Enhanced Virtual Synchronous Machine With Online Grid Impedance Estimation

Abstract

In the growing context of integrating renewable sources and storage systems in the electric grid, the virtual synchronous machine (VSM) concept represents a valid solution to embed grid support functionalities in grid-tied converters. The knowledge of grid impedance is required to exploit properly the VSM features in providing grid ancillary services, such as inertial behavior, harmonics compensation, as well as short-circuit current injection during faults. Therefore, this article proposes a closed-loop grid impedance estimator, specifically designed for VSM applications, which is directly integrated into the VSM model. The grid resistance and inductance estimations are performed in real time, without the need of data elaboration through complex mathematical tools, which are usually required by methods available in the literature. The proposed solution leverages the VSM dependency on grid parameters when an amount of current is directly injected into the grid without being processed by the VSM itself. The proposed estimator inherently rejects the noise on voltage and current measurements, thus providing reliable results for real world operating conditions affected by disturbances. It can be easily tuned according to the desired filtering capability and estimation process duration. Experimental tests on a 15 kVA grid-tied inverter validate the proposed solution.