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.

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.

The Back-to-Back (BTB) converter is one of the most popular converter topologies for the control of electrical machines, power transmission systems, and power quality applications. The conventional cascade control structure is commonly used due to its simple design procedure and reliable operation. However, the bandwidth of the DC-link voltage controller has to be limited in order to avoid instability issues and be restrictive in high performance applications. This paper presents a differential and common-current (power)-based state-feedback control for BTB converters. This controller features a fast control of active and reactive powers, and a stiff regulation of the DC-link voltage. A theoretical analysis of the proposed controller along with a strategy for current limiting is presented. Its robustness was tested against variations of the filter parameters, the DC capacitance, the grid inductance and the grid voltage values. The controller was experimentally validated under nominal operation, voltage sags, and connected to a weak grid by using a 15 kVA BTB converter in a laboratory test network.

In recent years the penetration of distributed generators (DGs) with power electronics interfaces has rapidly increased and this fact has created new challenges for the operation of power systems. In some cases, when DGs are connected via long distribution cables an interaction between their control systems and the network may produce undesired resonant effects. This emphasises the importance of more accurate dynamic modelling of the system to predict and prevent such events. This paper presents an integrated small-signal model of Voltage Source Converters (VSCs) that includes the dead-time and time delays implementation effects that are typically not considered in the literature. An approach based on Eigenvalue and Singular Value Decomposition (SVD) is used to evaluate the impact of the network impedances on the robustness of the power system. The accuracy of the integrated small-signal model was validated by using a dedicated laboratory test network composed of 15~kVA VSCs connected to the grid via emulated distribution lines. The analysis of the implementation effects revealed their significant impact on the accuracy of the small-signal models as well as on the overall system stability. The two models that include and exclude the implementation effects are critically compared and the experimental results provided.