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Virtual synchronous machines (VSM) are inverters that behave towards the power grid like synchronous generators. Hence, they can be used as grid forming inverters and they can support the grid with inertia, droops, fault ride-through and more. Synchronverters are a much studied type of VSM. In this paper we present several improvements and innovations to the synchronverter control algorithm. We offer a complete design procedure for this algorithm. One change is meant to mitigate the fact that earlier designs are very sensitive to grid voltage measurement errors and processing delay, which may cause harmonic distortion and fluctuating amplitude of the grid-side currents. We propose to include a fast current controller as the internal control loop of the inverter. The design of this current controller involves delicate issues of compensating the delays and eliminating the DC components of the currents. The new design enables also a natural way for distortion-less current limitation. We present a smooth start?up procedure. Our simulations and experiments show that the current controller leads to a dramatic reduction of the sensitivity of the VSM to measurement errors, processing delay and grid voltage imbalance.

Virtual synchronous machines (VSM) are inverters that behave towards the power grid like synchronous generators. One popular way to realize such inverters are synchronverters, whose control algorithm has evolved over time, but both theoretical analysis and practical observations show that the output currents of a synchronverter are very sensitive to grid voltage measurement errors and processing delay, as well imprecisions in the PWM process. To overcome this problem (of excessive sensitivity), we propose in this paper to use a different type of control to realize a VSM, that includes a fast current controller as the internal control loop of the inverter. Our simulations and experiments show that this results in a dramatic reduction of the sensitivity of the VSM to various kinds of measurement errors, noise and imprecision, and hence to the proper operation of such inverters. In particular, it leads to a large reduction of the total harmonic distortion of the grid currents.

Virtual synchronous machines are inverters with a control algorithm that causes them to behave towards the power grid like synchronous generators. A popular way to realize such inverters are synchronverters. Their control algorithm has evolved over time, but all the different formulations in the literature share the same "basic control algorithm". We investigate the equilibrium points and the stability of a synchronverter described by this basic algorithm, when connected to an infinite bus. We formulate a fifth order model for a grid-connected synchronverter and derive a necessary and sufficient condition for the existence of equilibrium points. We show that the set of equilibrium points with positive field current is a two-dimensional manifold that can be parametrized by the corresponding pair $(P,Q)$, where $P$ is the active power and $Q$ is the reactive power. This parametrization has several surprizing geometric properties, for instance, the prime mover torque, the power angle and the field current can be seen directly as distances or angles in the $(P,Q)$ plane. In addition, the stable equilibrium points correspond to a subset of a certain angular sector in the $(P,Q)$ plane. Thus, we can predict the stable operating range of a synchronverter from its parameters and from the grid voltage and frequency. Our stability result is based on the intrinsic two time scales property of the system, using tools from singular perturbation theory. We illustrate our theoretical results with two numerical examples.

The synchronverter algorithm is a way to control a switched mode power converter that connects a DC energy source to the AC power grid. The main features of this algorithm are frequency and voltage droops as well as synthetic inertia, so that the inverter resembles a synchronous generator (SG). Many versions of this algorithm have been proposed and tested, but all share the same “basic control algorithm”, which is based on the equations of a SG. We analyze the sensitivity of the output currents of a synchronverter, with respect to the measurement errors. We show that some of the sensitivity functions exhibit high gains at the relevant frequencies, leading to distorted grid currents, which makes the use of this inverter control algorithm problematic. We then do a similar analysis assuming that we have controlled current sources available at the grid output of the converter, that we control using virtual currents generated in the algorithm. The virtual currents are flowing through virtual output inductors, that we can choose to be significantly larger than the actual output inductors. We show that using the current sources reduces the sensitivity considerably, thus indicating a better approach to synchronverter design.