Method and system for fault ride-through of m3c converter station during asymmetric grid voltage sag

Abstract

The application discloses a kind of M3C converter station's fault ride-through method and system when grid voltage is asymmetric drop, comprising: the active fluctuation and negative sequence current of frequency side and frequency division side are inhibited respectively, and the positive and negative sequence current reference value of frequency side and the positive and negative sequence current reference value of frequency division side are obtained;Based on the compensation current of frequency side, the compensation amount of current reference value of frequency side, the compensation amount of current reference value of frequency division side and the compensation current of frequency division side, the circulating voltage control amount of frequency and the circulating voltage control amount of frequency division are obtained;Further obtain the voltage control amount of frequency bridge arm and the voltage control amount of frequency division bridge arm;The voltage control amount of frequency bridge arm and the voltage control amount of frequency division bridge arm are used as M3C frequency output voltage modulation wave, through level approximation and carrier phase-shifted PWM method, and trigger signal is obtained.The application keeps frequency division wind power grid-connected system to keep grid-connected operation by grid-side control strategy and bridge arm power compensation control strategy and provides active and reactive power support to grid, and improves the fault ride-through capability of M3C converter station.

Description

Technical Field

[0001] This invention belongs to the field of electrical control technology and relates to a fault ride-through method and system for an M3C converter station when the grid voltage is asymmetrically dropped. Background Technology

[0002] As offshore wind power construction gradually moves to the far reaches of the sea, the connection between offshore wind farms and the onshore power grid is becoming increasingly important. On the one hand, the performance indicators of the transmission system have a significant impact on the efficiency of the wind power system; on the other hand, the economic cost of the transmission system generally accounts for the second largest proportion of the total cost after the wind farm investment, and has a huge impact on the economics of the wind power system.

[0003] Fault ride-through control refers to the ability of wind turbines or wind farms to maintain continuous operation without disconnecting from the grid and smoothly transition to normal operation within a certain voltage or frequency range and duration when the voltage or frequency at the grid connection point exceeds the standard allowable normal operating range due to power system accidents or disturbances. It is a key issue that must be addressed in the research of frequency-division offshore wind power systems. The fault ride-through capability of frequency-division wind power grid-connected systems is crucial to the safe and stable operation of the power grid. If a frequency-division wind power grid-connected system disconnects from the grid upon encountering a grid fault, it will increase the difficulty of recovering from local grid faults, jeopardize grid stability, and may even lead to system collapse.

[0004] Defects and shortcomings of existing technology:

[0005] Traditional back-to-back M3C technology separates the frequency converter into rectification and inversion stages. This circuit structure does not conform to the "n-1" principle; if any bridge arm fails and goes out of service, the entire converter station will be forced to shut down. Since the polarity of the voltage across each bridge arm remains constant during normal operation, back-to-back M3C can use half-bridge modules. However, this eliminates the converter station's ability to withstand DC-side faults, potentially leading to serious consequences. Conversely, to achieve DC-side fault tolerance, at least half of the half-bridge modules must be replaced with full-bridge modules, significantly increasing the converter station's economic costs.

[0006] Compared to traditional back-to-back MMC (Multi-Module Control) technology, the M3C (Multi-Module Control) scheme has smaller individual converter valve and module capacitor capacities, and a smaller total capacity for converter valves and energy storage. The M3C scheme also boasts higher circuit redundancy, uniquely satisfying the "n-1" rule, enabling uninterrupted maintenance and offering high reliability. It is the optimal technology for frequency converters in frequency-division transmission systems. While systematic research has been achieved in mathematical modeling and control design for M3C, research on grid fault ride-through control remains lacking. Summary of the Invention

[0007] The purpose of this invention is to address the lack of research on grid fault ride-through control in the existing technology, and to provide a fault ride-through method and system for M3C converter stations when the grid voltage is asymmetrically dropped.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] Fault ride-through methods for M3C converter stations during grid voltage asymmetry dips include:

[0010] The active power fluctuation and negative sequence current on the power frequency side and the active power fluctuation and negative sequence current on the frequency division side are suppressed respectively, and the reference values ​​of positive and negative sequence current on the power frequency side and the frequency division side are obtained.

[0011] Based on the reference values ​​of positive and negative sequence voltage on the power frequency side, positive and negative sequence current on the power frequency side, positive and negative sequence voltage on the frequency division side, and positive and negative sequence current on the frequency division side, the compensation current on the power frequency side and the compensation current on the frequency division side are output respectively.

[0012] Based on the reference values ​​of positive and negative sequence currents on the power frequency side and the reference values ​​of positive and negative sequence currents on the frequency division side, the positive sequence voltage of the bridge arm on the power frequency side, the control quantity of the negative sequence voltage on the power frequency side, the positive sequence voltage of the bridge arm on the frequency division side, and the control quantity of the negative sequence voltage on the frequency division side are obtained respectively.

[0013] Based on the M3C basic bridge arm voltage equalization control, the compensation amount of the current reference values ​​on the power frequency side and the frequency division side is obtained;

[0014] Based on the power frequency side compensation current, the compensation amount of the power frequency side current reference value, the compensation amount of the frequency division side current reference value, and the frequency division side compensation current, the power frequency circulating current voltage control amount and the frequency division circulating current voltage control amount are obtained.

[0015] Based on the positive sequence voltage of the power frequency side bridge arm, the negative sequence voltage control quantity of the power frequency side, the circulating current voltage control quantity of the power frequency side, the positive sequence voltage of the frequency division side bridge arm, the negative sequence voltage control quantity of the frequency division side, and the circulating current voltage control quantity of the frequency division, the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are obtained.

[0016] The mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are used as the M3C mains frequency division output voltage modulation wave. The trigger signal is obtained through level approximation and carrier phase shift PWM method.

[0017] A further improvement of the present invention is that:

[0018] Furthermore, by suppressing active power fluctuations and negative sequence currents on the power frequency side and on the frequency division side respectively, reference values ​​for positive and negative sequence currents on the power frequency side and the frequency division side are obtained; specifically:

[0019] Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the power frequency side, the reference values ​​of positive and negative sequence currents on the power frequency side are obtained.

[0020] Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the frequency division side, the reference values ​​of positive and negative sequence currents on the frequency division side are obtained.

[0021] Furthermore, grid-side control strategies to suppress active power fluctuations on the power frequency side or suppress negative sequence current; specifically:

[0022] The strategy for suppressing active power fluctuations on the power frequency side is as follows:

[0023]

[0024] Where, Δi dLα refB , Δi dLβ ref B The command value for the frequency-division positive sequence circulating current component required to compensate for DC power bias; the power compensation circulating current required for the grid-side control strategy to suppress active power fluctuations is 0.

[0025] Strategy for suppressing negative sequence components of current on the power frequency side:

[0026]

[0027] Furthermore, grid-side control strategies to suppress active power fluctuations on the frequency division side or suppress negative sequence current; specifically:

[0028] Frequency division side active power fluctuation suppression strategy:

[0029]

[0030] Among them, the power compensation circulating current required by the grid-side control strategy to suppress active power fluctuations is 0;

[0031] Strategy for suppressing negative sequence current components on the frequency division side:

[0032]

[0033] Furthermore, based on the reference values ​​of the positive and negative sequence voltages and currents on the power frequency side, and the reference values ​​of the positive and negative sequence voltages and currents on the frequency division side, the compensation current on the power frequency side and the compensation current on the frequency division side are output respectively; specifically:

[0034] Power compensation on the power frequency side is performed based on the reference values ​​of positive and negative sequence voltage and positive and negative sequence current on the power frequency side, and the power frequency side compensation current is output.

[0035] The power on the frequency divider side is accumulated based on the reference values ​​of the positive and negative sequence voltage and current on the frequency divider side, and the compensation current on the frequency divider side is output.

[0036] Furthermore, based on the reference values ​​of positive and negative sequence currents on the power frequency side and the frequency division side, the control quantities of the positive sequence voltage of the bridge arm on the power frequency side, the negative sequence voltage of the bridge arm on the power frequency side, and the positive sequence voltage of the bridge arm on the frequency division side and the negative sequence voltage of the bridge arm on the frequency division side are obtained, respectively; specifically:

[0037] By setting the reference values ​​of positive and negative sequence currents on the power frequency side to the power frequency positive sequence common mode current controller and the power frequency negative sequence common mode current controller, the control quantities of the positive sequence voltage and negative sequence voltage of the power frequency side bridge arm are obtained.

[0038] The reference values ​​of the positive and negative sequence currents on the frequency division side are given to the frequency division positive sequence common mode current controller and the frequency division negative sequence common mode current controller to obtain the control quantities of the positive sequence voltage and the negative sequence voltage of the frequency division side bridge arm.

[0039] Furthermore, based on the power frequency side compensation current, the compensation amount of the power frequency side current reference value, the compensation amount of the frequency division side current reference value, and the frequency division side compensation current, the power frequency circulating current voltage control amount and the frequency division circulating current voltage control amount are obtained; specifically:

[0040] The compensation amounts of the power frequency side compensation current and the power frequency side current reference value are accumulated and given to the power frequency circulating current controller to obtain the power frequency circulating current voltage control amount.

[0041] The compensation amount of the frequency division side current reference value and the compensation current of the frequency division side are accumulated and given to the frequency division circulating current suppressor to obtain the circulating current voltage control amount of the frequency division.

[0042] Furthermore, based on the positive-sequence voltage of the power frequency side bridge arm, the negative-sequence voltage control quantity of the power frequency side, the circulating current voltage control quantity of the power frequency side, the positive-sequence voltage of the frequency division side bridge arm, the negative-sequence voltage control quantity of the frequency division side, and the circulating current voltage control quantity of the frequency division, the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are obtained, specifically:

[0043] The positive sequence voltage of the power frequency side bridge arm, the negative sequence voltage of the power frequency side, and the circulating current voltage control quantity of the power frequency are accumulated to obtain the power frequency bridge arm voltage control quantity.

[0044] The positive sequence voltage of the frequency division side bridge arm, the negative sequence voltage of the frequency division side, and the circulating current voltage control quantity of the frequency division are accumulated to obtain the frequency division side bridge arm voltage control quantity.

[0045] The fault ride-through system of the M3C converter station during grid voltage asymmetry drops includes:

[0046] The suppression module is used to suppress active power fluctuations and negative sequence currents on the power frequency side and on the frequency division side, respectively, to obtain reference values ​​for positive and negative sequence currents on the power frequency side and on the frequency division side.

[0047] The compensation current acquisition module outputs the compensation current on the power frequency side and the compensation current on the frequency division side based on the positive and negative sequence voltage on the power frequency side, the reference value of the positive and negative sequence current on the power frequency side, the positive and negative sequence voltage on the frequency division side, and the reference value of the positive and negative sequence current on the frequency division side.

[0048] The voltage control quantity acquisition module obtains the positive sequence voltage of the bridge arm on the power frequency side, the negative sequence voltage control quantity on the power frequency side, the positive sequence voltage of the bridge arm on the power frequency side, and the negative sequence voltage control quantity on the frequency division side, respectively, based on the positive and negative sequence current reference values ​​on the power frequency side and the positive and negative sequence current reference values ​​on the frequency division side.

[0049] The compensation amount acquisition module acquires the compensation amount of the reference current values ​​on the power frequency side and the frequency division side based on the M3C basic bridge arm voltage equalization control.

[0050] The circulating current voltage control quantity acquisition module obtains the circulating current voltage control quantity at the power frequency and the circulating current voltage control quantity at the frequency division based on the power frequency side compensation current, the compensation quantity of the power frequency side current reference value, the compensation quantity of the frequency division side current reference value, and the frequency division side compensation current.

[0051] The bridge arm voltage control quantity acquisition module acquires the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity based on the power frequency side positive sequence voltage, power frequency side negative sequence voltage control quantity, power frequency circulating current voltage control quantity, frequency division side positive sequence voltage, frequency division side negative sequence voltage control quantity and frequency division circulating current voltage control quantity.

[0052] The output module is used to take the mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity as the M3C mains frequency division output voltage modulation wave, and obtain the trigger signal through level approximation and carrier phase shift PWM method.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] This invention suppresses active power fluctuations and negative sequence currents on the power frequency side and the frequency division side, respectively, to obtain reference values ​​for positive and negative sequence currents on the power frequency side and the frequency division side. Based on these reference values, the positive sequence voltage of the power frequency side bridge arm, the control quantity of the negative sequence voltage on the power frequency side, the positive sequence voltage of the frequency division side bridge arm, and the control quantity of the negative sequence voltage on the frequency division side are obtained. Based on the compensation current on the power frequency side, the compensation quantity of the current reference value on the power frequency side, the compensation quantity of the current reference value on the frequency division side, and the compensation current on the frequency division side, the circulating current voltage control quantity of the power frequency side and the circulating current voltage control quantity of the frequency division side are obtained. Furthermore, the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are acquired. These are used as the modulation wave of the M3C power frequency and frequency division output voltages, and a trigger signal is obtained through level approximation and carrier phase-shifting PWM methods. This invention enables the frequency-division wind power grid-connected system to maintain grid-connected operation and provide active and reactive power support to the grid through grid-side control strategies and bridge arm power compensation control strategies, thereby enhancing grid resilience and improving the fault ride-through capability of the M3C converter station. Attached Figure Description

[0055] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 This is a diagram showing the grid connection structure of an offshore wind farm containing an M3C converter station via a frequency-division transmission system in this invention.

[0057] Figure 2 This is a topology diagram of the M3C converter in this invention;

[0058] Figure 3 This is a flowchart of the fault ride-through method for the M3C converter station when the grid voltage is asymmetrically dropped, according to the present invention.

[0059] Figure 4 This is a structural diagram of the fault ride-through system of the M3C converter station under grid voltage asymmetry drop according to the present invention;

[0060] Figure 5 This is a system block diagram of the control strategy for the fault ride-through method of the M3C converter station when the grid voltage is asymmetrically dropped, as described in this invention.

[0061] Figure 6 This is a simulation system topology diagram used in the embodiments of the present invention;

[0062] Figure 7The figures show a comparison of the current waveforms on the power frequency side and the frequency division side during the simulation test. (a) shows the power frequency positive sequence current and reference value; (b) shows the power frequency negative sequence current and reference value; (c) shows the frequency division positive sequence current and reference value; (d) shows the frequency division negative sequence current and reference value; (e) shows the power frequency three-phase current when suppressing active power fluctuations; (f) shows the power frequency three-phase current when suppressing the negative sequence component of the current; (g) shows the frequency division three-phase current when suppressing active power fluctuations; and (h) shows the frequency division three-phase current when suppressing the negative sequence component of the current. In figures (e), (f), (g), and (h), a, b, and c represent the three phase lines of the AC current, respectively.

[0063] Figure 8 The following are simulation waveform comparison diagrams with and without power compensation: (a) is a schematic diagram of the power frequency circulating current component without power compensation; (b) is a schematic diagram of the frequency division circulating current component without power compensation; (c) is a schematic diagram of the differential mode component of the capacitor voltage without power compensation; (d) is a schematic diagram of the average module capacitor voltage without power compensation; (e) is a schematic diagram of the average power of the bridge arm without power compensation; (f) is a schematic diagram of the power frequency circulating current component with power compensation; (g) is a schematic diagram of the frequency division circulating current component with power compensation; (h) is a schematic diagram of the differential mode component of the capacitor voltage with power compensation; (i) is a schematic diagram of the average module capacitor voltage with power compensation; and (j) is a schematic diagram of the average power of the bridge arm with power compensation. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0065] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0066] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0067] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0068] Furthermore, the use of the term "horizontal" does not imply that the component must be absolutely horizontal, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0069] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0070] The invention will be further described below with reference to the accompanying drawings.

[0071] See Figure 1 The offshore wind power frequency sub-system comprises a power frequency system and a frequency sub-system. The frequency sub-system consists of the offshore wind farm, the offshore data collection system, the frequency sub-step-up transformer, and the frequency sub-transmission lines. The offshore wind farm generates electricity using wind power. This electricity is fed into the offshore data collection system and stepped up by the frequency sub-step-up transformer. The stepped-up electricity is then transmitted to land via the frequency sub-transmission lines. The power frequency system includes the M3C converter station and the power frequency system. The M3C converter station changes the frequency of the electricity transmitted via the frequency sub-transmission lines from 50 / 3 Hz to 50 Hz before finally integrating it into the power frequency system.

[0072] See Figure 2 The M3C converter's structure diagram includes nine bridge arms. The three-phase output on the power frequency side of the M3C converter is connected to the three-phase input on the frequency divider side. Each bridge arm consists of one bridge arm reactor and n full-bridge modules connected in series. From the frequency divider side, the M3C converter is divided into three sub-converters a, b, and c; from the power frequency side, it is divided into three sub-converters u, v, and w. The M3C's bridge arms do not have concentrated DC links, resulting in high conversion efficiency. The pure full-bridge module bridge arm structure enables the M3C to suppress fault currents on any side.

[0073] See Figure 3 This invention discloses a fault ride-through method for an M3C converter station during a grid voltage asymmetry drop, comprising:

[0074] S101, suppresses active power fluctuations and negative sequence currents on the power frequency side and on the frequency division side respectively, to obtain reference values ​​for positive and negative sequence currents on the power frequency side and on the frequency division side.

[0075] Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the power frequency side, the reference values ​​of positive and negative sequence currents on the power frequency side are obtained.

[0076] Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the frequency division side, the reference values ​​of positive and negative sequence currents on the frequency division side are obtained.

[0077] The strategy for suppressing active power fluctuations on the power frequency side is as follows:

[0078]

[0079] Where, Δi dLα refB Δ idLβ refB The command value for the frequency-division positive sequence circulating current component required to compensate for DC power bias; the power compensation circulating current required for the grid-side control strategy to suppress active power fluctuations is 0.

[0080] Strategy for suppressing negative sequence components of current on the power frequency side:

[0081]

[0082] Frequency division side active power fluctuation suppression strategy:

[0083]

[0084] Among them, the power compensation circulating current required by the grid-side control strategy to suppress active power fluctuations is 0;

[0085] Strategy for suppressing negative sequence current components on the frequency division side:

[0086]

[0087] S102 outputs the power frequency side compensation current and the frequency division side compensation current based on the power frequency side positive and negative sequence voltage, the power frequency side positive and negative sequence current reference values, the frequency division side positive and negative sequence voltage, and the frequency division side positive and negative sequence current reference values.

[0088] Power compensation on the power frequency side is performed based on the reference values ​​of positive and negative sequence voltage and positive and negative sequence current on the power frequency side, and the power frequency side compensation current is output.

[0089] The power on the frequency divider side is accumulated based on the reference values ​​of the positive and negative sequence voltage and current on the frequency divider side, and the compensation current on the frequency divider side is output.

[0090] S103, based on the reference values ​​of positive and negative sequence currents on the power frequency side and the reference values ​​of positive and negative sequence currents on the frequency division side, respectively, obtain the positive sequence voltage of the bridge arm on the power frequency side, the control quantity of the negative sequence voltage on the power frequency side, the positive sequence voltage of the bridge arm on the frequency division side, and the control quantity of the negative sequence voltage on the frequency division side.

[0091] By setting the reference values ​​of positive and negative sequence currents on the power frequency side to the power frequency positive sequence common mode current controller and the power frequency negative sequence common mode current controller, the control quantities of the positive sequence voltage and negative sequence voltage of the power frequency side bridge arm are obtained.

[0092] The reference values ​​of the positive and negative sequence currents on the frequency division side are given to the frequency division positive sequence common mode current controller and the frequency division negative sequence common mode current controller to obtain the control quantities of the positive sequence voltage and the negative sequence voltage of the frequency division side bridge arm.

[0093] S104, based on the M3C basic bridge arm voltage equalization control, obtains the compensation amount of the current reference values ​​on the power frequency side and the frequency division side.

[0094] S105, based on the power frequency side compensation current, the compensation amount of the power frequency side current reference value, the compensation amount of the frequency division side current reference value, and the frequency division side compensation current, obtain the power frequency circulating current voltage control amount and the frequency division circulating current voltage control amount.

[0095] The compensation amounts of the power frequency side compensation current and the power frequency side current reference value are accumulated and given to the power frequency circulating current controller to obtain the power frequency circulating current voltage control amount.

[0096] The compensation amount of the frequency division side current reference value and the compensation current of the frequency division side are accumulated and given to the frequency division circulating current suppressor to obtain the circulating current voltage control amount of the frequency division.

[0097] S106, based on the positive sequence voltage of the power frequency side bridge arm, the negative sequence voltage control quantity of the power frequency side, the circulating current voltage control quantity of the power frequency side, the positive sequence voltage of the frequency division side bridge arm, the negative sequence voltage control quantity of the frequency division side, and the circulating current voltage control quantity of the frequency division, obtain the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity.

[0098] The positive sequence voltage of the power frequency side bridge arm, the negative sequence voltage of the power frequency side, and the circulating current voltage control quantity of the power frequency are accumulated to obtain the power frequency bridge arm voltage control quantity.

[0099] The positive sequence voltage of the frequency division side bridge arm, the negative sequence voltage of the frequency division side, and the circulating current voltage control quantity of the frequency division are accumulated to obtain the frequency division side bridge arm voltage control quantity.

[0100] S107 uses the mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity as the M3C mains frequency division output voltage modulation wave, and obtains the trigger signal through level approximation and carrier phase shift PWM method.

[0101] See Figure 4 This invention discloses a fault ride-through system for an M3C converter station during grid voltage asymmetry drops, comprising:

[0102] The suppression module is used to suppress active power fluctuations and negative sequence currents on the power frequency side and on the frequency division side, respectively, to obtain reference values ​​for positive and negative sequence currents on the power frequency side and on the frequency division side.

[0103] The compensation current acquisition module outputs the compensation current on the power frequency side and the compensation current on the frequency division side based on the positive and negative sequence voltage on the power frequency side, the reference value of the positive and negative sequence current on the power frequency side, the positive and negative sequence voltage on the frequency division side, and the reference value of the positive and negative sequence current on the frequency division side.

[0104] The voltage control quantity acquisition module obtains the positive sequence voltage of the bridge arm on the power frequency side, the negative sequence voltage control quantity on the power frequency side, the positive sequence voltage of the bridge arm on the power frequency side, and the negative sequence voltage control quantity on the frequency division side, respectively, based on the positive and negative sequence current reference values ​​on the power frequency side and the positive and negative sequence current reference values ​​on the frequency division side.

[0105] The compensation amount acquisition module acquires the compensation amount of the reference current values ​​on the power frequency side and the frequency division side based on the M3C basic bridge arm voltage equalization control.

[0106] The circulating current voltage control quantity acquisition module obtains the circulating current voltage control quantity at the power frequency and the circulating current voltage control quantity at the frequency division based on the power frequency side compensation current, the compensation quantity of the power frequency side current reference value, the compensation quantity of the frequency division side current reference value, and the frequency division side compensation current.

[0107] The bridge arm voltage control quantity acquisition module acquires the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity based on the power frequency side positive sequence voltage, power frequency side negative sequence voltage control quantity, power frequency circulating current voltage control quantity, frequency division side positive sequence voltage, frequency division side negative sequence voltage control quantity and frequency division circulating current voltage control quantity.

[0108] The output module is used to take the mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity as the M3C mains frequency division output voltage modulation wave, and obtain the trigger signal through level approximation and carrier phase shift PWM method.

[0109] See Figure 5 The fault ride-through technique for M3C converter stations during grid voltage asymmetry drops, as described in this invention, includes the following steps:

[0110] (1) Establish a sensor system that includes voltage measurement at PCC points on the power frequency and frequency division sides and output current measurement at the power frequency and frequency division ports of M3C to obtain the physical quantities required for control;

[0111] (2) Select a grid-side control strategy to suppress active power fluctuations on the power frequency side or suppress negative sequence current, and obtain reference values ​​for positive and negative sequence currents on the power frequency side.

[0112] (3) Perform power compensation on the power frequency side based on the positive and negative sequence voltages on the power frequency side and the current reference value obtained in step (2), and output the compensation current;

[0113] (4) The compensation amount of the reference values ​​of the current on the power frequency side and the frequency division side is obtained based on the voltage equalization control between the basic bridge arms of M3C.

[0114] (5) The reference value of the power frequency current obtained in step (2) is given to the power frequency positive sequence common mode current controller and the power frequency negative sequence common mode current controller to obtain the control quantities of the positive sequence voltage and negative sequence voltage of the power frequency side bridge arm;

[0115] (6) The compensation current obtained in step (3) and the compensation amount of the reference value of the power frequency side current obtained in step (4) are added together and given to the power frequency circulating current controller to obtain the power frequency circulating current voltage control amount.

[0116] (7) The positive sequence voltage, negative sequence voltage and circulating current voltage control quantities of the power frequency side bridge arm obtained in step (5) and step (6) are added together to obtain the power frequency bridge arm voltage control quantity.

[0117] (8) Select a grid-side control strategy that suppresses active power fluctuations or suppresses negative sequence currents on the frequency division side, and obtain reference values ​​for positive and negative sequence currents on the frequency division side.

[0118] (9) Accumulate the power on the frequency division side based on the positive and negative sequence voltages on the frequency division side and the current reference value obtained in step (8), and output the compensation current;

[0119] (10) The frequency division current reference value obtained in step (8) is given to the frequency division positive sequence common mode current controller and the frequency division negative sequence common mode current controller to obtain the frequency division side bridge arm positive sequence voltage and negative sequence voltage control quantity.

[0120] (11) The compensation amount of the frequency division side current reference value obtained in step (4) and the compensation current in step (9) are added together and given to the frequency division circulating current suppressor to obtain the frequency division circulating current voltage control amount.

[0121] (12) The positive sequence voltage, negative sequence voltage and circulating current voltage control quantities of the frequency division side bridge arm obtained in step (10) and step (11) are accumulated to obtain the frequency division side bridge arm voltage control quantity;

[0122] (13) The working frequency divider bridge arm voltage obtained from steps (7) and (12) is used as the M3C working frequency divider output voltage modulation wave. After being modulated by the nearest level approximation and carrier phase shift PWM method, the trigger signal is obtained.

[0123] Taking the power frequency grid-side controller as an example, the principle of this strategy is introduced. In a stationary coordinate system, when a negative sequence component appears in the three-phase voltage, the voltage vector can be expressed as the sum of a positive sequence voltage vector rotating in the positive direction and a negative sequence voltage vector rotating in the opposite direction:

[0124]

[0125] In the formula: E s For the power frequency grid voltage vector, ω s E is the power frequency angular frequency. dqS P E is the positive sequence voltage vector at power frequency. dqS N negative sequence power frequency

[0126] Similarly, three-phase current can be expressed as:

[0127]

[0128] In the formula: I s I is the power frequency grid current vector. dqS P I is the positive sequence current vector at power frequency. dqS N It is the negative sequence current vector at power frequency.

[0129] The apparent power of a three-phase system can be expressed as:

[0130] S s =P s +jQ s =E s I s * (3)

[0131] Substituting equations (1) and (2) into equation (3), we get:

[0132]

[0133] In the formula: P0 and Q0 are the DC components of active and reactive power, respectively. c2 P s2 Q is the frequency harmonic AC component of the active power. c2 Q s2 The frequency-doubled AC component in reactive power is expressed as shown in equation (5).

[0134]

[0135] In equation (5), e dS P e qS PThese are the d and q components of the positive-sequence voltage vector in the positive-sequence synchronous coordinate system, respectively, and e. dS N e qS N These are the d and q components of the negative-sequence voltage vector in the negative-sequence synchronous coordinate system, respectively. All four active and reactive power AC components are caused by the negative-sequence components of voltage and current. When the grid voltage is constant, power can only be regulated through current. However, the left side of the equation has six power components, while the right side only has four current components. Therefore, it is impossible to achieve complete control of all six power components; only a maximum of four power components can be selected as the control object, or a new control object can be created through weighted synthesis.

[0136] The network-side control strategies selected in this technology include:

[0137] (1) Control the average active and reactive power and suppress active power fluctuations;

[0138] Under this strategy, P0 and Q0 are set to the average active and reactive power setpoints, respectively. c2 P s2 Set to 0 to suppress active power fluctuations, Q c2 Q s2 Uncontrolled, the command values ​​for the four current components are:

[0139]

[0140] The control strategy of Equation (6) can achieve stable transmission of active power and control the average power factor of the converter, but there is a second harmonic fluctuation in reactive power.

[0141] (2) Control the average active and reactive power and suppress negative sequence current.

[0142] In this strategy, P0 and Q0 are set according to the average active and reactive power that the converter needs to output, and the current setpoint is determined by the following formula:

[0143]

[0144]

[0145] At this time, P c2 P s2 Q c2 and Q s2 Uncontrolled, i dS N and i qS N Since the value is set to 0, there is no negative sequence component in the three-phase current, maintaining three-phase symmetry. The average active and reactive power are controlled, but both exhibit second harmonic fluctuations.

[0146] Based on the aforementioned grid-side control strategy, the power of the bridge arms is analyzed. If the circulating current component is ignored, that is, if the line currents on both sides are evenly distributed among the bridge arms in steady state, the current of bridge arm xy (x = a, b, c; y = u, v, w) can be expressed as:

[0147]

[0148] Ignoring commutation losses, when the input and output power of the M3C are balanced, we have:

[0149]

[0150] Where the subscript L represents the frequency division side, taking the bridge arm au as an example, its instantaneous power can be expressed as:

[0151]

[0152] Therefore, p au The power output contains both DC and AC components. While the AC component may exacerbate voltage fluctuations in the module capacitors, it has no other impact. However, the DC component causes the bridge arms to be constantly charging or discharging, resulting in a continuous rise or fall in the module capacitor voltage. This degrades the power quality of the converter output and affects the safe and stable operation of the converter station. Therefore, the DC component of the bridge arm power must be controlled. The DC component can be expressed as:

[0153]

[0154] in, α This represents the initial phase angle difference between the positive and negative sequence components of the power frequency grid voltage. Similarly, the DC power bias of the other bridge arms caused by the negative sequence components of voltage and current can be calculated as follows:

[0155]

[0156] Where A = e dS P i dS N +e dS N i dS P B = e dS P i qS N -e dS N i qS P .

[0157] According to equation (13), for three bridge arms connected to the same phase on the power frequency side, the DC power bias caused by the negative sequence component of the power frequency side current is always equal. In other words, the negative sequence component of the power frequency side current has the same effect on the three bridge arms belonging to any sub-converter y; however, the negative sequence component of the power frequency side current has different effects on different sub-converters. The DC power bias caused by the asymmetry of grid voltage and current is related to the amplitude and phase of the negative sequence components of voltage and current, and is affected by the initial phase angle between the positive and negative sequence voltage components (related to the fault type of the power system). When the positive and negative sequence components of the grid voltage are constant, different outer loop control strategies have different effects on the DC power bias of the bridge arms by outputting different current command values, which need to be analyzed specifically. The power matrix formed by the two Clark transformations is obtained as follows:

[0158]

[0159] Among them, T αβ0 This is the Clark transformation matrix with equal amplitude.

[0160] Analysis shows that the negative sequence voltage and current components on the grid side will disrupt the power balance of some bridge arms in the M3C converter station. Some bridge arms will be in a constant charging or discharging state, and the module capacitor voltage will deviate significantly from the rated value, which will have an adverse effect on the output power quality of the converter station and even the safety of the equipment. To address the above problems, a bridge arm DC power bias compensation measure for grid voltage imbalance is proposed. The essence of the vertical differential mode component of power in equation (14) is the power differential mode component between the sub-converters u, v and w, which can be expressed as:

[0161]

[0162] Equation (15) shows that the power vertical differential mode component caused by the negative sequence component of the power frequency side current can be eliminated through three pathways: (1) frequency-divided positive sequence circulating current component; (2) frequency-divided negative sequence circulating current component; (3) power frequency line current negative sequence component. Obviously, the third method requires injecting an additional current component into the power frequency system, which changes the external characteristics of the M3C, and is therefore not suitable. If the second method is adopted, on the one hand, an additional frequency-divided negative sequence circulating current controller needs to be configured, which increases the complexity of the control system; on the other hand, this method depends on the amplitude of the frequency-divided negative sequence voltage component. When there is no asymmetric fault in the frequency-divided system, i.e., e dL N When the value is close to 0, this method is ineffective. Therefore, injecting a frequency-divided positive-sequence circulating current component into the M3C is the most suitable method. According to equation (15), the frequency-divided positive-sequence circulating current component required to eliminate the power vertical differential mode component is:

[0163]

[0164] Combining the two grid-side control strategies mentioned above, the current command values ​​from equations (6), (7), and (8) are substituted into equation (15) to obtain the results. and Then, by substituting into equation (16), the power compensation circulating current command value of the corresponding strategy can be obtained.

[0165] The strategy for suppressing active power fluctuations on the power frequency side is as follows:

[0166]

[0167] In the formula: Δi dLα refB , Δi dLβ refB The command value of the positive sequence circulating current component required to compensate for DC power bias. As shown in (17), the power compensation circulating current required by the grid-side control strategy to suppress active power fluctuations is 0. This is because the essence of suppressing active power fluctuations is to maintain the symmetry of the three-phase output power. Therefore, the power balance of each bridge arm inside the M3C is not broken, and power compensation is not required.

[0168] Strategy for suppressing negative sequence components of current on the power frequency side:

[0169]

[0170] When negative sequence occurs on the frequency division side, the required compensation circulating current command value can be derived in a similar way, as follows:

[0171] Frequency division side active power fluctuation suppression strategy:

[0172]

[0173] Strategy for suppressing negative sequence current components on the frequency division side:

[0174]

[0175] Therefore, the arm power compensation control strategy is characterized by controlling the instantaneous DC component of the power in the arm of the M3C converter during asymmetrical faults. By injecting a specific frequency-divided positive sequence circulating current component into the M3C, the DC power bias compensation of the arm is realized, the vertical differential mode component of the power is eliminated, and the stable operation of the M3C converter is ensured during asymmetrical faults.

[0176] Example 1

[0177] refer to Figure 6 This invention takes the M3C converter station in an offshore wind power frequency division system scenario as an example, constructs a digital simulation model, and verifies the theoretical analysis and control strategy of this invention. The system topology is shown in the figure, and the system parameters are shown in Table 1.

[0178] Table 1 Simulation System Parameters

[0179]

[0180]

[0181] In the simulation example of this invention, the active power control objective on the power frequency side of the converter station is to control the average module capacitor voltage of the M3C, while the active power control objective on the frequency division side is constant (average) active power; the reactive power control objective on both sides is to achieve (average) unity power factor operation. It is assumed that a single-phase ground fault occurs successively on the power frequency side and the frequency division side output bus of the converter station, and the phase voltages of phases u and a drop to 0.3pu respectively. The power frequency side fault occurs at time t = 0.2s, and the frequency division side fault occurs at time t = 0.4s; at time t = 0.8s, both faults are cleared, and the converter station resumes normal operation. During the fault period, the grid-side control strategy on both the power frequency side and the frequency division side is to suppress active power fluctuations, while at time t = 0.6s, the grid-side control strategy switches from suppressing active power fluctuations to suppressing the negative sequence component of the current.

[0182] refer to Figure 7 The current waveforms throughout the entire simulation test process are displayed. Figure 7 Figures (a)-(d) show the positive and negative sequence components of the current on the power frequency side and the frequency division side, respectively, and their reference values. As can be seen from the figures, each current component can quickly track changes in the command value, demonstrating a fast response speed. Figure 7 Figures (e)-(h) show the steady-state waveforms of the line currents on both sides under two control strategies: suppressing active power fluctuations and suppressing the negative sequence component of the current. It can be seen that under the control strategy of suppressing active power fluctuations, the three-phase currents are asymmetrical, and the amplitude of the line current is approximately inversely proportional to the voltage amplitude of the corresponding phase. However, under the control strategy of suppressing the negative sequence component of the current, the three-phase current waveforms maintain good symmetry. The simulation waveforms also show that fault ride-through on the power frequency side and the frequency division side are independent of each other and do not interfere with each other. In summary, the traditional control algorithm for three-phase voltage source converters under grid voltage asymmetry conditions achieves ideal control results on the M3C, realizing the preset control objectives and can be used for fault ride-through control in M3C converter stations.

[0183] Example 2

[0184] To verify the effectiveness of the arm power compensation control strategy, in the second simulation test, the fault clearing time was delayed to time t = 1.5s, while other operating parameters of the M3C remained unchanged.

[0185] refer to Figure 8 The waveforms of the waveforms without power compensation and with power compensation were compared. Figure 8(a)-(e) are the simulation waveforms without power compensation. It can be seen that in the absence of power compensation, although the horizontal and vertical voltage equalization controllers slow down the rate of increase of the voltage differential component to some extent and reduce the impact of DC power bias of the bridge arm caused by grid voltage asymmetry, the average module capacitor voltage of some bridge arms has deviated significantly from the command value. Therefore, the voltage equalization controller alone is insufficient to meet the demand for bridge arm power compensation during fault ride. Figure 8 (f)-(j) show the simulation waveforms with power compensation. The power compensation stage can predict the occurrence of DC bias in the bridge arm power based on the grid's operating conditions and inject corresponding compensation components into the circulating current command value. Simultaneously, the M3C's circulating current controller has a fast response speed, enabling it to quickly control and track changes in the command value, promptly offsetting the effects of grid voltage asymmetry. In summary, compared to... Figure 8 (a)-(e) and (f)-(j), the power compensation control strategy proposed in this invention can effectively suppress the generation of DC bias in the bridge arm power when the grid voltage drops asymmetrically, ensuring the stable operation of M3C and significantly improving the fault ride-through capability of M3C in response to such faults. It is an important auxiliary measure in the M3C control system.

[0186] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fault ride-through method for an M3C converter station when the grid voltage is asymmetrically dropped, characterized in that, include: The active power fluctuation and negative sequence current on the power frequency side and the active power fluctuation and negative sequence current on the frequency division side are suppressed respectively, and the reference values ​​of positive and negative sequence current on the power frequency side and the frequency division side are obtained. Based on the reference values ​​of positive and negative sequence voltage on the power frequency side, positive and negative sequence current on the power frequency side, positive and negative sequence voltage on the frequency division side, and positive and negative sequence current on the frequency division side, the compensation current on the power frequency side and the compensation current on the frequency division side are output respectively. Based on the reference values ​​of positive and negative sequence currents on the power frequency side and the reference values ​​of positive and negative sequence currents on the frequency division side, the positive sequence voltage of the power frequency side bridge arm, the control quantity of the negative sequence voltage on the power frequency side, and the positive sequence voltage of the frequency division side bridge arm and the control quantity of the negative sequence voltage on the frequency division side are obtained, respectively. Specifically, the reference values ​​of positive and negative sequence currents on the power frequency side are given to the power frequency positive sequence common-mode current controller and the power frequency negative sequence common-mode current controller to obtain the positive sequence voltage of the power frequency side bridge arm and the control quantity of the negative sequence voltage on the power frequency side; the reference values ​​of positive and negative sequence currents on the frequency division side are given to the frequency division positive sequence common-mode current controller and the frequency division negative sequence common-mode current controller to obtain the positive sequence voltage of the frequency division side bridge arm and the control quantity of the negative sequence voltage on the frequency division side. Based on the M3C basic bridge arm voltage equalization control, the compensation amount of the current reference values ​​on the power frequency side and the frequency division side is obtained; Based on the power frequency side compensation current, the compensation amount of the power frequency side current reference value, the compensation amount of the frequency division side current reference value, and the frequency division side compensation current, the power frequency circulating current voltage control amount and the frequency division circulating current voltage control amount are obtained; specifically: the power frequency side compensation current and the compensation amount of the power frequency side current reference value are accumulated and given to the power frequency circulating current controller to obtain the power frequency circulating current voltage control amount; the compensation amount of the frequency division side current reference value and the frequency division side compensation current are accumulated and given to the frequency division circulating current suppressor to obtain the frequency division circulating current voltage control amount. Based on the positive-sequence voltage of the power frequency side bridge arm, the negative-sequence voltage control quantity of the power frequency side, the circulating current voltage control quantity of the power frequency side, the positive-sequence voltage of the frequency division side bridge arm, the negative-sequence voltage control quantity of the frequency division side, and the circulating current voltage control quantity of the frequency division, the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are obtained; specifically, the positive-sequence voltage of the power frequency side bridge arm, the negative-sequence voltage of the power frequency side, and the circulating current voltage control quantity of the power frequency side are accumulated to obtain the power frequency bridge arm voltage control quantity; the positive-sequence voltage of the frequency division side bridge arm, the negative-sequence voltage of the frequency division side, and the circulating current voltage control quantity of the frequency division are accumulated to obtain the frequency division bridge arm voltage control quantity. The mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity are used as the M3C mains frequency division output voltage modulation wave. The trigger signal is obtained through level approximation and carrier phase shift PWM method.

2. The fault ride-through method for M3C converter stations during grid voltage asymmetry drops according to claim 1, characterized in that, The active power fluctuation and negative sequence current on the power frequency side and the active power fluctuation and negative sequence current on the frequency division side are suppressed respectively to obtain reference values ​​for positive and negative sequence current on the power frequency side and reference values ​​for positive and negative sequence current on the frequency division side. Specifically: Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the power frequency side, the reference values ​​of positive and negative sequence currents on the power frequency side are obtained. Based on the grid-side control strategy of suppressing active power fluctuations or suppressing negative sequence currents on the frequency division side, the reference values ​​of positive and negative sequence currents on the frequency division side are obtained.

3. The fault ride-through method for M3C converter stations during grid voltage asymmetry drops according to claim 2, characterized in that, The grid-side control strategy for suppressing active power fluctuations or negative sequence current on the power frequency side is as follows: The strategy for suppressing active power fluctuations on the power frequency side is as follows: in, , The command value of the frequency-division positive sequence circulating current component required to compensate for DC power bias; the power compensation circulating current required by the grid-side control strategy to suppress active power fluctuations is 0; Strategy for suppressing negative sequence components of current on the power frequency side: 。 4. The fault ride-through method for M3C converter stations during grid voltage asymmetry drops according to claim 1, characterized in that, The grid-side control strategy for suppressing active power fluctuations on the frequency division side or suppressing negative sequence current is as follows: Frequency division side active power fluctuation suppression strategy: Among them, the power compensation circulating current required by the grid-side control strategy to suppress active power fluctuations is 0; Strategy for suppressing negative sequence current components on the frequency division side: 。 5. The fault ride-through method for M3C converter stations during grid voltage asymmetry drops according to claim 1, characterized in that, Based on the reference values ​​of positive and negative sequence voltage and current on the power frequency side, and the reference values ​​of positive and negative sequence voltage and current on the frequency division side, the power frequency side compensation current and the frequency division side compensation current are output respectively; specifically: Power compensation on the power frequency side is performed based on the reference values ​​of positive and negative sequence voltage and positive and negative sequence current on the power frequency side, and the power frequency side compensation current is output. The power on the frequency divider side is accumulated based on the reference values ​​of the positive and negative sequence voltage and current on the frequency divider side, and the compensation current on the frequency divider side is output.

6. A fault ride-through system for an M3C converter station during asymmetrical voltage dips in the power grid, used to implement the fault ride-through method for an M3C converter station during asymmetrical voltage dips in claim 1, characterized in that, include: The suppression module is used to suppress active power fluctuations and negative sequence currents on the power frequency side and on the frequency division side, respectively, to obtain reference values ​​for positive and negative sequence currents on the power frequency side and on the frequency division side. The compensation current acquisition module outputs the compensation current on the power frequency side and the compensation current on the frequency division side based on the positive and negative sequence voltage on the power frequency side, the reference value of the positive and negative sequence current on the power frequency side, the positive and negative sequence voltage on the frequency division side, and the reference value of the positive and negative sequence current on the frequency division side. The voltage control quantity acquisition module obtains the positive sequence voltage of the bridge arm on the power frequency side, the negative sequence voltage control quantity on the power frequency side, the positive sequence voltage of the bridge arm on the power frequency side, and the negative sequence voltage control quantity on the frequency division side, respectively, based on the positive and negative sequence current reference values ​​on the power frequency side and the positive and negative sequence current reference values ​​on the frequency division side. The compensation amount acquisition module acquires the compensation amount of the reference current values ​​on the power frequency side and the frequency division side based on the M3C basic bridge arm voltage equalization control. The circulating current voltage control quantity acquisition module obtains the circulating current voltage control quantity at the power frequency and the circulating current voltage control quantity at the frequency division based on the power frequency side compensation current, the compensation quantity of the power frequency side current reference value, the compensation quantity of the frequency division side current reference value, and the frequency division side compensation current. The bridge arm voltage control quantity acquisition module acquires the power frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity based on the power frequency side positive sequence voltage, power frequency side negative sequence voltage control quantity, power frequency circulating current voltage control quantity, frequency division side positive sequence voltage, frequency division side negative sequence voltage control quantity and frequency division circulating current voltage control quantity. The output module is used to take the mains frequency bridge arm voltage control quantity and the frequency division bridge arm voltage control quantity as the M3C mains frequency division output voltage modulation wave, and obtain the trigger signal through level approximation and carrier phase shift PWM method.

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