This paper addresses the formulation problem of a power flow for controlled islanded microgrids including battery energy storage systems. The formulation considers droop-based primary controllers, probability-based wind distributions and power curves for the distributed generation units, and sigmoid-based models for the BESS operation and the load shedding. Two case studies are addressed where different BESS conditions are selected to highlight its role in the islanded microgrid, and its impact in the microgrid operative steady-state dynamics. The obtained results corroborate the proposed power flow formulation reliability for representing the controlled islanded microgrid and its usefulness for detailed operative planning and design.

For multibus wind power plants in microgrids, it is challenging to develop a reliable, effective, and robust harmonic suppression method for harmonic voltages and currents of all buses. This paper proposes a hierarchical harmonic control method to mitigate the harmonic voltages and currents of all buses in grid-forming wind power plants. The proposed method effectively reduces the terminal harmonic voltages and currents, PCC harmonic voltage, and grid harmonic current. It achieves adaptive harmonic mitigation and automatic harmonic current sharing between wind turbine units according to the rated capacity and feeder impedance of each unit. The effectiveness of the method is verified in the planned Turkish offshore wind power plant by simulation in Digsilent/PowerFactory.

The virtual synchronous generator (VSG) has been extensively applied for the integration of distributed generators. For the benefit of analysis, both the reduced-order small-signal model (RSM) and full-order small-signal model (FSM) have been proposed; however, because of the different modeling approaches, the modeling accuracy may differ a lot. To provide a guideline for selecting a suitable model, an accuracy assessment has been conducted in this paper, where different grid strength cases and equilibrium points are considered. More specific, the eigenvalue analysis is adopted for comparing the dominant dynamics of the RSM and FSM. Afterwards, the responses of the RSM and FSM are compared with the nonlinear model (NM) in the Digsilent/Powerfactory to show the modeling accuracy. It has been found that the RSM is mainly suitable for the weak and relatively strong grids, while the FSM shows better accuracy in the case of an extremely strong grid and around the equilibrium point that is close to VSG's rated output.

In order to emulate the frequency support characteristics of conventional synchronous generators (SGs), control algorithms based on the virtual synchronous generator (VSG) have been extensively applied to grid-tied converters. However, in the case that a large inertia is used in VSGs, active power oscillations can be introduced in the transients, due to the lack of enough damping effects. To avoid this phenomena, various solutions with additional damping terms have been proposed. This paper analyzes and compares six relevant fixed- parameter active-power-oscillation damping solutions in detail, where responses under set point and phase angle changes, the parameter sensitivity, and capabilities of limiting the rate of change of frequency (RoCoF) and attenuating power ripples are included. The results show the advantages and drawbacks of each solution, leading to the conclusion that extra damping terms may significantly degrade the inertial response of VSGs and have high dependency on the parameter estimation accuracy.

Wind turbine converters (WTCs) with grid forming (GFM) control can contribute to voltage and frequency support in the power system, by emulating synchronous machine (SM) dynamics, provided that there are sufficient buffers. Over the past few years, several control algorithms have been developed for the GFM operation of WTCs. This article investigates the state-of-the-art in the field of GFM control algorithms for WTCs, where the principle of each control algorithm is discussed in detail, and corresponding characteristics are summarized. Comparison results with respect to voltage build-up strategies, the synchronization of GFM WTCs, the interaction attenuation, the virtual inertia realization and the complexity of individual GFM algorithm are given in a table, which leads to the conclusion that mitigation of oscillations among WTCs, appropriate inertia emulation, and coordination with energy storage systems need to be further investigated in the future GFM WTCs.