A cascade converter test system and test method

Abstract

The present invention discloses a cascade converter test system and test method, the test system comprises a dual-winding transformer, a plurality of multi-winding transformers, a plurality of low-voltage converter modules and a plurality of cascade converter modules, the primary winding of the multi-winding transformer is connected to a low-voltage power grid, the secondary winding of the multi-winding transformer is connected to the low-voltage converter module through a low-voltage grid-connected reactor, simulating the battery cluster function; the low-voltage converter module is connected to the cascade converter module through a DC reactor, a DC fuse and a DC circuit breaker, the cascade converter module is connected to the secondary winding of the dual-winding transformer through a high-voltage grid-connected reactor after AC side series voltage boost, and the primary winding of the dual-winding transformer is connected to the low-voltage power grid. The present invention can realize aging test or working condition simulation under low-voltage power grid conditions, without battery cluster conditions or without large-capacity battery cluster conditions.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a cascade converter testing system and a testing method. Background Art

[0002] In recent years, the H-bridge-based high-voltage direct-mounted cascade energy storage system has received extensive attention in 100-megawatt energy storage power stations due to its features such as no step-up transformer, low switching frequency, high system efficiency, good safety, and cost advantages. The high-voltage direct-mounted cascade energy storage system adopts the energy and power splitting concept to reasonably split the hundreds of thousands of large-capacity battery cells and tens of megawatts of high-power converters in 100-megawatt energy storage power stations into small-capacity battery clusters (battery cells are only connected in series but not in parallel) and converters that match their power. Technologies such as one cluster one management, one cluster one conversion, fault intelligent bypass, and converter AC side series boost expansion are adopted to achieve the segmentation and control of massive battery cells in 100-megawatt energy storage power stations, effectively improving the safety of energy storage power stations and the utilization rate of battery cell capacity.

[0003] Since each phase of the cascaded multilevel converter consists of dozens of converters connected in series directly to the grid, the number of converters in a three-phase direct-connected cascaded energy storage system is nearly hundreds. In the absence of battery clusters and high-voltage power grids in the factory, how to efficiently, safely and quickly age hundreds of converters, simulate the performance, functions and intelligent fault handling effects of hundreds of converters after cascading, and ensure that the tested converter reorganization and cascaded performance functions meet the actual application requirements are key steps in the grid-connected operation of the cascaded energy storage system. Currently, cascaded energy storage converters are widely used, such as Figure 1 The full-bridge topology is shown.

[0004] At present, the aging test of direct-mounted cascade energy storage converters is usually sampled as follows: Figure 2 The test system shown. After the DC side filter of converter 1, battery cluster BT1 is connected. Converter 1 is connected to the AC side of converter 2 through reactor L1. After the DC side filter of converter 2, battery cluster BT2 is connected. In active test mode, converter 1 and battery cluster BT1 are first started at a constant voltage, and then converter 2 is started. The controller controls converter 2 to charge and discharge battery cluster BT2 to achieve active power circulation between the two converter systems (including battery clusters). In power-free test mode, the controller adjusts the voltage, current phase angle and amplitude on the AC side of the two converters to achieve reactive power circulation between the two converters. In this mode, the active power of the battery clusters on the DC side of the two converters is basically zero. Converter 1 and converter 2 both use Figure 1 The full-bridge topology is shown.

[0005] In order to improve the efficiency of aging test and simulate the performance function test of cascade energy storage converter, the existing technology adopts Figure 3The working condition simulation test system shown in the figure. Among them, A1, A2...An, B1, B2...Bn, C1, C2...Cn are Figure 1 The full-bridge topology shown. In order to maintain the independence of the DC side power supply of the cascade converter, a bidirectional isolated DC / DC converter is connected to the DC side of each converter. In this test system, the bidirectional AC / DC converter and the isolated bidirectional DC / DC converter on the left can simulate the battery cluster function. This method solves the problem of completing the performance function test of the cascade converter system, the large-scale aging test and / or the working condition simulation test of the cascade converter system without the battery cluster.

[0006] use Figure 2 The test system shown has the following disadvantages when performing aging tests:

[0007] 1. Only two converters can be tested for aging each time, resulting in low test efficiency.

[0008] 2. It is necessary to customize and develop aging test software to complete the aging test of the converter (including active test and reactive test modes).

[0009] 3. Each converter DC side needs to be equipped with a battery cluster to complete the test, which has low safety.

[0010] 4. The aging test system cannot simulate the performance and functional testing of multiple converters in cascade.

[0011] use Figure 3 When the working condition simulation test system shown in the figure performs industrial control simulation test, although it can realize the performance, function and working condition simulation test of the cascade energy storage system, it also has the following disadvantages:

[0012] 1. The addition of many bidirectional isolated DC / DC converters as DC sources has led to a sharp increase in the cost of the cascade converter test system.

[0013] 2. The control complexity of the bidirectional isolated DC / DC converter itself is extremely high, and it also involves coordinated control with the cascade-side converter. Therefore, the custom development of the control software for the working condition simulation test system is extremely difficult.

[0014] 3. The simulation test system does not take into account the actual situation of the 400V AC power grid in the factory. It is necessary to increase the low-voltage side transformation capacity to support the 10-megawatt cascade converter system test. Summary of the invention

[0015] In order to overcome the above technical defects, the present invention provides a cascade converter testing system and a testing method.

[0016] In order to solve the above problems, the first aspect of the present invention discloses a cascade converter test system, including a dual-winding transformer, a plurality of multi-winding transformers, a plurality of low-voltage converter modules and a plurality of cascade converter modules, wherein the primary winding of the multi-winding transformer is connected to the low-voltage power grid, the secondary winding of the multi-winding transformer is connected to the AC side of the low-voltage converter module, the DC side of the low-voltage converter module is connected to the DC side of the cascade converter module, and the AC side of the cascade converter module is connected to the secondary winding of the dual-winding transformer.

[0017] The low-voltage converter module includes a plurality of first full-bridge circuits, and the cascade converter module includes a plurality of second full-bridge circuits. The AC sides of the second full-bridge circuits are connected in series to form a cascade converter module; the AC sides of the first full-bridge circuits are connected to the secondary winding of the multi-winding transformer, the DC sides of the first full-bridge circuits are connected to the DC sides of the second full-bridge circuits through a DC reactor, a DC fuse and a DC circuit breaker in sequence, the AC side of the second full-bridge circuit is connected to the secondary winding of the dual-winding transformer through a high-voltage grid-connected reactor, and the primary winding of the dual-winding transformer is connected to the low-voltage power grid, wherein the number of the second full-bridge circuits is the same as the number of the first full-bridge circuits.

[0018] Furthermore, it also includes a host computer, a first control module and a second control module, the host computer is connected to the first control module and the second control module respectively, the first control module is connected to the first full-bridge circuit, and controls the first full-bridge circuit to work or stop working;

[0019] The second control module is connected to the second full-bridge circuit to control the second full-bridge circuit to work or stop working.

[0020] Furthermore, it also includes a low-voltage grid-connected reactor, and the AC side of the first full-bridge circuit is connected to the secondary winding of the multi-winding transformer through the low-voltage grid-connected reactor.

[0021] Furthermore, it also includes a bypass switch connected to both ends of the AC side of the second full-bridge circuit.

[0022] Furthermore, it also includes a multi-way selection switch, the fixed end of the multi-way selection switch is connected to the grid-connected reactor, and the selection end is respectively connected to three taps of the secondary winding of the double-winding transformer.

[0023] Furthermore, it also includes an AC circuit breaker, and the primary winding of the multi-winding transformer is connected to the low-voltage power grid through the AC circuit breaker.

[0024] The second aspect of the present invention further discloses a cascade converter test method, which is applied to the above-mentioned cascade converter test system and is characterized in that it comprises the following steps:

[0025] Obtaining a cascade voltage level selected by a user, and calculating the number of first full-bridge circuits and second full-bridge circuits required based on the cascade voltage level selected by the user;

[0026] Control the closing of AC circuit breaker;

[0027] The first control module controls a corresponding number of first full-bridge circuits to power on and operate;

[0028] According to the cascade voltage level, the tap connection between the multi-select switch and the secondary winding corresponding to the cascade voltage level is controlled to be closed;

[0029] The second control module controls a corresponding number of second full-bridge circuits to power on and operate;

[0030] The first control module monitors the DC side voltage of the first full-bridge circuit in real time, and the second control module monitors the DC side voltage of the second full-bridge circuit in real time, and controls the DC circuit breaker to close when the DC side voltage of the second full-bridge circuit is equal to the DC side voltage of the first full-bridge circuit;

[0031] The second control module sends an active power signal and / or reactive power to the second full-bridge circuit, and controls the DC side of the second full-bridge circuit to output or input a corresponding current to the DC side of the first full-bridge circuit.

[0032] Furthermore, when a fault occurs in the second full-bridge circuit during the test, the bypass switches at both ends of the AC side of the second full-bridge circuit are controlled to close.

[0033] Further, the step of calculating the number of the first full-bridge circuit and the second full-bridge circuit required based on the cascade voltage level selected by the user comprises the steps of:

[0034] Calculate the number of links per phase power requirement based on the cascade voltage level selected by the user;

[0035] The number of the required first full-bridge circuits and the second full-bridge circuits is calculated according to the number of links, wherein the number of the first full-bridge circuits is the same as the number of the second full-bridge circuits.

[0036] Furthermore, when the test is an aging test, the test is conducted continuously for 48 hours at a temperature of 40°C and a relative humidity of 93%;

[0037] Or, conduct continuous testing for 48 hours at a temperature of -15°C and a relative humidity of 0%.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The present invention uses a multi-winding transformer and a first full-bridge circuit to simulate a battery cluster, and provides a DC source for a cascade converter formed by connecting the second full-bridge circuit in series on the AC side, so that the cascade converter formed by the second full-bridge circuit can be subjected to an aging test or a working condition simulation test under low-voltage grid conditions, without a battery cluster, or without a large-capacity battery cluster.

[0040] (2) The present invention uses a multi-winding transformer to achieve converter DC isolation, and a double-winding transformer is used on the cascade side of the cascade converter to reduce the high voltage to a low voltage, which can solve the problem of voltage mismatch between the low-voltage power supply in the factory and the high-voltage output of tens of kilovolts after cascading;

[0041] (3) In the present invention, the DC side of the first full-bridge circuit is connected to the DC side of the second full-bridge circuit, so that power can circulate internally when the two full-bridge circuits are working. When the cascade converter formed by the cascade connection of the second full-bridge circuit discharges, the first full-bridge circuit can realize power feedback. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings, wherein:

[0043] Figure 1 The current cascade energy storage converter described in the background technology;

[0044] Figure 2 The current direct-mounted cascade energy storage converter aging test system described in the background technology;

[0045] Figure 3 The three-phase cascade converter test system described in Example 1;

[0046] Description of symbols: 100, host computer; 110, first control module; 120, second control module. DETAILED DESCRIPTION

[0047] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0048] Example 1

[0049] like Figure 3 This embodiment discloses a three-phase cascade converter test system. The cascade converter adopts a star connection method. The three-phase cascade converter test system provided in this embodiment is also applicable to a delta connection method. The three-phase cascade converter test system includes a double-winding transformer T1, an A-phase multi-winding transformer TA, a B-phase multi-winding transformer TB, a C-phase multi-winding transformer TC, a plurality of A-phase low-voltage converter modules, a plurality of A-phase cascade converter modules, a plurality of B-phase low-voltage converter modules, a plurality of B-phase cascade converter modules, a plurality of C-phase low-voltage converter modules and a plurality of C-phase cascade converter modules. The following is an explanation by taking an A-phase multi-winding transformer, an A-phase low-voltage converter module and an A-phase cascade converter module as examples.

[0050] The primary winding of the A-phase multi-winding transformer TA is connected to the 400V AC low-voltage power grid, the secondary winding of the A-phase multi-winding transformer TA is connected to the AC side of the A-phase low-voltage converter module, the DC side of the A-phase low-voltage converter module is connected to the DC side of the A-phase cascade converter module, and the AC side of the A-phase cascade converter module is connected to the secondary winding of the double-winding transformer T1. Specifically, the A-phase low-voltage converter module includes a plurality of first full-bridge circuits (switches Q1-Q4 form a first full-bridge circuit), and the A-phase cascade converter module includes a plurality of second full-bridge circuits (switches Q5-Q8 form a second full-bridge circuit), wherein the number of the second full-bridge circuits is the same as the number of the first full-bridge circuits, and the AC sides of the second full-bridge circuits are connected in series to form a cascade converter.

[0051] The AC side of the first full-bridge circuit is connected to the secondary winding of the A-phase multi-winding transformer TA, the DC side of the first full-bridge circuit is connected to the DC side of the second full-bridge circuit through a DC reactor L, a DC fuse F and a DC circuit breaker S in sequence, the AC side of the second full-bridge circuit is connected to the secondary winding of the dual-winding transformer T1 through a high-voltage grid-connected reactor L_A, and the primary winding T1 of the dual-winding transformer is connected to a 400V AC low-voltage power grid.

[0052] Specifically, the double-winding transformer T1 is a double-winding three-tap transformer, and the three taps of the secondary winding of the double-winding transformer T1 can be connected to the outputs of 6kV, 10kV and 35kV cascade converters respectively.

[0053] In one of the embodiments, a multi-way selection switch S1 is further included, wherein a fixed end of the multi-way selection switch S1 is connected to the high-voltage grid-connected reactor L_A, and a selection end is respectively connected to three taps of the secondary winding of the double-winding transformer T1. Specifically, the corresponding tap connection is selected according to the output voltage of the cascade converter.

[0054] In one embodiment, it also includes a host computer 100, a first control module 110 and a second control module 120, the host computer 100 is connected to the first control module 110 and the second control module 120 respectively, the first control module 110 is connected to the first full-bridge circuit to control the first full-bridge circuit to work or stop working; the second control module 120 is connected to the second full-bridge circuit to control the second full-bridge circuit to work or stop working. Specifically, the host computer 100, the first control module 110 and the second control module 120 use DSP and FPGA, and dual-port RAM is used between DSP and FPGA to realize data interaction and collaborative work, giving full play to the advantages of DSP in complex sampling calculation and FPGA in data logic operation; the first control module 110 and the second control module 120 receive test software and / or test instructions from the host computer 100, and the first control module 110 and the second control module 120 realize data interaction and collaboration through optical fiber.

[0055] Specifically, it also includes a low-voltage grid-connected reactor Ln_A, and the AC side of the first full-bridge circuit is connected to the secondary winding of the A-phase multi-winding transformer TA through the low-voltage grid-connected reactors L1_A, L2_A...Ln_A. The first full-bridge circuit and the low-voltage grid-connected reactor are combined to provide the required DC power for the cascade converter module. The current of the voltage source can flow in both directions, and the voltage can be adjusted according to actual needs, thereby achieving the function of simulating a battery cluster.

[0056] Specifically, it also includes a plurality of bypass switches K1 connected to both ends of the AC side of each second full-bridge circuit. When the second full-bridge circuit fails, the bypass switch is closed to cut off the failed second full-bridge circuit.

[0057] Specifically, it also includes an A-phase AC circuit breaker SA, and the primary winding of the A-phase multi-winding transformer TA is connected to the 400V AC low-voltage power grid through the AC circuit breaker SA.

[0058] The connection relationship between the B-phase multi-winding transformer, the B-phase low-voltage converter module and the B-phase cascade converter module, and the connection relationship between the C-phase multi-winding transformer, the C-phase low-voltage converter module and the C-phase cascade converter module are the same as the connection relationship between the above-mentioned A-phase multi-winding transformer, the A-phase low-voltage converter module and the A-phase cascade converter module, and will not be repeated here.

[0059] The secondary winding of the multi-winding transformer can be connected to multiple first full-bridge circuits according to actual needs to ensure that the DC side power supplies of the cascaded converters are independent and insulated from each other. In addition, the first full-bridge circuit can feed back the active power absorbed by the cascaded AC converter formed by the cascaded AC side of the second full-bridge circuit to the power grid to ensure the stability of the DC bus voltage. At the same time, in order to reduce the capacity of the first full-bridge circuit, a unit power factor grid-connected control method can be used for the first full-bridge circuit to ensure that the transformer only bears the active power capacity of the device; in order to reduce the primary ripple current of the first full-bridge circuit, a carrier phase shift control can be used for the first full-bridge circuit.

[0060] The second full-bridge circuit adopts active power and reactive power decoupling control to simulate the active power and reactive power consumed by the actual load. The second full-bridge circuit usually adopts carrier phase shift control to control the size and phase of the cascade current output and increase the equivalent switching frequency to improve the grid-connected voltage and current waveform quality.

[0061] The power devices of the first full-bridge circuit and the second full-bridge circuit are all the same, and the software algorithms both use carrier phase-shift control algorithms, which greatly reduces the difficulty of related software and hardware development.

[0062] The three-phase cascade converter test system provided in this embodiment can provide multiple independent and isolated DC power supplies at the same time by using a multi-winding transformer and a first full-bridge circuit to simulate a battery cluster as the DC source of the cascade converter, thereby realizing the testing of multiple cascade converters at the same time and improving the testing efficiency of the cascade converter; and by using a multi-winding transformer and a first full-bridge circuit to simulate a battery cluster, it can solve the problem that the cascade converter is difficult to test under low-voltage power grid conditions and without a large-capacity battery cluster. The DC side of the first full-bridge circuit is connected to the DC side of the second full-bridge circuit to realize the internal circulation of the power of the two, and the first full-bridge circuit can realize power feedback when the cascade converter is discharged.

[0063] Since a multi-winding transformer and a first full-bridge circuit are used to simulate a battery cluster, in an extreme short-circuit condition, the first full-bridge circuit will not cause major safety problems such as battery short circuit, nor will there be problems of short battery life and self-discharge.

[0064] A multi-winding transformer is used to achieve converter DC isolation, and a double-winding transformer is used on the cascade side of the second full-bridge circuit to reduce the high voltage to a low voltage, which can solve the problem of low-voltage test cascade high-voltage output in the field, and there is no need to use a high-frequency bidirectional isolation DC / DC converter for isolation, nor is there any need to carry out targeted development for high-frequency bidirectional isolation DC / DC converters, which reduces the difficulty of developing related software algorithms for the three-phase cascade converter test system. The three-phase cascade converter test system provided in this embodiment uses the same model of the first full-bridge circuit and the second full-bridge circuit, and there is no need to carry out targeted development for components of different models, which reduces the difficulty of developing related software algorithms for the three-phase cascade converter test system.

[0065] Example 2

[0066] The reliability and stability of the converter directly affect the safe operation of the energy storage system. Therefore, the converter needs to be subjected to long-term, high-temperature, high-humidity and full-load aging tests before it leaves the factory to test the converter's reliability and stability.

[0067] This embodiment discloses a cascade converter aging test method, which is applied to the phase cascade converter test system described in Example 1. According to the requirements of GB / T-34133-2023, a continuous test is performed for 48 hours in an environment with a temperature of 40°C and a relative humidity of 93%, and a cascade voltage of 35 kV. Specifically, the method includes the following steps:

[0068] Control the AC circuit breakers SA, SB, SC and the multi-way selector switch S1 to close.

[0069] The host computer 100 sends a test signal to the first control module 110 , and the first control module 110 controls the first full-bridge circuit to power on and operate, so as to simulate an actual battery cluster.

[0070] The multi-selection switch is controlled to be closed in accordance with the cascade voltage level and the tap connection of the secondary winding corresponding to the cascade voltage level is closed.

[0071] The host computer 100 sends a rated charging current instruction to the second control module 120, and the second control module 120 controls the second full-bridge circuits corresponding to the first full-bridge circuit to power on and charge the DC side of the first full-bridge circuit with a rated current for 48 hours.

[0072] Specifically, when a fault occurs in the second full-bridge circuit during the test, the bypass switch K1 at both ends of the AC side of the second full-bridge circuit is controlled to close.

[0073] In one embodiment, it also includes:

[0074] The first control module 110 and the second control module 120 are first powered on and self-checked. When a fault is found in the self-check, the test is stopped and the user is notified to eliminate the fault.

[0075] During the aging test, not only the second full-bridge circuit was subjected to a charging aging test, but the first full-bridge circuit was also subjected to a discharging aging test.

[0076] By using a multi-winding transformer and a first full-bridge circuit to simulate a battery cluster as a voltage source, a second full-bridge circuit and its series connection form a cascade converter module, so that the cascade converter module can realize charging and discharging of the battery cluster, battery balancing management (including inter-phase balancing and intra-phase balancing), fault intelligent bypass, and grid-connected power quality control.

[0077] Example 3

[0078] This embodiment discloses a cascade converter working condition simulation test method, which is applied to the phase cascade converter test system described in Example 1. Taking the working condition simulation test of a 35kV-25MW-50MWh star-shaped cascade energy storage system as an example, the test method specifically includes the following steps:

[0079] Select lithium iron phosphate cells with a capacity of 314Ah, and connect 300 cells in series into a cluster, that is, the rated voltage of the battery cluster is 960V, and the voltage range is: 780V~1095V.

[0080] The cascade voltage level selected by the user is obtained as 35kV. Based on the cascade voltage level selected by the user and the rated voltage of the battery cluster, the number of links N for each phase power requirement is calculated: Where Pt is the system rated power, Qbat is the rated capacity of the battery cell, Vbat is the rated voltage of the battery cluster, and 0.5 means that the battery cluster is charged and discharged at a current of 0.5C; thus, N=55.3, which is rounded up to 56 levels. Considering the redundancy of the cascade system, the number of cascades is 60.

[0081] The total number of the required first full-bridge circuits and second full-bridge circuits is calculated according to the number of links: 60*3*2=360, wherein the number of the first full-bridge circuits is the same as the number of the second full-bridge circuits.

[0082] Control the AC circuit breakers SA, SB, and SC to close.

[0083] The constant voltage output is set to 960V, and the first control module 110 controls 180 first full-bridge circuits to power on and operate.

[0084] According to the cascade voltage level 35 kV, the multi-select switch S1 is controlled to be connected and closed to the tap 3 (35 kV) of the secondary winding corresponding to the cascade voltage level.

[0085] The second control module 120 controls 180 second full-bridge circuits to power on and operate in standby mode.

[0086] The first control module 110 monitors the DC side voltage of the first full-bridge circuit in real time, and the second control module 120 monitors the DC side voltage of the second full-bridge circuit in real time. When the DC side voltage of the second full-bridge circuit is equal to the DC side voltage of the first full-bridge circuit, the DC circuit breaker S is controlled to close.

[0087] The first control module 110 exchanges data with the second control module 120 , and the first control module 110 controls the DC voltage drop of the first full-bridge circuit to 780V.

[0088] The second control module 120 sends a constant current charging signal to the second full-bridge circuit, controls the DC side of the second full-bridge circuit to charge the DC side of the first full-bridge circuit at a constant current of 157A for 2 hours, simulates the charging of the energy storage system, and makes the voltage of the first full-bridge circuit reach 1095V.

[0089] Specifically, when a fault occurs in the second full-bridge circuit during the test, the bypass switch K1 at both ends of the AC side of the second full-bridge circuit is controlled to close.

[0090] In one embodiment, it also includes:

[0091] The first control module 110 and the second control module 120 are first powered on and self-checked. The first control module 110 and the second control module 120 are powered by UPS. When a fault is found in the self-check, the test is stopped and the user is notified to troubleshoot.

[0092] Specifically, the steps include:

[0093] After the test is finished, the second control module controls the second full-bridge circuit to stop working first, and the second control module shares data with the first control module.

[0094] When the second full-bridge circuit completely stops working, the first control module controls the first full-bridge circuit to stop working.

[0095] The cascade converter test system and test method provided by the present invention can solve the problem of implementing cascade converter aging test and working condition simulation test under the condition that neither battery cluster nor high-voltage power grid is available. The test system can perform full power cycle test internally, and the system is connected to the low-voltage power grid externally to only provide the loss energy of the transformer and converter, without the need to apply for capacity expansion of the factory transformer or direct grid connection of the cascade high voltage.

[0096] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Therefore, any modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A cascade converter test method, applied to a cascade converter test system, characterized in that: The steps include: Control the closing of AC circuit breaker; The first control module controls the first full-bridge circuit to power on and operate; According to the cascade voltage level, the tap connection between the multi-select switch and the secondary winding corresponding to the cascade voltage level is controlled to be closed; The second control module controls the second full-bridge circuits corresponding to the number of the first full-bridge circuits to power on and operate; The first control module monitors the DC side voltage of the first full-bridge circuit in real time, and the second control module monitors the DC side voltage of the second full-bridge circuit in real time, and controls the DC circuit breaker to close when the DC side voltage of the second full-bridge circuit is equal to the DC side voltage of the first full-bridge circuit; The second control module sends an active power signal and / or a reactive power signal to the second full-bridge circuit to control the DC side of the second full-bridge circuit to output or input a corresponding current to the DC side of the first full-bridge circuit; The cascade converter test system comprises a dual-winding transformer, a plurality of multi-winding transformers, a plurality of low-voltage converter modules and a plurality of cascade converter modules, wherein the primary winding of the multi-winding transformer is connected to the low-voltage power grid, the secondary winding of the multi-winding transformer is connected to the AC side of the low-voltage converter module, the DC side of the low-voltage converter module is connected to the DC side of the cascade converter module, and the AC side of the cascade converter module is connected to the secondary winding of the dual-winding transformer. The low-voltage converter module includes a plurality of first full-bridge circuits, and the cascade converter module includes a plurality of second full-bridge circuits. The AC sides of the second full-bridge circuits are connected in series to form a cascade converter module; the AC sides of the first full-bridge circuits are connected to the secondary winding of the multi-winding transformer, the DC sides of the first full-bridge circuits are connected to the DC sides of the second full-bridge circuits through a DC reactor, a DC fuse and a DC circuit breaker in sequence, the AC side of the second full-bridge circuit is connected to the secondary winding of the dual-winding transformer through a high-voltage grid-connected reactor, and the primary winding of the dual-winding transformer is connected to the low-voltage power grid, wherein the number of the second full-bridge circuits is the same as the number of the first full-bridge circuits.

2. The cascade converter testing method according to claim 1, characterized in that: The cascade converter test system also includes a host computer, a first control module and a second control module, the host computer is connected to the first control module and the second control module respectively, the first control module is connected to the first full-bridge circuit, and controls the first full-bridge circuit to work or stop working; The second control module is connected to the second full-bridge circuit to control the second full-bridge circuit to work or stop working.

3. The cascade converter testing method according to claim 1, characterized in that: The cascade converter test system further comprises a low-voltage grid-connected reactor, and the AC side of the first full-bridge circuit is connected to the secondary winding of the multi-winding transformer through the low-voltage grid-connected reactor.

4. The cascade converter testing method according to claim 1, characterized in that: The cascade converter test system further includes a bypass switch connected to both ends of the AC side of the second full-bridge circuit.

5. The cascade converter testing method according to claim 1, characterized in that: The cascade converter test system also includes a multi-way selection switch, a fixed end of the multi-way selection switch is connected to the high-voltage grid-connected reactor, and a selection end is respectively connected to three taps of the secondary winding of the double-winding transformer.

6. The cascade converter testing method according to claim 1, characterized in that: The cascade converter test system further comprises an AC circuit breaker, and the primary winding of the multi-winding transformer is connected to the low-voltage power grid via the AC circuit breaker.

7. The cascade converter testing method according to claim 1, characterized in that: When a fault occurs in the second full-bridge circuit during the test, the bypass switches at both ends of the AC side of the second full-bridge circuit are controlled to close.

8. The cascade converter testing method according to claim 1, characterized in that: The step of calculating the number of the first full-bridge circuit and the second full-bridge circuit required based on the cascade voltage level selected by the user comprises the steps of: Calculate the number of links per phase power requirement based on the cascade voltage level selected by the user; The number of the required first full-bridge circuits and the second full-bridge circuits is calculated according to the number of links, wherein the number of the first full-bridge circuits is the same as the number of the second full-bridge circuits.

9. The cascade converter testing method according to claim 1, characterized in that: When the test is an aging test, the test is carried out continuously for 48 hours at a temperature of 40°C and a relative humidity of 93%; Or, conduct continuous testing for 48 hours at a temperature of -15°C and a relative humidity of 0%.

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