Method and system for controlling off-grid test power supply of large-capacity medium-voltage chain energy storage system

Abstract

The application discloses a large-capacity medium-voltage chain energy storage system off-grid test power supply control method and system, and the method comprises the following steps: obtaining PCS off-grid stable operation parameters and battery cluster information and judging whether the chain energy storage system can start off-grid; when it is judged that the chain energy storage system can start off-grid, charging the energy storage converter from the battery side to start in the constant AC voltage and frequency control mode; controlling the AC voltage and frequency of the energy storage converter to reach the off-grid stable operation parameters, and realizing the unlocking operation of the chain energy storage system; after the chain energy storage system is stably operated off-grid, obtaining test parameters of power grid adaptability and fault ride-through capability and performing various tests. The application is applied to the medium-voltage off-grid test power supply, one-key automatic off-grid starting is realized, human intervention operation is greatly reduced, the safe and stable operation of the energy storage system is ensured, one-key automatic test function is realized, and the required waveforms of the power grid adaptability and fault ride-through capability test required by the composite standard are provided.

Description

Technical Field

[0001] This invention belongs to the field of power system control technology, and more specifically, relates to a method and system for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system. Background Technology

[0002] As new energy sources rapidly increase their share in the overall energy system, the large-scale application of energy storage systems with a capacity of 1MW or more can effectively reduce the peak-to-valley difference between day and night, improve grid stability, flexibility, and power quality, and promote the large-scale integration of new energy into the grid, thus effectively addressing the increasingly prominent issue of large-scale new energy consumption. Among these, large-capacity battery energy storage power stations offer advantages such as short construction cycles, small footprint, and flexible deployment. They can meet millisecond-level dispatch response requirements, enabling rapid response and cutoff for power auxiliary services and demand-side management, and have broad application prospects in the power grid.

[0003] Cascaded power conversion systems (PCS) can be directly connected to medium-voltage power grids without transformers, effectively reducing power loss. They can cascade the appropriate number of modules according to capacity requirements, have large single-unit capacity, and offer advantages such as multi-module redundancy and high safety. Furthermore, cascaded power conversion systems (PCS) achieve better harmonic characteristics at low switching frequencies and have high operating efficiency.

[0004] Medium-voltage cascaded large-capacity energy storage systems are often integrated within containers. They can be used more flexibly and conveniently as test power supplies for wind turbine generators and other equipment under test through various mobile methods such as vehicle-mounted or ship-mounted installations. They are primarily used to provide the voltage waveforms required for grid adaptability testing and fault ride-through capability of the wind turbine generators under test. The amplitude, frequency, and waveform of this voltage can be adjusted as needed to meet standard requirements. Existing technology, "A Test Power Supply and Method for Wind Turbine Generators with Energy Storage and Simulated Load" (CN104868503B), discloses a test power supply and method for wind turbine generators with energy storage and simulated load. It provides an energy storage inverter power supply system that combines supercapacitor and lithium battery energy storage, integrates a high-power simulated load, and combines a three-phase bridge inverter and a Buck / Boost converter. This system is applied to wind turbine generator test power supply devices to complete the commissioning work of various systems before the wind farm is connected to the grid. Existing technologies are only applicable to AC 380V voltage levels and can only test one wind turbine unit. However, test power supplies for 10kV inverters in medium-voltage systems are all non-chain energy storage systems. Furthermore, medium-voltage test power supplies for Modular Multilevel Converters (MMCs) require stable grid voltage support. Due to the large number of controlled devices in the test system and the rapid changes in operating conditions, the difficulty of manual operation is increased. Especially when providing test power for test operations, a large number of target value conversions and mode switching operations are concentrated, resulting in high operational intensity for operators.

[0005] Currently, the most widely used energy storage system access solution in engineering applications is still low-voltage parallel aggregation boosting energy storage access technology, which meets the capacity / power requirements of energy storage power stations. This involves building a common AC bus on the low-voltage side of the low-voltage energy storage system to aggregate and boost the voltage for grid connection. However, low-voltage parallel aggregation technology suffers from low power conversion efficiency, making it difficult to guarantee simultaneous response to dispatch commands during black start-up and prone to power oscillations. In contrast, large-capacity medium-voltage chain energy storage technology, with single-unit capacities reaching tens of megawatts, effectively solves these problems, offering higher system conversion efficiency and better response consistency. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a control method and system for a large-capacity medium-voltage chained energy storage system used as an off-grid test power supply. This method, designed for off-grid test power supply applications at medium voltage levels such as 10kV, enables a single energy storage system to test multiple low-voltage loads such as wind turbines. Voltage regulation at the medium-voltage feeder location provides a more realistic simulation of grid faults. The medium-voltage chained energy storage system features one-button automatic off-grid startup, significantly reducing human intervention and ensuring safe and stable operation. It also provides a one-button automatic testing function, offering waveforms required for grid adaptability and fault ride-through capability testing that meet standard requirements.

[0007] The present invention adopts the following technical solution.

[0008] This invention proposes a method for controlling a large-capacity medium-voltage chain energy storage system as an off-grid test power source, comprising:

[0009] Step 1: Obtain the off-grid stable operation parameters of the energy storage converter;

[0010] Step 2: Obtain battery cluster information from the battery management system; determine whether the chain energy storage system can start off-grid based on the battery cluster information; if it is determined that the chain energy storage system can start off-grid, proceed to step 3; otherwise, repeat step 2.

[0011] Step 3: When the off-grid start command of the chain energy storage system is received, the battery side charges the energy storage converter and starts it according to the constant AC voltage and frequency control mode; and controls the AC voltage and frequency of the energy storage converter to reach the off-grid stable operation parameters, so as to realize the unlocking operation of the chain energy storage system.

[0012] Step 4: After the chain energy storage system is stably operating off-grid, it is used as an off-grid test power source to obtain test parameters for grid adaptability and fault ride-through capability, and to conduct various tests.

[0013] Preferably, in step 1, the off-grid stable operation parameters include: AC voltage target value and frequency target value.

[0014] Preferably, in step 2, the battery cluster information includes: total battery cluster voltage, total battery cluster current, battery cluster state of charge, battery cluster health status, battery cluster maximum charging power, battery cluster maximum discharging power, battery cluster communication status, battery cluster fault status, and connection switch status between the battery and the energy storage converter power module.

[0015] Preferably, step 3 includes:

[0016] Step 3.1: Determine whether the chain energy storage system is in a ready state based on the battery cluster information. That is, if the connection switch between the battery cluster and the power module of the energy storage converter is closed and the voltage of all battery clusters reaches the start-up voltage, then close the pre-charge switch of all power modules of the energy storage converter, and the battery clusters charge the power modules. The chain energy storage system changes from the shutdown state to the DC side pre-charge state; otherwise, repeat step 3.1.

[0017] Step 3.2: When the capacitor voltage of the energy storage converter power module meets the charging completion condition, that is, when the capacitor voltage of the energy storage converter power module reaches the charging completion voltage, the connection switch of the energy storage converter power module is closed, and the chain energy storage system is switched from the DC side pre-charging state to the DC side connection state.

[0018] Step 3.3: When the capacitor voltage of the energy storage converter power module reaches the charging completion voltage and remains so for 10 seconds without any faults, the pre-charge switch of the energy storage converter power module is turned off, and the chain energy storage system switches from DC side connection state to DC side operation state.

[0019] Step 3.4: When the power module of the energy storage converter is in DC side operation state for 10 seconds and there is no fault, close the AC side off-grid outgoing switch and AC bypass switch, and the chain energy storage system changes from DC side operation state to locked state.

[0020] Step 3.5: When the power module of the energy storage converter meets the unlocking conditions, it will automatically unlock and control the AC voltage and frequency of the energy storage converter to reach the off-grid stable operating parameters. The chain energy storage system will then switch from the locked state to the unlocked state.

[0021] Furthermore, in step 3.5, the unlocking conditions include: the energy storage converter power module is ready, the AC incoming switch is closed, the AC side bypass switch is closed, the DC side connection switch is closed, the phase control is ready, there is no fault tripping, and the energy storage converter is in a locked state.

[0022] Preferably, in step 4, the test parameters include: test voltage amplitude, rate, and test time, which are used to indicate grid adaptability and fault ride-through capability.

[0023] In various grid adaptability tests, the off-grid stable chain energy storage system provides test power to the device under test and collects the actual output value of the device under test; the actual output value is compared with the test parameters, and the comparison result is sent to the monitoring system by the test personnel.

[0024] Preferably, the control method further includes:

[0025] Step 5: After all tests are completed, the chain energy storage system maintains a constant AC voltage and frequency output.

[0026] In another aspect, this invention also proposes a control system for a large-capacity medium-voltage chain energy storage system used as an off-grid test power supply, which is used to realize a control method for a large-capacity medium-voltage chain energy storage system used as an off-grid test power supply.

[0027] The control system includes multiple energy storage links, which are connected by a chain structure.

[0028] Each energy storage link includes: one energy storage converter power module, one cluster of energy storage batteries and a corresponding battery management system; the output side of the energy storage battery cluster is connected to the DC side of the energy storage converter power module, and the battery management system controls the energy storage batteries.

[0029] The control system also includes: grid-connected switchgear, off-grid switchgear, and starter cabinet;

[0030] Among them, one end of the grid-connected switchgear is connected to the bus grid connection point, one end of the off-grid switchgear outputs off-grid test power, the other end of the grid-connected switchgear is connected to one end of the starter cabinet and the other end of the off-grid switchgear, and the other end of the starter cabinet is connected to the energy storage link on each phase.

[0031] The starter cabinet includes: a pre-charge circuit with a resistor; the pre-charge circuit with a resistor is used for AC side charging of the power module.

[0032] The beneficial effects of this invention are that, compared with the prior art,

[0033] This invention proposes an off-grid test power supply control system based on a medium-voltage chain energy storage system. It integrates a battery management system (BMS), a power supply unit (PCS), a cooling system, and a fire suppression system. Each power module of the PCS drives a battery cluster in a cascaded configuration. Through the design of an off-grid loop, it provides a test power supply that meets standard grid adaptability and fault ride-through capabilities, simulating more realistic grid faults for the device under test. Functional reuse saves on the hardware cost of a complete test power supply.

[0034] The medium-voltage chain energy storage system proposed in this invention is suitable for providing off-grid test power to multiple medium-voltage wind turbine units such as AC10kV at the same time. Since it directly uses medium voltage, it simulates grid faults more realistically.

[0035] This invention also proposes an off-grid test power supply control method based on a medium-voltage chain energy storage system, enabling one-click off-grid unlocking and operation of the chain energy storage system. This significantly reduces human intervention and fundamentally lowers the possibility of human error. It achieves one-click unlocking and operation of the medium-voltage chain energy storage system without human intervention, ensuring safety and stability; off-grid power output provides a pre-defined waveform with a single click; using medium-voltage chain energy storage as an off-grid test power supply is an innovative application.

[0036] This invention addresses grid adaptability and fault ride-through capability testing, imposing higher operational requirements on the test power supply. It meets the waveform requirements for grid adaptability and fault ride-through capability testing in the GB / T 36994-2018 standard, provides a one-click testing function, and outputs waveforms that meet design requirements, thereby improving the stability, accuracy, and ease of use of the test power supply.

[0037] The control method proposed in this invention automatically switches to maintaining a stable AC voltage / frequency output after the grid adaptability or fault ride-through capability test is completed. This process requires no human intervention, improving the usability and safety of the entire system. Attached Figure Description

[0038] Figure 1 This is a flowchart of a power control method for off-grid testing of a large-capacity medium-voltage chain energy storage system proposed in this invention;

[0039] Figure 2 This is a structural diagram of a large-capacity medium-voltage chain energy storage system used as an off-grid test power control system proposed in this invention;

[0040] Figure 3 This is a topology diagram of the connection between the PCS power module and the battery cluster in the off-grid test power control system of a large-capacity medium-voltage chain energy storage system proposed in this invention.

[0041] Figure 4 This is a test flowchart for providing a test power supply in an embodiment of the present invention;

[0042] Figure 2 , 3 The annotations in the accompanying drawings are explained as follows:

[0043] 100 - Energy storage link; 101 - 1 cluster of energy storage batteries; 102 - Power module of energy storage converter;

[0044] 1-Grid-connected switchgear; 2-Off-grid switchgear; 3-Starter cabinet;

[0045] QF0 - First circuit breaker; QF1 - Second circuit breaker; QF2 - Third circuit breaker;

[0046] QE0 - First grounding switch; QE1 - Second grounding switch; QE2 - Third grounding switch;

[0047] QS1 - First disconnecting switch; R - Resistor in the pre-charge circuit; Detailed Implementation

[0048] The present application will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and should not be construed as limiting the scope of protection of the present application.

[0049] This invention proposes a method for controlling a large-capacity medium-voltage chain energy storage system as an off-grid test power source, such as... Figure 1 ,include:

[0050] Step 1: Obtain the off-grid stable operation parameters of the energy storage converter.

[0051] The PCS off-grid voltage regulation operation parameters are necessary for unlocking operation and are issued by the operation personnel through the monitoring system. See Table 1 for details.

[0052] Table 1 PCS Off-Grid Voltage Stabilization Operation Parameters

[0053] Serial Number Parameter name Parameter Description 1 Uac_ref AC voltage target value 2 F_ref Frequency target value

[0054] In this embodiment, the off-grid stable operation parameters include: target AC voltage value and target frequency value. It is worth noting that those skilled in the art can select different off-grid stable operation parameters according to the test scenario; the selection in this embodiment is a non-limiting but preferred choice.

[0055] Step 2: Obtain battery cluster information from the battery management system; determine whether the chain energy storage system can start off-grid based on the battery cluster information; if it is determined that the chain energy storage system can start off-grid, proceed to step 3; otherwise, repeat step 2.

[0056] Battery cluster information is one of the conditions for determining whether an energy storage system is capable of off-grid startup. It is obtained through communication between the PCS and BMS systems, using the Modbus / IEC104 communication protocol. See Table 2 for detailed battery cluster information.

[0057] Serial Number Parameter name Parameter Description 1 BCMS_U Total voltage of battery cluster 2 BCMS_I Total current of battery cluster 3 BCMS_SOC Battery Cluster SOC 4 BCMS_SOH Battery clusters SOH 5 BCMS_Pmax_char Maximum charging power of battery cluster 6 BCMS_Pmax_dischar Maximum discharge power of battery cluster 7 BCMS_comm_status Battery cluster communication status 8 BCMS_fault_status Battery cluster fault status 9 BCMS_br_status Battery cluster connection switch status

[0058] Table 2 Battery Cluster Information of BMS System

[0059] In this embodiment, the battery cluster information includes: total battery cluster voltage, total battery cluster current, battery cluster state of charge, battery cluster health status, maximum battery cluster charging power, maximum battery cluster discharging power, battery cluster communication status, battery cluster fault status, and connection switch status between the battery and the energy storage converter power module. It is worth noting that those skilled in the art can select different battery cluster information based on the test scenario; the selection in this embodiment is a non-limiting but preferred choice.

[0060] Step 3: When the off-grid start command of the chain energy storage system is received, the battery side charges the energy storage converter and starts it according to the constant AC voltage and frequency control mode; and controls the AC voltage and frequency of the energy storage converter to reach the off-grid stable operation parameters, so as to realize the unlocking operation of the chain energy storage system.

[0061] Specifically, step 3 includes:

[0062] Step 3.1: Determine whether the chain energy storage system is in a ready state based on the battery cluster information. That is, if the connection switch between the battery cluster and the power module of the energy storage converter is closed and the voltage of all battery clusters reaches the start-up voltage, then close the pre-charge switch of all power modules of the energy storage converter, and the battery clusters charge the power modules. The chain energy storage system changes from the shutdown state to the DC side pre-charge state; otherwise, repeat step 3.1.

[0063] Step 3.2: When the capacitor voltage of the energy storage converter power module meets the charging completion condition, that is, when the capacitor voltage of the energy storage converter power module reaches the charging completion voltage, the connection switch of the energy storage converter power module is closed, and the chain energy storage system is switched from the DC side pre-charging state to the DC side connection state.

[0064] Step 3.3: When the capacitor voltage of the energy storage converter power module reaches the charging completion voltage and remains so for 10 seconds without any faults, the pre-charge switch of the energy storage converter power module is turned off, and the chain energy storage system switches from DC side connection state to DC side operation state.

[0065] Step 3.4: When the power module of the energy storage converter is in DC side operation state for 10 seconds without fault, close the AC side off-grid outgoing switch and AC bypass switch, and the chain energy storage system changes from DC side operation state to locked state.

[0066] Step 3.5: When the power module of the energy storage converter meets the unlocking conditions, it will automatically unlock and control the AC voltage and frequency of the energy storage converter to reach the off-grid stable operating parameters. The chain energy storage system will then switch from the locked state to the unlocked state.

[0067] Furthermore, in step 3.5, the unlocking conditions include: the energy storage converter power module is ready, the AC incoming switch is closed, the AC side bypass switch is closed, the DC side connection switch is closed, the phase control is ready, there is no fault tripping, and the energy storage converter is in a locked state.

[0068] The pre-charge switch is a charging resistor connected in series in the circuit connecting the battery and the power module. This resistor limits the charging current and prevents direct charging. The connection switch bypasses the pre-charge resistor, allowing the battery to connect directly to the module.

[0069] This invention also proposes an off-grid test power supply control method based on a medium-voltage chain energy storage system, enabling one-click off-grid unlocking and operation of the chain energy storage system. This significantly reduces human intervention and fundamentally lowers the possibility of human error. It achieves one-click unlocking and operation of the medium-voltage chain energy storage system without human intervention, ensuring safety and stability; off-grid power output provides a pre-defined waveform with a single click; using medium-voltage chain energy storage as an off-grid test power supply is an innovative application.

[0070] Step 4: After the chain energy storage system is stably operating off-grid, it is used as an off-grid test power source to obtain test parameters for grid adaptability and fault ride-through capability, and to conduct various tests.

[0071] The parameters required for power grid adaptability and fault ride-through capability testing are detailed in Table 3.

[0072] Table 3 Test parameters for power grid adaptability and fault ride-through capability

[0073]

[0074]

[0075]

[0076] Power grid adaptability testing involves providing test power to the device under test and outputting necessary parameters such as voltage and frequency as required, for the aforementioned test target values, which are issued by the test personnel through the monitoring system.

[0077] In this embodiment, the test parameters include: test voltage amplitude, rate, and test time, which indicate grid adaptability and fault ride-through capability. It is worth noting that those skilled in the art can select different test parameters according to the test scenario; the selection in this embodiment is a non-limiting but preferred choice.

[0078] In various grid adaptability tests, the off-grid stable chain energy storage system provides test power to the equipment under test and collects the actual output value of the equipment under test; the actual output value is compared with the test parameters, and the comparison result is sent to the monitoring system by the test personnel.

[0079] This invention addresses grid adaptability and fault ride-through capability testing, imposing higher operational requirements on the test power supply. It provides waveforms required for grid adaptability and fault ride-through capability testing according to the GB / T36994-2018 standard, offering a one-click testing function and outputting waveforms that meet design requirements, thereby improving the stability, accuracy, and ease of use of the test power supply.

[0080] Taking voltage interlocking faults as an example, after receiving the start notification of the voltage interlocking fault test, the voltage interlocking test is automatically completed without human intervention. The method includes the following steps, as shown in the appendix. Figure 4 As shown:

[0081] ① Upon receiving the voltage interlock test start, control the AC voltage to the initial output voltage value and maintain the initial voltage value for the duration.

[0082] ② The AC voltage steps up to the first step value of the output voltage and maintains the first step value of the output voltage for a certain duration.

[0083] ③ The AC voltage steps to the second step value of the output voltage and maintains the second step value of the output voltage for a certain duration.

[0084] ④ Control the AC voltage to the output voltage termination value.

[0085] ⑤ Follow this process to complete the required test items.

[0086] ⑥ After the grid adaptability or fault ride-through capability test is completed, it automatically switches to maintaining a stable AC voltage / frequency output.

[0087] Furthermore, the control method also includes: Step 5, after all tests are completed, the chain energy storage system maintains a constant AC voltage and frequency output.

[0088] The control method proposed in this invention automatically switches to maintaining a stable AC voltage / frequency output after the grid adaptability or fault ride-through capability test is completed. This process requires no human intervention, improving the usability and safety of the entire system.

[0089] In another aspect, this invention also proposes a control system for a large-capacity medium-voltage chain energy storage system used as an off-grid test power supply, which is used to realize a control method for a large-capacity medium-voltage chain energy storage system used as an off-grid test power supply.

[0090] The control system includes multiple energy storage links, such as Figure 2 As shown, each energy storage link 100 on each phase is connected by a chain structure.

[0091] Among them, such as Figure 2 Each energy storage link includes: a power module of an energy storage converter, a cluster of energy storage batteries, and a corresponding battery management system; such as Figure 3 As shown, the output side of a cluster of energy storage batteries 101 is connected to the DC side of the power module 102 of the energy storage converter, and the battery management system controls the energy storage batteries 101.

[0092] The battery system serves as a stable voltage source for the PCS, charging the DC side of the power module. Through the start-up cabinet and off-grid switch cabinet, it outputs a stable 10kV AC voltage / frequency.

[0093] like Figure 2 The control system also includes: grid-connected switchgear 1, off-grid switchgear 2, and starter cabinet 3;

[0094] Among them, one end of the grid-connected switchgear is connected to the 10kV bus grid connection point, and one end of the off-grid switchgear outputs 10kV off-grid power, which is a 10kV off-grid test power. The other end of the grid-connected switchgear is connected to one end of the starter cabinet and the other end of the off-grid switchgear. The other end of the starter cabinet is connected to the energy storage link on each phase.

[0095] Grid-connected switchgear 1 includes: a first circuit breaker QF0 and a first grounding switch QE0;

[0096] Off-grid switchgear 2 includes: a second circuit breaker QF1 and a second grounding switch QE1;

[0097] Starter cabinet 3 includes: third circuit breaker QF2, pre-charge circuit with resistor R, first disconnect switch QS1, and third grounding switch QE2;

[0098] The resistor-equipped pre-charge circuit is used for AC side charging of the power module; the three-phase outgoing lines of the starter cabinet are connected to the energy storage links on each phase through reactors.

[0099] from Figure 2It can be seen that there are many controlled devices and the operating conditions change rapidly, which increases the difficulty of manual operation. Especially when it is used as a test power source to provide test operations, a large number of target value conversions and mode switching operations are concentrated, which makes the operation intensity of the operators high. Therefore, this invention proposes a control method for the application of energy storage system as off-grid test power source, which automatically completes the entire process of power source test of grid adaptability and fault ride-through capability after startup.

[0100] This invention proposes an off-grid test power supply control system based on a medium-voltage chain energy storage system. It integrates a battery management system (BMS), a power supply unit (PCS), a cooling system, and a fire suppression system. Each power module of the PCS drives a battery cluster in a cascaded configuration. Through the design of an off-grid loop, it provides a test power supply that meets standard grid adaptability and fault ride-through capabilities, simulating more realistic grid faults for the device under test. Functional reuse saves on the hardware cost of a complete test power supply.

[0101] The medium-voltage chain energy storage system proposed in this invention is suitable for providing off-grid test power to multiple medium-voltage wind turbine units such as AC10kV at the same time. Since it directly uses medium voltage, it simulates grid faults more realistically.

[0102] The applicant of this invention has provided a detailed description of the embodiments of the invention in conjunction with the accompanying drawings. However, those skilled in the art should understand that the above embodiments are merely preferred embodiments of the invention. The detailed description is only intended to help readers better understand the spirit of the invention and is not intended to limit the scope of protection of the invention. On the contrary, any improvements or modifications made based on the inventive spirit of the invention should fall within the scope of protection of the invention.

Claims

1. A method for controlling a large-capacity medium-voltage chain energy storage system as an off-grid test power supply, characterized in that, The control method includes: Step 1: Obtain the off-grid stable operation parameters of the energy storage converter; Step 2: Obtain battery cluster information from the battery management system; determine whether the chain energy storage system can start off-grid based on the battery cluster information; if it is determined that the chain energy storage system can start off-grid, proceed to step 3; otherwise, repeat step 2. Step 3: Upon receiving the off-grid start command for the chain energy storage system, the battery side charges the energy storage converter and starts it according to the constant AC voltage and frequency control mode; and controls the AC voltage and frequency of the energy storage converter to reach the stable off-grid operating parameters, thereby unlocking and operating the chain energy storage system; including: Step 3.1: Determine whether the chain energy storage system is in a ready state based on the battery cluster information. That is, if the connection switch between the battery cluster and the power module of the energy storage converter is closed and the voltage of all battery clusters reaches the start-up voltage, then close the pre-charge switch of all power modules of the energy storage converter, and the battery clusters charge the power modules. The chain energy storage system changes from the shutdown state to the DC side pre-charge state; otherwise, repeat step 3.

1. Step 3.2: When the capacitor voltage of the energy storage converter power module meets the charging completion condition, that is, when the capacitor voltage of the energy storage converter power module reaches the charging completion voltage, the connection switch of the energy storage converter power module is closed, and the chain energy storage system is switched from the DC side pre-charging state to the DC side connection state. Step 3.3: When the capacitor voltage of the energy storage converter power module reaches the charging completion voltage and remains so for 10 seconds without any faults, the pre-charge switch of the energy storage converter power module is turned off, and the chain energy storage system switches from DC side connection state to DC side operation state. Step 3.4: When the power module of the energy storage converter is in DC side operation state for 10 seconds without fault, close the AC side off-grid outgoing switch and AC bypass switch, and the chain energy storage system changes from DC side operation state to locked state. Step 3.5: When the power module of the energy storage converter meets the unlocking conditions, it will automatically unlock and control the AC voltage and frequency of the energy storage converter to reach the off-grid stable operating parameters. The chain energy storage system will change from the locked state to the unlocked state. The unlocking conditions include: the power module of the energy storage converter is ready, the AC incoming switch is closed, the AC side bypass switch is closed, the DC side connection switch is closed, the phase control is ready, there is no fault tripping, and the energy storage converter is in the locked state. Step 4: After the chain energy storage system is stably operating off-grid, it is used as an off-grid test power source to obtain test parameters for grid adaptability and fault ride-through capability, and to conduct various tests.

2. The method for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system according to claim 1, characterized in that, In step 1, the off-grid stable operation parameters include: AC voltage target value and frequency target value.

3. The method for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system according to claim 1, characterized in that, In step 2, the battery cluster information includes: total battery cluster voltage, total battery cluster current, battery cluster state of charge, battery cluster health status, battery cluster maximum charging power, battery cluster maximum discharging power, battery cluster communication status, battery cluster fault status, and connection switch status between the battery and the energy storage converter power module.

4. The method for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system according to claim 1, characterized in that, In step 4, the test parameters include: test voltage amplitude, rate, and test time, which are used to indicate grid adaptability and fault ride-through capability.

5. The method for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system according to claim 4, characterized in that, In various grid adaptability tests, the off-grid stable chain energy storage system provides test power to the equipment under test and collects the actual output value of the equipment under test; the actual output value is compared with the test parameters, and the comparison result is sent to the monitoring system by the test personnel.

6. The method for controlling the off-grid test power supply of a large-capacity medium-voltage chain energy storage system according to claim 5, characterized in that, The control method further includes: Step 5: After all tests are completed, the chain energy storage system maintains a constant AC voltage and frequency output.

7. A control system for a large-capacity medium-voltage chain energy storage system used as an off-grid test power supply, for implementing the control method for the large-capacity medium-voltage chain energy storage system used as an off-grid test power supply as described in any one of claims 1 to 6, characterized in that, The system includes multiple energy storage links, which are connected by a chain structure. Each energy storage link includes: one energy storage converter power module, one cluster of energy storage batteries and a corresponding battery management system; the output side of the energy storage battery cluster is connected to the DC side of the energy storage converter power module, and the battery management system controls the energy storage batteries.

8. The off-grid test power control system for the large-capacity medium-voltage chain energy storage system according to claim 7, characterized in that, The system also includes: grid-connected switchgear, off-grid switchgear, and starter cabinet; Among them, one end of the grid-connected switchgear is connected to the bus grid connection point, one end of the off-grid switchgear outputs off-grid test power, the other end of the grid-connected switchgear is connected to one end of the starter cabinet and the other end of the off-grid switchgear, and the other end of the starter cabinet is connected to the energy storage link on each phase. The starter cabinet includes: a pre-charge circuit with a resistor; the pre-charge circuit with a resistor is used for AC side charging of the power module.

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