Frequency Dynamical Behaviour and Frequency Equilibrium Point of Multi-VSC Systems

Abstract

With replacement of synchronous generator (SG) by voltage source converter (VSC) interfaced renewables, power system dynamics is undergoing significant changes. This paper investigates the frequency dynamical behaviour of new-generation power systems composed solely of grid-following (GFL) and/or grid-forming (GFM) VSCs, without an infinitely strong bus or SG. It is found that for a system composed solely of GFL-VSC, as it lacks a frequency equilibrium point after a certain disturbance, the whole system exhibits an unusual phenomenon of frequency drifting. On the other hand, for a hybrid system composed of both GFM-VSC and GFL-VSC, as it has a frequency equilibrium point, the system can settle down to a new frequency steady-state, and the GFM-VSC and GFL-VSC show completely different behaviours in the transient process. The GFM-VSC plays a key role in the frequency dynamics, similar to the SG. Based on the inertia-centre frequency dynamics, it is observed that the GFM-VSC determines the equivalent damping of the system, and both GFM-VSC and GFL-VSC contribute to the equivalent inertia. All these findings are well supported and verified by our theoretical analysis and time-domain simulations, and they can provide physical insights in the bulk frequency dynamical behaviour of new-generation power systems dominated by converters.