A double sequence synchronous stability control method for a multi-wind farm system under asymmetric grid faults

Abstract

The application discloses a double-sequence synchronous stability control method for a multi-wind farm system under an asymmetric fault of a power grid. During an asymmetric short-circuit fault of the power grid, preset values of positive and negative sequence reactive currents and preset values of positive and negative sequence active currents injected by each wind farm according to the grid code are calculated in sequence. For a permanent magnet direct drive type wind farm, the obtained positive and negative sequence active and reactive current preset values are respectively sent to current controllers of grid-side converters of the permanent magnet direct drive type wind farm, and for a double-fed type wind farm, the obtained positive and negative sequence active and reactive current preset values are used as total current instructions of rotor-side converters and grid-side converters of the double-fed type wind farm. According to the method, the double-sequence synchronous stability of the multi-wind farm system can be improved by controlling each grid-side converter of the permanent magnet direct drive type wind farm and / or each rotor-side converter and grid-side converter of the double-fed type wind farm without increasing equipment influence and changing a phase-locked loop control structure, and transient instability can be avoided.

Description

Technical Field

[0001] This invention relates to a dual-sequence synchronous stability control method for multi-wind farm systems under asymmetrical grid faults. It is applicable to doubly-fed and permanent magnet direct-drive multi-wind farm systems under asymmetrical short-circuit faults in AC grids. This method can significantly improve the dual-sequence synchronous stability of multi-wind farm systems under asymmetrical short-circuit faults in AC grids and solve the transient instability problem of multi-wind farm systems under grid faults. Background Technology

[0002] With the increasing penetration rate of new energy sources, wind turbines and other new energy power generation equipment are gradually becoming the main power source of the power system, which will lead to fundamental changes in the power system synchronization mechanism. Since most wind farms use phase-locked loop (PLL) synchronization, when an asymmetrical short-circuit fault occurs in the power grid, the PLL output angle is easily affected by grid faults and current injection, leading to dual-sequence synchronization instability. For multi-wind farm systems, when a wind farm experiences transient instability, it may cause large-scale disconnection of other wind farms in the system, seriously affecting the safe and stable operation of the power grid. Therefore, improving the dual-sequence synchronization stability of multi-parallel wind farm systems under asymmetrical grid faults is a key issue in the current development of new energy. Currently, scholars at home and abroad have conducted relevant research, as shown in the following published literature:

[0003] X.He,H.Geng,J.Xi,and JMGuerrero.ResynchronizationAnalysis andImprovement ofGrid-Connected VSCs During Grid Faults[J].IEEE Journal ofEmerging and Selected Topics in Power Electronics.2021,9(1):438-450.

[0004] O.Goksu, R.Teodorescu, CLBak, and et al. Instability of Wind TurbineConverters During Current Injection to Low Voltage Grid Faults and PLL Frequency Based Stability Solution [J]. IEEE Transactions on Power Systems, 2014, 29(4): 1683-1691.

[0005] Reference [1] identified the integral regulator of the phase-locked loop as the key factor affecting synchronization using the modified equal area criterion, and proposed a variable structure phase-locked loop method to remove the integral regulator of the phase-locked loop during grid faults. However, this method can only be used when the system has a balance point, and cannot ensure the synchronization stability of the system without a balance point. Reference [2] proposed an active current regulation method based on the phase-locked loop frequency, which suppresses the influence of voltage offset term on synchronization stability by adjusting the active current output. However, the above references are all for single-machine infinite bus systems under symmetrical grid faults, and do not consider the interaction between negative sequence components and multiple wind farms under asymmetrical grid faults. In fact, when an asymmetrical grid fault occurs, the coupling of positive and negative sequence components and the coupling between wind farms may cause a certain wind farm to lack a balance point, thereby causing the output components of other wind farms to oscillate and the system to experience dual-sequence synchronization instability. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to propose a dual-sequence synchronous stability control method for multi-wind farm systems under asymmetrical grid faults. This method can enhance the dual-sequence synchronous stability of multi-wind farm systems without increasing the impact of equipment or changing the phase-locked loop control structure.

[0007] The technical solution of this invention is implemented as follows:

[0008] A dual-sequence synchronous stability control method for a multi-wind farm system under asymmetrical grid faults, comprising the following steps:

[0009] A1) During an asymmetrical short-circuit fault in the power grid, the preset values ​​of positive and negative sequence reactive current injected into wind farms 1, 2, ... n as required by the power grid guidelines shall be calculated according to the following formula:

[0010]

[0011] In the formula, These are the preset values ​​for positive and negative sequence reactive currents of n wind farms, i = 1, 2, ..., n; K + K - These are the positive and negative sequence reactive current proportionality coefficients, respectively; U + U - These are the positive and negative sequence components of the grid connection point voltage, respectively.

[0012] A2) Based on the preset values ​​of positive and negative sequence reactive currents of the n wind farms obtained in step A1), calculate the preset values ​​of positive and negative sequence active currents injected into wind farms 1, 2, ... n according to the following formula:

[0013]

[0014]

[0015] In the formula, Preset values ​​for the positive sequence active current of each of the n wind farms; These are the preset values ​​for the negative sequence active current of n wind farms; |K1| and |K2| are both positive and negative sequence coupling impedance values; Let |X1| and |R1| be the impedance angles of the positive and negative sequence coupling impedances K1 and K2, respectively; |X1| and |R1| are the positive and negative sequence coupling reactance and resistance values, respectively; |X L1 |,|X L2 |,…|X Ln |and|R L1 |,|R L2 |,…|R Ln | represents the reactance and resistance values ​​of the branch transmission lines from the outlet ends of n wind farms to the common junction point; θ f + and θ f - It is the phase angle of the voltage at the fault point;

[0016] Among them, the positive and negative sequence coupling impedances K1 and K2, as well as the positive and negative sequence coupling reactances and resistances X1 and R1, are calculated according to the following formulas considering different types of asymmetrical short-circuit faults:

[0017] During a single-phase ground fault:

[0018]

[0019] During a two-phase ground fault:

[0020]

[0021] During a two-phase short circuit fault:

[0022] In the formula and These are the positive, negative, and zero-sequence impedance values ​​of the power grid, respectively. and Z represents the positive-sequence and zero-sequence impedance values ​​of the transmission line from the fault point to the point of common coupling; F The impedance value of the faulty branch;

[0023] A3) For permanent magnet direct-drive wind farms in multi-wind farm systems, the results obtained in steps A1) and A2) The current controllers of the grid-side converters of permanent magnet direct-drive wind farms, i = 1, 2, ..., n, respectively send the command values ​​of the positive and negative sequence current components of the d / q axis of the grid-side converters. Set as follows:

[0024]

[0025] For a doubly-fed wind farm in a multi-wind farm system, the results obtained in steps A1) and A2) As the total current command for the rotor-side converter and grid-side converter of a doubly-fed wind farm, i = 1, 2, ..., n; its rotor-side converter d / q-axis positive and negative sequence current commands and command values ​​of d / q axis current components of the grid-side converter Determine using the following formula:

[0026]

[0027] in, and U dc K represents the given and actual values ​​of the DC bus voltage, respectively. p and τ i Here, represents the proportional gain and integral time constant of the PI controller in the DC voltage control loop of the grid-side converter, respectively, and s is the Laplace operator. L represents the positive and negative sequence q-axis components of the stator flux linkage. s and L m These are the equivalent inductance and equivalent mutual inductance of the stator windings, respectively.

[0028] The dual-sequence synchronization stability of a multi-wind farm system can be improved by controlling the grid-side converter of each permanent magnet direct-drive wind farm and / or the rotor-side converter and grid-side converter in each doubly-fed wind farm, thus avoiding transient instability.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] This invention enhances the dual-sequence synchronization stability of multi-wind farm systems by optimizing the allocation of positive and negative sequence current command values ​​for each wind farm, without increasing equipment impact or altering the phase-locked loop control structure. Furthermore, this method guarantees that all wind farms in a multi-wind farm system possess a positive and negative sequence equilibrium point, significantly improving the system's dual-sequence synchronization stability and providing guiding suggestions for enhancing the transient operational capabilities of multi-wind farm systems. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the topology of a multi-wind farm system.

[0032] Figure 2 The simulation waveform diagram shows the system instability caused by a two-phase short-circuit ground fault in the power grid's public line, where both the positive and negative sequence voltages at the fault point are 0.487 pu and the current command is inappropriate.

[0033] Figure 3The simulation waveform diagram shows the system stability when the proposed control method is applied to a two-phase short-circuit ground fault occurring on a power grid public line, with both the positive and negative sequence voltages at the fault point being 0.487 pu. Detailed Implementation

[0034] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0035] Taking a doubly-fed multi-wind farm system as an example, Figure 1 This is a schematic diagram of the topology of a system of three 200MW doubly-fed wind farms connected in parallel.

[0036] This invention relates to a dual-sequence synchronous stability control method for multiple wind farms during grid asymmetric faults, used to improve the dual-sequence synchronous stability of doubly-fed and permanent magnet direct-drive multi-wind farm systems during grid asymmetric faults. This method involves the optimized allocation of command values ​​for the positive and negative sequence dq-axis current components of each wind farm in the multi-wind farm system.

[0037] The specific implementation steps of this invention are as follows:

[0038] A1) During an asymmetrical short-circuit fault in the power grid, the preset values ​​of positive and negative sequence reactive current injected into wind farms 1, 2, ... n as required by the power grid guidelines shall be calculated according to the following formula:

[0039]

[0040] In the formula, These are the preset values ​​for positive and negative sequence reactive currents of n wind farms; K + K - These are the positive and negative sequence reactive current proportionality coefficients, respectively; U + U - These are the positive and negative sequence components of the grid connection point voltage, respectively.

[0041] A2) Based on the preset values ​​of positive and negative sequence reactive currents of the n wind farms obtained in step A1), calculate the preset values ​​of positive and negative sequence active currents injected into wind farms 1, 2, ... n according to the following formula:

[0042]

[0043]

[0044] In the formula, Preset values ​​for the positive sequence active current of each of the n wind farms; These are the preset values ​​for the negative sequence active current of n wind farms; |K1| and |K2| are both positive and negative sequence coupling impedance values; Let |X1| and |R1| be the impedance angles of the positive and negative sequence coupling impedances K1 and K2, respectively; |X1| and |R1| are the positive and negative sequence coupling reactance and resistance values, respectively; |X L1 |,|XL2 |,…|X Ln |and|R L1 |,|R L2 |,…|R Ln | These represent the reactance and resistance values ​​of the branch transmission lines from the outlet ends of n wind farms to the common connection point; and It is the phase angle of the voltage at the fault point;

[0045] Among them, the positive and negative sequence coupling impedances K1 and K2, as well as the positive and negative sequence coupling reactances and resistances X1 and R1, are calculated according to the following formulas considering different types of asymmetrical short-circuit faults:

[0046] During a single-phase ground fault:

[0047]

[0048] During a two-phase ground fault:

[0049]

[0050] During a two-phase short circuit fault: In the formula and These are the positive, negative, and zero-sequence impedance values ​​of the power grid, respectively. and Z represents the positive-sequence and zero-sequence impedance values ​​of the transmission line from the fault point to the point of common coupling; F The impedance value of the faulty branch;

[0051] A3) For permanent magnet direct-drive wind farms in multi-wind farm systems, the results obtained in steps A1) and A2) The current controllers of the grid-side converters of permanent magnet direct-drive wind farms, i = 1, 2, ..., n, respectively send the command values ​​of the positive and negative sequence current components of the d / q axis of the grid-side converters. Set as follows:

[0052]

[0053] For a doubly-fed wind farm in a multi-wind farm system, the results obtained in steps A1) and A2) As the total current command for the rotor-side converter and grid-side converter of a doubly-fed wind farm, i = 1, 2, ..., n; its rotor-side converter d / q-axis positive and negative sequence current commands and command values ​​of d / q axis current components of the grid-side converter Determine using the following formula:

[0054]

[0055] in, and U dc K represents the given and actual values ​​of the DC bus voltage, respectively. p and τ i Here, represents the proportional gain and integral time constant of the PI controller in the DC voltage control loop of the grid-side converter, respectively, and s is the Laplace operator. L represents the positive and negative sequence q-axis components of the stator flux linkage. s and L m These are the equivalent inductance and equivalent mutual inductance of the stator windings, respectively.

[0056] The dual-sequence synchronization stability of a multi-wind farm system can be improved by controlling the grid-side converter of each permanent magnet direct-drive wind farm and / or the rotor-side converter and grid-side converter in each doubly-fed wind farm, thus avoiding transient instability.

[0057] Description of the effects of this invention:

[0058] Figure 2 and Figure 3 Simulation waveforms of a multi-wind farm system are presented respectively when a two-phase short-circuit ground fault occurs on the power grid's common line, with both the positive and negative sequence voltages at the fault point being 0.487 pu, and when the current command is inappropriate and the current command is given using the control method of this invention. Figure 2 It can be seen that when the current setpoint of the multi-wind farm system is inappropriate, wind farm 3 experiences positive-sequence synchronous instability, the positive-sequence q-axis terminal voltage cannot be adjusted to 0, and its own negative-sequence voltage and the positive-sequence voltage of other wind farms fluctuate around 0. Figure 3 It is understood that when a multi-wind farm system adopts the control method proposed in this invention, by coordinating the setting of the positive and negative sequence dq-axis current component ratios for each wind farm, each wind farm has an equilibrium point, and the positive and negative sequence q-axis voltages are adjusted to 0. At this time, the multi-parallel wind farm system can safely and stably achieve low-voltage ride-through.

[0059] In summary, the dual-sequence synchronous stability control method for multi-wind farm systems under asymmetrical grid faults described in this invention can ensure that the system has an equilibrium point under asymmetrical grid faults, avoid transient instability, and improve the safe and stable operation capability of the power grid.

[0060] Finally, it should be noted that the above examples of the present invention are merely illustrative and not intended to limit the implementation of the invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art can make other variations and modifications based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A dual-sequence synchronous stability control method for a multi-wind farm system under asymmetrical grid faults, characterized in that: The steps are as follows: A1) During an asymmetrical short-circuit fault in the power grid, the preset values ​​of positive and negative sequence reactive current injected into wind farms 1, 2, ... n as required by the power grid guidelines shall be calculated according to the following formula: In the formula, These are the preset values ​​for positive and negative sequence reactive currents of n wind farms, i = 1, 2, ..., n; K + K - These are the positive and negative sequence reactive current proportionality coefficients, respectively; U + U - These are the positive and negative sequence components of the grid connection point voltage, respectively. A2) Based on the preset values ​​of positive and negative sequence reactive currents of the n wind farms obtained in step A1), calculate the preset values ​​of positive and negative sequence active currents injected into wind farms 1, 2, ... n according to the following formula: In the formula, Preset values ​​for the positive sequence active current of each of the n wind farms; These are the preset values ​​of negative sequence active current for n wind farms; |K1| and |K2| are both positive and negative sequence coupling impedance values. Let |X1| and |R1| be the impedance angles of the positive and negative sequence coupling impedances K1 and K2, respectively; |X1| and |R1| are the positive and negative sequence coupling reactance and resistance values, respectively; |X L1 |,|X L2 |,…|X Ln |and|R L1 |,|R L2 |,…|R Ln | These represent the reactance and resistance values ​​of the branch transmission lines from the outlet ends of n wind farms to the common junction point; and It is the phase angle of the voltage at the fault point; Among them, the positive and negative sequence coupling impedances K1 and K2, as well as the positive and negative sequence coupling reactances and resistances X1 and R1, are calculated according to the following formulas considering different types of asymmetrical short-circuit faults: During a single-phase ground fault: During a two-phase ground fault: During a two-phase short circuit fault: In the formula and These are the positive, negative, and zero-sequence impedance values ​​of the power grid, respectively. and Z represents the positive-sequence and zero-sequence impedance values ​​of the transmission line from the fault point to the point of common coupling; F The impedance value of the faulty branch; A3) For permanent magnet direct-drive wind farms in multi-wind farm systems, the results obtained in steps A1) and A2) The current controllers of the grid-side converters of permanent magnet direct-drive wind farms, i = 1, 2, ..., n, respectively send the command values ​​of the positive and negative sequence current components of the d / q axis of the grid-side converters. Set as follows: For a doubly-fed wind farm in a multi-wind farm system, the results obtained in steps A1) and A2) As the total current command for the rotor-side converter and grid-side converter of a doubly-fed wind farm, i = 1, 2, ..., n; its rotor-side converter d / q-axis positive and negative sequence current commands and command values ​​of d / q axis current components of the grid-side converter Determine by the following formula: in, and U dc K represents the given and actual values ​​of the DC bus voltage, respectively. p and τ i Here, represents the proportional gain and integral time constant of the PI controller in the DC voltage control loop of the grid-side converter, respectively, and s is the Laplace operator. L represents the positive and negative sequence q-axis components of the stator flux linkage. s and L m These are the equivalent inductance and equivalent mutual inductance of the stator windings, respectively. The dual-sequence synchronization stability of a multi-wind farm system can be improved by controlling the grid-side converter of each permanent magnet direct-drive wind farm and / or the rotor-side converter and grid-side converter in each doubly-fed wind farm, thus avoiding transient instability.

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