An M3C low-frequency converter and its fault ride-through method and system

Abstract

This invention discloses an M3C low-frequency converter and its fault ride-through method and system under asymmetrical faults in a power frequency grid. The M3C low-frequency converter connects the power frequency grid and the low-frequency grid, and includes the following steps: the M3C converter samples the power frequency grid voltage in real time; the M3C converter detects the percentage deviation between the d-axis component of the positive sequence voltage of the power frequency grid and the rated positive sequence voltage of the power frequency grid in real time; compares this deviation with a threshold; and maintains the low-frequency side output AC voltage at the rated output voltage u. y_额定 Alternatively, the output AC voltage on the low-frequency side can be kept unchanged, while the output AC current on the low-frequency side can be increased. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy dissipation device of the wind turbine converter will be enabled. This method can ensure the power supply and demand balance, safety, and stability of the M3C under asymmetrical faults in the power frequency grid, and avoid damage to the M3C converter due to overvoltage and overcurrent during the fault. The method of this invention improves the steady-state performance of the M3C low-frequency converter and has practical value.

Description

Technical Field

[0001] This invention relates to control strategies for high-power power conversion devices, and more particularly to a fault ride-through method for an M3C low-frequency converter under asymmetrical faults in a power frequency grid. Background Technology

[0002] The Modular Multilevel Matrix Converter (M3C) is a new type of AC-AC frequency converter characterized by perfectly harmonic-free input current, adjustable power factors at both input and output, and high control flexibility. It is widely used in practical engineering, playing a crucial role in asynchronous grid interconnection, offshore wind power, and long-distance low-frequency power transmission.

[0003] There is limited research on fault ride-through control of converters (M3C) under asymmetrical faults in existing technologies. However, in actual power grids, sudden faults such as short circuits often cause power systems to operate in an asymmetrical state. When the three-phase system is asymmetrical, negative sequence components will appear in the system. These negative sequence components will increase the effective value of the current, leading to overcurrent in the system. At the same time, a large number of non-characteristic harmonics will be generated in the AC system connected to the M3C. These harmonics may, at best, affect the controller's performance and worsen the control results, and at worst, cause overvoltage / overcurrent in the converter components, or even burn out the components. All of these will seriously threaten the safe and stable operation of the entire system. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a fault ride-through method for M3C low-frequency converters under asymmetrical faults in power frequency grids. This method can ensure the power supply and demand balance, safety and stability of M3C under asymmetrical faults in power frequency grids, and avoid damage to the M3C converter due to overvoltage and overcurrent during the fault.

[0005] Technical Solution: A fault ride-through method for an M3C low-frequency converter under asymmetrical faults in a power frequency grid, wherein the M3C low-frequency converter is connected to both the power frequency grid and the low-frequency grid. The method steps are as follows:

[0006] (1) Sampling of power frequency grid voltage;

[0007] (2) Real-time detection of the d-axis component of the positive sequence voltage of the power grid by the M3C converter and the d-axis component of the rated power frequency positive sequence voltage percentage of deviation

[0008] (3) If k is the M3C current overload factor, then the low-frequency side output AC voltage is kept at the rated output voltage u. y_额定 Unchanged; if This reduces the low-frequency side output AC voltage and increases the low-frequency side output AC current. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy dissipation device of the wind turbine converter will be enabled.

[0009] Furthermore, the M3C low-frequency converter employs constant M3C capacitor voltage average value stabilization control, low-frequency side power control, and three-phase current symmetrical control on the power frequency side, and constant AC voltage control, constant frequency control, and three-phase current symmetrical control on the low-frequency side.

[0010] Furthermore, in step (3), the low-frequency side output AC voltage is reduced to

[0011] Furthermore, in step (2), the M3C converter detects the d-axis component of the positive sequence voltage of the power grid in real time. The specific data collection steps are as follows:

[0012] (2.1) The collected three-phase voltages of the power frequency grid are transformed by coordinates to obtain the power frequency grid voltage components u in a two-phase stationary coordinate system. xα and u xβ ;

[0013] (2.2) The positive sequence components of the power frequency grid voltage in the two-phase stationary coordinate system are obtained using the 1 / 4 time delay method. and negative order components

[0014]

[0015] (2.3) After coordinate transformation, the positive sequence components of the power frequency grid voltage in the two-phase rotating coordinate system are obtained.

[0016] A system for fault ride-through of M3C low-frequency converters under asymmetrical faults in power frequency grids includes: a sampling module: real-time acquisition of three-phase AC power from the power frequency grid, followed by coordinate transformation to obtain the positive-sequence voltage d-axis component in the rotating coordinate system of the power frequency grid.

[0017] Detection module: Calculates the d-axis component of the positive sequence voltage of the power frequency grid. and the desired positive sequence voltage d-axis component percentage of deviation

[0018] Fault identification and handling module: compares the deviation percentage with the critical value. Compare, if If the value is less than the critical value, the low-frequency side output AC voltage will be maintained at the rated output voltage u. y_额定If it remains unchanged, then reduce the low-frequency side output AC voltage and increase the low-frequency side output AC current. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy discharge device of the wind turbine converter will be enabled.

[0019] Furthermore, a DC energy dissipation device is connected in parallel to the DC side of the wind turbine converter.

[0020] Furthermore, the threshold for engaging the parallel DC energy dissipation device on the DC side of the wind turbine converter is k times the rated value of the low-frequency current, and the threshold for disengaging it is the rated value of the low-frequency current.

[0021] An M3C low-frequency converter including the above system is used to connect the power frequency grid and the low-frequency grid. The M3C low-frequency converter consists of three sub-converters, each sub-converter has three bridge arms, each bridge arm consists of N cascaded H-bridge units and one AC inductor L, and the three neutral points of the three sub-converters are the three phases of the AC system; each H-bridge unit consists of four IGBTs and one DC capacitor.

[0022] Furthermore, the frequency of the low-frequency power grid is 15Hz to 30Hz.

[0023] Furthermore, the M3C low-frequency converter is built on land and connected to the offshore centralized booster platform via existing submarine power frequency cables.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: Based on the change of the positive sequence voltage component of the power frequency and the overload capacity of the M3C converter, the output voltage of the M3C converter is reduced, causing the current of the wind turbine converter to rise to the threshold, and the energy dissipation resistor is activated to dissipate energy, thereby completing the fault ride-through of the M3C converter. This ensures that the power supply and demand of the M3C is balanced, safe and stable under the asymmetrical fault of the power frequency grid, avoids damage to the M3C converter due to overvoltage and overcurrent during the fault, and improves the steady-state performance of the system. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the wind power transmission system structure of the present invention;

[0026] Figure 2 This is the simulated waveform of the power frequency high-voltage side phase voltage of the present invention;

[0027] Figure 3 This is the simulated waveform of the high-voltage phase current on the power frequency side of the present invention;

[0028] Figure 4 This is the simulated waveform of the low-voltage side line voltage at power frequency according to the present invention;

[0029] Figure 5 The simulated waveforms of the currents on both sides of the power frequency transformer of this invention;

[0030] Figure 6 This is the simulated waveform of the frequency division side voltage of the present invention;

[0031] Figure 7 This is a simulation waveform of the capacitor voltage of the present invention;

[0032] Figure 8 This is a flowchart of the method of the present invention;

[0033] Figure 9 This invention relates to a DC energy dissipation device. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0035] A typical structure of an offshore wind power transmission system based on the M3C low-frequency converter is as follows: Figure 1 As shown, the M3C converter station is built on land, connecting the onshore main power grid and the offshore low-frequency power grid. The M3C and the offshore wind farm are connected by submarine cables, and the frequency of the low-frequency power grid is 15Hz to 30Hz.

[0036] The main circuit structure of the M3C converter consists of three sub-converters, each with three bridge arms. Each bridge arm comprises N cascaded H-bridge units and one AC inductor L. The three neutral points of the three sub-converters correspond to the three phases of the AC system. Each H-bridge unit consists of four IGBTs and one DC capacitor.

[0037] The M3C low-frequency converter employs constant M3C capacitor voltage average value stabilization control, low-frequency side power control, and three-phase current symmetrical control on the power frequency side, and constant AC voltage amplitude control, constant frequency control, and three-phase current symmetrical control on the low-frequency side.

[0038] The flowchart of the fault ride-through method for M3C low-frequency converter under asymmetrical faults in the power frequency grid is as follows: Figure 8 As shown, the M3C converter detects the d-axis component of the positive sequence voltage of the power grid in real time. and the d-axis component of the rated power frequency positive sequence voltage percentage of deviation When an asymmetrical fault occurs in the onshore power frequency main grid, if (k is the M3C current overload factor), then the low-frequency side output AC voltage is kept at the rated output voltage u. y_额定 Unchanged; if This reduces the low-frequency side output AC voltage. Increasing the low-frequency side output AC current will enable the wind turbine converter's energy dissipation device when the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter. A system applicable to the fault ride-through method of M3C low-frequency converter under asymmetrical faults in the power frequency grid includes: a sampling module: real-time acquisition of the three-phase AC power grid data, followed by coordinate transformation to obtain the positive sequence voltage d-axis component in the rotating coordinate system of the power frequency grid.

[0039] Detection module: Calculates the d-axis component of the positive sequence voltage of the power frequency grid. and the desired positive sequence voltage d-axis component percentage of deviation

[0040] Fault identification and handling module: compares the deviation percentage with the critical value. Compare, if If the value is less than the critical value, the low-frequency side output AC voltage will be maintained at the rated output voltage u. y_额定 If it remains unchanged, then reduce the low-frequency side output AC voltage and increase the low-frequency side output AC current. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy discharge device of the wind turbine converter will be enabled.

[0041] Among them, the DC side of the wind turbine converter is connected in parallel with the DC energy dissipation device, such as Figure 9 As shown. The threshold for engaging the parallel DC energy dissipation device on the DC side of the wind turbine converter is k times the rated value of the low-frequency current, and the threshold for disengaging it is the rated value of the low-frequency current.

[0042] The simulation system parameters for the M3C converter station are shown in Table 1:

[0043] Table 1 Simulation parameters of M3C system

[0044]

[0045] During the simulation, a short-circuit fault occurred in phase A of the power frequency AC system at t=1s, and the fault lasted for 1s. The fault was cleared at t=2s. The simulation waveform is shown in the figure. After the short-circuit fault occurred in phase A of the power frequency AC system, the phase A voltage was zero, and energy was injected into the fault point by the power frequency system, resulting in a large short-circuit current (e.g., ...). Figure 2 , Figure 3 (as shown); Figure 4 and Figure 5 This indicates that the M3C generates a three-phase sinusoidal current on the power frequency side, but the amplitude does not exceed the maximum current of the system, which also injects a certain amount of energy into the fault point. Figure 6 This is the frequency divider side voltage of the M3C system. Due to a short circuit on the power frequency side, the system's transmission power is limited, so the M3C system will reduce the output amplitude of the frequency divider side voltage. Figure 7This indicates that during the fault, the voltage fluctuations of the submodule capacitors in the M3C system increased, but remained within a reasonable range and did not threaten the system's safety. (Based on simulation waveforms) Figures 2-7 It can be seen that by adopting the method of the present invention, when the grid voltage fails, the power supply and demand balance, safety and stability of M3C can be ensured under the asymmetrical fault of the power frequency grid, and the M3C converter can be prevented from being damaged due to overvoltage and overcurrent during the fault. Moreover, after the fault is cleared, the converter can quickly return to normal operation.

Claims

1. A fault ride-through method for an M3C low-frequency converter under asymmetrical faults in a power frequency grid, characterized in that, The steps for connecting the M3C low-frequency converter to the mains frequency grid and the low-frequency grid are as follows: (1) Sampling of power frequency grid voltage; (2) M3C converter detects the d-axis component of positive sequence voltage of power frequency grid in real time and the d-axis component of the rated power frequency positive sequence voltage percentage of deviation ; (3) If If k is the M3C current overload factor, then the low-frequency side output AC voltage is kept at the rated output voltage. Unchanged; if This reduces the low-frequency side output AC voltage, thus reducing the low-frequency side output AC voltage to... Increase the low-frequency side output AC current. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy discharge device of the wind turbine converter will be enabled.

2. The fault ride-through method for M3C low-frequency converter under asymmetrical faults in a power frequency grid according to claim 1, characterized in that, The M3C low-frequency converter employs constant M3C capacitor voltage average value stabilization control, low-frequency side power control, and three-phase current symmetrical control on the power frequency side, and constant AC voltage control, constant frequency control, and three-phase current symmetrical control on the low-frequency side.

3. The fault ride-through method for M3C low-frequency converter under asymmetrical faults in a power frequency grid according to claim 1, characterized in that, In step (2), the M3C converter detects the d-axis component of the positive sequence voltage of the power frequency grid in real time. The specific data collection steps are as follows: (2.1) The collected three-phase voltages of the power frequency grid are transformed by coordinate transformation to obtain the power frequency grid voltage components in a two-phase stationary coordinate system. and ; . (2.2) The positive sequence components of the power frequency grid voltage in the two-phase stationary coordinate system are obtained by using the 1 / 4 time delay method. , and negative order components , ; (2.3) After coordinate transformation, the positive sequence components of the power frequency grid voltage in the two-phase rotating coordinate system are obtained. .

4. A system for a fault ride-through method of an M3C low-frequency converter under asymmetrical faults in a power frequency grid according to claim 1, characterized in that, Includes: Sampling module: Real-time acquisition of three-phase AC power grid data from the power frequency grid, followed by coordinate transformation to obtain the positive sequence voltage d-axis component in the rotating coordinate system of the power frequency grid. ; Detection module: Calculates the d-axis component of the positive sequence voltage of the power frequency grid. and the desired positive sequence voltage d-axis component of the power frequency percentage of deviation ; Fault identification and handling module: compares the deviation percentage with the critical value. Compare, if If the value is less than the threshold, the low-frequency side output AC voltage will be maintained at the rated output voltage. If it remains unchanged, then reduce the low-frequency side output AC voltage and increase the low-frequency side output AC current. When the low-frequency AC current reaches the overcurrent threshold of the wind turbine converter, the energy discharge device of the wind turbine converter will be enabled.

5. A system for a fault ride-through method of an M3C low-frequency converter under an asymmetrical fault in a power frequency grid according to claim 4, characterized in that, A DC energy dissipation device is connected in parallel to the DC side of the wind turbine converter.

6. A system for a fault ride-through method of an M3C low-frequency converter under an asymmetrical fault in a power frequency grid according to claim 4, characterized in that, The threshold for engaging the parallel DC energy dissipation device on the DC side of the wind turbine converter is k times the rated value of the low-frequency current, and the threshold for disengaging it is the rated value of the low-frequency current.

7. An M3C low-frequency converter for asymmetrical faults in a power frequency grid, characterized in that, The M3C low-frequency converter includes the system of claim 4. The M3C low-frequency converter is used to connect the power frequency grid and the low-frequency grid. The M3C low-frequency converter consists of three sub-converters. Each sub-converter has three bridge arms. Each bridge arm consists of N cascaded H-bridge units and an AC inductor L. The three neutral points of the three sub-converters are the three phases of the AC system. Each H-bridge unit consists of four IGBTs and a DC capacitor.

8. An M3C low-frequency converter under asymmetrical fault conditions in a power frequency grid according to claim 7, characterized in that, The low-frequency power grid has a frequency of 15Hz to 30Hz.

9. An M3C low-frequency converter under asymmetrical faults in a power frequency grid according to claim 7, characterized in that, The M3C low-frequency converter is built on land and connected to the offshore centralized booster platform via existing submarine power frequency cables.

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