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.

This paper analyzes a 10MW medium voltage Back-to-Back (BTB) Modular Multilevel Converter (MMC) without a DC-Link capacitor with half-bridge submodules. It focusses on the system behavior under single-line-to-ground (SLG) fault when there is no capacitor on the DC-Link. The fault current is computed numerically and compared with the simulated results. The converter control behaviour with regards to the injection of the positive sequence currents in the presence of unbalanced terminal voltages has been investigated. Finally, the converter protection using cell level chopper control strategy is presented to prevent DC overvoltages in the sub-modules during faults.

This paper develops a complete control scheme for the Modular Multilevel Converter (MMC) in a STATCOM application. The focus is laid on the internal balancing of the converter and its reliable operation under internal mismatches as well as under external unbalanced conditions. The leg energy controller ensures that the sum of the capacitor voltages of each phase are equal. An intuitive arm energy controller is also implemented which allows the decoupled control of the energy levels of every arm. The control performance of the STATCOM was evaluated under asymmetrical grid faults and the simulation results verified the capability to provide maximum reactive power to the AC grid while maintaining internal balance through the individual control of each arm energy level.

In this paper, an optimal power flow problem is formulated in order to minimize the total operation cost by considering real-time pricing in DC microgrids. Each generation resource in the system, including the utility grid, is modeled in terms of operation cost, which combines the cost-efficiency of the system with the demand response requirements of the utility. By considering the primary (local) control of the grid-forming converters of a microgrid, optimal parameters can be directly applied to the control of this level, thus achieving higher control accuracy and faster response. The optimization problem is solved in a heuristic way by using genetic algorithms. In order to test the proposed algorithm, a six-bus droop-controlled DC microgrid is used as a case-study. The obtained simulation results show that under variable renewable generation, load, and electricity prices, the proposed method can successfully dispatch the resources in the microgrid with lower total operation costs.

In some networks Distributed Generators (DGs) are phasing out conventional power plants in terms of power production but still large efforts are required for providing ancillary services. In this paper the usage of fast response units like a Micro Turbines (MT) and a stationary Electric Vehicle Battery (EVB) is proposed for providing primary regulation in grid connected mode and for hierarchically manage an islanded LV distribution feeder. The unit models are described and a novel EVB model directly based on manufacturer's data is proposed and evaluated comparing its performances with SimPowerSystems library block. Moreover a voltage dependant power term is applied to the Voltage-Source Converter (VSC) control scheme of the EVB for improving the performances of the islanded feeder. The control is tested in case of under frequency and consequent load shedding occurring at the residential feeder of CIGRE C6.04.02 benchmark network.

Modular multilevel converter (MMC) is the latest converter topology suitable for the transformerless applications in HVDC transmission. HVDC systems are required to remain connected during grid faults, provide grid support and completely decouple the healthy side from the faulty one. The MMC converter weak points are challenged by this particular condition and by these demands. This paper demonstrates the effect of negative and zero sequence current control in MMC-HVDC during asymmetric grid faults. A current limitation strategy for MMC is derived and verified through simulations.