Synchronverters With Damper Windings to Attenuate Power Oscillations in Grids

Synchronous generators (SGs) have damper windings on their rotor that help to dampen the oscillations. Virtual synchronous machines (VSMs) are inverters that emulate the behavior of SGs. Usually, these have no damper windings, instead they make use of fast frequency droop to help maintain the power balance in the grid and also for the damping of unwanted oscillations. The drawback is that the inverter is required to be able to inject or absorb extra power, depending on the grid frequency. We propose a simple model of a damper winding that can be added to the control algorithm of a VSM. A precise model of the damper windings present in an SG is complicated and necessities several additional state variables. Instead, we propose a very simple but effective approximation, where the virtual damper winding torque Tw is derived directly from the existing state variables of the VSM and its terminal voltages. There is no need for a phase-locked loop (PLL) to estimate the grid frequency ωg, even though, Tw depends on the difference between the internal frequency ω and ωg. The analysis is based on a simplified model of a synchronverter. The proposed algorithm has been verified both by simulations and microgrid experiments. The performance of the proposed algorithm has been compared to the performance of two other recently proposed algorithms, which use a damping torque in addition to frequency droop, and which also do not rely on a PLL for this.