A method for transient stability control of a doubly-fed wind power single-machine grid-connected system under asymmetric grid faults

Abstract

This invention discloses a transient stability control method for a doubly-fed induction generator (DFIG) wind turbine grid-connected system under asymmetrical grid faults. By calculating the positive and negative sequence active and reactive currents output by the DFIG wind turbine grid-connected system during asymmetrical grid faults, and optimizing these outputs, transient stability control of the DFIG wind turbine grid-connected system under asymmetrical grid faults is achieved. This method considers the requirements of grid guidelines and, without adding hardware, ensures that the stable equilibrium point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state. This minimizes the unbalanced transient energy accumulated by the system during the quasi-steady state and reduces the probability of transient instability.

Description

Technical Field

[0001] This invention relates to a transient stability control method for a doubly fed wind power unit grid-connected system under asymmetrical grid faults. It is applicable to improving the transient stability operation capability of a doubly fed wind power unit grid-connected system under asymmetrical short-circuit faults in AC power grids and belongs to the field of new energy power transmission and distribution technology. Background Technology

[0002] The large-scale development and long-distance transmission of new energy sources, while improving the absorption capacity of new energy, have also made the power system highly susceptible to asymmetrical short-circuit faults. Due to the weak damping and inertia support capabilities of power electronic converters, phase-locked synchronous new energy power generation equipment using power electronic converters as grid-connected interfaces is prone to transient instability problems such as transient loss of synchronization, cascading faults, and even grid disconnection during asymmetrical short-circuit faults in the power grid. This is especially true for doubly-fed induction generators (DFIGs) whose stators are directly connected to the grid; the negative sequence components caused by asymmetrical short-circuit faults in the power grid are more likely to intrude into the main circuit and control loop of the power generation equipment, leading to transient instability in the DFIG wind power grid-connected system due to multiple stresses exceeding limits, such as DC voltage, electromagnetic torque, and current. Therefore, it is urgent to propose a transient stability control method for doubly-fed wind power grid-connected systems under asymmetrical grid faults to improve the transient stability of the system during faults, thereby enhancing the system's transient stable operation capability. Currently, scholars at home and abroad have conducted a series of related studies, such as the following published literature:

[0003] [1]YI XT,PENG YL,ZHOU Q,et al.Transient synchronization stability analysis and enhancement of paralleled converters considering differentcurrent injection strategies[J].IEEE Transactions on Sustainable Energy, 2022,13(4):1957-1968.

[0004] [2]XU HL,ZHANG YF,LI Z,et al.Reactive current constraints and coordinated control of DFIG's RSC and GSC during asymmetric grid condition[J].IEEE Access,2020,8:184339-184349.

[0005] Reference [1] proposes a stable control strategy for a doubly fed wind power single-unit grid-connected system under asymmetrical short-circuit faults in the power grid, realizing flexible control of multiple objectives. However, this control strategy mainly focuses on the support of positive-sequence reactive current during the fault period, which is difficult to fully meet the requirements of existing grid connection guidelines for negative-sequence reactive current support. Reference [2] proposes a stable control strategy for a new energy power generation system under asymmetrical faults in the power grid, based on meeting the requirements of existing grid connection guidelines. However, the control strategy mainly targets the small-disturbance stable control problem of the new energy power generation system, and the adaptability of the proposed control strategy to the transient stable control of the system needs further investigation. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention proposes a transient stability control method for a doubly-fed induction generator (DFIG) wind turbine grid-connected system under asymmetrical grid faults. This method considers the requirements of grid guidelines and, without adding hardware equipment, optimizes the positive and negative sequence active and reactive currents output by the DFIG wind turbine grid-connected system during asymmetrical grid faults. This ensures that the stable equilibrium point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state, thereby minimizing the unbalanced transient energy accumulated by the system during the quasi-steady state and reducing the probability of transient instability.

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

[0008] A transient stability control method for a single-unit doubly-fed induction generator (DFIG) wind power grid-connected system under asymmetrical grid faults, comprising the following specific steps:

[0009] A1) Calculate the positive-sequence reactive current output of a single doubly-fed wind turbine connected to the grid during a grid asymmetric fault using the following formula. and negative sequence reactive current

[0010]

[0011] Among them, K + K is the dynamic positive sequence reactive current proportionality coefficient of the wind farm; - I is the dynamic negative sequence reactive current proportionality coefficient of the wind farm; N This is the rated current of the wind farm. This is the effective value of the positive sequence stator voltage. This represents the effective value of the negative sequence stator voltage.

[0012] A2) The equivalent impedance matrix of the system under a single-phase-to-ground short-circuit fault is calculated using the following formula:

[0013]

[0014] in, and These are the positive-sequence network-side impedance and the transmission line impedance, respectively. and These are the negative sequence network-side impedance and the transmission line impedance, respectively. and These are the zero-sequence network-side impedance and the transmission line impedance, respectively, Z f Z1 and Z3 are the grounding impedances; Z2 and Z4 are the positive-sequence equivalent impedances and negative-sequence equivalent impedances, respectively; Z1 and Z3 are the negative-sequence coupling impedances and positive-sequence coupling impedances, respectively. For Z i The impedance angle of (i = 1, 2, 3, 4) and They are respectively and impedance angle, and These are the initial angles of the positive and negative equivalent work angles, respectively; X 2×2 R is the equivalent reactance matrix. 2×2 This is the equivalent resistance matrix;

[0015] A3) Calculate the positive-sequence active current output of a single doubly-fed wind turbine connected to the grid during a grid asymmetric fault using the following formula. and negative sequence active current

[0016]

[0017] in, The positive sequence grid voltage is given by K1 and K2 respectively. Equivalent drop depth in positive and negative sequence synchronous reference coordinate systems and The phase angles of K1 and K2 are respectively;

[0018] During asymmetrical grid faults, the transient stability control of the doubly fed wind turbine grid-connected system can be achieved by optimizing the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This method takes into account the requirements of the power grid guidelines and optimizes the positive and negative sequence active and reactive current output of a doubly-fed induction generator (DFIG) wind turbine grid-connected system during asymmetrical power grid faults without adding hardware equipment. This makes the stable equilibrium point of the system's positive and negative sequence equivalent power angles coincide with the initial operating point during the quasi-steady state, thereby minimizing the unbalanced transient energy accumulated by the system during the quasi-steady state. Consequently, it significantly improves the transient stability of the DFIG wind turbine grid-connected system during asymmetrical short-circuit faults in the power grid and reduces the risk of transient instability of the system. Attached Figure Description

[0021] Figure 1 A flowchart for optimizing the active and reactive current control of a single doubly fed wind turbine grid-connected system under grid asymmetric faults.

[0022] Figure 2 The figure shows the simulation results of using the traditional control scheme under a two-phase-to-ground short-circuit fault.

[0023] Figure 3 The figure shows the simulation results when the control scheme proposed in this invention is used under a two-phase-to-ground short-circuit fault. Detailed Implementation

[0024] This invention is used to improve the transient stability of a doubly fed wind power unit grid-connected system during asymmetrical short-circuit faults in the power grid. Figure 1 This invention presents a flowchart for the optimized control of active and reactive currents in a doubly-fed induction generator (DFIG) wind turbine grid-connected system under grid asymmetric fault conditions. By optimizing the positive and negative sequence active and reactive currents output by the system, this invention can minimize the unbalanced transient energy accumulated during the quasi-steady-state period, thereby improving the system's transient stability.

[0025] The specific implementation steps of this invention are as follows.

[0026] A1) Calculate the positive-sequence reactive current output of a single doubly-fed wind turbine connected to the grid during a grid asymmetric fault using the following formula. and negative sequence reactive current

[0027]

[0028] Among them, K + K is the dynamic positive sequence reactive current proportionality coefficient of the wind farm; - I is the dynamic negative sequence reactive current proportionality coefficient of the wind farm; N This is the rated current of the wind farm. This is the effective value of the positive sequence stator voltage. This represents the effective value of the negative sequence stator voltage.

[0029] A2) The equivalent impedance matrix of the system under a single-phase-to-ground short-circuit fault is calculated using the following formula:

[0030]

[0031] in, and These are the positive-sequence network-side impedance and the transmission line impedance, respectively. and These are the negative sequence network-side impedance and the transmission line impedance, respectively. and These are the zero-sequence network-side impedance and the transmission line impedance, respectively, Z fZ1 and Z3 are the grounding impedances; Z2 and Z4 are the positive-sequence equivalent impedances and negative-sequence equivalent impedances, respectively; Z1 and Z3 are the negative-sequence coupling impedances and positive-sequence coupling impedances, respectively. For Z i The impedance angle of (i = 1, 2, 3, 4) and They are respectively and impedance angle, and These are the initial angles of the positive and negative equivalent work angles, respectively; X 2×2 For the equivalent reactance matrix, R 2×2 This is the equivalent resistance matrix.

[0032] A3) Calculate the positive-sequence active current output of a single doubly-fed induction generator (DFIG) grid-connected system during a grid asymmetric fault using the following formula. and negative sequence active current

[0033]

[0034] in, The positive sequence grid voltage is given by K1 and K2 respectively. Equivalent drop depth in positive and negative sequence synchronous reference coordinate systems. and These are the phase angles of K1 and K2, respectively.

[0035] During asymmetrical grid faults, the transient stability control of the doubly fed wind turbine grid-connected system can be achieved by optimizing the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system.

[0036] This invention optimizes the positive and negative sequence active and reactive currents output by a doubly-fed induction generator (DFIG) wind turbine grid-connected system during asymmetrical grid faults, so that the stable equilibrium point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state. This minimizes the unbalanced transient energy accumulated by the system during the quasi-steady state, thereby significantly improving the transient stability of the DFIG wind turbine grid-connected system during asymmetrical grid short-circuit faults and reducing the risk of transient instability.

[0037] Description of the effects of this invention:

[0038] Figure 2 and Figure 3 This paper compares the simulation results of the traditional control scheme and the control scheme proposed in this invention under a two-phase-to-ground short-circuit fault. Figure 2 and Figure 3 In the middle, a two-phase ground fault occurred on the 220kV AC transmission line at 1.5s, causing U fabThe voltage dropped to 0.25 pu. During the quasi-steady-state period, the unit first provides positive and negative sequence reactive current support to the grid according to the grid connection guidelines, and the remaining converter current capacity provides positive sequence active current support to the grid. Under this operating condition, the system experienced transient instability, such as... Figure 2 As shown. (Through) Figure 3 It can be seen that when the control strategy proposed in this invention is adopted, the DC voltage and positive and negative sequence equivalent power angles of the system can quickly recover transient stability during the quasi-steady state. Therefore, the transient stability control method for a doubly-fed induction generator (DFIG) single-unit grid-connected wind power system under asymmetrical grid faults proposed in this invention can effectively improve the transient stability of the system under asymmetrical short-circuit faults, reduce the risk of transient instability, and enhance the system's safe and stable operation capability.

[0039] 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 transient stability control method for a doubly-fed induction generator (DFIG) wind power unit grid-connected system under asymmetrical grid faults, characterized in that... The specific steps are as follows: A1) Calculate the positive-sequence reactive current output of a single doubly-fed wind turbine connected to the grid during a grid asymmetric fault using the following formula. and negative sequence reactive current Among them, K + K is the dynamic positive sequence reactive current proportionality coefficient of the wind farm; - I is the dynamic negative sequence reactive current proportionality coefficient of the wind farm; N This is the rated current of the wind farm. This is the effective value of the positive sequence stator voltage. This represents the effective value of the negative sequence stator voltage. A2) The equivalent impedance matrix of the system under a single-phase-to-ground short-circuit fault is calculated using the following formula: in, and These are the positive-sequence network-side impedance and the transmission line impedance, respectively. and These are the negative sequence network-side impedance and the transmission line impedance, respectively. and These are the zero-sequence network-side impedance and the transmission line impedance, respectively, Z f Z1 and Z3 are the grounding impedances; Z2 and Z4 are the positive-sequence equivalent impedances and negative-sequence equivalent impedances, respectively; Z1 and Z3 are the negative-sequence coupling impedances and positive-sequence coupling impedances, respectively. For Z i The impedance angle of (i = 1, 2, 3, 4) and They are respectively and The impedance angle, δ1 + and δ1 - These are the initial angles of the positive and negative equivalent work angles, respectively; X 2×2 R is the equivalent reactance matrix. 2×2 This is the equivalent resistance matrix; A3) Calculate the positive-sequence active current output of a single doubly-fed induction generator (DFIG) grid-connected system during a grid asymmetric fault using the following formula. and negative sequence active current in, The positive sequence grid voltage is given by K1 and K2 respectively. Equivalent drop depth in positive and negative sequence synchronous reference coordinate systems and The phase angles of K1 and K2 are respectively; During asymmetrical grid faults, the transient stability control of the doubly fed wind turbine grid-connected system can be achieved by optimizing the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system.

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