Impact factors on the active power flow recovery in multi-terminal HVDC systems after DC fault clearing

Multi-terminal HVDC systems have been proposed as a promising solution to enable the large-scale integration of offshore wind power plants. Due to the envisaged amount of installed generation capacity in the range of several gigawatts, a high availability of these DC grids is required, as a loss of power transfer from the HVDC grid to the AC grid could endanger the overall system stability. Consequently, a fast, and secure fault handling strategy for faults in the DC grid is needed. So far, no standard DC grid protection system is available for HVDC grids. Several fault clearing strategies have been proposed, with most publications focussing on selectivity and breaker design and stresses for the different fault clearing strategies. This paper shifts the focus to the power flow recovery after fault clearing to identify recovery times and the main impact factors on it. To this end, a selective fault clearing strategy based on HVDC circuit breakers is evaluated in PSCAD | EMTDC for varying fault scenarios and breaker opening times with regard to the recovery of the DC voltage and the active power transfer to the AC side. The results indicate, that fast fault clearing leads to a faster recovery, where changes in the range of few milliseconds in the fault clearing may have a huge impact on the power flow deviations.