Dead–Time Effect on Two–Level Grid–Forming Virtual Synchronous Machines

The concept of Virtual Synchronous Machine (VSM) is a promising solution to integrate renewable energy sources into the grid. Thanks to this approach, renewable energy sources can provide grid services (e.g., inertial behavior), grid support during faults and island operation. Moreover, VSMs can enhance the voltage quality at the point of connection to the grid by behaving as harmonic and unbalance sinks under non–ideal conditions (i.e., grid harmonic distortion and unbalance). However, the inverter dead–time affects the harmonic and unbalance sink capability of grid–forming VSMs with no closed loop current control. Although the literature contains several papers concerning the dead–time issues, the negative influence of the switching dead–time on the harmonic and unbalance sink capability of VSMs has not been analyzed. Therefore, this article demonstrates through experiments that: (1) the inverter dead–time effect limits the harmonic and unbalance sink capability of grid–forming VSMs under non–ideal grid voltage conditions and (2) a dead–time compensation is mandatory to make grid–forming VSMs behave according to the theoretical analysis. A first experimental test shows that both the dead–time and its compensation do not influence the VSM response under normal operating conditions (i.e., symmetrical and sinusoidal grid voltage). Next, two experimental tests under a 5 $\%$ grid voltage unbalance and a 10 $\%$ grid voltage fifth harmonic distortion validate the negative influence of the dead–time and the beneficial effect of its compensation.