A coordinated power flow control strategy to enhance the reliability of hybrid ac/dc power grids during cascading faults

Cascading fault is a newly emerging challenge in hybrid ac/dc grids including current source converters (CSCs) which commences from a dc or a long-lasting severe inverter ac fault and ultimately provokes a total blackout at the inverter ac side. Nevertheless, since the cascading faults emanating from dc faults are not accompanied by commutation failure, the prevailing commutation failure elimination strategies are unable to inhibit these contingencies. To obviate such a challenge, this paper proposes a coordinated power flow control strategy which can preclude the blackouts caused by cascading faults initiated from either dc or inverter ac faults, thereby improving the reliability of hybrid ac/dc power grids. To achieve this, a back-to-back voltage source converter (B2B-VSC) is employed which is responsible for controlling both the inverter ac bus voltage and the active power flow of its connected ac line. This would not only mitigate the probability of commutation failure due to the reinforced inverter ac bus voltage, but also enhance the power flow flexibility of the power grid during both normal and fault conditions. In addition, the cost associated with implementation of the proposed strategy is much less than installing several backup parallel lines which are useless under no-fault circumstances. Finally, the practical viability of the proposed strategy is scrutinized through laboratory testing using co-simulation of RTDS and Opal-RT simulators. The obtained results show that the developed strategy can effectively manage the power flow of B2B-VSC and CSC-HVDC systems during different possible fault cases so that the power outage at the inverter ac side is minimized.