Low-frequency transmission test system

Abstract

The embodiments of this specification specifically disclose a low-frequency power transmission test system, which integrates the M3C test circuit and the DRU-MMC test circuit into one circuit. The system can verify the control and protection strategies when the sending-end busbar fault, the step-up transformer side fault and the receiving-end AC outgoing line fault occur in the M3C and DRU-MMC transmission system projects, thereby saving the number of test equipment and the construction cost of the system. At the same time, by setting the incoming line switch and the outgoing line switch of the power supply branch and the outgoing line switch on the M3C and DRU-MMC test circuit, different access methods of the incoming line circuit and the outgoing line circuit can be realized. By opening or closing the switch, the wind power converter module is used to isolate the sending-end AC incoming line and the receiving-end AC outgoing line, thereby realizing the decoupling of the AC incoming line and the AC outgoing line of the closed-loop system, solving the problem that the AC outgoing line fault of the traditional closed-loop system will cause multiple faults, and improving the accuracy of the test results.

Description

Technical Field

[0001] This specification belongs to the field of low-frequency power transmission technology, and in particular to a low-frequency power transmission test system. Background Art

[0002] With the increasing emphasis on ecological and environmental protection, the development of clean energy as a key means of energy conservation and emission reduction has become a hot topic in the energy industry. Offshore wind power boasts abundant resources, stable wind energy, high power generation hours, no land occupation, suitability for large-scale development, and minimal environmental impact. Furthermore, offshore wind reserves are relatively abundant, with only 1.3% of these reserves currently being developed. Therefore, the research and development of offshore wind power technologies is particularly important.

[0003] At present, the common offshore wind power transmission technologies in the world mainly include: high-voltage AC transmission technology (HVAC), high-voltage direct current transmission technology (HVDC), and fractional frequency transmission system (FFTS).

[0004] However, HVAC technology is difficult to apply to long-distance and large-capacity wind power transmission and grid connection, making it unsuitable for the large-scale, offshore development of offshore wind power. For medium- and long-distance transmission, the current mainstream technology for offshore wind power transmission and grid connection is Flexible High Voltage Direct Current (VSC-HVDC). However, the technical difficulty and investment cost of constructing offshore converter stations are far higher than those of onshore converter stations. Furthermore, offshore converter stations are located in complex environments and require significant maintenance, making them less economical. Compared to these two options, FFTS significantly increases the power transmission distance and capacity by reducing the frequency, thereby increasing the current capacity of the cable. Furthermore, the elimination of offshore converter stations significantly reduces investment and maintenance costs. Therefore, the prospects of FFTS are more aligned with the development trends of offshore wind power.

[0005] Fractional Frequency Transmission Technology (FFTS), a technology for offshore wind power transmission systems, reduces transmission frequency, reduces line reactance, shortens line electrical distances, enhances line transmission capacity, and extends cable life without increasing voltage levels. This breaks away from the traditional AC transmission approach of increasing voltage levels to increase transmission capacity. Furthermore, this technology solution eliminates the need for offshore converter stations, reducing overall system investment costs and meeting cost reduction requirements. Based on the concept of fractional frequency transmission, some scholars have proposed low-frequency alternating current (LFAC) technology to achieve offshore wind power transmission.

[0006] However, low-frequency transmission technology is less mature than HVDC, and no actual wind power projects have adopted this theoretical approach. Therefore, there is an urgent need for a technical solution capable of conducting low-frequency transmission tests that can realistically simulate the key functions and performance indicators of low-frequency systems. This can help identify engineering issues in advance, mitigate engineering risks, provide forward-looking practical guidance for projects, and ensure their feasibility and manageable risks. Summary of the Invention

[0007] The purpose of the embodiments of this specification is to provide a low-frequency power transmission test system, which realizes the steady-state characteristic test and dynamic performance test of the low-frequency M3C AC power transmission system and the low-frequency DRU-MMC power transmission system, and can also constitute a low-frequency power transmission test loop with M3C-DRU-MMC in series to verify its steady-state characteristics.

[0008] On the one hand, an embodiment of this specification provides a low-frequency power transmission test system, the system comprising: an AC bus, an AC power supply module, a power supply branch, a cable simulation device, an M3C test circuit, and a DRU-MMC test circuit;

[0009] The AC power supply module, the power supply branch, the cable simulation device, and the M3C test circuit are sequentially connected in series to the AC bus, and the DRU-MMC test circuit is connected in parallel to both ends of the M3C test circuit;

[0010] The power supply branch includes a first power supply branch, a second power supply branch and a third power supply branch connected in parallel. A branch control switch S1 is connected between the outgoing end of the first power supply branch and the second power supply branch and the third power supply branch. A decoupling outgoing line is provided at the parallel node of the outgoing end of the DRU-MMC test loop and the M3C test loop. The decoupling outgoing line is connected between the third power supply branch and the AC power supply module. A third branch incoming line switch S3.1 is provided on the third power supply branch. The third branch incoming line switch S3.1 is provided between the decoupling outgoing line and the AC power supply module; a first outgoing line switch S9 is provided between the parallel node and the AC busbar, and a second outgoing line switch S8 is provided between the parallel node and the third branch incoming line switch S3.1;

[0011] The system further comprises at least one fault simulation device connected at a fault detection location of the system.

[0012] Furthermore, the first power supply branch includes a first branch incoming line switch S1.1, a first wind power converter module W1, a booster T1.1 and a first branch outgoing line switch S1.2 connected in series in sequence; the second power supply branch includes a second branch incoming line switch S2.1, a second branch incoming line transformer T2.1, a second wind power converter module W2, a second branch outgoing line transformer T2.2 and a second branch outgoing line switch S2.2 connected in series in sequence; the third power supply branch includes a third branch incoming line switch S3.1, a third branch incoming line transformer T3.1, a third wind power converter module W3, a third branch outgoing line transformer T3.2 and a third branch outgoing line switch S3.2 connected in series in sequence.

[0013] Furthermore, the M3C test circuit includes: an M3C test incoming line switch S3, an M3C test incoming line transformer T2, an M3C converter A, an M3C test outgoing line transformer T3, and an M3C test outgoing line switch S4, which are sequentially connected in series.

[0014] Furthermore, the DRU-MMC test circuit includes: a DRU-MMC test incoming line switch S5, a DRU-MMC test incoming line transformer T4, a diode converter valve B, an MMC converter C, a DRU-MMC test outgoing line transformer T5, and a DRU-MMC test outgoing line switch S6 connected in series in sequence.

[0015] Furthermore, a boost control switch S2 and a transformer T1 are sequentially connected between the power supply branch and the cable simulation device.

[0016] Furthermore, the fault simulation device is arranged at at least one of the following positions: the AC incoming busbar between the power supply branch and the cable simulation device, the step-up and high-voltage transformer between the cable simulation device and the M3C test circuit, and the AC outgoing line of the M3C test circuit and the DRU-MMC test circuit.

[0017] Furthermore, the AC power supply module adopts any one of the following modes for power supply: direct power supply from the power grid, power supply from a synchronous motor group, and power supply from a combination of the power grid and the synchronous motor group.

[0018] Furthermore, the M3C converter in the M3C test loop adopts a full-bridge topology power module.

[0019] Furthermore, the diode converter valve in the DRU-MMC test loop adopts a series pulse diode rectifier, and the MMC converter adopts a half-bridge topology, a full-bridge topology, or a full-half-bridge hybrid power module.

[0020] Furthermore, the first power supply branch includes one branch or multiple parallel branches.

[0021] The low-frequency transmission test system provided in this specification integrates the M3C test circuit and the DRU-MMC test circuit into one circuit to verify the control and protection strategies when a sending-end busbar fault, a step-up transformer side fault, and a receiving-end AC outgoing line fault occur in the M3C transmission system project and the DRU-MMC transmission system project. Tests with different requirements are carried out on one system, which saves the number of test equipment and thus saves the construction cost of the low-frequency transmission test system. At the same time, the embodiments of this specification can realize different access methods of the incoming line circuit and the outgoing line circuit by setting the incoming line switch and the outgoing line switch on the power supply branch and the M3C test circuit and the DRU-MMC test circuit. More importantly, the sending-end AC incoming line and the receiving-end AC outgoing line can be isolated by using the wind power converter module by opening or closing the switch, thereby realizing the decoupling of the AC incoming line and the AC outgoing line of the closed-loop system, solving the problem that the AC outgoing line fault of the closed-loop system will cause multiple faults, and improving the accuracy of the test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of this specification or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in this specification. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0023] Figure 1 This is a schematic diagram of the structure of the low-frequency power transmission test system provided in the embodiments of this specification;

[0024] Figure 2 is a schematic diagram of the structure of M3C in one embodiment of this specification;

[0025] Figure 3 This is a schematic diagram of the topological structure of a DRU in one embodiment of this specification;

[0026] Figure 4 is a schematic diagram of the topological structure of the MMC in one embodiment of this specification;

[0027] Figure 5 This is a schematic diagram of the connection process of a typical M3C test circuit for AC input and output line coupling in one embodiment of this specification;

[0028] Figure 6 This is a flow chart of a sending-end busbar fault test using an M3C typical test circuit coupled with AC input and output lines in one embodiment of this specification;

[0029] Figure 7This is a schematic diagram of the connection process of the M3C test circuit for AC input and output line decoupling in one embodiment of this specification;

[0030] Figure 8 This is a flow chart of a receiving-end busbar fault test using an M3C test circuit with decoupled AC input and output lines in one embodiment of this specification;

[0031] Figure 9 This is a schematic diagram of the connection process of a typical DRU-MMC test circuit with AC input and output line coupling in one embodiment of this specification;

[0032] Figure 10 This is a flow chart of a typical DRU-MMC test circuit for performing a sending-end busbar fault test using AC input and output line coupling in one embodiment of this specification;

[0033] Figure 11 This is a schematic diagram of the connection process of the DRU-MMC test circuit with AC input and output line decoupling in one embodiment of this specification;

[0034] Figure 12 This is a flow chart of a receiving-end busbar fault test using a DRU-MMC test circuit with AC input and output line decoupling in one embodiment of this specification;

[0035] Figure 13 This is a schematic diagram of the connection process of a test loop in which an M3C branch and a DRU-MMC branch are connected in series in one embodiment of this specification;

[0036] Figure 14 It is a flow chart of a test using a series-connected M3C branch and a DRU-MMC test loop in one embodiment of this specification. DETAILED DESCRIPTION

[0037] To help those skilled in the art better understand the technical solutions in this specification, the following will provide a clear and complete description of the technical solutions in the embodiments of this specification, in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of this specification, not all of them. All other embodiments derived by those skilled in the art based on the embodiments in this specification without creative effort shall fall within the scope of protection of this specification.

[0038] As a new power transmission method, the Low Frequency AC Transmission System (LFAC) reduces the system operating frequency f. On the one hand, the line inductive reactance XL decreases as the frequency decreases, significantly reducing the transmission line impedance and effectively shortening the line's electrical distance. On the other hand, the line capacitive reactance XC increases as the frequency decreases, which can reduce the charging reactive power of the cable line and greatly increase the line's transmission capacity. It is one of the grid-connected power transmission methods with future development prospects.

[0039] The embodiment of this specification provides a low-frequency test system that integrates the M3C test circuit and the DRU-MMC test circuit into one circuit. By controlling the closing and opening of the corresponding switches, steady-state characteristic tests and dynamic performance tests of the low-frequency M3C AC transmission system and the low-frequency DRU-MMC transmission system can be carried out respectively. A low-frequency transmission test circuit of M3C-DRU-MMC series can also be formed to verify its steady-state characteristics. For example, it can be used to verify the control and protection strategies when a sending-end busbar fault, a step-up transformer side fault, and a receiving-end AC outgoing line fault occur in the M3C low-frequency AC transmission system project and the DRU-MMC transmission system project. At the same time, the wind power converter module is used to isolate the sending-end AC incoming line and the receiving-end AC outgoing line, thereby realizing the decoupling of the AC incoming line and the AC outgoing line of the closed-loop system, solving the problem that the AC outgoing line fault of the closed-loop system will cause multiple faults, and improving the accuracy of the test results.

[0040] Although this specification provides the method operation steps or device structure as shown in the following embodiments or drawings, more or fewer operation steps or module units may be included in the method or device based on routine or no creative labor. In the steps or structures where there is no necessary causal relationship logically, the execution order of these steps or the module structure of the device is not limited to the execution order or module structure shown in the embodiments or drawings of this specification. When the method or module structure is applied to an actual device, server or terminal product, it can be executed sequentially or in parallel according to the method or module structure shown in the embodiment or drawings (for example, in an environment of parallel processors or multi-threaded processing, or even in an implementation environment of distributed processing or server clusters).

[0041] Figure 1 This is a schematic diagram of the structure of the low-frequency power transmission test system provided in the embodiment of this specification. Figure 1As shown, in one embodiment of the low-frequency transmission test system provided in this specification, the low-frequency transmission test system may include: an AC bus, an AC power supply module, a power supply branch, a cable simulation device (used to simulate the working conditions of the cables in the transmission system), an M3C test circuit, and a DRU-MMC test circuit. Among them, M3C (modular multilevel matrix converter) can be understood as a modular multilevel matrix converter, DRU (Diode rectifier unit) can be understood as a diode rectifier unit, and MMC (modular multilevel converter) can be listed as a modular multilevel converter. Figure 1 As shown, the AC power supply module, power supply branch, cable simulation device, and M3C test circuit are connected in series on the AC busbar, and the DRU-MMC test circuit is connected in parallel at both ends of the M3C test circuit. The embodiment of this specification integrates the M3C test circuit and the DRU-MMC test circuit into one circuit, allowing two low-frequency power transmission tests, the M3C test circuit and the DRU-MMC test circuit, to be performed in one circuit, saving equipment and reducing the construction cost of the test system.

[0042] like Figure 1 As shown, in some embodiments of this specification, the M3C test circuit may include: M3C test incoming line switches connected in series, namely Figure 1 The switches S3 and M3C in the test line transformer are Figure 1 The T2 and M3C converters are Figure 1 The converter A and M3C test outgoing transformer are Figure 1 The T3 and M3C test outlet switches are Figure 1 By setting the incoming and outgoing switches of the M3C test, you can control the access status of the M3C test loop and implement M3C tests in different scenarios. Figure 2 This is a schematic diagram of the structure of M3C in one embodiment of the present invention. Figure 2 As shown, in some embodiments of this specification, the M3C converter in the M3C test loop can use a full-bridge topology power module. The M3C converter can use output voltage control as the outer loop of the bridge arm current control.

[0043] like Figure 1 As shown, the DRU-MMC test circuit may include: DRU-MMC test incoming line switches connected in series, namely Figure 1 The switch S5 and DRU-MMC test line transformer are Figure 1 T4 and diode commutation valve in Figure 1 The diode valve B and MMC converter in Figure 1Converter C and DRU-MMC test outgoing transformer are Figure 1 T5, DRU-MMC test outlet switch Figure 1 Switch S6 in. Figure 3 This is a schematic diagram of the topological structure of a DRU in an embodiment of this specification. Figure 4 Schematic diagram of the topology of MMC in one embodiment of this specification, such as Figure 3-Figure 4 As shown, in some embodiments of this specification, the diode converter valve in the DRU-MMC test loop can use a series pulse diode rectifier such as a 6-pulse or 12-pulse diode rectifier, the MMC converter can use a power module with a half-bridge topology or a full-bridge or full-half-bridge hybrid power module, and the MMC converter can use a constant DC voltage control. The function of a single DRU and MMC device in the DRU-MMC test loop is to achieve AC to DC or DC to AC conversion. In the embodiments of this specification, the main purpose is to perform AC voltage fault test detection, such as Figure 1 The electrical locations of fault simulation devices F2 and F3 are both AC voltage. This embodiment of the specification uses a DRU and MMC in a stacked configuration to achieve AC-to-AC connectivity. The specific models of the diode commutator valve and MMC converter can be selected based on actual needs and are not specifically limited in this embodiment.

[0044] It should be noted that the control strategy of the M3C converter and the MMC converter is a recommended strategy, which can be used to simulate the wind power transmission system of the grid-type offshore wind turbine. The control strategy can also be adjusted according to different test requirements and is not limited to the strategy given in the embodiments of this specification.

[0045] like Figure 1 As shown, in one embodiment of this specification, the power supply branch may include a first power supply branch connected in parallel, namely Figure 1 Branch 1 and the second power supply branch are Figure 1 Branch 2 and the third power supply branch in Figure 1 In branch 3, a branch control switch is connected between the outlet end of the first power supply branch and the second power supply branch and the third power supply branch. Figure 1 Switch S1 is used in the circuit. Branches 2 and 3 can be connected in parallel with branch 1 to form the incoming line loop by closing switch S1. Alternatively, branch 2 and branch 3 can be connected in series to form the system's outgoing line loop by opening switch S1, thereby decoupling the closed-loop system's AC incoming and outgoing lines. In typical closed-loop test systems, since the system's AC incoming and outgoing lines share the same busbar, simulating an outgoing AC line fault can cause the same fault to occur on the AC incoming line, leading to multiple faults and making it impossible to control the type of test fault, ultimately impacting the test results.

[0046] In addition, if Figure 1 As shown, the parallel node of the outgoing terminal of the DRU-MMC test loop and the M3C test loop (i.e. Figure 1 The node at the left side of the DRU-MMC test loop and the M3C test loop, that is, Figure 1 A decoupling output line is provided at the node between the middle switch S7 and the switch S8, and the decoupling output line is connected between the third power supply branch and the AC power supply module. The third power supply branch is provided with a third branch incoming line switch, namely Figure 1 The switch S3.1 in the third branch incoming line switch is set between the decoupling outgoing line and the AC power supply module. In addition, the first outgoing line switch is also set between the parallel node and the AC bus. Figure 1 The switch S9 in the parallel node is also provided with a second outgoing line switch between the parallel node and the third branch incoming line switch. Figure 1 Switch S8 in the Figure 1 As shown, different access modes of the DRU-MMC test loop and the M3C test loop can be realized by controlling the closing and opening of switches such as switch S1, switch S3.1, switch S8, and switch S9, thereby realizing tests of different wiring modes for the DRU-MMC test loop and the M3C test loop.

[0047] like Figure 1 As shown, in some embodiments of this specification, the first power supply branch is Figure 1 The branch 1 in the circuit may include the first branch incoming line switch connected in series. Figure 1 S1.1, the first wind power converter module Figure 1 W1 in the booster is Figure 1 T1.1 and the first branch outgoing line switch Figure 1 S1.2 in the second power supply branch. Figure 1 The branch 2 in the circuit may include a second branch incoming line switch connected in series. Figure 1 S2.1 in the second branch incoming transformer is Figure 1 T2.1, the second wind power converter module Figure 1 W2 in the second branch outgoing transformer is Figure 1 T2.2 and the second branch outgoing line switch Figure 1 S2.2 in the third power supply branch is Figure 1 The branch 3 in the circuit may include a third branch incoming line switch connected in series. Figure 1 S3.1 in the third branch incoming transformer is Figure 1 T3.1, the third wind power converter module Figure 1 W3 in the third branch outgoing transformer is Figure 1 T3.2 and the third branch outgoing line switch Figure 1S3.2 in the figure. By setting up incoming and outgoing line switches on each of the three power supply branches, combined with switch S1, the access method of branches 2 and 3 can be flexibly controlled. This lays the data foundation for flexibly setting the incoming and outgoing line routes for subsequent M3C test loops and DRU-MMC test loops, thereby implementing tests with different functions. For specific line connection methods, please refer to the description of the test method section in the subsequent examples.

[0048] In addition, in some embodiments of this specification, the first power supply branch includes one branch or multiple parallel branches, that is, branch 1 can be composed of a single branch, or can be composed of multiple parallel incoming branch branches of the same structure to change the incoming line loop structure. The specific configuration can be based on actual needs and is not specifically limited in this specification. Figure 1 As shown, when the incoming and outgoing switches of branches 2 and 3 are both closed, branches 2 and 3 can be understood as parallel incoming branches to branch 1, which is equivalent to the parallel connection of multiple incoming branches described for branch 1. However, by coordinating the incoming and outgoing switches of branches 2 and 3 with the control of switch S1, branches 2 and 3 can be connected in series, functioning as system outgoing lines. This structure is the core of isolating the AC incoming and outgoing lines of a closed-loop system.

[0049] like Figure 1 As shown, in some embodiments of this specification, a boost control switch connected in sequence is further provided between the power supply branch and the cable simulation device. Figure 1 The switch S2 and transformer in Figure 1 By closing and opening the switch S2, the access of the cable simulation device can be controlled. Figure 1 As shown, a fault simulation device F1 can also be provided between the switch S2 and the transformer T1 to perform a fault test on the high voltage side of the step-up transformer T1.

[0050] like Figure 1 As shown, the system also includes at least one fault simulation device, which can be connected to the fault detection position of the system according to actual test requirements to test the system performance after faults occur at different positions.

[0051] In some embodiments of this specification, the fault simulation device can be set at at least one of the following locations: the AC incoming busbar between the power supply branch and the cable simulation device, that is, Figure 1 The step-up voltage change point between F1, cable simulation device and M3C test circuit is Figure 1 The AC outlet of the F2, M3C test circuit and the DRU-MMC test circuit is Figure 1F3 in the figure. Of course, the fault location can also be set at other locations depending on the test requirements, and this specification does not specifically limit this. By installing a fault simulation device at the location where fault detection is required, fault simulation tests can be performed at different locations in the low-frequency power transmission system, testing system performance and laying a data foundation for the application of low-frequency power transmission systems in actual production.

[0052] Of course, the low-frequency power transmission test system provided in the embodiments of this specification may also have other components, such as Figure 1 As shown, a switch S0 can also be set between the AC bus and the AC power supply module, and a switch S7 can be set between the outgoing line of the DRU-MMC test circuit and the M3C test circuit, etc. The specific settings can be made according to actual test requirements, and the embodiments of this manual do not make specific limitations.

[0053] The applicable scenarios of the low-frequency power transmission test system provided in the embodiments of this specification include: at least one application scenario in the steady-state operation test and dynamic test of the offshore wind power low-frequency transmission system and the onshore low-frequency AC power transmission system. The AC power supply module can adopt different power supply methods according to the requirements of the test for the power supply, such as grid power supply, synchronous motor group power supply, and any one of the grid and synchronous motor group power supply methods to meet the needs of different test scenarios. For example: the AC power supply module can be powered by a synchronous motor group with a rated output voltage of 690V and a rated power generation capacity of 3MW, and the wind turbine group is equipped with a simulated wind power generation motor and its controller, speed regulator and traction system. The AC power supply module uses a simulated wind turbine group to form a test system for the offshore low-frequency power transmission system. The system can also use other construction methods to construct the AC power supply module to simulate the AC power supply system under different scenarios.

[0054] In addition, the wind power converter module in the embodiments of this specification includes at least a wind power converter primary device and its control and protection unit, an external interface unit, etc. for configuring the module. The M3C converter may also include at least an M3C converter primary device and its control and protection unit, an external interface unit, etc. for configuring the module. The MMC converter may also include at least an MMC converter primary device and its control and protection unit, an external interface unit, etc. for configuring the module. The specific structure of the converter can be configured according to actual needs and is not specifically limited in the embodiments of this specification.

[0055] The parameters and control strategy of the wind power converter are a recommended strategy that can be used to simulate the wind power transmission system of the grid-type offshore wind turbine. The control strategy can also be adjusted according to different test requirements and is not limited to the strategy given in the embodiment. For example: the rated power of the wind power converter module W1 is 2.5MW, the rated voltage is 690V, and the grid-side frequency is 20Hz. Its machine-side converter control strategy can adopt a zero current control method to control its DC voltage and reactive power. Its grid-side converter performs fixed active power control by controlling the maximum power point tracking (MPPT) of the wind turbine to generate an AC voltage amplitude, and generates a voltage phase angle by fixing the reactive power, that is, fixed active power and reactive power control. The grid side of the wind power converter W1 is connected to the step-up transformer T1.1 to form branch 1. The low-frequency transmission test system provided in the embodiment of this specification integrates the M3C test circuit and the DRU-MMC test circuit into one circuit to verify the control and protection strategies when the sending-end busbar fault, the step-up transformer side fault and the receiving-end AC outgoing line fault occur in the M3C transmission system project and the DRU-MMC transmission system project. Tests with different requirements are carried out on one system, which saves the number of test equipment and thus saves the construction cost of the low-frequency transmission test system. At the same time, the embodiment of this specification can realize different access methods of the incoming line circuit and the outgoing line circuit by setting the incoming line switch and the outgoing line switch on the power supply branch and the M3C test circuit and the DRU-MMC test circuit. More importantly, the sending-end AC incoming line and the receiving-end AC outgoing line can be isolated by using the wind power converter module by opening or closing the switch, thereby realizing the decoupling of the AC incoming line and the AC outgoing line of the closed-loop system, solving the problem that the AC outgoing line fault of the traditional closed-loop system will cause multiple faults (causing both the AC outgoing line fault and the AC incoming line fault), and improving the accuracy of the test results.

[0056] By using the low-frequency power transmission test system provided in the embodiment of this specification, the M3C circuit and the DRU-MMC circuit can be tested separately through the coordination of switches, and the M3C, DRU, and MMC series circuits can also be tested. Moreover, when testing the M3C circuit or the DRU-MMC circuit alone, the AC outgoing line fault in the closed-loop system can be isolated from the AC incoming line fault, realizing the decoupling of the AC outgoing line and the AC incoming line of the closed-loop test system, thus solving the problem that the AC outgoing line fault of the closed-loop system may cause multiple faults. Figure 1 The system structure diagram shown in the figure specifically introduces different wiring modes and test methods of the low-frequency power transmission test system in the embodiment of this specification:

[0057] Figure 5 FIG. 1 is a schematic diagram of the connection process of a typical M3C test circuit for AC input and output line coupling in an embodiment of this specification. Figure 5As shown, the embodiment of this specification can connect an M3C typical test loop in the low-frequency power transmission test system through the following process:

[0058] Step 1: Confirm that all switches are initially open, close switch S0, and connect to the AC bus.

[0059] Step 2: Close switches S1.1 and S1.2 to build the incoming line loop of branch 1.

[0060] Step 3: Close switches S2.1, S2.2, S3.1, S3.2, and S1 to connect branches 2 and 3 in parallel with branch 1 through S1 to form an incoming line loop.

[0061] Step 4: Close switch S2 to connect the incoming line loop to transformer T1.

[0062] Step 5: Close switches S3 and S4 to connect the M3C test circuit to the test system.

[0063] Step 6: Close switch S9 to complete the test circuit.

[0064] It can be seen that based on Figure 5 As shown in the figure, after controlling the switches and other components in the low-frequency transmission test system, a typical M3C test circuit was constructed. In this test circuit, S2.1, S2.2, S3.1, S3.2, and S1 are closed, S8 is open, and the wind power converter module is connected to the T1 transformer (the electrical location of the fault module F1) to supply power to M3C. At this time, the incoming and outgoing lines of the test circuit are the same AC bus, which is a typical closed-loop experimental circuit.

[0065] Figure 6 FIG. 1 is a flow chart of a typical M3C test circuit for performing a sending-end busbar fault test using an AC input / output line coupling in an embodiment of this specification. Figure 6 As shown, the test process is as follows:

[0066] Step 1: Construct a typical M3C test circuit. Figure 5 The construction method shown.

[0067] Step 2: Start the AC power supply module, the wind power converter module W1 is energized, the wind power converter operates with a control strategy of fixed DC voltage and reactive power on the generator side, and operates with fixed active power and fixed reactive power on the grid side, unlocking the wind power converter (i.e., the wind power converter module in the above embodiment).

[0068] Step 3: The busbar boost transformer T1 and the connecting transformer T2 are energized in sequence, and the inlet end of the M3C converter valve is energized.

[0069] Step 4: The M3C converter is unlocked and operated with a constant output voltage control strategy, so that the system operates in a steady state.

[0070] Step 5: Connect the fault simulation device F1 and conduct a sending-end busbar fault test.

[0071] In addition, in some embodiments of this specification, the M3C typical test circuit with AC input and output line coupling can be used to perform the high voltage side fault test of the step-up transformer T1. The specific process can refer to the above Figure 6 Repeat the sending end busbar fault test process in Figure 6 In step 5, connect the fault simulation device F2 to perform the high-voltage side fault test of the step-up transformer T1.

[0072] Figure 7 FIG. 1 is a schematic diagram of the connection process of the M3C test circuit for AC input and output line decoupling in one embodiment of this specification. Figure 7 As shown, the embodiment of this specification can connect an M3C test loop with decoupled AC input and output lines in a low-frequency power transmission test system through the following process:

[0073] Step 1: Confirm that all switches are initially open, close switch S0, and connect to the AC bus.

[0074] Step 2: Close switches S1.1 and S1.2 to build the incoming line loop of branch 1.

[0075] Step 3: Close switch S2 to connect the incoming line loop to transformer T1.

[0076] Step 4: Close switches S3 and S4 to connect the M3C test circuit to the test system.

[0077] Step 5: Switch S8 is closed, connecting to transformer T3.1.

[0078] Step 6: Close switches S3.2, S2.2, and S2.1 to connect branch 2 and branch 3 in series, forming an outgoing line loop.

[0079] Step 7: Complete the test circuit.

[0080] By coordinating various switches, the test system is reconstructed. At this time, the series connection of branch 2 and branch 3 is regarded as the outgoing line of the test system. The AC outgoing line of the M3C converter is connected to the outgoing line formed by the series connection of branch 2 and branch 3 to construct a test circuit with decoupled AC input and output lines. When the AC outgoing line test (fault point corresponding to the fault simulation device F3) is carried out under the decoupled topology, since the AC incoming line and outgoing line have been decoupled, that is, the AC incoming line and outgoing line are not the same line ( Figure 1At this time, the AC busbar at the fault point corresponding to the fault simulation device F3 is isolated), so there is no double fault problem, and the test detection of the AC outgoing line fault can be achieved.

[0081] Figure 8 FIG. 1 is a flow chart of a busbar fault test at a receiving end using an M3C test circuit with decoupled AC input and output lines in one embodiment of the present specification. Figure 8 As shown, the test process is as follows:

[0082] Step 1: Construct a test circuit for decoupling the M3C AC input and output lines. For details, please refer to the above embodiment. Figure 7 Based on the records, an M3C test circuit with decoupling of AC input and output lines was constructed.

[0083] Step 2: Start the AC power supply module, power the wind turbine converter module W1, and operate the wind turbine converter with a constant DC voltage and reactive power control strategy on the generator side. The grid side operates with a constant active power and constant reactive power, and the wind turbine converter is unlocked.

[0084] Step 3: The busbar boost transformer T1 and the connecting transformer T2 are energized in sequence, and the inlet end of the M3C converter valve is energized.

[0085] Step 4: The M3C converter is unlocked and operated with a constant output voltage control strategy.

[0086] Step 5: The wind power converter W3 operates with a constant DC voltage and reactive power control strategy on the transformer T3.1 side, and operates with a constant active power and reactive power control strategy on the transformer T3.2 side.

[0087] Step 6: The wind power converter W2 operates with a constant DC voltage and reactive power control strategy on the transformer T2.2 side, and operates with a constant active power and reactive power control strategy on the transformer T2.1 side, so that the system operates in a steady state.

[0088] Step 7: Connect the fault simulation device F3 to perform the receiving end AC outgoing line fault test.

[0089] In addition, in some embodiments of this specification, the M3C test circuit with AC input and output line decoupling can be used to perform the sending end busbar fault test. The specific process can be referred to the above Figure 8 The receiving end busbar fault test process in the above can be repeated Figure 8 In step 1 to step 6, in step 7, the fault simulation device F1 is connected to simulate the sending-end busbar fault.

[0090] Similarly, you can also refer to the above Figure 8 The receiving-end busbar fault test process in the above example is to use the M3C test circuit with AC input and output line decoupling to perform the high-voltage side fault test of the step-up transformer T1. Figure 8 In step 7, the fault simulation device F2 is connected to perform a fault test on the high-voltage side of the step-up transformer T1.

[0091] By controlling the switches to reconstruct the low-frequency transmission system, an M3C test circuit with decoupling of the AC input and output lines was constructed. Different converters and fault detection devices were connected through the control switches to implement tests when a busbar fault occurred at the sending end, a fault on the high-voltage side of the step-up transformer T1, or a fault on the AC output line at the receiving end. This enabled the system to establish steady-state operation of the M3C AC input and output line decoupling test circuit.

[0092] Similarly, the DRU-MMC test circuit can be accessed in different ways by closing and opening different switches in the control system. Figure 9 FIG. 1 is a schematic diagram of the connection process of a typical DRU-MMC test circuit for AC input and output line coupling in an embodiment of this specification. Figure 9 As shown, the embodiment of this specification can connect a typical DRU-MMC test loop with AC input and output line coupling in the low-frequency power transmission test system through the following process:

[0093] Step 1: Confirm that all switches are initially open, close switch S0, and connect to the AC bus;

[0094] Step 2: Close switches S1.1 and S1.2 to establish the incoming line loop of branch 1;

[0095] Step 3: Close switches S2.1, S2.2, S3.1, S3.2, and S1, so that branches 2 and 3 are connected in parallel with branch 1 through S1 to form an incoming line loop.

[0096] Step 4: Close switch S2 to connect the incoming line circuit to transformer T1;

[0097] Step 5: Close switches S5 and S6 to connect the DRU-MMC test circuit to the test system;

[0098] Step 6: Close switches S7 and S9 to complete the test circuit.

[0099] It can be seen that based on Figure 9 As shown in the figure, after controlling the switches and other components in the low-frequency power transmission test system, a typical DRU-MMC test circuit is constructed. At this time, the incoming and outgoing lines of the test circuit are the same AC bus, that is, a typical closed-loop experimental circuit.

[0100] Figure 10 FIG. 1 is a flow chart of a typical DRU-MMC test circuit for performing a sending-end busbar fault test using an AC input / output line coupling in an embodiment of this specification. Figure 10 As shown, the test process is as follows:

[0101] Step 1: Construct a typical DRU-MMC test circuit with AC input and output line coupling. Specifically, the above embodiment can be used. Figure 9 Constructed according to the method shown.

[0102] Step 2: Start the AC power supply module, power the wind turbine converter module W1, and operate the wind turbine converter with a control strategy of constant DC voltage and reactive power on the generator side and constant active power and reactive power on the grid side. Unlock the wind turbine converter.

[0103] Step 3: The busbar step-up transformer T1 and the connecting transformer T2 are energized in sequence, and the incoming line end of the DRU-MMC test circuit is energized;

[0104] Step 4: The MMC inverter can use constant DC voltage control. The active power input to the DRU is determined by the DRU AC bus voltage. Unlock MMC converter C to put the system into steady-state operation.

[0105] Step 5: Connect the fault simulation device F1 to perform the sending-end busbar fault test.

[0106] You can also refer to the above Figure 10 , use the DRU-MMC typical test circuit with AC input and output line coupling to conduct the high voltage side fault test of the step-up transformer T1. Figure 10 After the system is in steady-state operation through steps 1 to 4, the fault simulation device F2 is connected in step 5 to perform a fault test on the high-voltage side of the step-up transformer T1.

[0107] Figure 11 FIG. 1 is a schematic diagram of the connection process of the DRU-MMC test circuit with AC input and output line decoupling in one embodiment of this specification. Figure 11 As shown, the embodiment of this specification can connect a DRU-MMC test loop with AC input and output line decoupling in the low-frequency power transmission test system through the following process:

[0108] Step 1: Confirm that all switches are initially open, close switch S0, and connect to the AC bus.

[0109] Step 2: Close switches S1.1 and S1.2 to build the incoming line loop of branch 1.

[0110] Step 3: Close switch S2 to connect the incoming line loop to transformer T1.

[0111] Step 4: Close switches S5 and S6 to connect the DRU-MMC test circuit to the test system.

[0112] Step 5: Close switches S7 and S8 and connect to transformer T3.1.

[0113] Step 6: Close switches S3.2, S2.2, and S2.1 to connect branch 2 and branch 3 in series, forming an outgoing line loop.

[0114] Step 7: Complete the test circuit.

[0115] It can be seen that based on Figure 11 As shown in the figure, after controlling the switches and other components in the low-frequency power transmission test system, an AC input and output line decoupling DRU-MMC test circuit is constructed. At this time, the input and output lines of the test circuit are not the same AC bus.

[0116] Figure 12 FIG. 1 is a flow chart of a receiving-end busbar fault test using a DRU-MMC test circuit with AC input and output line decoupling in one embodiment of this specification, as shown in FIG. Figure 12 As shown, the test process is as follows:

[0117] Step 1: Construct a DRU-MMC AC input and output line decoupling test circuit. For details, please refer to the above embodiment. Figure 11 Constructed according to the method shown.

[0118] Step 2: Start the AC power supply module, power the wind turbine converter module W1, and operate the wind turbine converter with a constant DC voltage and reactive power control strategy on the generator side. The grid side operates with a constant active power and constant reactive power, and the wind turbine converter is unlocked.

[0119] Step 3: The busbar boost transformer T1 and the connecting transformer T2 are energized in sequence, and the incoming line end of the DRU-MMC test circuit is energized.

[0120] Step 4: The MMC inverter can adopt constant DC voltage control. The active power input to the DRU is determined by the DRU AC bus voltage, unlocking the MMC converter C.

[0121] Step 5: The wind power converter W3 operates with a constant DC voltage and reactive power control strategy on the transformer T3.1 side, and operates with a constant active power and reactive power control strategy on the transformer T3.2 side.

[0122] Step 6: The wind power converter W2 operates with a constant DC voltage and reactive power control strategy on the transformer T2.2 side, and operates with a constant active power and reactive power control strategy on the transformer T2.1 side, so that the system operates in a steady state.

[0123] Step 7: Connect the fault simulation device F3 to perform the receiving end AC outgoing line fault test.

[0124] In addition, in some embodiments of this specification, the DRU-MMC test circuit with AC input and output decoupling can be used to perform the sending end busbar fault test. The specific process can be referred to the above Figure 12 The receiving end busbar fault test process in the above can be repeated Figure 12 In step 1 to step 6, in step 7, the fault simulation device F1 is connected to simulate the sending-end busbar fault.

[0125] Similarly, you can also refer to the above Figure 12 The receiving-end busbar fault test process in the above example is to use the DRU-MMC test circuit with AC input and output decoupling to perform the high-voltage side fault test of the step-up transformer T1. Figure 12 In step 7, the fault simulation device F2 is connected to perform a fault test on the high-voltage side of the step-up transformer T1.

[0126] By controlling the switches to reconstruct the low-frequency transmission system, a DRU-MMC test circuit with decoupling of the AC input and output lines is constructed. Different converters and fault detection devices are connected through the control switches to implement tests when a busbar fault occurs at the sending end, a fault occurs on the high-voltage side of the step-up transformer T1, and a fault occurs at the receiving end AC output line. This enables the system to establish steady-state operation of the DRU-MMC AC input and output line decoupling test circuit.

[0127] Figure 13 FIG. 1 is a schematic diagram of the connection process of a test circuit in which the M3C branch and the DRU-MMC branch are connected in series in one embodiment of this specification. Figure 13 As shown, the embodiment of this specification can connect an M3C branch and a DRU-MMC branch in series to form a test loop in the low-frequency power transmission test system through the following process:

[0128] Step 1: Confirm that all switches are initially disconnected;

[0129] Step 2: Close switches S9, S7, S4, and S6, and transformers T3 and T5 are energized;

[0130] Step 3: Unlock the M3C converter and close switches S3 and S5;

[0131] Step 4: Unlock the MMC converter to form a complete test circuit.

[0132] Figure 14 FIG. 1 is a flow chart of a test using a series-connected M3C branch and a DRU-MMC test loop in one embodiment of this specification. Figure 14 As shown, the test process is as follows:

[0133] Step 1: Construct a test loop in which the M3C branch and the DRU-MMC branch are connected in series. For details, see the above embodiment. Figure 13 Constructed according to the method shown.

[0134] Step 2: The M3C converter is unlocked using a fixed output voltage control strategy;

[0135] Step 3: The MMC inverter can use constant DC voltage control. The active power input to the DRU is determined by the DRU AC bus voltage. Unlock MMC converter C.

[0136] Step 4: The system operates in steady state and the steady-state operation characteristics of the system are collected.

[0137] The embodiment of this specification discloses a test system with low-frequency AC transmission and low-frequency DRU-MMC transmission, which integrates the M3C test loop and the DRU-MMC test loop into one loop. Compared with the independent M3C test system and DRU-MMC test system, it saves the number of equipment and reduces the construction cost of the test system. By isolating the AC input and output lines, the AC input and output lines are decoupled, which solves the problem that the AC output line failure of the closed-loop system will cause multiple failures. Using the low-frequency transmission test system provided by the embodiment of this specification, the steady-state characteristic test and dynamic performance test of the low-frequency M3C AC transmission system and the low-frequency DRU-MMC transmission system can be carried out respectively, and a low-frequency transmission test loop of M3C-DRU-MMC in series can also be constructed to verify its steady-state characteristics. It can be used to verify the control and protection strategy when the sending-end busbar fault, the step-up transformer side fault and the receiving-end AC output line fault occur in the low-frequency AC transmission system project and the DRU-MMC transmission system project. Moreover, through the coordination of switches, coordinated control of multiple converters of the low-frequency M3C transmission system and the low-frequency DRU-MMC transmission test system can be achieved.

[0138] The various embodiments in this specification are described in a progressive manner. Similar portions between the various embodiments can be referenced to each other. Each embodiment focuses on the differences between the other embodiments. In particular, the hardware + program embodiments are generally similar to the method embodiments, so their description is relatively simple. For relevant portions, reference can be made to the description of the method embodiments.

[0139] Although one or more embodiments of this specification provide method operation steps such as embodiments or flow charts, more or fewer operation steps may be included based on conventional or non-creative means. The order of steps listed in the embodiments is only one way of executing the order of many steps and does not represent the only execution order. When the device or terminal product in practice is executed, it can be executed in sequence or in parallel according to the method shown in the embodiments or the drawings (for example, a parallel processor or a multi-threaded processing environment, or even a distributed resource data update environment). The term "comprise", "include" or any other variant thereof is intended to cover non-exclusive inclusion, so that the process, method, product or equipment including a series of elements includes not only those elements, but also includes other elements that are not clearly listed, or also includes elements inherent to such process, method, product or equipment. In the absence of more restrictions, it is not excluded that there are other identical or equivalent elements in the process, method, product or equipment including the elements. Words such as first and second are used to represent names and do not represent any particular order.

[0140] For the convenience of description, the above devices are described in terms of functions divided into various modules. Of course, when implementing one or more of the present specifications, the functions of each module can be implemented in the same or multiple software and / or hardware, or the module that implements the same function can be implemented by a combination of multiple sub-modules or sub-units, etc. The device embodiments described above are merely schematic. For example, the division of units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0141] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between the various embodiments can be referenced to each other, and each embodiment focuses on the differences from the other embodiments. In particular, since the system embodiments are generally similar to the method embodiments, their description is relatively simple, and relevant parts can be referenced to the partial description of the method embodiments. Throughout this specification, reference to the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of this specification. In this specification, the schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, those skilled in the art may combine and integrate the different embodiments or examples, and features of different embodiments or examples, described in this specification, without conflict.

[0142] The above is merely an example of one or more embodiments of this specification and is not intended to limit the one or more embodiments of this specification. It will be apparent to those skilled in the art that various modifications and variations may be made to one or more embodiments of this specification. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this specification shall be included within the scope of the claims.

Claims

1. A low-frequency power transmission test system, characterized in that: The system includes: an AC bus, an AC power supply module, a power supply branch, a cable simulation device, an M3C test circuit, and a DRU-MMC test circuit; The AC power supply module, the power supply branch, the cable simulation device, and the M3C test circuit are sequentially connected in series to the AC bus, and the DRU-MMC test circuit is connected in parallel to both ends of the M3C test circuit; The power supply branch includes a first power supply branch, a second power supply branch and a third power supply branch connected in parallel, a branch control switch (S1) is connected between the outgoing end of the first power supply branch and the second power supply branch and the third power supply branch, a decoupling outgoing line is provided at the parallel node between the outgoing end of the DRU-MMC test loop and the M3C test loop, the decoupling outgoing line is connected between the third power supply branch and the AC power supply module, a third branch incoming line switch (S3.1) is provided on the third power supply branch, and the third branch incoming line switch (S3.1) is provided between the decoupling outgoing line and the AC power supply module; a first outgoing line switch (S9) is provided between the parallel node and the AC busbar, and a second outgoing line switch (S8) is provided between the parallel node and the third branch incoming line switch (S3.1); The system further comprises at least one fault simulation device connected at a fault detection location of the system.

2. The low-frequency power transmission test system according to claim 1, characterized in that: The first power supply branch includes a first branch incoming line switch (S1.1), a first wind power converter module (W1), a booster (T1.1) and a first branch outgoing line switch (S1.2) connected in series in sequence; the second power supply branch includes a second branch incoming line switch (S2.1), a second branch incoming line transformer (T2.1), a second wind power converter module (W2), a second branch outgoing line transformer (T2.2) and a second branch outgoing line switch (S2.2) connected in series in sequence; the third power supply branch includes a third branch incoming line switch (S3.1), a third branch incoming line transformer (T3.1), a third wind power converter module (W3), a third branch outgoing line transformer (T3.2) and a third branch outgoing line switch (S3.2) connected in series in sequence.

3. The low-frequency power transmission test system according to claim 1, characterized in that: The M3C test circuit comprises: an M3C test incoming line switch (S3), an M3C test incoming line transformer (T2), an M3C converter (A), an M3C test outgoing line transformer (T3), and an M3C test outgoing line switch (S4) which are sequentially connected in series.

4. The low-frequency power transmission test system according to claim 1, characterized in that: The DRU-MMC test circuit includes: a DRU-MMC test incoming line switch (S5), a DRU-MMC test incoming line transformer (T4), a diode converter valve (B), an MMC converter (C), a DRU-MMC test outgoing line transformer (T5), and a DRU-MMC test outgoing line switch (S6) connected in series.

5. The low-frequency power transmission test system according to claim 1, characterized in that: A boost control switch (S2) and a transformer (T1) are sequentially connected between the power supply branch and the cable simulation device.

6. The low-frequency power transmission test system according to claim 1, characterized in that: The fault simulation device is arranged at at least one of the following positions: the AC incoming busbar between the power supply branch and the cable simulation device, between the cable simulation device and the M3C test circuit, and at the AC outgoing line of the M3C test circuit and the DRU-MMC test circuit.

7. The low-frequency power transmission test system according to claim 1, characterized in that: The AC power supply module adopts any one of the following modes for power supply: direct power supply from the power grid, power supply from a synchronous motor group, and power supply from a combination of the power grid and the synchronous motor group.

8. The low-frequency power transmission test system according to claim 3, characterized in that: The M3C converter in the M3C test loop adopts a full-bridge topology power module.

9. The low-frequency power transmission test system according to claim 4, characterized in that: The diode converter valve in the DRU-MMC test loop adopts a series pulse diode rectifier, and the MMC converter adopts a power module with a half-bridge topology, a full-bridge topology, or a full-half-bridge hybrid topology.

10. The low-frequency power transmission test system according to claim 1, characterized in that: The first power supply branch includes one branch or multiple parallel branches.

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