Dynamic Adjustment of a VSC's Reactive Current Limit to Improve Frequency Stability

Future renewable energy sources will need to provide all ancillary network services, previously provided by the synchronous generators that are being decommissioned. The voltage sourced converter (VSC), with its ability to control active and reactive power independently, can be adapted to provide voltage support. The ability to provide voltage support can require the sacrifice of active power, which can be detrimental to the frequency stability. This paper presents three methods to limit the quantity of reactive power injection that can occur if the frequency has been perturbed. The aim is to provide support to the system frequency by limiting the quantity of active power reduction from the VSC. The three options presented use frequency deviation, rate of change of frequency, or a combination of both, to reduce the maximum reactive current limit. In this work it is shown that using the rate of change of frequency to reduce the maximum reactive current limit, provides the greatest benefit to the frequency stability. All options presented, allow the VSC to provide continually operating voltage support without having to prioritize one form of ancillary service over another.