As a new emerging approach, Virtual Synchronous Generator (VSG) control for interfacing inverters in renewable distributed generation (DG)-based microgrids has attracted significant research interest. However, VSG control cannot ensure proportional reactive power sharing among DG units when unequal transmission line impedances are involved, although active power sharing remains proportional among DGs. In this paper, an adaptive virtual impedance-based VSG control approach for grid-connected and islanded microgrids is proposed to alleviate impedance difference at the inverter output and improve proportional reactive power sharing among DGs. The virtual impedance consists of an adaptive virtual resistance and a fixed virtual inductance, where the virtual resistance is designed adaptively based on operation points of the microgrid. In this paper, the proposed adaptive virtual impedance control is implemented on a modified VSG control, known as Fuzzy secondary controller (FSC)-based VSG control, offering excellent voltage and frequency regulation at the PCC. Its performance is demonstrated through case and sensitivity studies using MATLAB/Simulink simulation; and further validated by comparing with an existing method.

With increasing penetration of wind power, it is recognized that manufacturer-specific models of wind turbines are not appropriate for power system studies, and the equivalent model of wind farms required by dynamic modeling poses a complicated technical challenge. In this paper, a practical equivalent method for wind farms with doubly-fed induction generator (DFIG) wind turbines is proposed. Three critical steps are involved in this method. Step 1, modeling of a wind turbine, the model incorporating the complete fault ride through process is constructed and validated for an operational DFIG wind turbine. Step 2, clustering of wind turbines, a practical four-machine equivalent method is proposed. Initially, dynamic responses of active power of a DFIG wind turbine operating at various wind speeds are investigated to obtain three clusters of wind turbines, then four clusters are further determined by improving equivalency error at some typical wind speed scenarios. Step 3, an analytical method is proposed to determine the equivalent collector network parameters for each of the four equivalent machines. The proposed method is validated by comparing with the single-machine representation and the detailed model. Simulation results show that the proposed method has good performance for different fault durations, voltage dips and changing wind scenarios.

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected offshore wind farms could suffer serious challenges: DC overvoltage during a fault and post-fault DC voltage restoration. In this paper, to mitigate DC overvoltage, a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction, by coordinating with a voltage-dependent active current control of individual wind turbines. The voltage drop depths are designed by considering the offshore de-loading efficiency and the stability of WTs. To improve the post-fault DC voltage recovery performance, an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power. The recovery trajectory of DC voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs. Case studies have been conducted on two test systems to validate the proposed method.

An improved single-machine equivalent method is proposed for wind power plants (WPPs) by calibrating the post-fault power recovery behaviors. A real WPP is simulated with extensive wind scenarios to evaluate the traditional single-machine equivalent model, and it is found that its equivalent error is mainly resulted from the post-fault recovery process. The simulation analysis further indicates that a wind turbine operating at different wind speeds restores to its pre-fault active power at a certain ramp rate after fault clearance, with different starting power and different recovery time. Taking a two-machine WPP as an example, the analytical expression for active power of the WPP is derived for the fault ride through process, and thus the equivalent error of traditional single-machine model is found to be resulted from two aspects. One is the mismatch of the starting power of the post-fault recovery process, and another is the mismatch of the power recovery rates between the complete WPP and its equivalent model. The analytical expressions are extended to multi-machine WPP to calibrate the starting power and the ramp rates of the post-fault recovery process. Simulation results show that the proposed method has good performance for various voltage drops, wind scenarios and grid characteristics.

In this paper, an analytical two-machine equivalent method of DFIG-based wind power plants (WPPs) is proposed. Through simulations of a 49.5 MW WPP, the root causes of errors of the single-machine equivalent are identified. Taking a four-machine WPP as an example, analytical expressions of fault-ride through (FRT) behaviors of the WPP model and its single-machine equivalent model are derived, and root causes of errors are revealed in theory including: inconsistent transient power behaviors in the initial FRT stage; unequal quasi-steady-state active power during a fault; and mismatching initial points and ramp rates of active power after the fault clearance. To eliminate equivalent errors, an improved equivalent method is proposed with two steps: 1) Group wind turbines into two clusters by the crowbar actions and represent them as two equivalent machines; 2) Correct their controller reference settings based on the derived FRT behavior expressions of the detailed WPP. Simulation results verify that the proposed method can accurately reproduce FRT behaviors of the WPP under various wind scenarios and voltage dips.

This paper proposes advanced control strategies to enhance fault ride-through capability of modular multilevel converter-based high voltage direct current (MMC-HVDC) connected doubly-fed induction generator (DFIG)-based wind farms. A stepwise voltage drop control is proposed for the sending-end MMC to suppress dc line voltage fluctuations by demagnetizing DFIGs. Meanwhile, a modified transient current control is proposed for the DFIG, where a voltage-dependent active current control is integrated into the rotor-side converter to improve synchronization stability of the converter, and a feedforward transient stator current control structure is designed to eliminate dc component in the stator current. The proposed control strategies are validated by comparing with three typical methods through time-domain simulations.