In a distributed generation system, the all-pass-filter phase-locked loop (APF-PLL) is a commonly used method for grid synchronization. However, the coupling effect between APF-PLL and current control loop increases the risk of oscillation instability for the inverter in the weak grid. At present, there are few effective methods to solve the adverse effect of APF-PLL on the inverter-grid interconnection system in the weak grid. Therefore, a small-signal impedance model of the inverter considering the dual d-q frame brought by APF-PLL is first established. Then the reason for the inverter instability caused by APF-PLL in the weak grid is analyzed. Subsequently, an impedance reshaping method based on a modified first-order filter PLL with a complex coefficient filter (CCF-MFOF-PLL) and its parameter optimization design method are proposed. Finally, the experimental results verify that the proposed method widens the stable range of the inverter and ensures the stable operation of the inverter even with the large grid impedance.

In some key areas, fault-tolerant control is usually needed in order to enable the motor to operate continuously in fault mode. Given that it is difficult to detect the zero-sequence current of the open winding permanent magnet synchronous motor after the phase break fault occurs, the traditional zero-sequence current suppression strategy is no longer applicable after the phase break fault occurs. Therefore, a zero-sequence current suppression strategy for a common DC bus under a phase break fault is proposed in this paper. By establishing the mathematical model between the current component in the synchronous coordinate system and the current component and the zero-sequence current in the static coordinate system, the relationship between the non-fault phase current and the zero-sequence current in the open phase fault is analyzed. A method of suppressing the zero-sequence current by using proportional integral double resonance in a zero-sequence current control loop is proposed. In addition, according to the large number of calculations in traditional space vector modulation (SVPWM)—such as sector judgment and coordinate transformation—a decoupling modulation algorithm is proposed to modulate the reference voltage vector. Finally, the experimental platform for the common DC bus open winding permanent magnet synchronous motor is built, and the zero-sequence current suppression method for the common DC bus OW-PMSM under phase break fault is verified experimentally.

In distributed generation systems, the inverter is the main power interface and its stability directly determines the reliable operation of the grid-connected system. As a typical topology for a three-phase four-wire inverter, the LCL-type three-phase four-wire split capacitor inverter (LCL-TFSCI) is taken as the research subject of this paper. Compared with the three-phase three-wire inverter, there is an additional zero-sequence path in the LCL-TFSCI. Therefore, it is not only necessary to consider the stability of the positive and negative sequence system, but there is also the need to consider the stability of the zero-sequence system when performing stability analysis for the LCL-TFSCI. In this paper, a small-signal impedance model considering the zero-sequence loop of LCL-TFSCI is firstly established. Subsequently, the instability risk is revealed when LCL-TFSCI is connected to the grid with parallel compensation capacitors. Through instability analysis, an impedance-reshaping method based on the complex filter and combined differential elements is proposed, which can reshape the impedance characteristic of LCL-TFSCI within the wide frequency range and expand the stability domain of the grid-connected system. Finally, the proposed method is verified by simulation and experiment.

With the large-scale integration of grid-forming wind turbines via LCC-HVDC transmission, the large-disturbance stability of such systems has become a critical concern due to complex interactions among multiple power electronic converters. This paper proposes a unified stability analysis framework based on the mixed potential function theory for a VSG-DFIG fed LCC-HVDC system under AC voltage sags at the point of common coupling. By incorporating the physical dynamics and constant-current control of the rectifier, and equivalently representing the PI controllers of the VSG-DFIG as RLC circuits, an equivalent circuit model of the entire system is established. The large-signal stability criterion is then derived, which explicitly reveals the influences of control parameters, circuit parameters, and the depth of AC voltage sags on the stability boundary. To enhance system stability, a virtual resistor strategy is introduced in the LCC-HVDC control loop. Finally, time-domain simulations in Simulink are carried out, and the results fully validate the theoretical analysis as well as the effectiveness of the proposed virtual resistor scheme.

Doubly fed induction generators (DFIGs) may suffer from excessive rotor deceleration during frequency support and secondary frequency drop (SFD) during rotor speed recovery. To address these issues, this paper proposes a two-stage frequency response strategy for DFIGs based on adaptive speed–frequency coordinated optimization. In the frequency support stage, the strategy simultaneously considers the initial rotor speed and real-time speed, thereby dynamically characterizing the remaining kinetic energy of the wind turbine and its release process. This allows the turbine to improve the system frequency secondary frequency drop and extend the effective support time under high-wind-speed conditions, while actively limiting support intensity under low-wind-speed conditions. In the rotor speed recovery stage, a DFIG rotor speed recovery method based on dynamic adaptive adjustment of recovery time is proposed. This method converts the conventional Rate of Change of Frequency (RoCoF) security constraint into a ramp rate constraint on the withdrawal of wind power and, combined with the turbine’s kinetic energy recovery capability, iteratively determines the recovery time online. This coordination ensures that the power withdrawal process aligns with the primary frequency regulation response of synchronous generators, thereby reducing the risk of SFD. Simulation results validate the effectiveness of the proposed strategy.