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

Abstract

The invention provides a low-frequency-side fault ride-through control method for a half-wave frequency conversion modular multilevel converter, and the method comprises the steps: detecting the d-q-axis components of the positive-sequence and negative-sequence voltages and the d-q-axis components of the positive-sequence current at the low-frequency network side of a current HWC-MMC after the fault of a low-frequency-side power grid occurs, and judging the type of an asymmetric fault of the power grid based on the d-q-axis components of the positive-sequence and negative-sequence voltages; calculating a low-frequency grid-side current phase, a low-frequency side equivalent grid-connected inductance voltage amplitude and a low-frequency side port voltage phase of the converter, and according to the asymmetric fault type, the low-frequency grid-side current phase, the low-frequency side equivalent grid-connected inductance voltage amplitude, the low-frequency grid-side voltage amplitude and the low-frequency side port voltage phase; performing control switching on the low-frequency side voltage reference value of the HWC-MMC after the fault; according to the asymmetric fault type, the low-frequency network side voltage amplitude and the low-frequency side port voltage phase, a corresponding zero-sequence voltage reference value is injected into an HWC-MMC low-frequency side control loop, and switching is carried out. According to the invention, the bridge arm energy balance and fault ride-through of the HWC-MMC can be realized under the condition that negative sequence current and overvoltage are not introduced.

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 the AC transmission line from the industrial frequency of 50Hz to the low frequency of 50 / 3Hz or 20Hz, thereby reducing the electrical distance of the AC transmission line and improving the transmission power capacity of the transmission line. It has broad application prospects in the efficient and large-capacity collection and transmission of new energy such as offshore wind power and "Shagohuang" onshore wind power and photovoltaic power.

[0003] High-voltage and large-capacity AC converters are the core equipment of low-frequency power transmission technology. As the main AC converter topology, modular multilevel matrix converter (M3C) has been demonstrated and applied in flexible low-frequency power transmission projects in Hangzhou and Taizhou, Zhejiang Province, my country. However, M3C has the disadvantages of a large number of sub-modules and a large volume and weight. A series of lightweight modular AC converter topologies such as Hexverter, Y-type modular multilevel converter (Y-MMC), and enneagonal converter have been proposed. Among them, the number of sub-modules and the total number of main components of Y-MMC and Hexverter are equivalent, both of which are 75% of M3C, but there are no circulating current problems and reactive power constraint problems, and the control is relatively simple; the enneagonal converter can be regarded as an extension of Hexverter, which has advantages in the three-port AC / AC conversion application scenario. From the perspective of sinusoidal waveform transformation, the above-mentioned new topologies all belong to full-wave transformation, that is, the bridge arm needs to be modulated to generate a complete sinusoidal wave or a heterofrequency combination wave, so the number of required sub-modules is still large, and there is room for further improvement in the power density of high-voltage and high-power AC converters.

[0004] Half-wave commutated modular multilevel converter (HWC-MMC) is different from the full-wave frequency conversion principle of the above-mentioned converter. It is a new type of AC converter based on the idea of ​​half-sine half-wave frequency conversion. The bridge arm only needs to be modulated to generate a combination of a sine half wave or a sine half wave with different frequencies. The number of required sub-modules is halved, and the total number of devices is reduced by more than 35% compared with M3C at the same capacity. The traditional M3C bridge arm can achieve balance naturally within the base frequency cycle. Unlike M3C, when HWC-MMC is applied to low-frequency transmission scenarios, when there is an asymmetric fault on the low-frequency grid side at the sending end, the traditional constant AC voltage control makes the low-frequency valve side voltage present an asymmetric characteristic, resulting in the AC voltage amplitude ratio on both sides of the HWC-MMC deviating from the natural energy balance constraint of the bridge arm, and the stable operation of the converter faces challenges. During the fault period, the valve side overvoltage is coupled with the HWC-MMC energy balance problem. It is necessary to comprehensively consider the overvoltage suppression and energy balance control strategies on the basis of clarifying the energy balance mechanism of the bridge arm. 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 of 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 which the asymmetric fault type of the grid is determined, and 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 grid are calculated;

[0009] According to the asymmetric fault type, low-frequency grid-side current phase, low-frequency side equivalent grid-connected inductance voltage amplitude, low-frequency grid-side voltage amplitude and low-frequency side port voltage phase, the voltage reference value of the HWC-MMC low-frequency side after the fault is controlled and switched;

[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 low-frequency grid side of the current HWC-MMC 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 low-frequency grid side of the grid are determined, and the equivalent grid-connected inductance voltage amplitude and the current phase of the low-frequency grid side of the grid 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 low-frequency grid side of the current HWC-MMC;

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

[0014]

[0015] In the formula, k f is the fault type signal, u ssd+ 、u ssd- They 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 calculated by HWC-MMC s+ Phase angle θ I And the low-frequency side equivalent grid-connected inductor voltage amplitude U Lm It is expressed as:

[0017]

[0018] In the formula, 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, 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 control strategy of the voltage on the low-frequency side of the HWC-MMC after the fault is switched, 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 The reference amplitude U fm Set it as a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of 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 voltage phase relationship on the low-frequency side. 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] In the formula, 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 frequency conversion modular multi-level converter, the method comprising: 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 network side after a low-frequency side grid fault occurs, judging the asymmetric fault type of the grid based on the positive sequence and calculating the low-frequency side equivalent grid-connected inductor voltage amplitude and the grid low-frequency network side current phase; according to the asymmetric fault type, the low-frequency network side current phase, the low-frequency side equivalent grid-connected inductor voltage amplitude, the low-frequency network side voltage amplitude and the low-frequency side port voltage phase, controlling and switching the HWC-MMC low-frequency side voltage reference value after the fault; according to the asymmetric fault type, the low-frequency network side voltage amplitude and the low-frequency side port voltage phase, injecting the corresponding zero-sequence voltage reference value into the HWC-MMC low-frequency side control loop and switching. This method can realize HWC-MMC bridge arm energy balance and fault ride-through without introducing negative sequence current and overvoltage, and the control is simple and does not require the addition of 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 drawings required for use in the embodiments will be briefly introduced below. 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 creative labor.

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

[0034] Figure 2 It is a schematic diagram of the energy accumulation of the bridge arm of the topology HWS-MMC under the operation state of different power factor angles under the asymmetric fault on the low frequency side before and after the zero-sequence voltage injection by 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 A 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 It is a 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 A 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 average capacitor voltage waveforms during a HWS-MMC fault period before and after the method of the present invention is adopted;

[0041] Fig. 9 A schematic diagram of transformer grid-side voltage and low-frequency-side grid-side current under a phase-to-phase short-circuit fault on the low-frequency side of the HWS-MMC after the method of the present invention is adopted;

[0042] Fig.10 It is a schematic diagram of the average value of the input power on the low-frequency side of the HWS-MMC under the interphase short-circuit fault on the low-frequency side before and after the method of the present invention is adopted;

[0043] Fig.11 A schematic diagram of transformer grid-side voltage and low-frequency-side grid-side current under a phase-to-phase short-circuit fault on the low-frequency side of the HWS-MMC after the method of the present invention is adopted;

[0044] Fig.12 A flow chart of a method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work 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 multi-level converter, which can achieve HWC-MMC bridge arm energy balancing and fault ride-through without introducing negative sequence current and overvoltage, and the control is simple 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, HWC-MMC is introduced. 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, HWC-MMC needs to install an additional single-phase transformer group on the low-frequency side. The half-bridge arm inside 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 a 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 a plurality of IGBTs connected in series to withstand the grid voltage.

[0049] Fig.12 A flow chart of a method provided by 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 Fig.12 and Figure 5 As shown, the present invention provides a low-frequency side fault ride-through control method of 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 inductance on the low-frequency side and the current phase of the low-frequency grid side of the grid are calculated;

[0052] Step 300: Control switching of the voltage reference value of the low-frequency side of the HWC-MMC after the fault according to the asymmetric fault type, the low-frequency grid-side current phase, the low-frequency side equivalent grid-connected inductance voltage amplitude, the low-frequency grid-side voltage amplitude and the low-frequency side port voltage phase;

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

[0054] To summarize briefly, the core strategy of the present invention is: to judge the single-phase grounding fault and two-phase asymmetric fault type according to the positive-sequence and negative-sequence voltage feedback values, and to suppress the negative-sequence current based on the positive-sequence current separation control, wherein the positive-sequence voltage reference value 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 a joint control scheme of zero-sequence voltage injection is adopted 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 low-frequency grid side of the current HWC-MMC 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 low-frequency grid side of the grid are calculated, and the equivalent grid-connected inductance voltage amplitude and the current phase of 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 low-frequency grid side of the current HWC-MMC;

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

[0058]

[0059] In the formula, k f is the fault type signal, u ssd+ 、u ssd- They 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 calculated by HWC-MMC s+ Phase angle θ I And the low-frequency side equivalent grid-connected inductor voltage amplitude U Lm It is expressed as:

[0061]

[0062] In the formula, 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, 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 control strategy of the voltage on the low-frequency side of the HWC-MMC after the fault is switched, 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 The reference amplitude U fm Set it as a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of 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 voltage phase relationship on the low-frequency side. 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 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:

[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] In the formula, 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 power factor angle on the low frequency side is as follows: Figure 2 As shown in the figure: using the zero-sequence voltage injection energy balancing strategy at different power factor angles, the energy imbalance degree of the full-bridge and half-bridge bridge arms is symmetrical, the energy accumulation of the single-phase fault bridge arm is 0 at the power factor angle π / 6, and the phase-to-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 diagram of which is as follows: 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 industrial frequency side of the HWC-MMC is connected to the 50Hz power grid. L is the equivalent grid-connected inductance 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 low-frequency side of the HWC-MMC be maintained at a fixed value during a fault. Figure 4 The positive sequence voltage u at the low-frequency side port calculated to the grid side of the transformer 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 graph shown in the figure, 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] In order to further demonstrate the feasibility and correctness of the implementation plan, the present invention simulates the single-phase grounding short-circuit fault calculation example of the low-frequency side power grid based on Matlab / Simulink, and the system simulation parameters are as follows: the rated power of the wind farm is 2.5MW, the transmission distance of the submarine cable is 50km; the frequency of the low-frequency side is 20Hz, and the effective value of the line voltage is 10kV; the submodule capacitance is 20mF, the average capacitance voltage of the submodule is 1500V, the number of bridge arm submodules is 15, and the bridge arm inductance is 5mH; the frequency of the power frequency side is 50Hz, and the effective value of the line voltage is 10kV. Before the grid fault occurs, the wind farm outputs the rated power and the system operates at a unity power factor; at 0.9s, a single-phase grounding fault / phase-to-phase grounding fault occurs in the low-frequency grid side grid voltage, which lasts for 700ms, and the grid voltage recovers at 1.6s.

[0078] The grid-side voltage and current are as follows: Figure 6 As shown in Figure 1, the negative sequence current is suppressed and the three-phase current is 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 as follows: Figure 7 As shown in Figure 1, 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, when the energy balancing strategy is not adopted, the average value of the capacitor voltage of all submodules of the three-phase half-bridge arm deviates from the rated value by about 15% after the fault 700ms, and the three phases deviate to different degrees, the u phase deviates from the rated value by about 30%, and the v and w phases deviate from the rated value by about 35%. The energy imbalance phenomenon is serious and the fault ride-through requirement cannot be met. After adopting the energy balancing control strategy described in the present invention, the average value of the capacitor voltage of the submodule of the three-phase half-bridge arm is kept near the rated value, and the average value of the capacitor voltage of each phase submodule deviates from the rated value within a transition time of about 0.1s after the fault. After the fault stabilizes, the average value of the capacitor voltage basically remains stable. After the fault is stabilized, the deviation from the rated value voltage is within 5% after 700ms, which is conducive to the realization of 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 the energy balancing strategy is not adopted, the average value of the capacitor voltage of all submodules of the three-phase full-bridge arm deviates from the rated value by about 15% after the fault 700ms, and the deviation degree of each phase is different. The C phase has the largest deviation, which is about 45% of the rated value. The overall energy imbalance phenomenon is serious and cannot meet the fault crossing requirements. After adopting the energy balancing control strategy described in the present invention, the average value of the capacitor voltage of the submodule of the three-phase full-bridge arm is kept near the rated value, and the average value of the capacitor voltage of each phase submodule deviates from the rated value during the transition time after the fault. After the fault is stabilized, the average value of the capacitor voltage basically remains stable. After 700ms after the fault, the deviation from the rated value voltage is within 5%, which is conducive to the realization of fault crossing and fault recovery.

[0081] When the low-frequency line BC phase is short-circuited, the simulation results of HWC-MMC are as follows: Figure 9-11 As shown, the grid-side voltage and current are as follows: Fig. 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 phase-to-phase fault is as follows: Fig.10 As shown in the figure. After the fault, 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 is reduced, and it is completely symmetrical after 1.2s. The simulation results of the average value of the capacitor voltage of the three-phase and single-phase full-bridge and half-bridge bridge arm submodules are shown in the figure. Fig.11 As shown in the figure, the deviation degree of each phase is different, the energy imbalance phenomenon is serious, and the fault ride-through requirement cannot be met. After about 0.1s of adopting the energy balance control strategy described in this article, the average value of the capacitor voltage of the bridge arm submodule basically remains stable. After 700ms after the fault, the deviation rated voltage is within 10%, which is conducive to the realization of fault ride-through and fault 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] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood 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: 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 which the asymmetric fault type of the grid is determined, and 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 grid are calculated; According to the asymmetric fault type, low-frequency grid-side current phase, low-frequency side equivalent grid-connected inductance voltage amplitude, low-frequency grid-side voltage amplitude and low-frequency side port voltage phase, the voltage reference value of the HWC-MMC low-frequency side after the fault is controlled and switched; 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.

2. The method according to claim 1, characterized in that Detect the positive-sequence and negative-sequence voltage dq-axis components and positive-sequence current dq-axis components at the low-frequency grid side of the current HWC-MMC, based on which determine the asymmetric fault type of the grid, and calculate the voltage amplitude of the equivalent grid-connected inductance on the low-frequency side of the grid and the current phase on the low-frequency grid side 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 low-frequency grid side of the current HWC-MMC; Based on the positive and negative sequence voltage d-axis components at the current HWC-MMC low-frequency grid side, the asymmetric fault type of the power grid is determined as follows: In the formula, k f is the fault type signal, u ssd+ 、u ssd- They 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 after the fault is calculated by HWC-MMC s+ Phase angle θ I And the low-frequency side equivalent grid-connected inductor voltage amplitude U Lm It is expressed as: In the formula, 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.

3. The method according to claim 2, characterized in that 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 control strategy of the voltage on the low-frequency side of the HWC-MMC after the fault is switched, specifically: 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 grid side positive sequence voltage u si The reference amplitude U fm Set it as a fixed value of 0.58pu and calculate the voltage u at the low-frequency side port of 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 voltage phase relationship on the low-frequency side. 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: 。 4. The method according to claim 3, characterized in that 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: 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: In the formula, ω s is the low-frequency side corner frequency.

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