Asymmetric series single-chain MMC solid-state transformer and control method thereof

Abstract

The present invention discloses an asymmetric series single-chain MMC solid-state transformer and a control method thereof, comprising an asymmetric series single-chain MMC unit, a DAB unit and a low-voltage converter unit; the asymmetric series single-chain MMC unit comprises a half-bridge submodule and a fault clearing submodule, and the half-bridge submodule and the fault clearing submodule comprise a DC capacitor; by asymmetrically arranging a DC isolation module on the three-phase AC side, it provides a DC isolation capacitor voltage during normal operation, and absorbs fault energy or supports line voltage during fault crossing to fully utilize it to meet normal working requirements and cope with DC short circuit faults. The solid-state transformer composed of the asymmetric series single-chain MMC unit, the DAB circuit and the low-voltage converter can not only provide multiple ports, but also further optimize energy management, which is helpful to improve the control performance of the solid-state transformer and enhance the operation safety of the system and equipment.

Description

Technical Field

[0001] The invention relates to an asymmetric series single-chain MMC solid-state transformer and a control method thereof, belonging to the technical field of modular multi-level converters. Background Art

[0002] In recent years, DC distribution networks have been widely used in the field of renewable energy due to their low cost and high flexibility. Multi-port solid-state transformers (SSTs) are the central equipment for energy transmission and conversion in AC / DC hybrid power grids. In order to connect to medium and high voltage DC power grids, SSTs often use modular multilevel converters (MMCs) that can provide medium and high voltage DC ports and have superior output characteristics, low switching frequency and modular characteristics as medium and high voltage side converters. They are gradually becoming the main topology of medium voltage DC distribution networks. However, the application of MMCs in medium voltage DC distribution networks also faces challenges, such as a large number of power equipment and the need to increase power density. In order to solve the above problems, several improved MMC topologies have been proposed, which are mainly divided into parallel, series and hybrid structures. Among them, the topology represented by the series single-chain MMC performs better in terms of power density and equipment volume optimization. Compared with the traditional three-phase MMC topology, the series single-chain MMC structure reduces the number of sub-modules (SMs) by 2 / 3. In addition, since the three-phase voltage output is coupled at the medium and high voltage AC ports of the series single-chain MMC, there is no need to connect a three-phase transformer to the medium and high voltage AC side.

[0003] DC short circuit fault is an important problem that must be faced in DC system. The existing MMC DC fault riding strategies mainly include blocking protection and voltage regulation operation. Blocking protection refers to blocking the driving signals of all switching devices and controlling and eliminating the fault current by using sub-modules with DC fault current blocking capability. Blocking protection can effectively absorb fault energy and limit fault current, but the sub-module capacitor voltage will cause serious imbalance and cause problems such as restart difficulty. The voltage regulation operation control strategy is to make the medium and high voltage DC side of the MMC work at 0 or even lower voltage, which depends on the use of bidirectional sub-modules that can output positive and negative voltages. Although these technologies have been studied a lot on MMC, and the topology and control method of the series single-chain MMC are different from those of the existing MMC, they cannot be directly applied. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide an asymmetric series single-chain MMC solid-state transformer and a control method thereof. For the existing MMC multi-port solid-state transformer, many sub-modules are used. Although improved MMCs with parallel, series and mixed structures have appeared, the topological structures and control methods of the traditional MMCs are not completely the same. In addition, the important problem of DC short-circuit faults must be faced, with the main goal of using fewer switching devices and giving full play to the role of each sub-module in different situations.

[0005] The present invention provides an asymmetric series single-chain MMC solid-state transformer, comprising an asymmetric series single-chain MMC unit, a DAB unit and a low-voltage converter unit; the asymmetric series single-chain MMC unit comprises a half-bridge submodule and a fault clearing submodule, and the half-bridge submodule and the fault clearing submodule comprise a DC capacitor and a power electronic switch;

[0006] The DAB unit includes a DAB circuit, a DC capacitor C LV The front-stage DC side of the DAB circuit is connected in parallel with the DC capacitors of the half-bridge submodule and the fault clearing submodule respectively; the rear-stage DC sides of all DAB circuits are connected in parallel to form a low-voltage DC port, and a DC capacitor C is connected in parallel to the low-voltage DC port. LV The low-voltage DC port is connected in parallel with the DC side of the low-voltage converter unit, and the AC side of the low-voltage converter unit serves as the low-voltage AC port of the solid-state transformer.

[0007] Preferably, the asymmetric series single-chain MMC unit includes a medium-high voltage AC port phase A, a medium-high voltage AC port phase B, a medium-high voltage AC port phase C, a medium-high voltage DC port, a bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 ; Bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 The two ends of the circuit formed in series serve as the medium and high voltage DC ports of the asymmetric series single chain MMC unit, and the medium and high voltage DC ports are connected in parallel with capacitors, and the bridge arm ARM AC1 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules, the bridge arm ARM AB Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM BC Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM AC2 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules; bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARMBC 、Bridge arm ARM AC2 The three connection points between the two are connection point A', connection point B' and connection point C'; the connection point A' is connected to the F end of the DC isolation module IDSM, the N end of the DC isolation module IDSM, the coupling inductor L d Connected to the medium and high voltage AC port A of the asymmetric series single-chain MMC unit, the connection point B' is connected through the coupling inductor L d Connected to the medium and high voltage AC port B of the asymmetric series single-chain MMC unit, the connection point C' is connected in sequence through the N end of the DC isolation module IDSM, the F end of the DC isolation module IDSM, the coupling inductor L d It is connected to the medium and high voltage AC port C of the asymmetric series single-chain MMC unit; each of the half-bridge submodule, fault clearing submodule and DC isolation module includes a DC capacitor and a power electronic switch.

[0008] Preferably, the rated voltage of the DC blocking capacitors included in the DC blocking modules on the medium- and high-voltage AC port phase A and the medium- and high-voltage AC port phase C of the solid-state transformer is 1 / 3 of the medium- and high-voltage DC port voltage of the solid-state transformer.

[0009] Preferably, the control method of the asymmetric series single-chain MMC solid-state transformer is based on an asymmetric series single-chain MMC solid-state transformer as described in any one of the items, all fault clearing submodules and DC isolation modules use submodules with fault clearing functions, when all fault clearing submodules and DC isolation modules have a DC short circuit fault on the high-voltage side of the solid-state transformer, a blocking method is used to absorb the fault energy, and the DC capacitor voltage of the fault clearing submodule does not exceed the rated voltage U cN When it is the main control target, the following steps are included:

[0010] Step 1: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, T1 of the unidirectional full-bridge submodule FCSM remains on, and the power electronic switches of all DC isolation modules IDSM remain on, that is, T2 and T3 of the DC isolation module IDSM remain on; the bridge arm ARM AC1 、Bridge arm ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BCProvide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side of the solid-state transformer and the low-voltage AC side of the solid-state transformer;

[0011] Detect whether there is a DC short circuit fault. If there is no DC short circuit fault, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, all DC isolation modules and all DAB circuits, and start charging the DC capacitors of all fault-clearing submodules.

[0012] Step 2: Determine whether the DC capacitor voltage in all fault clearing submodules exceeds the rated voltage U cN If it exceeds, the switches of all DAB circuits are controlled by the phase shift control mode, wherein the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit fault clearing submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the asymmetric series single-chain MMC unit fault clearing submodule = rated voltage U cN ;

[0013] The switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each DC capacitor voltage in the half-bridge submodule of the asymmetric series single-chain MMC unit is made equal to the average value of the DC capacitor voltage in the half-bridge submodule of all asymmetric series single-chain MMC units at the current sampling moment;

[0014] Step 3: Determine whether the DC short-circuit fault current is cleared, if not, execute steps 1 and 2, otherwise terminate the operation;

[0015] The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

[0016] Preferably, the phase shift control mode includes: controlling the phase difference between the control signal of the front switch of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule and the control signal of the rear switch, so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the asymmetric series single-chain MMC unit fault clearing submodule = rated voltage U cN ;

[0017] By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule, the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each capacitor voltage of the asymmetric series single-chain MMC unit half-bridge submodule is made equal to the average value of the capacitor voltage of all asymmetric series single-chain MMC unit half-bridge submodules at the current sampling moment;

[0018] Preferably, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U cN The difference is calculated by the PI or PR controller and then subtracted.

[0019] Preferably, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC voltage of the DC capacitor connected to the front stage of each DAB circuit and the average DC capacitor voltage in all asymmetric series single-chain MMC unit half-bridge sub-modules at the current sampling moment is calculated by the PI or PR controller and then subtracted to obtain the result.

[0020] Preferably, the control method of an asymmetric series single-chain MMC solid-state transformer is based on an asymmetric series single-chain MMC solid-state transformer as described in any one of the items, all fault clearing submodules and DC isolation modules use submodules with fault clearing functions, when all fault clearing submodules and DC isolation modules use a blocking method to absorb fault energy after a DC short circuit fault occurs on the high-voltage side of the solid-state transformer and take the DC capacitor voltage balance of all submodules as the main control target, the following steps are performed:

[0021] Step 11: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, T1 of the unidirectional full-bridge submodule FCSM is kept on, and T2 and T3 of the DC isolation module IDSM are kept on; the bridge arm ARM AC1 ,ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side of the solid-state transformer;

[0022] Detect whether there is a DC short circuit fault. If not, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, the switch control signals of all DC isolation modules and the switch control signals of all DAB circuits, and start charging the DC capacitors of all fault clearing submodules.

[0023] Step 12: Use the phase shift control mode to control the switches of all DAB circuits. The control goal is to make the low voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the DC capacitor voltage of each submodule in the asymmetric series single-chain MMC unit is equal to the average value of the DC capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment;

[0024] Step 13: Determine whether the fault current is cleared, if not, execute steps 11 and 12, otherwise terminate the operation;

[0025] The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

[0026] Preferably, the phase shift control mode includes: controlling the phase difference between the control signal of the front-stage switch and the control signal of the rear-stage switch in each DAB circuit to achieve the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN The voltage of each DC capacitor in the asymmetric series single-chain MMC unit = the average value of the DC capacitor voltages of all sub-modules of the asymmetric series single-chain MMC unit at the current sampling moment.

[0027] Preferably, the phase difference between the switch control signals of the previous and next stages of each DAB circuit is determined by the low voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage of the submodule connected to the front stage of each DAB circuit and the average value of the capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment is calculated by the PI or PR controller and then subtracted.

[0028] Preferably, the control method of the asymmetric series single-chain MMC solid-state transformer is based on an asymmetric series single-chain MMC solid-state transformer as described in any one of the items, all fault clearing submodules and DC isolation modules adopt bidirectional submodules that can operate in bidirectional mode, and when all fault clearing submodules and DC isolation modules have a DC short circuit fault on the high-voltage side of the solid-state transformer, the fault clearing submodules and DC isolation modules of the asymmetric series single-chain MMC units are operated in bidirectional mode to cross the fault as the main control target, and the following steps are performed:

[0029] Step 21: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, and the power electronic switches of all DC isolation modules IDSM remain turned on; the bridge arm ARM AC1 、Bridge arm ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BCProvide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control all their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side; detect whether there is a DC short-circuit fault, if not, continue to cycle to detect whether there is a DC short-circuit fault, otherwise all asymmetric series single-chain MMC unit fault clearing submodules and DC isolation modules IDSM operate in bidirectional operation mode, the output voltage of the DC isolation module IDSM on the A phase of the medium and high voltage AC port is equal to the AB two-phase line voltage, the output voltage of the DC isolation module IDSM on the C phase of the medium and high voltage AC port is equal to the BC two-phase line voltage, the bridge arm ARM AC1 、Bridge arm ARM AC2 The sum of the output voltages of the two DC isolation modules IDSM is equal to the AC two-phase line voltage; bypassing the half-bridge submodules of all asymmetric series single-chain MMC units;

[0030] Step 22: Control the switches of all DAB circuits using a phase shift control mode, wherein the switches of all DAB circuits connected to the DC capacitors in the fault clearing submodule capable of bidirectional operation with the asymmetric series single-chain MMC unit are controlled so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN To maintain the low voltage DC side voltage, the asymmetric series single chain MMC unit adopts a bidirectional fault clearing submodule with a DC capacitor voltage = rated voltage U cN ; Control the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule so that the voltage of each DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule = rated voltage U cN ;

[0031] Step 23: Determine whether the DC short-circuit fault current is cleared, if not, execute steps 21 and 22, otherwise terminate the operation;

[0032] The control strategy of the bidirectionally operable fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a full-bridge submodule, a cross-series twin submodule and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a full-bridge submodule, a cross-series twin submodule and a T-type full-bridge submodule.

[0033] Preferably, the phase shift control mode includes: controlling the phase difference between the control signal of the front switch of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule and the control signal of the rear switch, so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN, while making the asymmetric series single-chain MMC unit bidirectionally operable, the DC capacitor voltage in the fault clearing submodule = rated voltage U cN By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule, the capacitor voltage of the asymmetric series single-chain MMC unit half-bridge submodule = the rated voltage U cN .

[0034] Preferably, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitors in each bidirectionally operable fault clearing submodule of the asymmetric series single-chain MMC unit is determined by the low-voltage DC side output voltage U cLV Rated voltage U of the low voltage DC side of the solid-state transformer LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U c The difference is calculated by the PI controller or the PR controller and then subtracted.

[0035] Preferably, the phase difference of the front and rear stage switch control signals of the DAB circuit connected to the DC capacitors in each half-bridge submodule of the asymmetric series single-chain MMC unit is determined by the difference of the DC capacitor voltages in all the asymmetric series single-chain MMC unit half-bridge submodules and the rated voltage U cN The difference is calculated by the PI controller or the PR controller and then subtracted.

[0036] Preferably, if the DC short circuit fault on the medium and high voltage side is a permanent fault and the solid-state transformer cannot be shut down, all fault clearing sub-modules and DC isolation modules must use sub-modules that can operate in both directions, and must continue to operate in bidirectional mode after the fault is cleared; the medium and high voltage DC port of the asymmetric series single-chain MMC solid-state transformer adopts a bidirectional switch that can conduct in both directions and is composed of multiple fully controlled power electronic switching devices connected in reverse series. After the DC short circuit fault is cleared, the DC short circuit fault on the medium and high voltage side is bypassed, and the medium and high voltage DC port of the solid-state transformer is disconnected from the fault line by a switch.

[0037] Preferably, the present invention provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of any one of the methods described in the first aspect when executing the program.

[0038] Preferably, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of any one of the methods described in the first aspect.

[0039] The beneficial effects achieved by the present invention are:

[0040] 1) The present invention sets a DC isolation module asymmetrically on the three-phase AC side, which provides a DC isolation capacitor voltage during normal operation, and absorbs fault energy or supports line voltage during fault ride-through to fully utilize it to meet normal working requirements and deal with DC short-circuit faults. The solid-state transformer composed of asymmetric series single-chain MMC units, DAB circuits and low-voltage converters can not only provide multiple ports, but also further optimize energy management, which helps to improve the control performance of the solid-state transformer and enhance the operating safety of the system and equipment. The present invention fully utilizes the characteristics of the series single-chain MMC and uses fewer switching devices to achieve the DC short-circuit fault ride-through function.

[0041] 2) The proposed fault ride-through control method can effectively avoid capacitor overvoltage and improve the safety of system and equipment operation;

[0042] 3) Capacitor voltage balance can be achieved during fault clearing, facilitating restart;

[0043] 4) Low hardware complexity, low control complexity, and easy to implement;

[0044] 5) Different sub-modules can be used for various application scenarios, which is easy to promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the embodiments are briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0046] Figure 1 It is a topological structure diagram of the asymmetric series single-chain MMC solid-state transformer of the present invention;

[0047] Figure 2 It is a flow chart of the control method of the asymmetric series single-chain MMC solid-state transformer of the present invention;

[0048] Figure 3 for Figure 1 The topological structure diagram of the half-bridge submodule HBSM, the unidirectional full-bridge submodule FCSM and the DC isolation module IDSM is shown;

[0049] Figure 4 For Figure 3 Replacement circuit diagram for each module of asymmetric series single-chain MMC type solid-state transformer. DETAILED DESCRIPTION

[0050] To facilitate the technical solution of the application, some concepts involved in the application are first explained below.

[0051] See also Figure 1 , the present application discloses an asymmetric series single-chain MMC type solid-state transformer including an asymmetric series single-chain MMC unit, a DAB unit and a low-voltage converter unit; the asymmetric series single-chain MMC unit includes a half-bridge submodule and a fault clearing submodule, and the half-bridge submodule and the fault clearing submodule include a DC capacitor;

[0052] The DAB unit includes a DAB circuit, a DC capacitor C LV The front-stage DC side of the DAB circuit is connected in parallel with the DC capacitors of the half-bridge submodule and the fault clearing submodule respectively; the rear-stage DC sides of all DAB circuits are connected in parallel to form a low-voltage DC port, and a DC capacitor C is connected in parallel to the low-voltage DC port. LV The low-voltage DC port is connected in parallel with the DC side of the low-voltage converter unit, and the AC side of the low-voltage converter unit serves as the low-voltage AC port of the solid-state transformer.

[0053] The asymmetric series single-chain MMC unit includes a medium-high voltage DC port phase A, a medium-high voltage DC port phase B, a medium-high voltage DC port phase C, and a bridge arm ARM. AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 , medium and high voltage DC port, bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 The two ends of the circuit connected in series serve as the medium and high voltage DC ports of the asymmetric series single chain MMC unit, and the medium and high voltage DC ports are connected in parallel with capacitors, and the bridge arm ARM AC1 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules, the bridge arm ARM AB Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM BC Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM AC2 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules; bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 The three connection points between the two are connection point A', connection point B' and connection point C'; the connection point A' is connected to the F end of the DC isolation module IDSM, the N end of the DC isolation module IDSM, the coupling inductor L d Connected to the medium and high voltage AC port A of the asymmetric series single-chain MMC unit, the connection point B' is connected through the coupling inductor L dConnected to the medium and high voltage AC port B of the asymmetric series single-chain MMC unit, the connection point C' is connected in sequence through the N end of the DC isolation module IDSM, the F end of the DC isolation module IDSM, the coupling inductor L d It is connected to the medium and high voltage AC port C of the asymmetric series single-chain MMC unit; each of the half-bridge submodule, fault clearing submodule and DC isolation module includes a DC capacitor and a power electronic switch.

[0054] In the embodiment of the present application, all fault clearing submodules and DC isolation modules use submodules with fault clearing functions. When a DC short circuit fault occurs on the high-voltage side of the solid-state transformer, all fault clearing submodules and DC isolation modules use a blocking method to absorb the fault energy, and the DC capacitor voltage of the fault clearing submodule does not exceed the rated voltage U cN When it is the main control target, the following steps are included:

[0055] Step 1: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 All modulation and control methods of the existing series single-chain MMC are used, and all fault clearing submodules are used as half-bridge submodules, that is, T1 of the unidirectional full-bridge submodule FCSM is kept turned on, and the power electronic switches of all DC isolation modules IDSM are kept turned on, that is, T2 and T3 of the DC isolation module IDSM are kept turned on; the bridge arm ARM AC1 、Bridge arm ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side of the solid-state transformer and the low-voltage AC side of the solid-state transformer;

[0056] Detect whether there is a DC short circuit fault. If there is no DC short circuit fault, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, all DC isolation modules and all DAB circuits, start charging the DC capacitors of all fault clearing submodules, and go to step 2.

[0057] Step 2: Determine whether the DC capacitor voltage in all fault clearing submodules exceeds the rated voltage U cNIf it does not exceed, no additional control is performed. If it exceeds, the switches of all DAB circuits are controlled using the phase shift control mode. Among them, the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit fault clearing submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , in order to maintain the low-voltage DC side voltage of the solid-state transformer, and at the same time make the DC capacitor voltage in the asymmetric series single-chain MMC unit fault clearing submodule = rated voltage U cN ;

[0058] The switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each DC capacitor voltage in the half-bridge submodule of the asymmetric series single-chain MMC unit is made equal to the average value of the DC capacitor voltage in the half-bridge submodule of all asymmetric series single-chain MMC units at the current sampling moment, and then the process goes to step 3;

[0059] Step 3: Determine whether the DC short-circuit fault current has been cleared. If not, execute steps 1 and 2. Otherwise, terminate the operation and exit the fault clearing process.

[0060] The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

[0061] In the embodiment of the present application, the phase shift control mode includes:

[0062] By controlling the phase difference between the control signal of the front switch and the control signal of the rear switch of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule, the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the asymmetric series single-chain MMC unit bidirectional submodule = rated voltage U cN ;

[0063] By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule, the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each capacitor voltage of the asymmetric series single-chain MMC unit half-bridge submodule is made equal to the average value of the capacitor voltage of all asymmetric series single-chain MMC unit half-bridge submodules at the current sampling moment;

[0064] In the embodiment of the present application, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U c The difference is obtained by subtracting it through the PI controller or PR controller.

[0065] In the embodiment of the present application, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC voltage of the DC capacitor connected to the front stage of each DAB circuit and the average DC capacitor voltage in all asymmetric series single-chain MMC unit half-bridge sub-modules at the current sampling moment is calculated by a controller such as PI or PR and then subtracted.

[0066] In the embodiment of the present application, when all fault clearing submodules and DC isolation modules adopt a blocking method to absorb fault energy after a DC short circuit fault occurs on the high voltage side of the solid-state transformer and take the DC capacitor voltage balance of all submodules as the main control target, the following steps are performed:

[0067] Step 11: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 All modulation and control methods of the existing series single-chain MMC are used, and all fault clearing submodules are used as half-bridge submodules, that is, T1 of the unidirectional full-bridge submodule FCSM is kept turned on, and the power electronic switches of all DC isolation modules IDSM are kept turned on, that is, T2 and T3 of the DC isolation module IDSM are kept turned on; the bridge arm ARM AC1 ,ARM AC2The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side of the solid-state transformer;

[0068] Detect whether there is a DC short circuit fault. If not, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, the switch control signals of all DC isolation modules, and the switch control signals of all DAB circuits, start charging the DC capacitors of all fault clearing submodules, and go to step 12.

[0069] Step 12: Use the phase shift control mode to control the switches of all DAB circuits. The control goal is to make the low voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the DC capacitor voltage of each submodule in the asymmetric series single-chain MMC unit is made equal to the average value of the DC capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment, and go to step 13;

[0070] Step 13: Determine whether the fault current has been cleared, if not, execute steps 11 and 12, otherwise terminate the operation and exit the fault clearing process;

[0071] The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

[0072] In the embodiment of the present application, the phase shift control mode includes:

[0073] The low voltage DC side output voltage U is achieved by controlling the phase difference between the control signal of the front switch and the control signal of the rear switch in each DAB circuit. cLV = Rated voltage U of solid-state transformer low voltage DC side LVN The voltage of each DC capacitor in the asymmetric series single-chain MMC unit = the average value of the DC capacitor voltages of all sub-modules of the asymmetric series single-chain MMC unit at the current sampling moment.

[0074] In the embodiment of the present application, the phase difference between the switch control signals of the previous and next stages of each DAB circuit is determined by the low voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage of the submodule connected to the front stage of each DAB circuit and the average value of the capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment is calculated by a controller such as PI or PR and then subtracted.

[0075] In the embodiment of the present application, all fault clearing submodules and DC isolation modules use bidirectional submodules running in bidirectional mode, that is, the bidirectional submodule can output positive and negative levels. When all fault clearing submodules and DC isolation modules have a DC short circuit fault on the high-voltage side of the solid-state transformer, the fault clearing submodules and DC isolation modules of the asymmetric series single-chain MMC unit are operated in bidirectional mode to cross the fault as the main control target, and the following steps are performed:

[0076] Step 21: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 All modulation and control methods of the existing series single-chain MMC are used, all fault clearing submodules are used as half-bridge submodules, and the power electronic switches of all DC isolation modules IDSM are kept turned on; the bridge arm ARM AC1 ,ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control all their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side; at the same time, detect whether there is a DC short-circuit fault. If not, continue to cyclically detect whether there is a DC short-circuit fault. Otherwise, all asymmetric series single-chain MMC unit fault clearing submodules and DC isolation module IDSM operate in bidirectional operation mode. The output voltage of the DC isolation module IDSM is equal to the AB two-phase line voltage, and the output voltage of the DC isolation module IDSM is equal to the BC two-phase line voltage. The bridge arm ARM AC1 、Bridge arm ARM AC2 The sum of the output voltages of the two DC isolation modules IDSM is equal to the AC two-phase line voltage; at the same time, all the half-bridge sub-modules of the asymmetric series single-chain MMC units are bypassed, and the process goes to step 22;

[0077] Step 22: Control the switches of all DAB circuits using a phase shift control mode, wherein the switches of all DAB circuits connected to the DC capacitors in the fault clearing submodule capable of bidirectional operation with the asymmetric series single-chain MMC unit are controlled so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN To maintain the low voltage DC side voltage, the asymmetric series single chain MMC unit adopts a bidirectional fault clearing submodule with a DC capacitor voltage = rated voltage U cN ; Control the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule so that the voltage of each DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule = rated voltage U cN , go to step 23;

[0078] Step 23: Determine whether the DC short-circuit fault current has been cleared. If not, maintain the current control state and execute steps 21 and 22. Otherwise, terminate the operation and exit the fault clearing process.

[0079] In the embodiment of the present application, the phase shift control mode includes:

[0080] By controlling the phase difference between the control signal of the front switch and the control signal of the rear switch of all DAB circuits connected to the DC capacitor in the fault clearing submodule capable of bidirectional operation with the asymmetric series single-chain MMC unit, the output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the bidirectional fault clearing submodule of the asymmetric series single-chain MMC unit is equal to the rated voltage U cN ;

[0081] By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule, the capacitor voltage of the asymmetric series single-chain MMC unit half-bridge submodule = the rated voltage U cN .

[0082] In the embodiment of the present application, the phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitors in each bidirectionally operable fault clearing submodule of the asymmetric series single-chain MMC unit is determined by the low-voltage DC side output voltage U cLV Rated voltage U of the low voltage DC side of the solid-state transformer LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U c The difference is calculated by PI or PR controller and then subtracted.

[0083] In the embodiment of the present application, the phase difference of the front and rear stage switch control signals of the DAB circuit connected to the DC capacitors in each half-bridge submodule of the asymmetric series single-chain MMC unit is determined by the difference in the DC capacitor voltages in all the asymmetric series single-chain MMC unit half-bridge submodules and the rated voltage U cN The difference is calculated by PI or PR controller and then subtracted.

[0084] In an embodiment of the present application, the control strategy of the bidirectionally operable fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a full-bridge submodule (FBSM), a clamped twin submodule (CDSM), a series twin submodule (SDSM), a cross-series twin submodule (CCDSM) and a T-type full-bridge submodule (TFBSM). In addition to adopting at least one of a dual-buck circuit, a full-bridge submodule (FBSM), a clamped twin submodule (CDSM), a series twin submodule (SDSM), a cross-series twin submodule (CCDSM) and a T-type full-bridge submodule (TFBSM), the DC isolation module can meet the needs of different application scenarios.

[0085] In an embodiment of the present application, if the DC short circuit fault on the medium and high voltage side is a permanent fault and the solid-state transformer cannot be shut down, all fault clearing submodules and DC isolation modules need to continue to operate in bidirectional mode after the fault is cleared; the medium and high voltage DC port of the asymmetric series single-chain MMC solid-state transformer adopts a bidirectional switch that can be conducted in both directions and is composed of a plurality of fully controlled power electronic switching devices connected in reverse series. After the DC short circuit fault is cleared, the DC short circuit fault on the medium and high voltage side is bypassed, and the medium and high voltage DC port of the solid-state transformer is disconnected from the fault line by a switch to isolate the fault.

[0086] In an embodiment of the present application, the rated voltage of the DC isolation capacitor contained in the DC isolation module on the medium- and high-voltage AC port phase A and the medium- and high-voltage AC port phase C of the solid-state transformer is 1 / 3 of the medium- and high-voltage DC port voltage of the solid-state transformer. The switching device in the DC isolation module can use only one DC isolation module on the premise of meeting the voltage stress requirements. If the requirements are not met, multiple DC isolation modules can be used in series to meet the requirements of the rated voltage of the DC isolation capacitor.

[0087] In the control strategy described in the fault clearing submodule in the asymmetric series single-chain MMC unit, in addition to the unidirectional full-bridge submodule FCSM, various circuits such as the full-bridge submodule (FBSM), the clamped twin submodule (CDSM), the series twin submodule (SDSM), the cross-series twin submodule (CCDSM) and the T-type full-bridge submodule (TFBSM) can also be used. In addition to the dual-buck circuit, the DC isolation module can also use the above-mentioned submodules to meet the needs of different applications.

[0088] The fault clearing submodule in the asymmetric series single-chain MMC unit requires the use of a bidirectional submodule in the control strategy. In addition to the full-bridge submodule, a full-bridge submodule (FBSM), a cross-series dual submodule (CCDSM) and a T-type full-bridge submodule (TFBSM) and other circuits can also be used. In addition to the full-bridge submodule, the DC isolation module can also use the above-mentioned submodules to meet the needs of different applications.

[0089] In an embodiment of the present application, the present invention provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of any of the above methods when executing the program.

[0090] In an embodiment of the present application, the present invention provides a computer-readable storage medium having a computer program stored thereon, and when the computer program is executed by a processor, the steps of any of the above methods are implemented.

[0091] The various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0092] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the invention invented herein. This application is intended to cover any variations, uses or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art that are not invented by the present invention. The specification and examples are to be considered exemplary only, and the true scope and spirit of the present invention are indicated by the following claims.

[0093] The above specific implementation methods further illustrate the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above are only specific implementation methods of the present application and are not used to limit the protection scope of the present application. Any modifications, equivalent substitutions, improvements, etc. made on the basis of the technical solutions of the present application should be included in the protection scope of the present application.

Claims

1. An asymmetric series single-chain MMC solid-state transformer, characterized in that: It includes an asymmetric series single-chain MMC unit, a DAB unit and a low-voltage converter unit; the asymmetric series single-chain MMC unit includes a half-bridge submodule and a fault clearing submodule, and the half-bridge submodule and the fault clearing submodule include a DC capacitor and a power electronic switch; The DAB unit includes a DAB circuit, a DC capacitor C LV The front-stage DC side of the DAB circuit is connected in parallel with the DC capacitors of the half-bridge submodule and the fault clearing submodule respectively; the rear-stage DC sides of all DAB circuits are connected in parallel to form a low-voltage DC port, and a DC capacitor C is connected in parallel to the low-voltage DC port. LV , the low-voltage DC port is connected in parallel with the DC side of the low-voltage converter unit, and the AC side of the low-voltage converter unit serves as a low-voltage AC port of the solid-state transformer; The asymmetric series single-chain MMC unit includes a medium-high voltage AC port phase A, a medium-high voltage AC port phase B, a medium-high voltage AC port phase C, a medium-high voltage DC port, a bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 ; Bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 The two ends of the circuit formed in series serve as the medium and high voltage DC ports of the asymmetric series single chain MMC unit, and the medium and high voltage DC ports are connected in parallel with capacitors, and the bridge arm ARM AC1 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules, the bridge arm ARM AB Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM BC Including series-connected bridge arm inductors and N half-bridge sub-modules, bridge arm ARM AC2 It includes a series-connected bridge arm inductor and N / 2 fault clearing submodules; bridge arm ARM AC1 、Bridge arm ARM AB 、Bridge arm ARM BC 、Bridge arm ARM AC2 The three connection points between the two are connection point A', connection point B' and connection point C'; the connection point A' is connected to the F end of the DC isolation module IDSM, the N end of the DC isolation module IDSM, the coupling inductor L d Connected to the medium and high voltage AC port A of the asymmetric series single-chain MMC unit, the connection point B' is connected through the coupling inductor L d Connected to the medium and high voltage AC port B of the asymmetric series single-chain MMC unit, the connection point C' is connected in sequence through the N end of the DC isolation module IDSM, the F end of the DC isolation module IDSM, the coupling inductor L d It is connected to the medium and high voltage AC port C of the asymmetric series single-chain MMC unit; each of the half-bridge submodule, fault clearing submodule and DC isolation module includes a DC capacitor and a power electronic switch.

2. The asymmetric series single-chain MMC solid-state transformer according to claim 1, characterized in that: The rated voltage of the DC blocking capacitor included in the DC blocking module on the medium-high voltage AC port phase A and the medium-high voltage AC port phase C of the solid-state transformer is 1 / 3 of the medium-high voltage DC port voltage of the solid-state transformer.

3. A control method for an asymmetric series single-chain MMC solid-state transformer, characterized in that: Based on an asymmetric series single-chain MMC solid-state transformer according to any one of claims 1-2, all fault clearing submodules and DC isolation modules use submodules with fault clearing functions, and when a DC short circuit fault occurs on the high-voltage side of the solid-state transformer, all fault clearing submodules and DC isolation modules use a blocking method to absorb fault energy, and the DC capacitor voltage of the fault clearing submodule does not exceed the rated voltage U cN When it is the main control target, the following steps are included: Step 1: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, T1 of the unidirectional full-bridge submodule FCSM remains on, and the power electronic switches of all DC isolation modules IDSM remain on, that is, T2 and T3 of the DC isolation module IDSM remain on; the bridge arm ARM AC1 、Bridge arm ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side of the solid-state transformer and the low-voltage AC side of the solid-state transformer; Detect whether there is a DC short circuit fault. If there is no DC short circuit fault, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, all DC isolation modules and all DAB circuits, and start charging the DC capacitors of all fault-clearing submodules. Step 2: Determine whether the DC capacitor voltage in all fault clearing submodules exceeds the rated voltage U cN If it exceeds, the switches of all DAB circuits are controlled by the phase shift control mode, wherein the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit fault clearing submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the asymmetric series single-chain MMC unit fault clearing submodule = rated voltage U cN ; The switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule are controlled to make the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each DC capacitor voltage in the half-bridge submodule of the asymmetric series single-chain MMC unit is made equal to the average value of the DC capacitor voltage in the half-bridge submodule of all asymmetric series single-chain MMC units at the current sampling moment; Step 3: Determine whether the DC short-circuit fault current is cleared, if not, execute steps 1 and 2, otherwise terminate the operation; The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

4. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 3 is characterized in that: The phase shift control mode includes: controlling the phase difference between the control signal of the front switch of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule and the control signal of the rear switch, so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN At the same time, the DC capacitor voltage in the asymmetric series single-chain MMC unit fault clearing submodule = rated voltage U cN ; By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule, the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the target value of each capacitor voltage of the asymmetric series single-chain MMC unit half-bridge sub-module is made equal to the average capacitor voltage of all asymmetric series single-chain MMC unit half-bridge sub-modules at the current sampling moment.

5. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 4, characterized in that: The phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U cN The difference is calculated by the PI or PR controller and then subtracted.

6. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 4, characterized in that: The phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule is determined by the low-voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC voltage of the DC capacitor connected to the front stage of each DAB circuit and the average DC capacitor voltage in all asymmetric series single-chain MMC unit half-bridge sub-modules at the current sampling moment is calculated by the PI or PR controller and then subtracted to obtain the result.

7. A control method for an asymmetric series single-chain MMC solid-state transformer, characterized in that: Based on an asymmetric series single-chain MMC solid-state transformer according to any one of claims 1 to 2, all fault clearing submodules and DC isolation modules use submodules with fault clearing functions. When all fault clearing submodules and DC isolation modules use a blocking method to absorb fault energy after a DC short circuit fault occurs on the high-voltage side of the solid-state transformer, and take the DC capacitor voltage balance of all submodules as the main control target, the following steps are performed: Step 11: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, T1 of the unidirectional full-bridge submodule FCSM is kept on, and T2 and T3 of the DC isolation module IDSM are kept on; the bridge arm ARM AC1 ,ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side of the solid-state transformer; Detect whether there is a DC short circuit fault. If not, continue to detect whether there is a DC short circuit fault in a loop. Otherwise, block the switch control signals of all submodules in the asymmetric series single-chain MMC unit, the switch control signals of all DC isolation modules and the switch control signals of all DAB circuits, and start charging the DC capacitors of all fault clearing submodules. Step 12: Use the phase shift control mode to control the switches of all DAB circuits. The control goal is to make the low voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , and at the same time, the DC capacitor voltage of each submodule in the asymmetric series single-chain MMC unit is equal to the average value of the DC capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment; Step 13: Determine whether the fault current is cleared, if not, execute steps 11 and 12, otherwise terminate the operation; The control strategy of the fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a unidirectional full-bridge submodule FCSM, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a double buck circuit, a full-bridge submodule, a clamping twin module, a series twin module, a cross-series twin module and a T-type full-bridge submodule.

8. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 7, characterized in that: The phase shift control mode includes: controlling the phase difference between the control signal of the front-stage switch and the control signal of the rear-stage switch in each DAB circuit to achieve the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN The voltage of each DC capacitor in the asymmetric series single-chain MMC unit = the average value of the DC capacitor voltages of all sub-modules of the asymmetric series single-chain MMC unit at the current sampling moment.

9. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 8, characterized in that: The phase difference between the front and rear switch control signals of each DAB circuit is determined by the low voltage DC side output voltage U cLV Rated voltage U of solid-state transformer low voltage DC side LVN The difference between the DC capacitor voltage of the submodule connected to the front stage of each DAB circuit and the average value of the capacitor voltage of the submodule in all asymmetric series single-chain MMC units at the current sampling moment is calculated by the PI or PR controller and then subtracted.

10. A control method for an asymmetric series single-chain MMC solid-state transformer, characterized in that: Based on an asymmetric series single-chain MMC solid-state transformer according to any one of claims 1-2, all fault clearing submodules and DC isolation modules adopt bidirectional submodules that can operate in bidirectional mode. When all fault clearing submodules and DC isolation modules have a DC short circuit fault on the high-voltage side of the solid-state transformer, the fault clearing submodules and DC isolation modules of the asymmetric series single-chain MMC units are operated in bidirectional mode to cross the fault as the main control target, and the following steps are performed: Step 21: When the solid-state transformer is working normally, the four bridge arms of the asymmetric series single-chain MMC unit AC1 ,ARM AB ,ARM BC and ARM AC2 Using the existing modulation and control method of the series single-chain MMC, all fault clearing submodules are used as half-bridge submodules, and the power electronic switches of all DC isolation modules IDSM remain turned on; the bridge arm ARM AC1 、Bridge arm ARM AC2 The capacitors connected in parallel with the medium and high voltage DC ports jointly provide and establish the AC two-phase line voltage, the bridge arm ARM AB Provide and establish AB two-phase line voltage, bridge arm ARM BC Provide and establish BC two-phase line voltage; all DAB circuits use phase-shift control mode to control all their switches; the low-voltage converter unit uses the existing grid-connected control strategy to manage the energy flow between the low-voltage DC side and the low-voltage AC side; detect whether there is a DC short-circuit fault, if not, continue to cycle to detect whether there is a DC short-circuit fault, otherwise all asymmetric series single-chain MMC unit fault clearing submodules and DC isolation modules IDSM operate in bidirectional operation mode, the output voltage of the DC isolation module IDSM on the A phase of the medium and high voltage AC port is equal to the AB two-phase line voltage, the output voltage of the DC isolation module IDSM on the C phase of the medium and high voltage AC port is equal to the BC two-phase line voltage, the bridge arm ARM AC1 、Bridge arm ARM AC2 The sum of the output voltages of the two DC isolation modules IDSM is equal to the AC two-phase line voltage; bypassing the half-bridge submodules of all asymmetric series single-chain MMC units; Step 22: Control the switches of all DAB circuits using a phase shift control mode, wherein the switches of all DAB circuits connected to the DC capacitors in the fault clearing submodule capable of bidirectional operation with the asymmetric series single-chain MMC unit are controlled so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN To maintain the low voltage DC side voltage, the asymmetric series single chain MMC unit adopts a bidirectional fault clearing submodule with a DC capacitor voltage = rated voltage U cN ; Control the switches of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule so that the voltage of each DC capacitor in the asymmetric series single-chain MMC unit half-bridge submodule = rated voltage U cN ; Step 23: Determine whether the DC short-circuit fault current is cleared, if not, execute steps 21 and 22, otherwise terminate the operation; The control strategy of the bidirectionally operable fault clearing submodule in the asymmetric series single-chain MMC unit adopts at least one of a full-bridge submodule, a cross-series twin submodule and a T-type full-bridge submodule, and the DC isolation module adopts at least one of a full-bridge submodule, a cross-series twin submodule and a T-type full-bridge submodule.

11. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 10, characterized in that: The phase shift control mode includes: controlling the phase difference between the control signal of the front switch of all DAB circuits connected to the DC capacitor in the asymmetric series single-chain MMC unit fault clearing submodule and the control signal of the rear switch, so that the low-voltage DC side output voltage U cLV = Rated voltage U of solid-state transformer low voltage DC side LVN , while making the asymmetric series single-chain MMC unit bidirectionally operable, the DC capacitor voltage in the fault clearing submodule = rated voltage U cN By controlling the phase difference between the front-stage switch control signal and the rear-stage switch control signal of all DAB circuits connected to the DC capacitors in the asymmetric series single-chain MMC unit half-bridge submodule, the capacitor voltage of the asymmetric series single-chain MMC unit half-bridge submodule = the rated voltage U cN .

12. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 11, characterized in that: The phase difference between the front and rear switch control signals of the DAB circuit connected to the DC capacitors in the fault clearing submodules of the asymmetric series single-chain MMC unit capable of bidirectional operation is determined by the low-voltage DC side output voltage U cLV Rated voltage U of the low voltage DC side of the solid-state transformer LVN The difference between the DC capacitor voltage connected to the front stage of each DAB circuit and the rated voltage U c The difference is calculated by the PI controller or the PR controller and then subtracted.

13. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 11, characterized in that: The phase difference of the front and rear stage switch control signals of the DAB circuit connected to the DC capacitors in each half-bridge submodule of the asymmetric series single-chain MMC unit is determined by the difference of the DC capacitor voltages in all the asymmetric series single-chain MMC unit half-bridge submodules and the rated voltage U cN The difference is calculated by the PI controller or the PR controller and then subtracted.

14. The control method of the asymmetric series single-chain MMC solid-state transformer according to claim 10, characterized in that: If the DC short circuit fault on the medium and high voltage side is a permanent fault and the solid-state transformer cannot be shut down, all fault clearing sub-modules and DC isolation modules must use sub-modules that can operate in both directions, and must continue to operate in bidirectional mode after the fault is cleared; the medium and high voltage DC port of the asymmetric series single-chain MMC solid-state transformer uses a bidirectional switch that can conduct in both directions and is composed of multiple fully controlled power electronic switching devices connected in reverse series. After the DC short circuit fault is cleared, the DC short circuit fault on the medium and high voltage side is bypassed, and the medium and high voltage DC port of the solid-state transformer is disconnected from the fault line by a switch.

15. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps of the method according to any one of claims 3 to 14 are implemented.

16. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 4 to 14 are implemented.

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