A method for transient stability control of a multi-machine parallel system of double-fed wind power under asymmetric fault of power grid

Abstract

This invention discloses a transient stability control method for a doubly-fed induction generator (DFIG) wind turbine parallel system under asymmetrical grid faults. First, the positive and negative sequence reactive currents output by each unit in the DFIG wind turbine parallel system are calculated during the asymmetrical grid fault. Then, the equivalent impedance matrix of the system under a single-phase ground fault is calculated. Finally, the positive and negative sequence active currents output by each unit in the DFIG wind turbine parallel system are calculated during the asymmetrical grid fault. This method considers the requirements of grid guidelines and optimizes the positive and negative sequence active and reactive currents output by each unit in the DFIG wind turbine parallel system during asymmetrical grid faults without adding hardware. This ensures that the stable equilibrium point of the equivalent power angle of each unit coincides with the initial operating point during the quasi-steady state, thereby minimizing the unbalanced transient energy accumulated by each unit during the quasi-steady state and reducing the probability of transient instability in the system.

Description

Technical Field

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

[0002] With the increasing penetration of power electronic renewable energy generation equipment in power systems, significant changes have occurred in the operation and control of power systems. Because power electronic converters lack the inertia and damping characteristics of traditional synchronous machines, the adaptability of grid-connected renewable energy generation equipment to asymmetrical short-circuit faults in the power grid is weakened. Secondly, due to the mutual coupling of positive and negative sequence impedances during asymmetrical faults, the interaction between renewable energy generation equipment and the grid becomes increasingly intense, making the renewable energy generation equipment highly susceptible to transient loss of synchronization under asymmetrical short-circuit faults, resulting in large-scale grid disconnection of renewable energy generation equipment and seriously threatening the safe and stable operation of the power system. Therefore, it is urgent to propose an active and reactive power optimization control method for doubly-fed induction generator (DFIG) wind power multi-unit parallel systems under asymmetrical 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] Wang Jilei, Zhang Xing. Transient stability analysis and transient current injection strategy of multi-inverter parallel system [J]. High Voltage Engineering, 2025, 51(02): 390-400.

[0004] [2]NEUMANN T,WIJNHOVEN T,DECONINCK G.et al.Enhanced dynamic voltagecontrol oftype 4wind turbines during unbalanced grid faults[J].IEEETransactions On Energy Conversion,2015,30(4):1650-1659.

[0005] Reference [1] analyzed the existence of equilibrium point in a multi-machine parallel system of new energy sources during grid faults based on the Newton-Raphson method and proposed a current optimization method for the multi-machine system. However, the control strategies in the above literature mainly focus on the transient stability of the multi-machine parallel system of new energy sources during symmetrical grid faults. Unlike symmetrical grid faults, there are also complex dynamic interaction behaviors between the positive and negative sequence components of different power generation equipment in the multi-machine parallel system of new energy sources during asymmetrical short-circuit faults of the grid. This makes it difficult to apply the transient stability and instability criteria and stability control methods of the multi-machine parallel system of new energy sources under symmetrical grid faults to asymmetrical grid faults. Reference [2] analyzed the impact of positive and negative sequence reactive current injection on the transient stability of the new energy power generation system during faults and proposed a stability control strategy for the new energy power generation system under asymmetrical short-circuit faults of the grid. However, the above studies mainly focus on the small-disturbance stability control problem of the new energy power generation system. The adaptability of the proposed control strategies to the transient stability control of the new energy power generation 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 power multi-unit parallel system under grid asymmetric 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 each unit in the DFIG wind power multi-unit parallel system during grid asymmetric faults. This ensures that the stable equilibrium point of the positive and negative sequence equivalent power angles of each unit coincides with the initial operating point during the quasi-steady state, thereby minimizing the unbalanced transient energy accumulated by each unit during the quasi-steady state and reducing the probability of system transient instability.

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

[0008] A transient stability control method for a doubly-fed induction generator (DFIG) wind power system with multiple turbines in parallel under asymmetrical grid faults, comprising the following steps:

[0009] A1) Calculate the positive-sequence reactive current output of each unit j (j=1,2,3...n) in a doubly fed wind power multi-unit parallel system 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 represents the effective value of the j-sequence stator voltage of the unit. This represents the effective value of the j-sequence stator voltage of the unit.

[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]

[0015] in, and These are the positive-sequence and negative-sequence network-side impedances, respectively. and These are the transmission line impedances of the positive and negative sequence branches of the unit, 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 sequence equivalent power angles of unit j, respectively; X 2n×2n R is the equivalent reactance matrix. 2n×2n This is the equivalent resistance matrix; and These are the positive-sequence and negative-sequence common transmission line impedances, respectively. and They are respectively and The impedance angle;

[0016] A3) Calculate the positive-sequence active current output of each unit in a doubly-fed wind power multi-unit parallel system during a grid asymmetric fault using the following formula. and negative sequence active current

[0017]

[0018]

[0019] 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 angles of K1 and K2, respectively.

[0020] During asymmetrical grid faults, the transient stability control of the doubly fed wind power multi-unit parallel system can be achieved by optimizing the positive and negative sequence active and reactive currents output by each unit.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] This method takes into account the requirements of the power grid guidelines and optimizes the positive and negative sequence active and reactive currents output by each unit in a doubly-fed induction generator (DFIG) multi-unit parallel system during asymmetrical power grid faults without adding hardware equipment. This ensures that the stable equilibrium point of the positive and negative sequence equivalent power angles of each unit coincides with the initial operating point during the quasi-steady state, thereby minimizing the unbalanced transient energy accumulated by each unit during the quasi-steady state. Consequently, it significantly improves the transient stability of the DFIG multi-unit parallel system during asymmetrical short-circuit faults in the power grid and reduces the risk of transient instability of the system. Attached Figure Description

[0023] Figure 1 This is a control structure diagram of a doubly fed wind power system with multiple turbines in parallel under grid asymmetric fault conditions.

[0024] Figure 2 The figure shows the simulation results of the traditional control scheme under a two-phase-to-ground short-circuit fault.

[0025] Figure 3 The figure shows the simulation results of the control scheme proposed in this invention under a two-phase ground short-circuit fault. Detailed Implementation

[0026] This invention is used to improve the transient stability of a doubly fed wind power system with multiple turbines in parallel during asymmetrical short-circuit faults in the power grid. Figure 1 This diagram illustrates the control structure of a doubly-fed induction generator (DFIG) wind power system with multiple generators operating in parallel under grid asymmetric fault conditions. By optimizing the positive and negative sequence active and reactive currents output by each generator, the unbalanced transient energy accumulated during the quasi-steady-state period can be minimized, thereby improving the system's transient stability.

[0027] The specific implementation steps of this invention are as follows.

[0028] A1) Calculate the positive-sequence reactive current output of each unit j (j=1,2,3...n) in a doubly fed wind power multi-unit parallel system during a grid asymmetric fault using the following formula. and negative sequence reactive current

[0029]

[0030] 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; NThis is the rated current of the wind farm. This represents the effective value of the j-sequence stator voltage of the unit. This represents the effective value of the j-sequence stator voltage of the unit.

[0031] A2) The equivalent impedance matrix of the system under a single-phase-to-ground short-circuit fault is calculated using the following formula:

[0032]

[0033]

[0034] in, and These are the positive-sequence and negative-sequence network-side impedances, respectively. and These are the transmission line impedances of the positive and negative sequence branches of the unit, 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 sequence equivalent power angles of unit j, respectively; X 2n×2n R is the equivalent reactance matrix. 2n×2n This is the equivalent resistance matrix; and These are the positive-sequence and negative-sequence common transmission line impedances, respectively. and They are respectively and The impedance angle.

[0035] A3) Calculate the positive-sequence active current output of each unit in a doubly-fed wind power multi-unit parallel system during a grid asymmetric fault using the following formula. and negative sequence active current

[0036]

[0037] 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 angles of K1 and K2, respectively.

[0038] By optimizing the positive and negative sequence active and reactive current output of each unit in a doubly fed wind power system during asymmetrical grid faults, the stable equilibrium point of the equivalent power angle of each unit 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 doubly fed wind power system during asymmetrical grid short-circuit faults and reducing the risk of transient instability.

[0039] Description of the effects of this invention:

[0040] Figure 3 and Figure 2 This paper compares the simulation results of the proposed control scheme and the traditional control scheme under a two-phase-to-ground short-circuit fault. The system experiences a two-phase short-circuit-to-ground fault at 1.5 seconds. U fab The value dropped to 0.25 pu. After the Crowbar circuit was disconnected, when each unit used the traditional control strategy to provide the maximum positive and negative sequence reactive current support to the grid, it was through... Figure 2 As can be seen, transient instability occurred in units 1-3 of the system during the fault. Under the same operating conditions, the current commands of each unit were optimized according to the control method proposed in this invention. Figure 3 As can be seen, compared with the traditional control strategy, after adopting the control strategy proposed in this invention, the positive and negative sequence transient imbalance energy of each unit during the quasi-steady state is significantly reduced, and the DC bus voltage and positive and negative sequence equivalent power angle of units 1-3 in the system can quickly recover to transient stability during the fault. Therefore, the stability control strategy proposed in this invention can effectively improve the transient stability of the system's DC bus voltage and reduce the risk of transient instability.

[0041] 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 system with multiple turbines operating in parallel under asymmetrical grid faults, characterized in that... The specific steps are as follows: A1) Calculate the positive-sequence reactive current output of each unit j (j=1,2,3...n) in a doubly fed wind power multi-unit parallel system 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 represents the effective value of the j-sequence stator voltage of the unit. This represents the effective value of the j-sequence stator voltage of the unit. 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 and negative-sequence network-side impedances, respectively. and These are the transmission line impedances of the positive and negative sequence branches of the unit, 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 sequence equivalent power angles of unit j, respectively; X 2n×2n R is the equivalent reactance matrix. 2n×2n This is the equivalent resistance matrix; and These are the positive-sequence and negative-sequence common transmission line impedances, respectively. and They are respectively and The impedance angle; A3) Calculate the positive-sequence active current output of each unit in a doubly-fed wind power multi-unit parallel 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 These are the angles of K1 and K2, respectively. During asymmetrical grid faults, the transient stability control of the doubly fed wind power multi-unit parallel system can be achieved by optimizing the positive and negative sequence active and reactive currents output by each unit.

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