Effect of RES current injection requirements on Voltage Stability Margin

Renewable energy sources (RES) are required by various grid codes to inject reactive current in response to low system voltages. This paper investigates the impact of these requirements on the long-term voltage stability margin (VSM) of a transmission system. VSM is determined in the paper using optimization for various cases: a) when the RES are operating under unity power factor and b) when following grid code requirements for reactive current injection. These are then compared with the optimal active and reactive injection that maximizes VSM, in order to determine rules to better tune injection current settings. Optimal injection is shown to depend strongly on the relative distance of the RES to the area experiencing voltage stability limitations. For this study a simple radial transmission corridor is assumed, on which a RES is connected at a different distance between the source and load each time to demonstrate the impact of proximity to the affected area.