Virtual Inertia Control for CHB-STATCOM in a Power Electronics Dominated Grid

Integration of renewable energy sources (RESs) leads the power system to be a low inertia system, resulting in frequency deviation in the grid. This article presents a cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) providing virtual synchronous inertial support. This topology achieves two functionalities. 1) It reduces the system frequency deviation in a power electronics dominated grid (PEDG) following the updated grid codes. 2) Without disturbing the normal operation of CHB-STATCOM, it provides reactive power support. This topology uses the cumulative stored energy in the distributed DC-bus capacitors of CHB-STATCOM to improve the frequency nadir/zenith and the rate of change of frequency (RoCoF), preventing frequency instability. A closed-loop control algorithm is developed, which injects an adequate amount of stored energy to prevent frequency variation. It also facilitates re-energizing the capacitors in a smooth fashion, so that subsequent frequency events can be taken care of. For this, a high-pass-filter (HPF) is designed for autonomous DC-link voltage restoration for CHB converter. Relationship between frequency variation and inertial energy support by CHB is analyzed in depth. Finally, the viability of the proposed theory is verified in OPAL-RT test bench for step-up and step-down load change.