Stability analysis of HVDC control loops

The influence of HVDC control loops on the dynamic stability of an overall HVDC-HVAC system is discussed. The eigenvalue decomposition is used for the analysis of system stability in the frequency range below 100 Hz. The use of direct current and fast power feedback at the rectifier side of the system is analysed and compared. It is found that DC feedback actually deteriorates the system stability around the first harmonic. At the inverter side, as the commonly used control strategies, constant beta control is analysed and compared with gamma minimum control, direct current feedback and direct voltage feedback. Reactive current feedback and CCCM control methods are also analysed as suggested alternative control techniques. Reactive current feedback is found to be the best control strategy from the stability point of view. The test system used is assumed to operate with weak and very weak AC systems at the inverter side