Low-frequency side fault ride-through control method for half-wave frequency conversion modular multilevel converter

Abstract

The present invention provides a low-frequency side fault ride-through control method for a half-wave variable frequency modular multilevel converter, the method comprising: detecting the positive-sequence and negative-sequence voltage d-q axis components and the positive-sequence current d-q axis components at the current HWC-MMC low-frequency grid side after a low-frequency grid fault occurs, determining the asymmetric fault type of the grid based on the positive-sequence and negative-sequence voltage d-q axis components and the positive-sequence current d-q axis components, calculating the converter low-frequency grid side current phase, the low-frequency side equivalent grid-connected inductor voltage amplitude, and the low-frequency side port voltage phase, controlling and switching the post-fault HWC-MMC low-frequency side voltage reference value based on the asymmetric fault type, the low-frequency grid side current phase, the low-frequency side equivalent grid-connected inductor voltage amplitude, the low-frequency grid side voltage amplitude, and the low-frequency side port voltage phase; and injecting a corresponding zero-sequence voltage reference value into the HWC-MMC low-frequency side control loop and switching the zero-sequence voltage reference value based on the asymmetric fault type, the low-frequency grid side voltage amplitude, and the low-frequency side port voltage phase. The present invention can achieve HWC-MMC bridge arm energy balancing and fault ride-through without introducing negative-sequence current and overvoltage.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible low-frequency power transmission control, and in particular to a low-frequency side fault ride-through control method of a half-wave frequency conversion modular multi-level converter. Background Art

[0002] The low-frequency transmission system (LFTS) reduces the operating frequency of AC transmission lines from the industrial frequency of 50 Hz to a low frequency of 50 / 3 Hz or 20 Hz, thereby reducing the electrical distance of AC transmission lines and improving the transmission power capacity of transmission lines. It has broad application prospects in scenarios where new energy sources such as offshore wind power and "Shagohuang" onshore wind power and photovoltaic power are efficiently and capacitively aggregated and transmitted.

[0003] High-voltage, high-capacity AC converters are core equipment for low-frequency power transmission technology. The modular multilevel matrix converter (M3C), currently the leading AC converter topology, has been demonstrated in flexible low-frequency power transmission projects in Hangzhou and Taizhou, Zhejiang Province, my country. However, the M3C suffers from the drawbacks of a large number of submodules and heavy volume. Consequently, a series of lightweight modular AC converter topologies, including the hexverter, Y-type modular multilevel converter (Y-MMC), and enneagonal converter, have been proposed. The Y-MMC and Hexverter have a similar number of submodules and total number of major components, both at 75% of the M3C, but avoid circulating current and reactive power constraints, making control relatively simple. The enneagonal converter, which can be considered an extension of the Hexverter, offers advantages in three-port AC / AC conversion applications. From the perspective of sinusoidal waveform transformation, the above-mentioned new topologies all belong to full-wave transformation, that is, the bridge arms need to be modulated to generate complete sinusoidal waves or heterogeneous frequency combination waves, so the number of required sub-modules is still relatively large, and there is room for further improvement in the power density of high-voltage and high-power AC converters.

[0004] The half-wave commutated modular multilevel converter (HWC-MMC) differs from the full-wave frequency conversion principle of the aforementioned converters. It is a novel AC converter based on the concept of half-sine and half-wave frequency conversion. The bridge arms only need to be modulated to generate a half-sine wave or a combination of half-sine waves at different frequencies. This reduces the number of required submodules by half, and the total number of components is reduced by over 35% compared to the M3C at the same capacity. Unlike the traditional M3C bridge arms, which naturally achieve balance within the fundamental frequency cycle, when the HWC-MMC is applied in low-frequency transmission scenarios, in the event of an asymmetric fault on the sending low-frequency grid side, traditional constant AC voltage control causes the low-frequency valve-side voltage to exhibit asymmetric characteristics. This causes the AC voltage amplitude ratio on both sides of the HWC-MMC to deviate from the natural energy balance constraint of the bridge arms, posing challenges to the stable operation of the converter. During a fault, valve-side overvoltages are coupled with the energy balance problem of the HWC-MMC. Therefore, it is necessary to comprehensively consider overvoltage suppression and energy balance control strategies based on a clear understanding of the bridge arm energy balance mechanism. Therefore, it is necessary to design a low-frequency side fault ride-through control method for a half-wave frequency conversion modular multilevel converter. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, an object of the present invention is to provide a low-frequency side fault ride-through control method for a half-wave frequency conversion modular multi-level converter.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] The present invention provides a low-frequency side fault ride-through control method for a half-wave frequency conversion modular multi-level converter, comprising:

[0008] After a low-frequency grid fault occurs, the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the low-frequency grid side of the HWC-MMC are detected. Based on these components, the type of asymmetric fault in the grid is determined, and the voltage amplitude of the equivalent grid-connected inductor on the low-frequency side and the current phase of the low-frequency grid side of the grid are calculated.

[0009] Control and switch the HWC-MMC low-frequency side voltage reference value after a fault based on the asymmetric fault type, low-frequency grid-side current phase, low-frequency side equivalent grid-connected inductor voltage amplitude, low-frequency grid-side voltage amplitude, and low-frequency side port voltage phase;

[0010] According to the asymmetric fault type, the voltage amplitude on the low-frequency grid side, and the voltage phase on the low-frequency side port, the corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC and switched.

[0011] Preferably, the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side are detected, and the asymmetric fault type of the grid is determined based on the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side. The equivalent grid-connected inductance voltage amplitude and the current phase of the grid low-frequency grid side are calculated, specifically:

[0012] Detect the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side;

[0013] Based on the d-axis components of the positive and negative sequence voltages at the current HWC-MMC low-frequency grid side, the asymmetric fault type of the power grid is determined as follows:

[0014]

[0015] Where k f is the fault type signal, u ssd+ 、u ssd- are the d-axis components of the measured positive-sequence voltage and negative-sequence voltage at the low-frequency grid side of the HWC-MMC, respectively;

[0016] The positive sequence current i of the low-frequency side port of the transformer grid side after the fault is converted from HWC-MMC s+ Phase angle θ I And the low-frequency side equivalent grid-connected inductor voltage amplitude U Lm Expressed as:

[0017]

[0018] Where i ssd+ 、i ssq+ are the dq axis components of the transformer low-frequency grid-side positive sequence current, ω s is the low-frequency side corner frequency, L s is the equivalent grid-connected inductance on the low-frequency side.

[0019] Preferably, the control strategy for the HWC-MMC low-frequency side voltage after the fault is switched according to the asymmetric fault type, the voltage amplitude of the equivalent grid-connected inductance on the low-frequency side, and the current phase of the low-frequency grid side of the power grid, specifically:

[0020] The low-frequency side port voltage u s+ , low-frequency side equivalent grid-connected inductor voltage u L All are returned to the transformer grid side, and the low-frequency side positive sequence voltage u si Reference amplitude U fm Set it to a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of the HWC-MMC s+ Phase angle θ s for:

[0021]

[0022] When a single-phase fault occurs, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for:

[0023]

[0024] When there is a two-phase fault, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for:

[0025]

[0026] Preferably, according to the asymmetric fault type, the voltage amplitude of the equivalent grid-connected inductance on the low-frequency side, and the current phase of the low-frequency grid side of the power grid, the corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC and switched, specifically:

[0027] The corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC, where the zero-sequence voltage reference value is expressed as:

[0028]

[0029] Where, ω s is the low-frequency side corner frequency.

[0030] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0031] The present invention provides a low-frequency side fault ride-through control method for a half-wave variable frequency modular multilevel converter. The method comprises: detecting the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side after a low-frequency grid fault occurs; determining the asymmetric fault type of the grid based on the detected positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components; calculating the low-frequency side equivalent grid-connected inductor voltage amplitude and the grid low-frequency grid-side current phase; controlling and switching the post-fault HWC-MMC low-frequency side voltage reference value based on the asymmetric fault type, the low-frequency grid-side current phase, the low-frequency side equivalent grid-connected inductor voltage amplitude, the low-frequency grid-side voltage amplitude, and the low-frequency side port voltage phase; and injecting a corresponding zero-sequence voltage reference value into the HWC-MMC low-frequency side control loop and switching the reference value based on the asymmetric fault type, the low-frequency grid-side voltage amplitude, and the low-frequency side port voltage phase. This method can achieve HWC-MMC bridge arm energy balancing and fault ride-through without introducing negative-sequence current and overvoltage, is simple to control, and does not require an additional controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 Schematic diagram of HWS-MMC topology and operation mechanism;

[0034] Figure 2 Schematic diagram of energy accumulation in the bridge arm of the HWS-MMC topology under different power factor angles in operation under an asymmetric fault on the low-frequency side before and after zero-sequence voltage injection using the method of the present invention;

[0035] Figure 3 This is a schematic diagram of the structure of an offshore wind power low-frequency transmission system scenario applicable to an embodiment of the method of the present invention;

[0036] Figure 4 This is the voltage vector relationship diagram of the HWS-MMC low-frequency side after the fault;

[0037] Figure 5 A schematic diagram of a method strategy provided by an embodiment of the present invention;

[0038] Figure 6 Schematic diagram of transformer grid-side voltage and low-frequency-side grid-side current under a single-phase grounding fault on the low-frequency side of an HWS-MMC after the method of the present invention is adopted;

[0039] Figure 7 Schematic diagram of the average value of the input power on the low-frequency side of the HWS-MMC under a single-phase grounding fault on the low-frequency side before and after the method of the present invention is adopted;

[0040] Figure 8 Schematic diagram showing the comparison of the three-phase average capacitor voltage waveforms of the half-bridge and full-bridge arms and the single-phase average capacitor voltage waveforms of the single-phase submodules during a HWS-MMC fault period before and after the method of the present invention is adopted;

[0041] Figure 9 Schematic diagram of transformer grid-side voltage and low-frequency-side grid-side current under HWS-MMC low-frequency-side interphase short-circuit fault after adopting the method of the present invention;

[0042] Figure 10 Schematic diagram of the average value of the low-frequency side input power of the HWS-MMC under the low-frequency side phase short circuit fault before and after the method of the present invention is adopted;

[0043] Figure 11 Schematic diagram of transformer grid-side voltage and low-frequency-side grid-side current under HWS-MMC low-frequency-side interphase short-circuit fault after adopting the method of the present invention;

[0044] Figure 12 A flowchart of a method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0046] The purpose of the present invention is to provide a low-frequency side fault ride-through control method for a half-wave frequency conversion modular multilevel converter, which can achieve HWC-MMC bridge arm energy balancing and fault ride-through without introducing negative sequence current and overvoltage, and has simple control and does not require an additional controller.

[0047] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] First, let’s introduce HWC-MMC. The topology of HWC-MMC is as follows: Figure 1 As shown, HWC-MMC connects two AC systems with different frequencies and amplitudes, and the voltage at the low-frequency side port of the converter is u sa 、u sb 、u sc , the current is i sa 、i sb 、i sc ; The voltage at the power frequency side port is u gu 、u gv 、u gw , the current is i gu 、i gv 、i gw . L arm is the internal bridge arm inductance of the topology, L s The equivalent grid-connected inductance on the low-frequency side is used. To achieve phase isolation, the HWC-MMC requires an additional single-phase transformer group to be installed on the low-frequency side. The half-bridge arm of the single-phase HWC-MMC is connected in parallel with the u-phase commutation H-bridge, and the full-bridge arm is connected in series with the a-phase commutation H-bridge. The half-bridge arm modulates the half-wave voltage waveform |u gu The full-bridge arm is responsible for modulating the difference between the half-wave voltage waveforms on both sides |u sa |-|u gu |, in superposition|u gu |After that, the internal port of phase a presents a low-frequency side half-wave voltage waveform|u sa |. The waveform conversion between half wave and full wave can be achieved by reversing switch K x1 , Kx2 , K x3 , K x4 (x=u, a) is implemented, wherein each commutation switch is composed of multiple IGBTs connected in series to withstand the grid voltage.

[0049] Figure 12 A flow chart of a method provided in an embodiment of the present invention, Figure 5 A schematic diagram of a method strategy provided by an embodiment of the present invention, such as Figure 12 and Figure 5 As shown, the present invention provides a low-frequency side fault ride-through control method for a half-wave frequency conversion modular multi-level converter, comprising:

[0050] Step 100: Determine whether a low-frequency power grid fault exists;

[0051] Step 200: After a low-frequency side grid fault occurs, the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side are detected, based on which the asymmetric fault type of the grid is determined and the voltage amplitude of the equivalent grid-connected inductor on the low-frequency side and the current phase of the grid low-frequency grid side are calculated;

[0052] Step 300: Control switching of the post-fault HWC-MMC low-frequency side voltage reference value based on the asymmetric fault type, low-frequency grid-side current phase, low-frequency side equivalent grid-connected inductor voltage amplitude, low-frequency grid-side voltage amplitude, and low-frequency side port voltage phase.

[0053] Step 400: According to the asymmetric fault type, the low-frequency grid-side voltage amplitude, and the low-frequency-side port voltage phase, inject the corresponding zero-sequence voltage reference value into the HWC-MMC low-frequency-side control loop and perform switching.

[0054] To summarize briefly, the core strategy of the present invention is: to judge the fault types of single-phase grounding fault and two-phase asymmetric fault according to the positive-sequence and negative-sequence voltage feedback values, and to suppress the negative-sequence current as the basis, wherein the positive-sequence voltage reference amplitude on the low-frequency side is switched from the rated value to the set value during a fault to suppress the fault overvoltage, and at the same time, the positive-sequence voltage dq-axis component reference value is changed according to the voltage phase relationship on the low-frequency side to keep the power factor angle on the low-frequency side at a specific angle and adopt a joint control scheme of zero-sequence voltage injection on the low-frequency side to ensure the energy balance of the bridge arm.

[0055] In step 200, the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side are detected, and the asymmetric fault type of the grid is determined based on the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side. The voltage amplitude of the equivalent grid-connected inductor on the low-frequency side of the grid and the phase of the current on the low-frequency grid side of the grid are calculated. Specifically,

[0056] Detect the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side;

[0057] Based on the d-axis components of the positive and negative sequence voltages at the current HWC-MMC low-frequency grid side, the asymmetric fault type of the power grid is determined as follows:

[0058]

[0059] Where k f is the fault type signal, u ssd+ 、u ssd- are the d-axis components of the measured positive-sequence voltage and negative-sequence voltage at the low-frequency grid side of the HWC-MMC, respectively;

[0060] The positive sequence current i of the low-frequency side port of the transformer grid side after the fault is converted from HWC-MMC s+ Phase angle θ I And the low-frequency side equivalent grid-connected inductor voltage amplitude U Lm Expressed as:

[0061]

[0062] Where i ssd+ 、i ssq+ are the dq axis components of the transformer low-frequency grid-side positive sequence current, ω s is the low-frequency side corner frequency, L s is the equivalent grid-connected inductance on the low-frequency side.

[0063] In step 300, the control strategy for the post-fault HWC-MMC low-frequency side voltage is switched based on the asymmetric fault type, the voltage amplitude of the equivalent grid-connected inductance on the low-frequency side, and the current phase of the low-frequency grid side of the power grid. Specifically,

[0064] The low-frequency side port voltage u s+ , low-frequency side equivalent grid-connected inductor voltage u L All are returned to the transformer grid side, and the low-frequency side positive sequence voltage u si Reference amplitude U fm Set it to a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of the HWC-MMC s+ Phase angle θ s for:

[0065]

[0066] When a single-phase fault occurs, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for:

[0067]

[0068] When there is a two-phase fault, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for:

[0069]

[0070] In step 400, according to the asymmetric fault type, the voltage amplitude of the equivalent grid-connected inductance on the low-frequency side, and the current phase of the grid low-frequency side, the corresponding zero-sequence voltage reference value is injected into the HWC-MMC low-frequency side control loop and switched. Specifically,

[0071] The corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC, where the zero-sequence voltage reference value is expressed as:

[0072]

[0073] Where, ω s is the low-frequency side corner frequency.

[0074] The energy balance of the half-wave variable frequency AC converter bridge arm requires that the voltage amplitude ratio of the power / low frequency side be maintained within a certain range. When an asymmetric fault occurs on the low frequency side, the bridge arm energy imbalance phenomenon will occur. After injecting the zero-sequence voltage described in the present invention, the energy accumulation ΔE generated by the three-phase half-bridge bridge arm and the full-bridge bridge arm within one bridge arm capacitance energy cycle fluctuation T under a theoretical single-phase short-circuit grounding fault is obtained. H , ΔE F The relationship with the low-frequency side power factor angle is as follows: Figure 2 As shown in the figure, the energy imbalance degree of the full-bridge and half-bridge bridge arms is symmetrical under different power factor angles using the zero-sequence voltage injection energy balancing strategy. The energy accumulation of the bridge arm for single-phase fault is simultaneously 0 at the power factor angle π / 6, and that for inter-phase fault is π / 2.

[0075] The present invention provides an embodiment of a low-frequency power transmission system for a wind farm, the structure of which is shown in FIG. Figure 3 As shown in the figure, the output power of the offshore wind farm is transmitted to the low-frequency side of the HWC-MMC through the low-frequency submarine cable, and the power frequency side of the HWC-MMC is connected to the 50Hz power grid. L is the equivalent grid-connected inductor voltage drop on the low-frequency side, u sti is the voltage on the commutation side of the low-frequency transformer, u ssi 、i ssi are the transformer low-frequency grid-side voltage and current, u swi 、V wgi are the low-frequency grid-side voltage of the wind farm and the grid-connected voltage of the wind farm, L w is the equivalent grid-connected inductance of the wind farm.

[0076] The present invention requires that the power factor angle of the HWC-MMC low-frequency side be maintained at a fixed value during a fault. Figure 4 The positive sequence voltage u of the low-frequency side port referred to the transformer grid side is shown. s+ , low-frequency side grid-connected inductor voltage u L , transformer commutation side positive sequence voltage u st+ The relationship between Figure 4 Under the relationship of the vector diagram shown, we can get u st+ The dq-axis component u std+ 、u stq+ The only solution is the transformer low-frequency grid-side voltage reference value u ssd+_ref 、u ssq+_ref The only solution of .

[0077] To further demonstrate the feasibility and correctness of the proposed solution, a Matlab / Simulink simulation was performed on a single-phase ground fault fault in the low-frequency grid. The system simulation parameters were as follows: a 2.5MW wind farm rated power, a 50km submarine cable transmission distance, a 20Hz low-frequency side frequency, and a 10kV line voltage (RMS). The submodule capacitance was 20mF, the average submodule capacitance voltage was 1500V, the number of bridge arm submodules was 15, and the bridge arm inductance was 5mH. The power frequency was 50Hz, and the line voltage was 10kV (RMS). Before the grid fault occurred, the wind farm was outputting rated power, and the system was operating at unity power factor. At 0.9s, a single-phase ground fault / interphase ground fault occurred in the low-frequency grid voltage, lasting 700ms. The grid voltage recovered after 1.6s.

[0078] Grid side voltage and current Figure 6 As shown in Figure 2, the negative sequence current is suppressed and the three-phase currents are basically symmetrical after 1.2s. The average input power of the HWC-MMC low-frequency side before and after the energy balancing strategy is adopted is shown in Figure 2. Figure 7 As shown in Figure 2, after the zero-sequence voltage is injected, the zero-sequence voltage offsets the three-phase unbalanced effect of the negative-sequence voltage on the active power, and the three-phase input power is symmetrical.

[0079] The simulation results of the average capacitor voltage of the HWC-MMC three-phase and single-phase half-bridge arm submodules are as follows: Figure 8As shown, without an energy balancing strategy, the average capacitor voltage across all submodules in the three-phase half-bridge arm deviates from the rated value by approximately 15% 700ms after the fault. The degree of deviation varies among the three phases, with the U phase deviating from the rated value by approximately 30% and the V and W phases deviating from the rated value by approximately 35%. This severe energy imbalance prevents the fault ride-through requirements from being met. With the energy balancing control strategy described in this invention, the average capacitor voltage across the three-phase half-bridge arm submodules remains near the rated value. The average capacitor voltage across each phase deviates from the rated value within a transition period of approximately 0.1s after the fault. After the fault stabilizes, the average capacitor voltage remains essentially stable. 700ms after the fault, the deviation from the rated voltage is within 5%, facilitating both fault ride-through and fault recovery.

[0080] The simulation results of the average capacitor voltage of the three-phase and single-phase full-bridge arm submodules are as follows: Figure 8 As shown, when no energy balancing strategy is implemented, the average capacitor voltage of all submodules in the three-phase full-bridge arm deviates from the rated value by approximately 15% 700ms after the fault, and the degree of deviation varies between phases. Phase C has the largest deviation, approximately 45% of the rated value. The overall energy imbalance is severe and cannot meet the fault ride-through requirements. After adopting the energy balancing control strategy described in the present invention, the average capacitor voltage of the submodules in the three-phase full-bridge arm remains near the rated value. The average capacitor voltage of each phase submodule deviates from the rated value during the post-fault transition time. After the fault stabilizes, the average capacitor voltage remains essentially stable. After 700ms after the fault, the deviation from the rated voltage is within 5%, which is conducive to achieving fault ride-through and fault recovery.

[0081] Under the low-frequency line BC phase short circuit fault, the HWC-MMC simulation results are as follows: Figure 9-11 As shown, the grid side voltage and current are as follows Figure 9 As shown in Figure 1, the negative sequence current is suppressed and the three-phase current is basically symmetrical after 1.2s. The average value of the capacitor voltage of the three-phase submodule under interphase fault is as follows: Figure 10 As shown in Figure 2. After the fault, both the negative sequence current and overvoltage are suppressed, and the total energy of the three-phase bridge arm is balanced. After the zero sequence voltage is injected, the difference between the three-phase input power decreases, and after 1.2 seconds, it is completely symmetrical. The simulation results of the average capacitor voltage of the three-phase and single-phase full-bridge and half-bridge bridge arm submodules are shown in Figure 2. Figure 11 As shown, the degree of deviation varies from phase to phase, resulting in severe energy imbalance and failure to meet fault ride-through requirements. After approximately 0.1 seconds of implementing the energy balancing control strategy described in this article, the average voltage of the bridge arm submodule capacitors remained essentially stable. 700 milliseconds after the fault, the voltage deviation from the rated value remained within 10%, facilitating both fault ride-through and recovery.

[0082] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0083] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A low-frequency side fault ride-through control method for a half-wave frequency conversion modular multi-level converter, characterized in that: include: Detect the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components on the low-frequency grid side of the current HWC-MMC. Based on these components, determine the asymmetric fault type of the grid and calculate the voltage amplitude of the line inductance on the low-frequency side and the current phase of the grid. Specifically, Detect the positive-sequence and negative-sequence voltage dq-axis components and the positive-sequence current dq-axis components at the current HWC-MMC low-frequency grid side; Based on the d-axis components of the positive and negative sequence voltages at the current HWC-MMC low-frequency grid side, the asymmetric fault type of the power grid is determined as follows: Where k f is the fault type signal, u ssd+ 、u ssd- are the d-axis components of the measured positive-sequence voltage and negative-sequence voltage at the low-frequency grid side of the HWC-MMC, respectively; The positive sequence current i of the low-frequency side port of the transformer grid side after the fault is converted from HWC-MMC s+ Phase angle θ I and the low-frequency side line inductance voltage amplitude U Lm Expressed as: Where i ssd+ 、i ssq+ are the dq axis components of the transformer low-frequency grid-side positive sequence current, ω s is the low-frequency side corner frequency, L s is the line inductance on the low-frequency side; Based on the asymmetric fault type, the voltage amplitude of the low-frequency line inductance, and the current phase of the power grid, the control strategy for the low-frequency side voltage of the HWC-MMC after the fault is switched. Specifically: The low-frequency side port voltage u s+ , low-frequency side line inductance voltage u L All are returned to the transformer grid side, and the low-frequency side positive sequence voltage u si Reference amplitude U fm Set it to a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of the HWC-MMC s+ Phase angle θ s for: When a single-phase fault occurs, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for: When there is a two-phase fault, the reference value u of the positive sequence voltage dq axis component is changed according to the phase relationship of the low-frequency side voltage. ssd+_ref 、u ssq+_ref for: According to the asymmetric fault type, the voltage amplitude of the line inductance on the low-frequency side, and the current phase of the grid, the corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC and switched.

2. The method according to claim 1, characterized in that According to the asymmetric fault type, the voltage amplitude of the low-frequency line inductance, and the current phase of the grid, the corresponding zero-sequence voltage reference value is injected into the low-frequency side control loop of the HWC-MMC and switched. Specifically: The corresponding zero-sequence voltage reference value is injected into the control loop on the low-frequency side of the HWC-MMC, where the zero-sequence voltage reference value is expressed as: Where, ω s is the low-frequency side corner frequency, is the power factor angle on the low-frequency side after the fault.

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