Assessment of droop and VSM equivalence considering the cascaded control dynamics

Abstract

Grid Forming (GFM) converters are changing the way that power converters interact with the network to resemble synchronous machines (SM). Several GFM technologies including the Synchronverter, a particular member of Virtual Synchronous Machine (VSM), and GFM droop, based on standard current control and droop, have been suggested. The controllers have been compared and found to be equivalent to each other in steady state. The equivalence generally focusses on individual operating conditions and fails to address differences in dynamic properties. This paper provides a clear tuning guide of both controls in terms of SM properties and a discussion of the analytical equivalence using the output of time-domain medium-voltage DC (MVDC) converter models. Initially, the GFM droop is found to be less damped despite the equivalent tuning in terms of damping ratio. A parametric sweep of the cascaded control present within the GFM droop finds that the Voltage Control bandwidth and proportional gain can be tuned to adjust the damping of the response. The Voltage Control parameters are also found to affect the GFM droop’s stability.