Stability and control of HVDC and AC transmissions in parallel

A generic concept combining transient angle and voltage stabilities of parallel AC/DC transmission is introduced. It proves that HVDC schemes in parallel operation with AC transmission are prone to both transient swing angle and voltage instabilities. This tendency is intensified as the parallel AC interconnection becomes stronger. The concept also explains why an HVDC scheme fitted with classical DC power controls does not contribute to system synchronizing torque and may increase the risks for instability. The concept offers a solid means to develop robust strategies to control the dynamic operation of DC and AC transmissions in parallel. With such strategies, the HVDC scheme can positively participate in the instantaneous rescheduling of power according to arising network conditions, particularly during disturbance conditions.