Control and Protection of MMC HVDC under AC and DC Network Fault Contingencies
The modular multilevel converter (MMC) has become the most attractive voltage source converter (VSC) topology for high power applications. AC or dc short circuit fault conditions cannot be avoided in high voltage direct current (HVDC) schemes, independent of the converter and the transmission link technology (e.g. cables or overhead lines) The probability, frequency, and characteristics of these faults depend on the employed transmission technology, converter technology (e.g. two level or MMC) and the grounding topology (e.g. direct, resistive, or high impedance) This chapter presents and compares the two level VSC HVDC and MMC HVDC fault characteristics and behaviors under ac and dc fault conditions. It discusses the impacts of different component failures, and presents related control and protection techniques. VSC HVDC terminals are designed to fulfill FRT requirements and remain synchronously connected to the network and provide the required ancillary services during the unbalanced ac fault conditions. Different techniques are applied to detect and protect the converter components from ac and dc fault conditions.