新型无功-电压控制策略提升不对称短路故障下GFM-GCI系统暂态同步稳定性

///'In recent years, new energy sources such as photovoltaic and wind power have been connected to the grid through grid-connected inverters (GCIs), leading to an increase in the penetration rate of power electronic devices in the power grid and a decrease in the dominance of synchronous generators (SGs) [1-4]. Traditional grid-following grid-connected inverters (GFL-GCIs) are characterized by a controlled current source with a high parallel impedance, which requires sampling of grid voltage at the connection point and synchronization with the grid through a phase-locked loop (PLL) [5-6]. The massive integration of new energy generation equipment into the grid through GFL-GCIs poses a serious threat to the safe and stable operation of the power system due to the decreasing grid strength caused by the increasing transmission line distances [7-9]. In order to improve the stability of power system frequency and voltage, grid-forming grid-connected inverters (GFM-GCIs) have been proposed by the academic and industrial communities. GFM-GCIs are characterized by a controlled voltage source with a low series impedance and adopt power self-synchronization control [10-13]. Under weak grid conditions, GFM-GCI systems have greater stability margin and can operate in island mode, independent of the main grid. Although GFM-GCIs have different control architectures, they all have the ability to establish AC output voltage without relying on grid voltage. Therefore, GFM-GCI systems can provide transient voltage and frequency support for high-proportion power electronic power systems and have attracted wide attention [14-16].//n//nCurrently, most research on the stability of GFM-GCI systems is based on steady-state operating conditions of the grid [17]. However, when the grid experiences major disturbances or faults, the system may experience transient instability similar to synchronous generators (SGs) [18]. In severe cases, GFM-GCIs may even actively disconnect from the grid or degrade into GFL-GCIs to avoid overvoltage and overcurrent, thus losing the advantages of GFM-GCI systems. Reference [19] proposed a current limitation strategy for GFM-GCI systems based on mode smooth switching, which switches the GFM-GCI to GFL-GCI mode during faults to achieve rapid fault limitation. However, the system cannot provide necessary frequency and voltage support to the grid during faults. With the increasing penetration rate of power electronic devices in the grid, GFM-GCIs should have strong fault adaptability to achieve grid-forming control throughout the entire process and provide necessary transient voltage and frequency support to the grid.//n//nCurrently, there have been many analyses and discussions on the transient synchronization stability of GFM-GCI systems during symmetrical short-circuit faults in the grid. Reference [20] used the Lyapunov direct method to analyze the influence of damping coefficient and reactive power droop coefficient on the transient stability of virtual synchronous generator (VSG) systems, and pointed out that the reactive power-voltage droop loop would reduce the rotor angle stability margin of VSG systems. Reference [21] analyzed the transient stability of VSG systems under large disturbances using the equal area method and proposed a mode adaptive power angle control method, which improves the transient stability of the system by controlling the switching between positive and negative feedback of the active power loop. However, this reference did not analyze the influence of damping coefficients on system stability, and the analysis results were conservative. Reference [22] used the phase plane method to analyze the transient synchronization stability of GFM-GCI systems under four control modes, and quantitatively analyzed the influence of control parameters on system stability, providing reference for the parameter design of GFM-GCI systems. Reference [23] considered current limiting during transmission line faults and voltage support after fault clearance, and proposed a two-stage synchronization control scheme to improve the transient stability of VSGs. Reference [24] studied the influence of reactive power-voltage loop on the transient stability of VSG systems in detail using the equal area method, and proposed an improved reactive power-voltage control structure, which improves the transient stability of the system during faults. The above research mainly studied the influence of active and reactive power loops on the transient stability of GFM-GCI systems during symmetrical short-circuit faults in the grid using the phase plane method, equal area method, and Lyapunov function method, and proposed some improved control strategies for the active and reactive power loops. However, in actual power systems, the probability of occurrence of asymmetrical short-circuit faults is greater, and during asymmetrical short-circuit faults, the positive sequence network and negative sequence network are interconnected, and there is coupling between the positive sequence component and the negative sequence component, leading to a more complex transient synchronization instability mechanism of GFM-GCI systems.//n//nTherefore, in order to investigate the transient synchronization stability of GFM-GCI systems during asymmetrical fault conditions in the grid, this paper first proposes a novel reactive power-voltage control strategy suitable for asymmetrical short-circuit faults, which can meet the requirements of positive and negative sequence reactive current injection during faults without switching control strategies. Based on this, a mathematical model of GFM-GCI systems under asymmetrical short-circuit faults in the grid is established. Secondly, based on the composite sequence network of the system, the positive and negative sequence power angle characteristic curves are obtained, and the transient synchronization stability of the positive and negative sequence systems is further analyzed using the equal area method. A stability criterion for evaluating system stability margin is proposed. Finally, the theoretical analysis is verified through simulation and hardware-in-the-loop experiments.///