed with the bipolar short circuit fault. The fault distance is calculated by monitoring the voltage across the additional capacitor and the current flowing through it, so as to facilitate timely repair. The present invention has a simple principle, convenient and flexible control, and can quickly measure the fault distance after a bipolar short circuit fault occurs, thereby reducing the loss caused by a system short circuit fault. The present invention is easy to install, has high reliability, and is low in cost.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current distribution network, and in particular to a bipolar short-circuit fault protection and distance measurement device and method for an MMC type distribution network. Background Art

[0002] With the continuous development of new energy technologies, distributed power sources have emerged, and DC loads have been connected in large numbers. DC distribution systems have more advantages than AC distribution systems. They have the advantages of large power supply capacity, high power quality, flexible control, low power loss, and excellent economy. In addition, the widespread application of power electronics technology has also accelerated the development of DC distribution systems. Compared with VSC, modular multilevel converters (MMCs) are easy to expand and have low switching losses. Therefore, they are very popular in the field of DC distribution networks. However, during the operation of DC distribution network systems, line disconnection faults, single-pole grounding faults and bipolar short-circuit faults often occur, and bipolar short-circuit faults occur the most frequently among the above three faults and are the most harmful. Fault location is one of the key technologies for reliable and high-quality power supply in DC distribution networks.

[0003] Since the DC transmission and distribution system has "low impedance" and "weak inertia", when a bipolar short circuit fault occurs in the DC line, its fault current will rise rapidly, causing a huge impact on the modular multilevel converter (MMC). If no measures are taken, it will even endanger the safe and stable operation of the entire system. Therefore, after a bipolar short circuit fault occurs in the DC system, it is necessary to limit the short circuit current in time, release the energy stored in the capacitor, accurately locate the fault, take measures, and eliminate the fault. Ensure that the system resumes normal and stable operation in a short time and reduce the losses caused by the fault. However, the traditional distance measurement method is affected by the branch current of the distribution network, which will cause a large error in the measured fault distance. In addition, the transmission distance of the DC distribution system is short, generally not exceeding 10km, so the traveling wave method fault distance measurement requires a higher frequency requirement and also has certain defects. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes an MMC type distribution network bipolar short-circuit fault protection and ranging device and method, which can quickly release the converter energy storage, prevent power electronic devices from being damaged, measure the fault distance, and prevent large-scale power outages after a DC bus bipolar short-circuit fault occurs.

[0005] On the one hand, an MMC type distribution network bipolar short-circuit fault protection and ranging device includes a central monitoring unit, a bipolar short-circuit energy release unit, a capacitor ranging unit, a multi-level converter, and a DC bus;

[0006] The first input end of the bipolar short-circuit energy release unit is connected to the output positive pole of the multi-level converter, and the first output end of the bipolar short-circuit energy release unit is connected to the output negative pole of the multi-level converter (MMC); the second input end of the bipolar short-circuit energy release unit is connected to the first output end of the central monitoring unit; the second output end of the bipolar short-circuit energy release unit is connected to the first input end of the central monitoring unit;

[0007] The first input end of the capacitive ranging unit is connected to the positive pole of the DC bus; the first output end of the capacitive ranging unit is connected to the negative pole of the DC bus; the second input end of the capacitive ranging unit is connected to the second output end of the central monitoring unit; and the second output end of the capacitive ranging unit is connected to the second input end of the central monitoring unit.

[0008] The bipolar short-circuit energy release unit comprises a first voltage transformer, a first current transformer, a first energy dissipation resistor, a second energy dissipation resistor, a third energy dissipation resistor, a first insulated gate bipolar transistor, a second insulated gate bipolar transistor, a third insulated gate bipolar transistor, a first inductor, a second inductor and a third inductor; wherein, one end of the first energy dissipation resistor is connected in series with the first inductor, and the other end of the first inductor is connected to the collector of the first insulated gate bipolar transistor, one end of the second energy dissipation resistor is connected in series with the second inductor, and the other end of the second inductor is connected to the collector of the second insulated gate bipolar transistor, one end of the third energy dissipation resistor is connected in series with the third inductor, and the other end of the third inductor is connected to the collector of the third insulated gate bipolar transistor, and the other ends of the first inductor, the second inductor and the third inductor are connected together as the first input end of the bipolar short-circuit energy release unit; the first insulated gate bipolar transistor The emitter of the first insulated gate bipolar transistor, the emitter of the second insulated gate bipolar transistor, and the emitter of the third insulated gate bipolar transistor are connected together as the first output end of the bipolar short-circuit energy release unit; the base of the first insulated gate bipolar transistor, the base of the second insulated gate bipolar transistor, and the base of the third insulated gate bipolar transistor serve as the second input end of the bipolar short-circuit energy release unit; one end of the first voltage transformer is connected to the positive pole of the DC bus, and the other end of the first voltage transformer is connected to the negative pole of the DC bus; one end of the first current transformer is connected to the emitter of the first insulated gate bipolar transistor, the emitter of the second insulated gate bipolar transistor, and the emitter of the third insulated gate bipolar transistor, and the other end of the first current transformer is connected in series to the negative pole output terminal of the DC side of the multilevel converter (MMC); the first current transformer and the first voltage transformer serve as the second output end of the bipolar short-circuit energy release unit.

[0009] The capacitive ranging unit includes a fourth insulated gate bipolar transistor, a fifth insulated gate bipolar transistor, a sixth insulated gate bipolar transistor, a first DC voltage source, a fourth inductor, a first capacitor, a fourth energy consumption resistor, a second current transformer, a third current transformer and a second voltage transformer; wherein the collector of the fourth insulated gate bipolar transistor is connected to the positive electrode of the first DC voltage source, the negative electrode of the first DC voltage source is connected to one end of the fourth energy consumption resistor and one end of the fourth inductor, the other end of the fourth energy consumption resistor is connected to one end of the third current transformer, the other end of the third current transformer is connected to the collector of the fifth insulated gate bipolar transistor, and the emitter of the sixth insulated gate bipolar transistor is connected to one end of the first capacitor. end, the other end of the first capacitor is connected to one end of the second current transformer, the emitter of the fourth insulated gate bipolar transistor, the collector of the sixth insulated gate bipolar transistor and one end of the second voltage transformer are connected together as the first input end of the capacitance ranging unit; the other end of the fourth inductor, the emitter of the fifth insulated gate bipolar transistor, the other end of the second current transformer and the other end of the second voltage transformer are connected together as the first output end of the capacitance ranging unit; the base of the fourth edge gate bipolar transistor, the base of the fifth insulated gate bipolar transistor and the base of the sixth insulated gate bipolar transistor serve as the second input end of the capacitance ranging unit; the second voltage transformer, the second current transformer and the third current transformer serve as the second output end of the capacitance ranging unit.

[0010] On the other hand, a MMC type distribution network bipolar short circuit fault protection and ranging method is implemented based on the aforementioned MMC type distribution network bipolar short circuit fault protection and ranging device, comprising the following steps:

[0011] Step 1: Use the central monitoring unit to control the internal components of the capacitance ranging unit. When the central monitoring unit receives a signal that the voltage value monitored by the second voltage transformer is 0, a control signal is sent to the fourth insulated gate bipolar transistor and the sixth insulated gate bipolar transistor inside the capacitance ranging unit to turn them on.

[0012] Step 2: Use a second current transformer and a second voltage transformer to monitor the voltage across the first capacitor and the current flowing through the first capacitor in real time;

[0013] Step 3: When the voltage across the first capacitor reaches the set value, the central monitoring unit sends a control signal to the fourth insulated gate bipolar transistor and the sixth insulated gate bipolar transistor to disconnect them, and the capacitor charging is completed. At the same time, the central monitoring unit sends a control signal to the fifth insulated gate bipolar transistor to turn it on.

[0014] Step 4: Use the third current transformer to monitor the current flowing through the fourth energy consumption resistor in real time. When the collected current is 0, the central monitoring unit sends a control signal to the fifth insulated gate bipolar transistor to disconnect it.

[0015] Step 5: Use the central monitoring unit to control the internal components of the bipolar short-circuit energy release unit. When a bipolar short-circuit fault occurs in the system, the central monitoring unit sends a control signal to the first insulated gate bipolar transistor, the second insulated gate bipolar transistor, and the third insulated gate bipolar transistor of the bipolar short-circuit energy release unit to turn it on.

[0016] Step 6: Use the first current transformer and the first voltage transformer inside the bipolar short-circuit energy release unit to collect the bus current and the voltage between the positive and negative buses in real time, and send the collected voltage and current values ​​to the central monitoring unit; when a bipolar short-circuit fault occurs, the energy stored in the modular multilevel converter (MMC) will be released through three energy consumption resistors;

[0017] Step 7: The central monitoring unit determines whether the voltage and current values ​​monitored by the first current transformer and the first voltage transformer are 0. If they are 0, execute step 8; otherwise, continue to execute step 6.

[0018] Step 8: The central monitoring unit sends a control signal to the first insulated gate bipolar transistor, the second insulated gate bipolar transistor, and the third insulated gate bipolar transistor to disconnect the bipolar short circuit energy release unit;

[0019] Step 9: The central monitoring unit sends a control signal to the sixth insulated gate bipolar transistor to turn on the capacitor ranging unit. At the same time, the second current transformer and the second voltage transformer monitor the voltage across the first capacitor and the current flowing through the capacitor in real time for ranging. The central monitoring unit processes the voltage and current data to obtain the bipolar short circuit fault distance.

[0020] The principle of the distance measurement is shown in formula (1) and formula (2):

[0021]

[0022]

[0023] Among them, u c (t) represents the voltage across the capacitor; i c (t) represents the current flowing through the capacitor; r0 represents the resistance per unit length of the DC bus; l0 represents the inductance per unit length of the DC bus; x represents the bipolar short-circuit fault distance of the DC bus; t represents time.

[0024] The beneficial effects produced by the present invention are:

[0025] The technical solution provides a bipolar short-circuit fault protection and distance measurement device and method for an MMC type distribution network. Compared with the traditional distance measurement method, the present invention first utilizes the energy dissipation resistor in the energy release circuit to release the energy stored in the converter due to the short-circuit fault, thereby avoiding damage to the multi-level converter (MMC) due to short-circuit overcurrent. Secondly, an additional capacitor with initial energy storage is inserted between the positive and negative busbars, and the voltage at both ends of the additional capacitor and the current flowing through it are calculated and processed to locate the fault in time so as to eliminate the fault. The invention has a simple structure, is easy to control flexibly, has a small size, has a low economic cost, and is convenient for modification and installation. Its unique structure determines that it has a low heat generation and is not prone to fuse failures, so its failure rate is low and its reliability is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of the MMC type distribution network bipolar short-circuit fault protection and distance measurement device of the present invention;

[0027] Figure 2 1 is a schematic diagram of the access structure of the bipolar short-circuit fault protection and distance measurement device according to an embodiment of the present invention;

[0028] Figure 3 is a circuit diagram of a bipolar short-circuit energy release unit according to an embodiment of the present invention;

[0029] Figure 4 is a circuit diagram of a capacitive ranging unit according to an embodiment of the present invention;

[0030] Figure 5 is a circuit schematic diagram of a signal conditioning circuit according to an embodiment of the present invention;

[0031] Figure 6 is a schematic diagram of a central monitoring unit circuit according to an embodiment of the present invention;

[0032] Figure 7 It is a flow chart of a bipolar short-circuit fault protection and ranging method applicable to a modular multilevel converter (MMC) DC power distribution system according to an embodiment of the present invention;

[0033] Figure 8 1 is a circuit diagram of releasing energy storage of a multi-level converter (MMC) according to an embodiment of the present invention;

[0034] Fig. 9 is a circuit diagram of a capacitor charging process according to an embodiment of the present invention;

[0035] Fig.10 1 is a schematic diagram of a capacitive ranging unit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0036] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0037] On the one hand, a MMC type distribution network bipolar short-circuit fault protection and ranging device, such as Figure 1 As shown, it includes a central monitoring unit, a bipolar short-circuit energy release unit, a capacitor distance measurement unit, a multi-level converter, and a DC bus;

[0038] In the embodiment of the present invention, Figure 2 As shown in the figure, when a bipolar short circuit fault occurs in the DC bus, the modular multilevel converter (MMC) releases the stored energy through the energy dissipation resistor in the bipolar short circuit energy release unit, and then connects the ranging capacitor between the positive and negative poles of the DC bus to complete the fault ranging by monitoring and calculating the voltage across the capacitor and the current flowing through the capacitor. The central monitoring unit adopts the TMS320F28335 model.

[0039] The first input end of the bipolar short-circuit energy release unit is connected to the output positive pole of the multi-level converter, and the first output end of the bipolar short-circuit energy release unit is connected to the output negative pole of the multi-level converter (MMC); the second input end of the bipolar short-circuit energy release unit is connected to the first output end of the central monitoring unit; the second output end of the bipolar short-circuit energy release unit is connected to the first input end of the central monitoring unit;

[0040] The first input end of the capacitive ranging unit is connected to the positive pole of the DC bus; the first output end of the capacitive ranging unit is connected to the negative pole of the DC bus; the second input end of the capacitive ranging unit is connected to the second output end of the central monitoring unit; and the second output end of the capacitive ranging unit is connected to the second input end of the central monitoring unit.

[0041] In this embodiment, Figure 3 , Figure 4 As shown, the bipolar short-circuit energy release unit includes a first voltage transformer PT1, a first current transformer CT1, a first energy consumption resistor R1, a second energy consumption resistor R2, a third energy consumption resistor R3, a first insulated gate bipolar transistor T1, a second insulated gate bipolar transistor T2, a third insulated gate bipolar transistor T3, a first inductor L1, a second inductor L2, and a third inductor L3. Among them, the first voltage transformer adopts the JSZGF-10 model, and the first current transformer adopts the LZZJ1-10 model. The capacitance ranging unit includes a fourth insulated gate bipolar transistor T4, a fifth insulated gate bipolar transistor T5, a sixth insulated gate bipolar transistor T6, a first DC voltage source E1, a fourth inductor L4, a first capacitor C1, a fourth energy consumption resistor R4, a second current transformer CT2, a third current transformer CT3, and a second voltage transformer PT2. The second current transformer adopts LZZJ1-10 model, the third current transformer adopts LZZJ1-10 model, and the second voltage transformer adopts JSZGF-10 model.

[0042] In this embodiment, one end of the first energy dissipation resistor R1 of the bipolar short-circuit energy release unit is connected in series with the first inductor L1, and the other end of the first inductor L1 is connected to the collector of the first insulated gate bipolar transistor T1; one end of the second energy dissipation resistor R2 is connected in series with the second inductor L2, and the other end of the second inductor L2 is connected to the collector of the second insulated gate bipolar transistor T2; one end of the third energy dissipation resistor R3 is connected in series with the third inductor L3, and the other end of the third inductor L3 is connected to the collector of the third insulated gate bipolar transistor T3; the other end of the first energy dissipation resistor R1, the other end of the second energy dissipation resistor R2, and the other end of the third energy dissipation resistor R3 are connected together as the first input end of the bipolar short-circuit energy release unit; the emitter of the first insulated gate bipolar transistor T1, the emitter of the second insulated gate bipolar transistor T2, and the emitter of the third insulated gate bipolar transistor T3 are connected together as the first output end of the bipolar short-circuit energy release unit. The collector of the fourth insulated gate bipolar transistor T4 in the capacitance ranging unit is connected to the positive electrode of the first DC voltage source E1, the negative electrode of the first DC voltage source E1 is connected to one end of the fourth energy consumption resistor R4 and one end of the fourth inductor L4, the other end of the fourth energy consumption resistor R4 is connected to the third current transformer CT3, the other end of the third current transformer CT3 is connected to the collector of the fifth insulated gate bipolar transistor T5, the emitter of the sixth insulated gate bipolar transistor T6 is connected to one end of the first capacitor C1, the other end of the first capacitor C1 is connected to one end of the second current transformer CT2, the emitter of the fourth insulated gate bipolar transistor T4, the collector of the sixth insulated gate bipolar transistor T6 and one end of the second voltage transformer PT2 are connected together as the first input end of the capacitance ranging unit. The other end of the fourth inductor, the emitter of the fifth insulated gate bipolar transistor T5, the other end of the second current transformer CT2 and the other end of the second voltage transformer PT2 are connected together as the first output end of the capacitance ranging unit.

[0043] Figure 5 FIG. 1 is a signal conditioning diagram. In the embodiment of the present invention, five signal conditioning circuits are included, and the structures are the same. The input terminals U / I of the five signal conditioning circuits are respectively connected to the output terminals of two voltage transformers and the output terminals of three current transformers. Figure 6 As shown, the signal conditioning circuit is connected to the TMS320F28335 DSP chip through the ADS174 data acquisition chip, wherein the output terminals AIPN and AINN of the signal conditioning circuit are connected to the AIPN1 and AINN1 terminals of the data acquisition chip in turn; the IOVDD, CLK, / FSYNC, SCLK, DOUT, MODE1, and MODE0 are respectively connected to DVDD, CLKOUT, McBSP-PORT, GPI03, and GPI05 of the TMS320F28335 model DSP chip.

[0044] On the other hand, a MMC type distribution network bipolar short circuit fault protection and ranging method is implemented based on the aforementioned MMC type distribution network bipolar short circuit fault protection and ranging device, such as Figure 7 As shown, the following steps are included:

[0045] Step 1: Use the central monitoring unit to control the internal components of the capacitance ranging unit. When the central monitoring unit receives a signal that the voltage value monitored by the second voltage transformer is 0, a control signal is sent to the fourth insulated gate bipolar transistor and the sixth insulated gate bipolar transistor inside the capacitance ranging unit to turn them on.

[0046] Step 2: Use a second current transformer and a second voltage transformer to monitor the voltage across the first capacitor and the current flowing through the first capacitor in real time;

[0047] Step 3: When the voltage across the first capacitor C1 reaches a set value, usually about 200V and not less than 170V, the central monitoring unit sends a signal to the fourth insulated gate bipolar transistor T4 and the sixth insulated gate bipolar transistor T6 to disconnect them, and the capacitor charging is completed. Fig. 9 At the same time, the central monitoring unit sends a closing signal to the fifth insulated gate bipolar transistor T5 to turn it on.

[0048] Step 4: The third current transformer CT3 is used to monitor the current flowing through the fourth energy consumption resistor R4 in real time. When the collected current is 0, the central monitoring unit sends a control signal to the fifth insulated gate bipolar transistor T5 to disconnect it.

[0049] Step 5: Use the central monitoring unit to control the internal components of the bipolar short-circuit energy release unit. When a bipolar short-circuit fault occurs in the system, the central monitoring unit sends a control signal to the first insulated gate bipolar transistor T1, the second insulated gate bipolar transistor T2, and the third insulated gate bipolar transistor T3 of the bipolar short-circuit energy release unit to turn it on.

[0050] Step 6: Use the first current transformer CT1 and the first voltage transformer PT1 inside the bipolar short-circuit energy release unit to collect the bus current and the voltage between the positive and negative buses in real time, and send the collected voltage and current values ​​to the central monitoring unit; when a bipolar short-circuit fault occurs, the energy stored in the modular multilevel converter (MMC) will be released through three energy consumption resistors, such as Figure 8 As shown, to avoid device damage caused by overcurrent due to short circuit fault.

[0051] Step 7: The central monitoring unit determines whether the voltage and current values ​​monitored by the first current transformer CT1 and the first voltage transformer PT1 are 0. If they are 0, execute step 8; otherwise, continue to execute step 6.

[0052] Step 8: The central monitoring unit sends a control signal to the first insulated gate bipolar transistor T1, the second insulated gate bipolar transistor T2, and the third insulated gate bipolar transistor T3 to disconnect the bipolar short circuit energy release unit;

[0053] Step 9: The central monitoring unit sends a control signal to the sixth insulated gate bipolar transistor to turn on the capacitance ranging unit. At the same time, the second current transformer CT2 and the second voltage transformer PT2 monitor the voltage across the first capacitor C1 and the current flowing through the capacitor in real time for ranging. The central monitoring unit processes the voltage and current data to obtain the bipolar short circuit fault distance.

[0054] like Fig.10 As shown, the principle of the distance measurement is shown in formula (1) and formula (2):

[0055]

[0056]

[0057] Among them, u c (t) represents the voltage across the capacitor; i c (t) represents the current flowing through the capacitor; r0 represents the resistance per unit length of the DC bus; l0 represents the inductance per unit length of the DC bus; x represents the bipolar short-circuit fault distance of the DC bus; t represents time; Since the distribution lines of the DC distribution network are short, generally not exceeding 10 km, the distributed capacitance of the DC lines can be ignored.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A MMC type distribution network bipolar short circuit fault protection and distance measurement device, characterized in that: It includes a central monitoring unit, a bipolar short-circuit energy release unit, a capacitor distance measurement unit, a multi-level converter, and a DC bus; The first input end of the bipolar short-circuit energy release unit is connected to the output positive pole of the multi-level converter, and the first output end of the bipolar short-circuit energy release unit is connected to the output negative pole of the multi-level converter MMC; the second input end of the bipolar short-circuit energy release unit is connected to the first output end of the central monitoring unit; the second output end of the bipolar short-circuit energy release unit is connected to the first input end of the central monitoring unit; The first input end of the capacitive distance measuring unit is connected to the positive pole of the DC bus; the first output end of the capacitive distance measuring unit is connected to the negative pole of the DC bus; the second input end of the capacitive distance measuring unit is connected to the second output end of the central monitoring unit; the second output end of the capacitive distance measuring unit is connected to the second input end of the central monitoring unit; The bipolar short-circuit energy release unit comprises a first voltage transformer, a first current transformer, a first energy dissipation resistor, a second energy dissipation resistor, a third energy dissipation resistor, a first insulated gate bipolar transistor, a second insulated gate bipolar transistor, a third insulated gate bipolar transistor, a first inductor, a second inductor and a third inductor; wherein, one end of the first energy dissipation resistor is connected in series with the first inductor, and the other end of the first inductor is connected to the collector of the first insulated gate bipolar transistor, one end of the second energy dissipation resistor is connected in series with the second inductor, and the other end of the second inductor is connected to the collector of the second insulated gate bipolar transistor, one end of the third energy dissipation resistor is connected in series with the third inductor, and the other end of the third inductor is connected to the collector of the third insulated gate bipolar transistor, and the other ends of the first inductor, the second inductor and the third inductor are connected together as the first input end of the bipolar short-circuit energy release unit; the first insulated gate bipolar transistor The emitter of the first insulated gate bipolar transistor, the emitter of the second insulated gate bipolar transistor, and the emitter of the third insulated gate bipolar transistor are connected together as the first output end of the bipolar short-circuit energy release unit; the base of the first insulated gate bipolar transistor, the base of the second insulated gate bipolar transistor, and the base of the third insulated gate bipolar transistor serve as the second input end of the bipolar short-circuit energy release unit; one end of the first voltage transformer is connected to the positive pole of the DC bus, and the other end of the first voltage transformer is connected to the negative pole of the DC bus; one end of the first current transformer is connected to the emitter of the first insulated gate bipolar transistor, the emitter of the second insulated gate bipolar transistor, and the emitter of the third insulated gate bipolar transistor, and the other end of the first current transformer is connected in series to the negative pole output terminal of the DC side of the multi-level converter MMC; the first current transformer and the first voltage transformer serve as the second output end of the bipolar short-circuit energy release unit; The capacitive ranging unit includes a fourth insulated gate bipolar transistor, a fifth insulated gate bipolar transistor, a sixth insulated gate bipolar transistor, a first DC voltage source, a fourth inductor, a first capacitor, a fourth energy consumption resistor, a second current transformer, a third current transformer and a second voltage transformer; wherein the collector of the fourth insulated gate bipolar transistor is connected to the positive electrode of the first DC voltage source, the negative electrode of the first DC voltage source is connected to one end of the fourth energy consumption resistor and one end of the fourth inductor, the other end of the fourth energy consumption resistor is connected to one end of the third current transformer, the other end of the third current transformer is connected to the collector of the fifth insulated gate bipolar transistor, and the emitter of the sixth insulated gate bipolar transistor is connected to one end of the first capacitor. end, the other end of the first capacitor is connected to one end of the second current transformer, the emitter of the fourth insulated gate bipolar transistor, the collector of the sixth insulated gate bipolar transistor and one end of the second voltage transformer are connected together as the first input end of the capacitance ranging unit; the other end of the fourth inductor, the emitter of the fifth insulated gate bipolar transistor, the other end of the second current transformer and the other end of the second voltage transformer are connected together as the first output end of the capacitance ranging unit; the base of the fourth edge gate bipolar transistor, the base of the fifth insulated gate bipolar transistor and the base of the sixth insulated gate bipolar transistor serve as the second input end of the capacitance ranging unit; the second voltage transformer, the second current transformer and the third current transformer serve as the second output end of the capacitance ranging unit.

2. A MMC type distribution network bipolar short circuit fault protection and ranging method, based on a MMC type distribution network bipolar short circuit fault protection and ranging device in claim 1, characterized in that: The following steps are involved: Step 1: Using a central monitoring unit to control the internal components of the capacitance ranging unit, when the central monitoring unit receives a signal that the voltage value monitored by the second voltage transformer is 0, a control signal is sent to the fourth insulated gate bipolar transistor and the sixth insulated gate bipolar transistor inside the capacitance ranging unit to turn them on; Step 2: Use a second current transformer and a second voltage transformer to monitor the voltage across the first capacitor and the current flowing through the first capacitor in real time; Step 3: When the voltage across the first capacitor reaches a set value, the central monitoring unit sends a control signal to the fourth insulated gate bipolar transistor and the sixth insulated gate bipolar transistor to disconnect them, and the capacitor charging is completed. At the same time, the central monitoring unit sends a control signal to the fifth insulated gate bipolar transistor to turn it on; Step 4: using the third current transformer to monitor the current flowing through the fourth energy consumption resistor in real time, when the collected current is 0, the central monitoring unit sends a control signal to the fifth insulated gate bipolar transistor to disconnect it; Step 5: Using a central monitoring unit to control the internal components of the bipolar short circuit energy release unit, when a bipolar short circuit fault occurs in the system, the central monitoring unit sends a control signal to the first insulated gate bipolar transistor, the second insulated gate bipolar transistor, and the third insulated gate bipolar transistor of the bipolar short circuit energy release unit to turn on the transistor; Step 6: Use the first current transformer and the first voltage transformer inside the bipolar short-circuit energy release unit to collect the bus current and the voltage between the positive and negative buses in real time, and send the collected voltage and current values ​​to the central monitoring unit; when a bipolar short-circuit fault occurs, the energy stored in the modular multilevel converter MMC will be released through three energy consumption resistors; Step 7: The central monitoring unit determines whether the voltage value and the current value monitored by the first current transformer and the first voltage transformer are 0. If they are 0, execute step 8; otherwise, continue to execute step 6; Step 8: The central monitoring unit sends a control signal to the first insulated gate bipolar transistor, the second insulated gate bipolar transistor, and the third insulated gate bipolar transistor to disconnect the bipolar short circuit energy release unit; Step 9: The central monitoring unit sends a control signal to the sixth insulated gate bipolar transistor to turn on the capacitor ranging unit. At the same time, the second current transformer and the second voltage transformer monitor the voltage across the first capacitor and the current flowing through the capacitor in real time for ranging. The central monitoring unit processes the voltage and current data to obtain the bipolar short circuit fault distance.

3. A MMC type distribution network bipolar short circuit fault protection and ranging method according to claim 2, characterized in that: The principle of ranging described in step 9 is shown in formula (1) and formula (2): Among them, u c (t) represents the voltage across the capacitor; i c (t) represents the current flowing through the capacitor; r0 represents the resistance per unit length of the DC bus; l0 represents the inductance per unit length of the DC bus; x represents the bipolar short-circuit fault distance of the DC bus; t represents time.

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