Comparative Analysis of Droop Control and VSG Control for Inverter-Based Resources

As the share of renewable energy sources increases in today's power system, inverters are requested to operate in grid-forming (GFM) mode and provide primary control of frequency and voltage in the replacement of large synchronous generators. To this end, droop and virtual synchronous generator (VSG) are the two most common methods adopted by GFM inverters. Comparing droop and VSG has always been a contentious topic, especially when regulators and power industries seek to standardize the operation of GFM inverters, and many papers have done this comparison in different perspective. This paper therefore presents a mathematical proof of the equivalence between droop and VSG controllers. Their equivalence in dynamic performance is also verified using numerical studies in different grid scenarios. We demonstrate that both droop and VSG mimic the same characteristics of a synchronous machine despite using different control parameters. Establishing that both controllers are equivalent in the physical sense, we then examine the effect of their damping towards the transient disturbances and grid stability. Numerical results provide insights for engineers when setting up GFM inverters in meshed and radial networks.