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To eliminate low-frequency oscillations, this paper proposes an active-damping method for multiple grid-tied virtual synchronous generators (VSGs) in a power plant. Firstly, using the Lyapunov's indirect method, the damping ratio of multiple VSGs in parallel is analyzed. The average damping ratio reveals that this multi-VSG power plant can be poorly damped in a wide range of inertia and damping settings. Then, self- and mutual-damping controllers are developed to suppress self- and mutually induced low-frequency power oscillations, respectively. For the practical implementation, an adaptive tuning algorithm that enables automatic realization is proposed. Through a reassessment, a remarkable damping-ratio improvement is validated. Moreover, the inertial response improvement is validated by the frequency response analysis. Finally, simulations in Digsilent/PowerFactory and experiments are performed to demonstrate the accuracy of the analyses and the effectiveness of the proposed method.

This paper presents a novel approach of employing hierarchical control to optimize the operation of islanded AC microgrids. The proposed method is an offline, centralized, power-flow-based energy management scheme which includes primary and secondary control dynamics in a modified power-flow formulation. The inner power-flow level maintains bus voltages and system frequency within the desired range and ensures power balance in the network. The outer optimization level ensures that the various components remain within their operational constraints, while optimizing a system-level objective. Two case studies are explored using a modified 14-bus medium voltage (MV) CIGRE benchmark microgrid to validate the proposed energy management algorithm. The first case includes minimization of conventional generator operating cost and renewable energy curtailment, both with linear and non-linear loads, and the second case includes minimization of conventional generator operating cost with load shedding. The results obtained from the case studies show the efficiency of the proposed energy management algorithm, and evidence its reliability for the optimal operation of islanded AC microgrids with multiple renewable energy sources.

The secondary control is applied in islanded Microgrids (MGs), after the primary control, in order to restore the buses voltage and frequency to specified values. The existing power flow methods can accurately calculate the power flow for droop-controlled islanded MGs, but in many cases, they cannot calculate the steady-state solution of the MG after the action of secondary controllers. The main challenge in the steady-state modelling of the secondary layer lies in that the secondary controllers consist of integral parts, which can integrate functions with different integral histories, and therefore, under certain circumstances, can imply inaccurate power sharing between the distributed generation units (DGs). This phenomenon is most pronounced under communication failures, as will be shown in the simulations. In this way, this paper proposes a power flow method for calculating, accurately, the steady-state solution of hierarchically controlled islanded AC MGs, including droop-based primary control and secondary control. The paper includes four main features: a) generalized implementation in several communication strategies e.g., centralized, decentralized, consensus, distributed averaging, b) precise simulation of communication links and integral parts of secondary controllers, c) low computation time, and d) accurate 4-wire network representation. Simulations were executed to validate the proposed method against Simulink and to highlight the importance of an accurate modelling of secondary control in the power flow method for islanded MGs

Due to the high penetration of renewable energy resources in microgrids (MGs), the grid inertia becomes low which leads to the grid to be vulnerable to large disturbances. The energy storage devices can play an important role to enhance the inertia of MGs. However, due to the high investment cost of storages or their dp/dt limitation, the installed energy storages cannot cover the challenge of high df/dt. A prominent solution to solve the problem is to use the inertia response of the wind generators. However, relatively high second frequency nadir is the main drawback of using the inertia response of the wind generators which may impose an extensive disturbance to MGs. Accordingly, a coordinated operation strategy for MGs between wind generator and hybrid energy storage (HES) system is proposed in this paper. In addition, to improve the inertia response of the MG; providing high-quality communication infrastructures with low delay and increasing the Ultracapacitor capacity have been paid attention. In this paper, the costs of the installed Ultracapacitor and quality of communication services are defined as the operation cost. Guaranteeing enough frequency damping for the MG with low operation cost are two conflict objectives. Therefore, a multi-objective optimization method is used to set the controllers’ values and reduce the operation cost. The results confirmed that the effectiveness of the proposed strategy to control hybrid power storage in coordination with the wind generator and the frequency recovery process is improved. Also, employing the optimum values guaranteed the frequency damping effectively with low operation cost. The Integral Absolute Error (IAE) value and operation cost are reduced by 13.6% and 32%, respectively. Also, the simulation results show that the maximum MG frequency deviation and maximum df/dt is well compatible with different standards in the presence of load perturbations and different wind speeds.

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.

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.

The proliferation of renewable energy sources in autonomous microgrids has led to the deterioration of system inertia. In the past decade, the decrease in system inertia was addressed through numerous virtual synchronous generator (VSG) topologies. Synchronverter is one such control technique exhibiting promising performance as that of synchronous machines (SMs). Recently, myriad other modifications were carried out to enhance the capability of the synchronverter for better dynamic response. However, the limitations of fixed virtual inertia and damping coefficient were left untouched, which can be optimized for better frequency regulation of microgrids. Hence, this paper proposes an adaptive synchronverter (ASV) by optimizing OSV parameters through fuzzy logic. Subsequently, the proposed ASV was employed to control a novel pico hydel and battery hybrid energy storage for frequency regulation of the microgrid. The small-signal model of parallel operating ASVs is presented along with the eigenvalue analysis to prove the system stability under parameter variation. The MATLAB/Simulink simulation results revealed that the proposed ASV exhibited a lower rate of change of frequency and frequency nadir compared to the original synchronverter (OSV) and vector control (VC).

This paper presents the power flow modelling for AC/DC hybrid islanded microgrids including droop-controlled distributed generation units, secondary frequency and voltage restoration control for the AC side of the microgrid, and secondary voltage restoration control for the DC side of the microgrid. The interlink converter between the AC and DC microgrids includes a frequency-voltage droop control, and considers the effect of the secondary control for the AC microgrid side. Two case studies are presented for the power flow model evaluation, in the first case a microgrid with linear loads and equal droop characteristic for the distributed generation units are used; in the second case, voltage dependent loads for both AC and DC microgrids are included, and different droop characteristic are chosen for each distributed generation unit. Comparisons between the power flow solutions through the proposed modelling and the professional simulator MATLAB/Simulink are presented. Additionally, the computational speed and convergence rate of the power flow method are shown. The obtained results corroborate the reliability and effectiveness of the proposed power flow modeling to represent the controlled AC/DC hybrid microgrid including hierarchical controllers.

Hybrid AC/DC distribution networks are promising candidates for future applications due to rapid advancement in power electronics technology. They use interface converters (IFCs) to link DC and AC distribution networks. However, they possess drawbacks of the AC voltage and frequency offsets when transferring from grid-tied to islanding modes. To tackle this problem, this paper proposes a simple but effective strategy based on reverse droop method. Initially, the power balance equation of the distribution system is derived, it reveals that the cause of voltage and frequency offsets is the mismatch between IFC output power and the rated load power. Then, the reverse droop control is introduced into the IFC controller. By using a voltage-active power / frequency-reactive power (U-P /f-Q) reverse droop loop, the IFC output power enables adaptive tracking of the rated load power. Therefore, the AC voltage offset and frequency offset are suppressed during the transfer process of operational modes. In addition, the universal parameter design method is discussed based on the stability limitations of the control system and the voltage quality requirements of AC critical loads. Finally, simulation and experimental results clearly validate the proposed control strategy and parameter design method.

This document presents a performance assessment of the three-phase two-level voltage source converter (VSC) model of the real-time digital simulator RTDS/RSCAD. This evaluation is conducted through two case studies. One case is a simple two-bus power electronic-based system, and the second case is a three-bus system with a frequency-dependent transmission line model and a closed-loop controlled VSC with communication delay. The results obtained reveal that the RTDS/RSCAD VSC model presents some issues on stability, harmonic spectrum, ripple, and power quality in general, as compared with the PSCAD VSC solutions. These results also suggest the need for better power electronic components models.