Coherence Analysis of System Characteristics and Control Parameters for Hybrid HVDC Transmission Systems Based on Small-Signal Modeling

Hybrid high-voltage direct current (HVdc) system, which comprises a line-commutated converter (LCC) at rectifier side and a voltage source converter (VSC) at inverter side, is drawing wide attention due to its high reliability and economic benefit. For such a new transmission technology, the system characteristics are not comprehensively studied, and inappropriate control parameters may be coherent to system instability. This article puts forward a novel coherence analysis procedure that analyzes system stability based on a verified hybrid HVdc small-signal model. For the first time, the procedure is proposed to capture the coherence relationships between the system characteristics and the control parameters. The analysis results reveal that the circuit and control parameters can highly affect the oscillatory modes and even cause instability. Moreover, the variation of grid short-circuit ratio (SCR) at LCC rectifier side trades off the response speed and damping of the oscillatory modes - under low SCR, large proportional and small integral coefficients of the LCC dc current control are suggested to enhance response performance. Also, the increase of VSC dc capacitance reduces the mode response speed - for a large capacitance, large proportional and small integral coefficients of the VSC dc voltage control shall be set to guarantee system stability.