Variable Virtual Impedance-based Overcurrent Protection For Grid-forming
Inverters: Small-Signal, Large-Signal Analysis and Improvement
Abstract
Grid-forming inverters are sensitive to large grid disturbances that may
engender overcurrent due to their voltage source behavior. To overcome
this critical issue and ensure the safety of the system, current
limitation techniques have to be implemented. In this context, the
variable virtual impedance (VI) appears as a suitable solution for this
problem. The design of the variable virtual impedance basically rests on
static considerations, while, its impact on the system stability and
dynamics considering both small-signal and large-signal aspects can be
significant. This paper proposes small-signal and nonlinear power models
to assess the impact of the virtual impedance parameters on the grid
current dynamics and on the angle stability. Thanks to the developed
theoretical approaches in this paper, the virtual impedance ratio X/R,
which is the unique degree of freedom has shown a contradictory effect
on the small and large signal stability. To overcome this constraint, a
Variable Transient Virtual Resistance (VRTVR) has been proposed as an
additional degree of freedom to the variable virtual impedance. It acts
as a transient damper to ensure the maximum angle stability margin
allowed by the variable virtual impedance. The effectiveness of the
proposed control has been proven through time-domain simulations.