A device for connecting an energy storage system to a DC microgrid and a method for controlling its fault current.

Abstract

This invention discloses an energy storage system access device for a DC microgrid and its fault current control method. It includes an energy storage system access module and a control module. The energy storage system access module is used to connect the energy storage system to the DC microgrid and realize voltage conversion between the two. When the energy storage system supplies power to the DC microgrid, the voltage is converted to a step-down converter; when the DC microgrid charges the energy storage system, the voltage is converted to a step-up converter. The control module is used to detect inter-electrode short-circuit faults in the DC microgrid and control the energy storage system access module based on the detection results. This invention makes the fault current of the energy storage system access module transparent and controllable through the energy storage system access method and its control scheme.

Description

Technical Field

[0001] This invention relates to fault current control, and more particularly to an energy storage system access device for a DC microgrid and a fault current control method thereof. Background Technology

[0002] With the increasing severity of the energy crisis and environmental pollution, renewable energy sources are being vigorously developed. Users are placing higher demands on distribution networks in terms of distributed power source integration, diversified load and electricity demand, power quality, and power supply reliability. DC microgrids, due to their inherent advantage in improving the efficiency of distributed power source absorption, have become a research hotspot both domestically and internationally.

[0003] Due to the randomness and intermittency of distributed power sources, energy storage systems with flexible adjustment capabilities are generally indispensable in microgrids. Figure 1 The diagram illustrates the existing energy storage system's access scheme and control strategy. Previously, energy storage systems primarily focused on power regulation or voltage support, typically consisting of low-voltage battery stacks and boost-type DC / DC converters. During grid-connected operation (i.e., connected to the mains), the main power source for the DC microgrid is generally the AC distribution network. The energy storage system is mainly used to regulate power, smooth power fluctuations from distributed sources, or optimize power flow for economical operation. During off-grid operation (i.e., disconnected from the mains), due to the randomness and volatility of distributed sources, they generally cannot serve as the main power source. In this case, the energy storage system acts as the main power source, employing a voltage control strategy to maintain voltage stability in the DC microgrid. Under this structure and control strategy, when an inter-electrode short-circuit fault occurs, the energy storage battery, acting as a voltage source, will provide a large short-circuit current to the fault point, jeopardizing system and equipment safety. Currently, the commonly used fault current-limiting measure in engineering is to add a current-limiting reactor on the energy storage system's output side. Its disadvantages include increased equipment requirements, the reactor's saturation under high current leading to loss of current-limiting function, limited current-limiting effect, difficulty in determining reactor values, and challenges in coordinating with protection systems. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide an energy storage system access device for DC microgrid and its fault current control method. Through the access method and control scheme of the energy storage system, the fault current of the energy storage system access module is made transparent and controllable.

[0005] The objective of this invention is achieved through the following technical solution: an energy storage system access device for a DC microgrid, comprising an energy storage system access module and a control module;

[0006] The energy storage system access module is used to connect the energy storage system to the DC microgrid and realize voltage conversion between the energy storage system and the DC microgrid; when the energy storage system supplies power to the DC microgrid, the voltage is converted to step-down conversion; when the DC microgrid charges the energy storage system, the voltage is converted to step-up conversion.

[0007] The control module is used to detect inter-electrode short-circuit faults in the DC microgrid and control the energy storage system access module based on the detection results.

[0008] The energy storage system access module adopts a DC / DC conversion circuit, including a first IGBT switch S1, a second IGBT switch S2, an inductor L, a first capacitor C1, a second capacitor C2, and a third capacitor C3;

[0009] Both the first IGBT switch S1 and the second IGBT switch S2 are reverse-conducting IGBT devices. The first IGBT switch S1 is composed of a first IGBT device M1 and a first diode D1 connected in reverse parallel, that is, the anode of the first diode D1 is connected to the source of the first IGBT device M1, and the cathode of the first diode D1 is connected to the drain of the first IGBT device M1. The second IGBT switch S2 is composed of a second IGBT device M2 and a first diode D2 connected in reverse parallel, that is, the anode of the second diode D2 is connected to the source of the second IGBT device M2, and the cathode of the second diode D2 is connected to the drain of the second IGBT device M2.

[0010] The drain of the first IGBT device M1 is connected to the first input terminal of the energy storage system access module. The source of the first IGBT device M1 is connected to the drain of the second IGBT device M2, and the source of the second IGBT device M2 is connected to the second input terminal of the energy storage system access module. One end of the first capacitor C1 is connected to the drain of the first IGBT device M1, and the other end is connected to the source of the second IGBT device M2 through the second capacitor C2. The first end of the inductor L is connected between the source of the first IGBT device M1 and the drain of the second IGBT device M2, and the second end of the inductor L1 is connected to the first output terminal of the energy storage system access module. The source of the second IGBT device M2 is also connected to the second output terminal of the energy storage system access module. One end of the third capacitor C3 is connected to the second end of the inductor L1, and the other end of the third capacitor is connected to the source of the second IGBT device M2.

[0011] The first and second input terminals of the energy storage system access module are used to connect to the energy storage system. When the energy storage system supplies power to the DC microgrid, the voltage between the first and second input terminals is the voltage supplied by the energy storage system to the energy storage system access module. When the DC microgrid charges the energy storage system, the voltage between the first and second input terminals is the voltage supplied by the energy storage system access module to the energy storage system.

[0012] The first and second output terminals of the energy storage system access module are used to connect to the DC bus of the DC microgrid. When the energy storage system supplies power to the DC microgrid, the voltage between the first and second output terminals is the voltage supplied to the DC microgrid. When the DC microgrid charges the energy storage system, the voltage between the first and second output terminals is the voltage supplied by the DC microgrid to the energy storage system access module.

[0013] A fault current control method for an energy storage system access device of a DC microgrid includes the following steps:

[0014] S1. The fault diagnosis unit detects the inter-electrode voltage and current of the energy storage system's connected modules and performs fault diagnosis accordingly.

[0015] When the inter-electrode voltage is less than the set value U set And the current is higher than the set value I set If a fault is detected, a fault diagnosis result of 1 is output; otherwise, a fault diagnosis result of 0 is output. The fault diagnosis result is then sent to the control mode switching unit.

[0016] S2. The control mode switching unit selects the control mode based on the fault diagnosis results:

[0017] When the fault diagnosis result of the fault diagnosis unit is 0, select control mode A. In this mode, use any of the following methods to control the energy storage system access module:

[0018] (1) When the energy storage system supplies power to the DC microgrid: the voltage output by the energy storage system access module to the DC microgrid is subtracted from the preset reference voltage, and the difference is then proportionally integrated through the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0019] Alternatively, when the DC microgrid charges the energy storage system, a pre-set reference voltage is used to subtract the voltage supplied by the DC microgrid to the energy storage system access module. The resulting difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0020] (2) When the energy storage system supplies power to the DC microgrid, the power transmitted to the DC microgrid by the energy storage system access module is subtracted by the preset reference power, and then the result is proportionally integrated by the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0021] Alternatively, when the DC microgrid charges the energy storage system, the power delivered by the DC microgrid to the energy storage system access module is subtracted from the preset reference power. The result is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0022] When the fault diagnosis result of the fault diagnosis unit is 1, select control mode B:

[0023] Using a pre-set reference current, the current of the energy storage system access module when an inter-electrode short-circuit fault occurs is subtracted. The difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch, which is used to control the energy storage system access module.

[0024] S3. Using the duty cycle obtained in step S2, control the first IGBT device M1 and the second IGBT device M2 in the energy storage access system, thereby realizing the on / off control of the first IGBT switch S1 and the second IGBT switch S2:

[0025] When the energy storage system supplies power to the DC microgrid, the duty cycle obtained in step S2 is used to control the second IGBT device in the energy storage access system, and the trigger signal of the first IGBT device is blocked.

[0026] When the DC microgrid charges the energy storage system, the duty cycle is obtained using step S2 to control the first IGBT device in the energy storage access system, and the trigger signal of the second IGBT device is blocked.

[0027] In this context, "locking" means not providing a trigger signal to turn on the switch, i.e., the duty cycle is 0.

[0028] The beneficial effects of the present invention are: (1) It significantly reduces the fault current and makes the fault current flexible and controllable: The access method and control method of this patent can achieve flexible and controllable short-circuit current during short-circuit faults through the voltage reduction operation strategy without affecting the normal operation of the system or increasing the cost, thereby significantly reducing the short-circuit current provided by the battery energy storage system and achieving a good current limiting effect.

[0029] (2) Simplified protection: By adopting the access method and control method proposed in this patent, the microgrid protection can be made simpler and easier to achieve differential coordination through precise control of the short-circuit current and coordination with the protection settings.

[0030] (3) Automatic switching of control mode: The access method and control method proposed in this patent can automatically detect faults, automatically limit current, and automatically recover after the fault is cleared.

[0031] (4) Wide range of applications: The access method and control method proposed in this patent can automatically realize fault current control in both grid-connected and off-grid states, and have a wide range of applications. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the energy storage system access principle of the present invention;

[0033] Figure 2 A schematic diagram of the circuit principle for the energy storage system access module;

[0034] Figure 3 A schematic diagram of the control logic for the energy storage system access module;

[0035] Figure 4 This is a schematic diagram illustrating the principle after connecting to a DC microgrid in the embodiment. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0037] Considering the voltage source characteristics of a battery energy storage system, a battery energy storage system connected to a DC microgrid via a DC / DC converter can be equivalent to an adjustable DC voltage source. When an inter-electrode short-circuit fault occurs in the system, the short-circuit current provided by the battery energy storage system can be expressed by the following formula:

[0038] i = U d / (Z+R f )

[0039] Among them, U d Z represents the equivalent voltage output of the battery energy storage system, Z represents the impedance between the fault point and the DC / DC converter, and R represents the impedance between the fault point and the DC / DC converter. f This represents the fault transition resistance. It is evident that the short-circuit current provided by the battery energy storage system primarily depends on the magnitude of the equivalent voltage Ud of the battery energy storage system. Therefore, controlling the equivalent voltage of the battery energy storage system controls the short-circuit current provided by that branch. The lower the equivalent voltage, the smaller the short-circuit current provided by the battery energy storage system. Consequently, this patent proposes an access method via a step-down DC / DC converter (from the energy storage side to the DC microgrid side; if from the DC microgrid side to the energy storage side, it is a step-up converter) to connect to the DC microgrid. This increases the number of series-connected batteries in the battery stack, ensuring that its minimum operating voltage at the port is higher than the DC microgrid bus voltage, and then connects to the DC bus of the DC microgrid via the step-down DC / DC converter. When an inter-electrode fault occurs, the step-down DC / DC converter is controlled to operate at a reduced voltage, greatly reducing Ud. d This value can significantly reduce the short-circuit current supplied by the battery energy storage system branch. Furthermore, by adjusting U... d The precise adjustment also enables precise control of the short-circuit current supplied to the branch.

[0040] It should be noted that in some high-resistance grounding systems, the fault current is very small during pole-to-ground faults and will not have a significant impact on system operation. Therefore, this patent is mainly designed for pole-to-pole faults.

[0041] The function of this patent is that, during operation, the battery energy storage system continues to be used for voltage or power regulation; in the event of a short-circuit fault, the battery energy storage system operates at a reduced voltage, flexibly controlling the short-circuit current it provides; after the fault is cleared, the battery energy storage system should resume its function in the microgrid, continuing to exert its power or voltage regulation capabilities. Therefore, during normal operation, the control method of the battery energy storage system in this patent is consistent with conventional methods, that is, power control is generally used when connected to the grid, and voltage control is generally used when disconnected from the grid. When a short-circuit fault occurs, the battery energy storage system in this patent switches to a reduced voltage operation mode, and current control can be used to achieve precise control of the short-circuit current. After the fault is cleared, it switches back to voltage or power control mode, specifically:

[0042] like Figure 1 As shown, an energy storage system access device for a DC microgrid includes an energy storage system access module and a control module.

[0043] The energy storage system access module is used to connect the energy storage system to the DC microgrid and realize voltage conversion between the energy storage system and the DC microgrid; when the energy storage system supplies power to the DC microgrid, the voltage is converted to step-down conversion; when the DC microgrid charges the energy storage system, the voltage is converted to step-up conversion.

[0044] The control module is used to detect inter-electrode short-circuit faults in the DC microgrid and control the energy storage system access module based on the detection results.

[0045] In the embodiments of this application, the energy storage system access module adopts a DC / DC conversion circuit, such as... Figure 2 The diagram shown is a schematic diagram of the circuit principle of the energy storage system access module. The energy storage system access module adopts a DC / DC conversion circuit, including a first IGBT switch S1, a second IGBT switch S2, an inductor L, a first capacitor C1, a second capacitor C2, and a third capacitor C3.

[0046] Both the first IGBT switch S1 and the second IGBT switch S2 are reverse-conducting IGBT devices. The first IGBT switch S1 is composed of a first IGBT device M1 and a first diode D1 connected in reverse parallel, that is, the anode of the first diode D1 is connected to the source of the first IGBT device M1, and the cathode of the first diode D1 is connected to the drain of the first IGBT device M1. The second IGBT switch S2 is composed of a second IGBT device M2 and a first diode D2 connected in reverse parallel, that is, the anode of the second diode D2 is connected to the source of the second IGBT device M2, and the cathode of the second diode D2 is connected to the drain of the second IGBT device M2.

[0047] The drain of the first IGBT device M1 is connected to the first input terminal of the energy storage system access module. The source of the first IGBT device M1 is connected to the drain of the second IGBT device M2, and the source of the second IGBT device M2 is connected to the second input terminal of the energy storage system access module. One end of the first capacitor C1 is connected to the drain of the first IGBT device M1, and the other end is connected to the source of the second IGBT device M2 through the second capacitor C2. The first end of the inductor L is connected between the source of the first IGBT device M1 and the drain of the second IGBT device M2, and the second end of the inductor L1 is connected to the first output terminal of the energy storage system access module. The source of the second IGBT device M2 is also connected to the second output terminal of the energy storage system access module. One end of the third capacitor C3 is connected to the second end of the inductor L1, and the other end of the third capacitor is connected to the source of the second IGBT device M2.

[0048] The first and second input terminals of the energy storage system access module are used to connect to the energy storage system. When the energy storage system supplies power to the DC microgrid, the voltage between the first and second input terminals is the voltage supplied by the energy storage system to the energy storage system access module. When the DC microgrid charges the energy storage system, the voltage between the first and second input terminals is the voltage supplied by the energy storage system access module to the energy storage system.

[0049] The first and second output terminals of the energy storage system access module are used to connect to the DC bus of the DC microgrid. When the energy storage system supplies power to the DC microgrid, the voltage between the first and second output terminals is the voltage supplied to the DC microgrid. When the DC microgrid charges the energy storage system, the voltage between the first and second output terminals is the voltage supplied by the DC microgrid to the energy storage system access module.

[0050] In the embodiments of this application, the energy storage system is a battery energy storage system, including a high-voltage battery stack formed by multiple batteries connected in series. The control module is connected to the gate of the first IGBT device M1 and the gate of the second IGBT device M2 in the energy storage system access module, respectively, and controls the entire energy storage system access module by controlling the conduction duty cycle of the first IGBT device M1 and the second IGBT device M2.

[0051] The control module includes:

[0052] The fault diagnosis unit is used to detect the inter-electrode voltage and current of the energy storage system's access module: when the inter-electrode voltage is less than the set value U... set And the current is higher than the set value I setIf a fault is detected, a fault diagnosis result of 1 is output; otherwise, a fault diagnosis result of 0 is output. The fault diagnosis result is then sent to the control mode switching unit. A fault diagnosis result of 0 indicates either that the inter-pole short circuit fault has not occurred or that the inter-pole short circuit fault has been cleared. A fault diagnosis result of 1 indicates that the fault has occurred and has not been cleared.

[0053] The normal operation control unit is used to determine the control scheme of the energy storage system access module according to the operating status of the DC microgrid: when the DC microgrid is connected to the grid, the energy storage system access module adopts power control; when the DC microgrid is isolated, the energy storage system access module adopts voltage control.

[0054] The short-circuit current control unit is used to determine the control scheme of the energy storage system access module based on the current of the energy storage system access module when an inter-electrode short-circuit fault occurs in the DC microgrid.

[0055] The control mode switching unit is used to control the energy storage system access module in control mode A when the fault diagnosis result of the fault diagnosis unit is 0; in control mode A, it switches to the normal operation control unit, and the control scheme determined by the normal operation control unit controls the energy storage system access module; when the fault diagnosis result of the fault diagnosis unit is 1, it switches to control mode B; in control mode B, it switches to the short-circuit current control unit, and the control scheme determined by the short-circuit current control unit controls the system access module.

[0056] Wherein, after the energy storage system access module is connected to the DC microgrid, the inter-electrode voltage is the voltage between the first and second output terminals of the energy storage system access module, and the current is the current of the energy storage system access module after the DC load is connected to the DC microgrid; the inter-electrode short-circuit fault refers to the fault caused by a short circuit in the DC load branch or power supply branch after the DC microgrid is connected to the DC load or power supply branch.

[0057] When determining the control scheme for the energy storage system, the normal operation control unit adopts any of the following methods:

[0058] (1) When the energy storage system supplies power to the DC microgrid: the voltage output by the energy storage system access module to the DC microgrid is subtracted from the preset reference voltage, and the difference is then proportionally integrated through the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0059] Alternatively, when the DC microgrid charges the energy storage system, a pre-set reference voltage is used to subtract the voltage supplied by the DC microgrid to the energy storage system access module. The resulting difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0060] (2) When the energy storage system supplies power to the DC microgrid, the power transmitted to the DC microgrid by the energy storage system access module is subtracted by the preset reference power, and then the result is proportionally integrated by the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0061] Alternatively, when the DC microgrid charges the energy storage system, a pre-set reference power is used to subtract the power delivered by the DC microgrid to the energy storage system access module. The result is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0062] The short-circuit current control unit determines the control scheme for the energy storage system access module as follows:

[0063] By subtracting the current of the energy storage system access module when an inter-electrode short-circuit fault occurs from the preset reference current, the difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0064] A fault current control method for a DC microgrid energy storage system access device, the control logic of which is as follows: Figure 3 As shown, it includes the following steps:

[0065] S1. The fault diagnosis unit detects the inter-electrode voltage and current of the energy storage system's connected modules and performs fault diagnosis accordingly.

[0066] When the inter-electrode voltage is less than the set value U set And the current is higher than the set value I set If a fault is detected, a fault diagnosis result of 1 is output; otherwise, a fault diagnosis result of 0 is output. The fault diagnosis result is then sent to the control mode switching unit.

[0067] S2. The control mode switching unit selects the control mode based on the fault diagnosis results:

[0068] When the fault diagnosis result of the fault diagnosis unit is 0, select control mode A. In this mode, use any of the following methods to control the energy storage system access module:

[0069] (1) When the energy storage system supplies power to the DC microgrid: the voltage output by the energy storage system access module to the DC microgrid is subtracted from the preset reference voltage, and the difference is then proportionally integrated through the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0070] Alternatively, when the DC microgrid charges the energy storage system, a pre-set reference voltage is used to subtract the voltage supplied by the DC microgrid to the energy storage system access module. The resulting difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0071] (2) When the energy storage system supplies power to the DC microgrid, the power transmitted to the DC microgrid by the energy storage system access module is subtracted by the preset reference power, and then the result is proportionally integrated by the PI controller to obtain the duty cycle of the switch, and the energy storage system access module is controlled.

[0072] Alternatively, when the DC microgrid charges the energy storage system, the power delivered by the DC microgrid to the energy storage system access module is subtracted from the preset reference power. The result is then proportionally integrated through a PI controller to obtain the duty cycle of the switch and control the energy storage system access module.

[0073] When the fault diagnosis result of the fault diagnosis unit is 1, select control mode B:

[0074] Using a pre-set reference current, the current of the energy storage system access module when an inter-electrode short-circuit fault occurs is subtracted. The difference is then proportionally integrated through a PI controller to obtain the duty cycle of the switch, which is used to control the energy storage system access module.

[0075] S3. Using the duty cycle obtained in step S2, control the first IGBT device M1 and the second IGBT device M2 in the energy storage access system, thereby realizing the on / off control of the first IGBT switch S1 and the second IGBT switch S2:

[0076] When the energy storage system supplies power to the DC microgrid, the duty cycle obtained in step S2 is used to control the second IGBT device in the energy storage access system, and the trigger signal of the first IGBT device is blocked.

[0077] When the DC microgrid charges the energy storage system, the duty cycle is obtained using step S2 to control the first IGBT device in the energy storage access system, and the trigger signal of the second IGBT device is blocked.

[0078] In this context, "locking" means not providing a trigger signal to turn on the switch, i.e., the duty cycle is 0.

[0079] like Figure 4As shown in the embodiments of this application, after the energy storage system access module is connected to the DC microgrid, the inter-electrode voltage is the voltage between the first and second output terminals of the energy storage system access module, and the current is the current output by the energy storage system access module after the DC load is connected to the DC microgrid; the inter-electrode short-circuit fault refers to the fault caused by a short circuit in the DC load branch or other power branch after the DC load or power branch is connected to the DC microgrid.

[0080] The above description represents preferred embodiments of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technical or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. An energy storage system access device for a DC microgrid, characterized in that: The energy storage system access module and the control module are connected to each other. The energy storage system access module is configured to access the energy storage system to the DC microgrid and realize voltage conversion between the energy storage system and the DC microgrid; when the energy storage system supplies power to the DC microgrid, the voltage conversion is a step-down conversion; when the DC microgrid charges the energy storage system, the voltage conversion is a step-up conversion. The control module is configured to detect an inter-pole short-circuit fault of the DC microgrid and control the energy storage system access module according to a detection result. The control module comprises: A fault diagnosis unit is configured to detect the voltage and current between poles of the energy storage system access module: when the voltage between poles is less than a set value U set and the current is higher than a set value I set , it is determined that a fault has occurred, and outputs a fault diagnosis result of 1, otherwise outputs a fault diagnosis result of 0; and the fault diagnosis result is given to the control mode switching unit; when the fault diagnosis result is 0, it includes two cases of no occurrence of a short circuit fault between poles or the short circuit fault between poles has been cleared; when the fault diagnosis result is 1, it means that a fault has occurred and has not been cleared; a normal operation control unit configured to determine a control scheme of the energy storage system access module according to an operation state of the DC microgrid; when the DC microgrid is grid-connected, the energy storage system access module adopts power control; when the DC microgrid is islanded, the energy storage system access module adopts voltage control; a short-circuit current control unit configured to determine a control scheme of the energy storage system access module according to a current of the energy storage system access module when the DC microgrid has an inter-pole short-circuit fault; a control mode switching unit configured to adopt control mode A to control the energy storage system access module when a fault diagnosis result of a fault diagnosis unit is 0; in control mode A, the control scheme determined by the normal operation control unit is adopted to control the energy storage system access module; adopt control mode B to control the energy storage system access module when the fault diagnosis result of the fault diagnosis unit is 1; in control mode B, the control scheme determined by the short-circuit current control unit is adopted to control the energy storage system access module. The inter-pole voltage is the voltage between the first output end and the second output end of the energy storage system access module, and the current is the current of the energy storage system access module after the DC microgrid accesses a DC load; the inter-pole short-circuit fault is a fault formed by a short circuit of a DC load branch or a power supply branch after the DC microgrid accesses a DC load or a power supply branch.

2. The energy storage system access device of a DC microgrid of claim 1, wherein: The energy storage system access module adopts a DC / DC conversion circuit comprising a first IGBT switch S1, a second IGBT switch S2, an inductor L, a first capacitor C1, a second capacitor C2, and a third capacitor C3. The first IGBT switch S1 and the second IGBT switch S2 are both reverse-conducting IGBT devices; the first IGBT switch S1 is composed of a first IGBT device M1 and a first diode D1 in reverse parallel connection, i.e., the anode of the first diode D1 is connected to the source of the first IGBT device M1, and the cathode of the first diode D1 is connected to the drain of the first IGBT device M1; the second IGBT switch S2 is composed of a second IGBT device M2 and a second diode D2 in reverse parallel connection, i.e., the anode of the second diode D2 is connected to the source of the second IGBT device M2, and the cathode of the second diode D2 is connected to the drain of the second IGBT device M2. The drain of the first IGBT device M1 is connected to the first input of the energy storage system access module, the source of the first IGBT device M1 is connected to the drain of the second IGBT device M2, and the source of the second IGBT device M2 is connected to the second input of the energy storage system access module; one end of the first capacitor C1 is connected to the drain of the first IGBT device M1, and the other end is connected to the source of the second IGBT device M2 through the second capacitor C2; the first end of the inductor L is connected between the source of the first IGBT device M1 and the drain of the second IGBT device M2, the second end of the inductor L1 is connected to the first output of the energy storage system access module, and the source of the second IGBT device M2 is also connected to the second output of the energy storage system access module; one end of the third capacitor C3 is connected to the second end of the inductor L1, and the other end of the third capacitor is connected to the source of the second IGBT device M2; The first input and the second input of the energy storage system access module are used to access the energy storage system, and when the energy storage system supplies power to the direct current microgrid, the voltage between the first input and the second input is the voltage delivered by the energy storage system to the energy storage system access module; when the direct current microgrid charges the energy storage system, the voltage between the first input and the second input is the voltage delivered by the energy storage system access module to the energy storage system. The first output and the second output of the energy storage system access module are used to access the direct current bus of the direct current microgrid, and when the energy storage system supplies power to the direct current microgrid, the voltage between the first output and the second output is the voltage delivered to the direct current microgrid; when the direct current microgrid charges the energy storage system, the voltage between the first output and the second output is the voltage delivered by the direct current microgrid to the energy storage system access module.

3. The energy storage system access device of a DC microgrid of claim 1, wherein: The energy storage system is a battery energy storage system, which includes a high-voltage battery stack formed by a plurality of batteries connected in series.

4. The energy storage system access device of a DC microgrid of claim 2, wherein: The control module is connected to the gate of the first IGBT device M1 and the gate of the second IGBT device M2 in the energy storage system access module, respectively, and controls the energy storage system access module by controlling the conduction duty cycle of the first IGBT device M1 and the second IGBT device M2.

5. The energy storage system access device of a DC microgrid of claim 1, wherein: When the normal operation control unit determines the control scheme of the energy storage system, any of the following methods is used: (1) When the energy storage system supplies power to the direct current microgrid: a reference voltage is set in advance, the voltage output by the energy storage system access module to the direct current microgrid is subtracted, and then the difference is proportional and integrated by a PI controller to obtain the duty cycle of switch conduction, and the energy storage system access module is controlled; Or, when the direct current microgrid charges the energy storage system, a reference voltage is set in advance, the voltage delivered by the direct current microgrid to the energy storage system access module is subtracted, and then the difference is proportional and integrated by a PI controller to obtain the duty cycle of switch conduction, and the energy storage system access module is controlled. (2) When the energy storage system supplies power to the DC microgrid, the preset reference power is subtracted from the power delivered by the energy storage system access module to the DC microgrid, and then the result is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; Or, when the DC microgrid charges the energy storage system, the preset reference power is subtracted from the power delivered by the DC microgrid to the energy storage system access module, and then the result is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module.

6. The energy storage system access device of a DC microgrid of claim 1, wherein: The short-circuit current control unit determines the control scheme of the energy storage system access module as follows: The preset reference current is subtracted from the current of the energy storage system access module when an inter-pole short-circuit fault occurs in the energy storage system access module, and then the difference is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module.

7. A fault current control method for an energy storage system access device of a DC microgrid, based on the device of any one of claims 1-6, characterized in that: The following steps are included: S1. The fault diagnosis unit detects the inter-pole voltage and current of the energy storage system access module and performs fault diagnosis based on the detection results: When the voltage between the poles is less than a set value U set and the current is higher than a set value I set , it is determined that a fault has occurred, and a fault diagnosis result is output as 1, otherwise the fault diagnosis result is output as 0; and the fault diagnosis result is given to the control mode switching unit; S2. The control mode switching unit selects the control mode based on the fault diagnosis results: When the fault diagnosis result of the fault diagnosis unit is 0, control mode A is selected, and in this mode, the energy storage system access module is controlled by any of the following methods: (1) When the energy storage system supplies power to the DC microgrid: the preset reference voltage is subtracted from the voltage output by the energy storage system access module to the DC microgrid, and then the difference is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; Or, when the DC microgrid charges the energy storage system, the preset reference voltage is subtracted from the voltage delivered by the DC microgrid to the energy storage system access module, and then the difference is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; (2) When the energy storage system supplies power to the DC microgrid, the preset reference power is subtracted from the power delivered by the energy storage system access module to the DC microgrid, and then the result is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; Or, when the DC microgrid charges the energy storage system, the preset reference power is subtracted from the power delivered by the DC microgrid to the energy storage system access module, and then the result is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; When the fault diagnosis result of the fault diagnosis unit is 1, control mode B is selected: The preset reference current is subtracted from the current of the energy storage system access module when an inter-pole short-circuit fault occurs in the energy storage system access module, and then the difference is proportionally integrated by a PI controller to obtain the duty cycle of switch conduction, thereby controlling the energy storage system access module; S3. Control the first IGBT device M1 and the second IGBT device M2 in the energy storage access system by using the duty cycle obtained in step S2, so as to realize the on-off control of the first IGBT switch S1 and the second IGBT switch S2: When the energy storage system supplies power to the DC microgrid, the duty cycle obtained in step S2 is used to control the second IGBT device in the energy storage access system, and the trigger signal of the first IGBT device is blocked; When the DC microgrid charges the energy storage system, the duty cycle obtained in step S2 is used to control the first IGBT device in the energy storage access system, and the trigger signal of the second IGBT device is blocked. Wherein, the blocking is not to give the switch on trigger signal, that is, the duty cycle is 0.

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