-The ongoing evolution of the power system towards a “SmartGrid” implies a dominant role of powerelectronic converters, but poses strict requirements on their control strategies to preserve stability andcontrollability. In this perspective, the definition of decentralized control schemes for power convertersthat can provide grid support and allow for seamless transition between grid-connected or islandedoperation is critical. Since these features can already be provided by synchronous generators, the conceptof Virtual Synchronous Machines (VSMs) can be a suitable approach for controlling power electronicsconverters. This paper starts with a discussion of the general features offered by the VSM concept inthe context of SmartGrids. A specific VSM implementation is then presented in detail together with itsmathematical model. The intended emulation of the synchronous machine characteristics is illustratedby numerical simulations. Finally, stability is assessed by analysing the eigenvalues of a small-signalmodel and their parametric sensitivities.© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicens

Control structures containing cascaded loops are used in several applications for the stand-alone and parallel operation of three-phase power electronic converters. Potential interactions between these cascaded loops and the complex functional dependence between the controller parameters and the system dynamics prevent the effective application of classical tuning methods in the case of converters operating with a low switching frequency. A tuning approach guided by the eigenvalue parametric sensitivities calculated from a linearized model of the converter and its control system is proposed in this paper. The method is implemented in the form of an iterative procedure enforcing the stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in the dynamic performance of the system when tuned with the presented algorithm compared with a conventional rule-based tuning method.

The concept of Virtual Synchronous Machines (VSMs) is emerging as a flexible approach for controlling power electronic converters in grid-connected as well as in stand-alone or microgrid applications. Several VSM implementations have been proposed, with the emulation of inertia and damping of a traditional Synchronous Machine (SM) as their common feature. This paper investigates a VSM implementation based on a Voltage Source Converter (VSC), where a virtual swing equation provides the phase orientation of cascaded voltage and current controllers in a synchronous reference frame. The control system also includes a virtual impedance and an outer loop frequency droop controller which is functionally equivalent to the governor of a traditional SM. The inherent capability of the investigated VSM implementation to operate in both grid-connected and islanded mode is demonstrated by numerical simulations. Then, a linearized small-signal model of the VSM operated in islanded mode while feeding a local load is developed and verified by comparing its dynamic response to the time-domain simulation of a nonlinear system model. Finally, this small-signal model is applied to identify the dominant modes of the system and to investigate their parametric sensitivity.2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/

Virtual Synchronous Machines (VSMs) have been introduced as a control concept for emulating the behavior of traditional synchronous machines with power electronic converters. This paper analyses a VSM control scheme where an outer loop for inertia emulation provides references for two cascaded voltage and current controllers. This scheme offers several advantages compared to alternative VSM implementations. However, the interactions between the cascaded control loops and the complex functional dependency of the system dynamics with respect to the controller parameters prevents the effective application of classical tuning methods to this scheme at low switching frequencies. Therefore, a tuning approach for the VSM guided by the eigenvalue sensitivity matrix of a linearized system model is proposed in the paper. The method is implemented in the form of an iterative optimization procedure enforcing stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in dynamic performance of the system when tuned with the presented algorithm compared to the results achieved by a more conventional tuning method.