Inverters usually employ several pulse-width modulation (PWM) controlled switches receiving energy from the DC side, followed by LC or LCL output filters to produce nearly sinusoidal currents injected to the power grid. The filter inductances are usually small, to avoid high costs, such that tiny voltage measurement-, delay-or PWM timing-errors can cause large deviations in the output currents. This is mitigated by employing fast output current controllers (OCCs) which allow to correct the effect of measurement errors and accurately track the desired output currents. Proportional integral (PI) controllers in a rotating reference frame are a common choice of implementation. In recent years, due to the more complex control strategies of grid forming (GFM) inverters, virtual impedances (admittances) have been proposed in order to obtain the reference currents for these PI controllers from certain virtual voltages produced by the control algorithm. While very efficient in strong grids, such control architectures face stability problems if the grid short circuit ratio (SCR) becomes low. In this paper we investigate the stability of an inverter, using such PI controllers and virtual impedances, and give recommendations for the tuning of all the parameters, for the positioning of the current probes inside the output filter and for a choice of the virtual impedance parameters to achieve stability in a wide range of grid conditions. We give recommendations for the selection of the LCL filter components and demonstrate the suggested tuning in an experimental setup. To enable automatic tuning, we have developed a method for measuring the grid impedance by injecting a small probing current into the grid, at a frequency that is of the same order of magnitude as the grid frequency.

Paper accepted for publication in JESTPE https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=6245517 Virtual synchronous machines (VSMs) have become popular solutions for grid-tied inverters, suited to work on AC-grids with an increasing share of distributed energy sources and storage. However, just like real synchronous machines, VSMs may experience power oscillations. Recently, a new damping concept called virtual friction (VF) has been proposed that makes use of real time communication between several inverters. VF introduces an additional damping torque in the swing equation of the VSM, proportional to the deviation of the virtual rotor frequency from the center of inertia (COI)-frequency. VF can apply high damping without imposing a strong response in the output power of the VSM during frequency transients. However, VF requires the transmission of frequencies over a communication network, which may suffer from transmission delays. We give an experimental proof of concept of VF both in an isolated microgrid and in grid connected operation of a 45kVA setup consisting of three VSMs realized by three-phase two-level IGBT-based inverters1. We consider continuous constant and discontinuous varying time delays and show theoretically and experimentally under what conditions delays can impact the output powers of the VSMs. Further, we show how to correctly estimate the COI-frequency with transmission delays using timestamps to align individual signals.

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.