Impact of Short-Circuit Ratio and Phase-Locked-Loop Parameters on the Small-Signal Behavior of a VSC-HVDC Converter

The impact of phase-locked loop (PLL) parameters on the dynamic and steady-state behavior of a voltage-source converter (VSC) in an HVDC transmission system is determined as a function of the system strength parameterized by the short-circuit ratio (SCR). This is achieved by using a linearized small-signal model of the converter system and its controls. The model is validated via electromagnetic transients simulation of the fully detailed large signal model. An interesting result from this study is that the maximum power transfer capability of the VSC-HVDC converter is affected by the PLL gains, and that the theoretical limit (obtained from static voltage stability analysis) is approachable as the PLL gains become very small. This paper shows that gains of the PLL, particularly at low SCRs, greatly affect the operation of the VSC-HVDC converter and that operation at low SCRs approaching 1.3 is very difficult.