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.

Abstract—Output current control is widely used as the inner?most control loop of inverters, in particular virtual synchronous machines (VSM). The realization of this controller is commonly based on a PI controller working in a synchronous reference frame. The controller should mitigate the grid currents distortion and the unsteady amplitudes caused by A/D converter imperfec?tions, measurement errors and processing delays. The elimination of any DC component in the output currents is another relevant demand. The literature indicates that the current controller parameters are usually chosen by experimental tuning. In this paper, we propose a fast current controller design procedure that provides compensation for the processing delays and eliminates the DC components of the grid-side currents. We propose to control also the zero component of the output currents (which helps in an unbalanced grid). We present the performance of the novel features of our current controller through a series of simulations, comparing it to the performance of the same con?troller without the proposed additional features. In addition, we compare our new current controller’s design with one proposed in the literature. The results show that the new current controller leads to a better performance of the VSM in terms of output active power oscillations and stability.