Most renewable energy sources (RESs) are interfaced to electricity grids via voltage-source converters (VSCs). To facilitate the integration, the controllers based on the emulation of synchronous generators have been increasingly used. This control technique is commonly referred to as Virtual Synchronous Machine (VSM) and recent studies have shown that VSMs are sensitive to variations in the network parameters if no countermeasures in form of virtual impedances are applied. However, a thorough and comprehensive methodology for the design of virtual impedance for VSMs has not been yet presented in the literature and the development of such procedure for the VSMs connected to weak grids is the principal objective of this paper. Each dynamic element of the VSM is modeled and all of them are then joined together forming a state-space model. The eigenvalue and singular value decomposition (SVD) analyzes were used then to evaluate the impact of the grid model and the virtual impedance on the VSM dynamics. From these results, a practical guide to design both the VSM and the virtual impedance parameters was proposed. All the theoretical developments were validated experimentally on a 15-kVA prototype of a VSM connected to a weak grid.

Virtual Synchronous Machine (VSM) term commonly refers to the emulation of synchronous machines operation by using power electronics converters. However, a wider application of this control technique is still pending on further refinement of its design aspects and digital implementation. In this paper a systematic approach to design of the VSM parameters is introduced. Firstly, a simplified third-order small-signal model of a VSM based on the conventional model of synchronous generators is developed. Unlike the existing works published in literature, the proposed model takes into account the effects of inductive and resistive weak grids. The model is then used to design the controller parameters by appropriately placing the poles of the system closed-loop. VSM robustness is studied for weak grids dominated by either inductance or resistance and the aspects of digital control implementation are addressed. All the control system features and improvements were tested on a 15~kW prototype of a battery-supported VSM connected to a weak grid.

Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator.

LCL filters are commonly used to connect Voltage Sourced Converters (VSCs) to the grid. This type of filter is cheaper than a single inductor for the same current THD, but it generates resonance problems if no active or passive damping method is applied. Active damping methods are becoming popular in the literature because they improve efficiency, but they are sometimes difficult to implement and additional measurements are required. This paper proposes an active damping method for VSCs connected to weak grids that is based on making zero the open-loop phase at the resonance frequency. It will be shown that this strategy provides adequate damping of oscillations and that it can be achieved in two different ways: at the design stage (if the design constraints make it possible) or with an all-pass filter in series with the current controller. Two methods to design the all-pass filter are proposed. Also, the proposed active damping technique is compared with three alternatives already proposed in the literature. All the control algorithms are verified by simulation and in a 15 kW prototype of a three-phase VSC connected to a configurable weak grid via a LCL filter.

In recent years synchronverters have become a preferred control solution for integration of renewable energy sources and battery systems to power networks. In this control strategy the converter acts as a fundamental frequency voltage source and, as such, is unable to deal with harmonic power quality issues. In the literature the harmonic control is frequently addressed by using virtual impedances, however, they have been seldom studied for synchronverters. This paper presents a detailed design procedure for harmonic virtual impedances applied to parallel-connected synchronverters. The proposed method offers a tailored-made design of the converter output impedance and large stability margins. The method used to decouple the dynamics of the synchronverter control and the proposed virtual-impedance is explained in detail. The obtained results showed the output impedance of each synchronverter can be designed to achieve a trade-off in power quality between current and voltage. In addition, the effect of frequency variations was explored and two solutions were provided and compared. All the control system improvements were experimentally validated by using two 15 kVA battery-supported synchronverters connected to a distorted weak grid.

Dynamic Voltage Restorers (DVRs) are a cost-effective solution to protect sensitive loads against voltage sags in medium- and high-power applications because the power required for the compensation is only a fraction of the load rated power. Voltage sag compensation with DVRs requires a robust sag detector that has to be able to address distorted-grid conditions. Also, sags should be detected fast in order to guarantee adequate protection of loads. In this paper, a sag detection algorithm for a DVR is proposed and comprehensively analysed. It is shown that this algorithm provides harmonic cancellation properties when the parameters are adequately tuned. Implementation aspects are addressed carefully and a method to adapt the algorithm when the grid frequency fluctuates is explained. The proposed algorithm was experimentally tested in a 5 kVA prototype of a DVR that protected a sensitive load (linear and non-linear) against voltage sags and voltage harmonics. The proposed algorithm was compared with three alternatives already proposed in the literature in terms of detection time, performance against voltage harmonics, and computational burden.