Transient stability control method for doubly-fed wind power single-machine grid-connected system under asymmetric fault of power grid

Abstract

The invention discloses a transient stability control method for a doubly-fed wind power single-machine grid-connected system under an asymmetric fault of a power grid, and the method comprises the steps: calculating positive and negative sequence active and reactive current outputted by the doubly-fed wind power single-machine grid-connected system during the asymmetric fault of the power grid, and optimizing the positive and negative sequence active and reactive current outputted by the doubly-fed wind power single-machine grid-connected system, therefore, transient stability control of the doubly-fed wind power single-machine grid-connected system under the asymmetric fault of the power grid is realized. According to the method, the requirement of a power grid guide rule is considered, and on the basis of not increasing hardware equipment, the stable equilibrium point of the positive and negative sequence equivalent power angle of the system coincides with the initial operation point in the quasi-steady-state period, so that unbalanced transient energy accumulated in the quasi-steady-state period of the system is reduced to the maximum extent, and the probability of transient instability of the system is reduced.

Description

Technical Field

[0001] The present invention relates to a transient stability control method for a doubly-fed wind turbine single-machine grid-connected system under an asymmetric grid fault, which is suitable for improving the transient stability operation capability of a doubly-fed wind turbine single-machine grid-connected system under an asymmetric short-circuit fault condition in an AC grid, and belongs to the field of new energy power transmission and distribution technology. Background Art

[0002] The large-scale development and long-distance transmission of renewable energy, while increasing the absorption capacity of renewable energy, also makes the power system extremely susceptible to asymmetric short-circuit faults. Due to the weak damping and inertia support capabilities of power electronic converters, phase-locked synchronous renewable energy power generation equipment using power electronic converters as grid-connected interfaces are prone to transient instability problems such as transient loss of step, cascading failures, and even grid disconnection during asymmetric short-circuit faults in the grid. In particular, for doubly-fed induction generators (DFIGs) whose stators are directly connected to the grid, the negative sequence components caused by asymmetric short-circuit faults in the grid are more likely to invade the main circuit and control links of the power generation equipment, causing transient instability in the DFIG wind power grid-connected system due to excessive DC voltage, electromagnetic torque, current, and other multiple stresses. Therefore, it is urgent to propose a transient stability control method for doubly-fed wind power grid-connected systems under asymmetric grid faults to improve the transient stability of the system during the fault period, thereby enhancing the system's transient stability operation capability. At present, scholars at home and abroad have carried out a series of related studies, such as the following published documents:

[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] proposed a stable control strategy for a doubly-fed wind turbine grid-connected system under asymmetric short-circuit faults based on the idea of balanced current control, achieving flexible control of multiple objectives. However, this control strategy mainly focuses on the support of the system's positive-sequence reactive current during the fault period, and it is difficult to fully meet the requirements of the existing grid-connected guidelines for negative-sequence reactive current support. Reference [2] proposed a stable control strategy for a renewable energy power generation system under asymmetric faults on the basis of meeting the requirements of the existing grid-connected guidelines. However, this control strategy mainly focuses on the small-disturbance stability control problem of the renewable energy power generation system, and the adaptability of the proposed control strategy in system transient stability control needs further exploration. Summary of the Invention

[0006] In response to the above-mentioned deficiencies in the prior art, the present invention proposes a transient stability control method for a doubly-fed wind turbine single-machine grid-connected system under asymmetric grid faults. This method takes into account the requirements of the grid guidelines and optimizes the positive and negative sequence active and reactive currents output by the doubly-fed wind turbine single-machine grid-connected system during asymmetric grid faults without adding hardware equipment, so that the stable balance point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state period, thereby minimizing the unbalanced transient energy accumulated in the system during the quasi-steady state period and reducing the probability of transient instability in the system.

[0007] The technical solution of the present invention is achieved as follows:

[0008] A transient stability control method for a doubly-fed wind turbine single-unit grid-connected system under asymmetric grid fault conditions is provided, and the specific steps are as follows:

[0009] A1) Calculate the positive sequence reactive current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault according to the following formula: and negative sequence reactive current

[0010]

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

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

[0013]

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

[0015] A3) Calculate the positive sequence active current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault by the following formula: and negative sequence active current

[0016]

[0017] in, is the positive sequence grid voltage, K1 and K2 are The equivalent drop depth in the positive and negative sequence synchronous reference frames, and are the phase angles of K1 and K2 respectively;

[0018] During asymmetric grid faults, the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system are optimized, thereby achieving transient stability control of the doubly fed wind turbine grid-connected system under asymmetric grid faults.

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

[0020] This method takes into account the requirements of the grid guidelines and optimizes the positive and negative sequence active and reactive currents output by the doubly fed wind turbine single-machine grid-connected system during asymmetric grid faults without adding hardware equipment, so that the stable balance point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state period, thereby minimizing the unbalanced transient energy accumulated in the system during the quasi-steady state period, and thus significantly improving the transient stability of the doubly fed wind turbine single-machine grid-connected system during asymmetric grid short-circuit faults and reducing the risk of transient instability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the active and reactive current optimization control flow chart of a doubly-fed wind turbine single-machine grid-connected system under asymmetric grid faults.

[0022] Figure 2 This is the simulation result diagram when using the traditional control scheme under a two-phase ground short circuit fault.

[0023] Figure 3 This is a simulation result diagram when the control scheme proposed in the present invention is used under a two-phase ground short circuit fault. DETAILED DESCRIPTION

[0024] The present invention is used to improve the transient stability of a double-fed wind power single-machine grid-connected system during an asymmetric short-circuit fault in the power grid. Figure 1 This is a flow chart for optimizing active and reactive current control in a doubly-fed wind turbine grid-connected system under asymmetric grid fault conditions. By optimizing the positive and negative sequence active and reactive currents output by the system, this method can minimize the unbalanced transient energy accumulated during the quasi-steady state, thereby improving the system's transient stability.

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

[0026] A1) Calculate the positive sequence reactive current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault according to the following formula: and negative sequence reactive current

[0027]

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

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

[0030]

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

[0032] A3) Calculate the positive sequence active current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault by the following formula: and negative sequence active current

[0033]

[0034] in, is the positive sequence grid voltage, K1 and K2 are Equivalent drop depth in the positive-sequence and negative-sequence synchronous reference frames. and are the phase angles of K1 and K2 respectively.

[0035] During asymmetric grid faults, the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system are optimized, thereby achieving transient stability control of the doubly fed wind turbine grid-connected system under asymmetric grid faults.

[0036] The present invention optimizes the positive and negative sequence active and reactive currents output by the doubly fed wind turbine single-machine grid-connected system during an asymmetric grid fault, so that the stable balance point of the system's positive and negative sequence equivalent power angles coincides with the initial operating point during the quasi-steady state period, thereby minimizing the unbalanced transient energy accumulated in the system during the quasi-steady state period, and further significantly improving the transient stability of the doubly fed wind turbine single-machine grid-connected system during an asymmetric grid short-circuit fault, thereby reducing the risk of transient instability of the system.

[0037] Effect description of the present invention:

[0038] Figure 2 and Figure 3 The simulation results of the traditional control scheme and the control scheme proposed by the present invention under two-phase ground short circuit fault are compared. Figure 2 and Figure 3 In the 220kV AC transmission line, a two-phase ground fault occurred at 1.5s, causing U fabDuring the quasi-steady state, 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 condition, the system has transient instability, such as Figure 2 As shown. Figure 3 It can be seen that when the control strategy proposed in this invention is adopted, the system's DC voltage and positive and negative sequence equivalent power angle can quickly restore transient stability during the quasi-steady state. Therefore, the transient stability control method for a doubly-fed wind turbine single-unit grid-connected system under asymmetric grid faults proposed in this invention can effectively improve the system's transient stability under asymmetric grid short-circuit faults, reduce the system's transient instability risk, and enhance the system's safe and stable operation capabilities.

[0039] Finally, it should be noted that the above examples of the present invention are merely illustrative of the present invention and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art will appreciate that other variations and modifications can be made based on the above description. It is not possible to enumerate all embodiments here. Any obvious variations or modifications derived from the technical solution of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for transient stability control of a doubly-fed wind turbine single-unit grid-connected system under asymmetric grid faults, characterized in that: The specific steps are as follows: A1) Calculate the positive sequence reactive current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault according to the following formula: and negative sequence reactive current Among them, K + K is the wind farm dynamic positive sequence reactive current proportional coefficient; - is the wind farm dynamic negative sequence reactive current proportional coefficient; I N is the rated current of the wind farm, is the positive sequence stator voltage effective value, is the effective value of the negative sequence stator voltage; A2) The equivalent impedance matrix of the system under a single-phase ground short-circuit fault is calculated using the following formula: in, and are the positive sequence grid side impedance and the transmission line impedance respectively, and are the negative sequence grid side impedance and the transmission line impedance respectively, and are the zero-sequence grid-side impedance and the transmission line impedance, Z f is the ground impedance; Z1 and Z3 are positive sequence equivalent impedance and negative sequence equivalent impedance respectively, Z2 and Z4 are negative sequence coupling impedance and positive sequence coupling impedance respectively; Z i The impedance angle of (i=1,2,3,4) and They are and The impedance angle, δ1 + and δ1 - are the initial angles of the positive and negative sequence equivalent power angles respectively; X 2×2 is the equivalent reactance matrix, R 2×2 is the equivalent resistance matrix; A3) Calculate the positive sequence active current output by the doubly-fed wind turbine grid-connected system during an asymmetric grid fault by the following formula: and negative sequence active current in, is the positive sequence grid voltage, K1 and K2 are The equivalent drop depth in the positive and negative sequence synchronous reference frames, and are the phase angles of K1 and K2 respectively; During asymmetric grid faults, the positive and negative sequence active and reactive currents output by the doubly fed wind turbine grid-connected system are optimized, thereby achieving transient stability control of the doubly fed wind turbine grid-connected system under asymmetric grid faults.

Images