Grid-connection of active-forced-commutated bridge: Power quality and DC fault protection

The active-forced-commutated (AFC) bridge for high-voltage (HV) DC transmission has recently been proposed, which operates low-loss thyristor valves in the manner of a voltage source converter (VSC) with the assistance of a self-commutated full-bridge (FB) chain-link (CL) circuit, creating a hybrid VSC solution which retains the key advantage of high-efficiency performance found in conventional line-commutated-converters (LCC). The AFC-bridge enables fully controlled turn-on/off and four-quadrant operation for a symmetrical thyristor bridge to synthesize independent AC voltage based on a given DC-link voltage. From a system-level perspective, it inherits the key functionalities of a standard VSC, including bidirectional current flow and independent active/reactive power control, in combination with a number of favorable features common to LCC-HVDC implementations, including the enhanced voltage scalability, reduced complexity, DC-fault blocking capability and bipolar DC voltage operability. However, the slow commutation speed of high-capacity thyristors results in power quality issues due to low frequency harmonics, which need a certain level of passive filtering to ensure grid code compliance. Continuing previous work, this paper addresses several AFC-bridge grid-connection issues, including minimization of the AFC-bridge total passive filter capacity by adopting a predefined selective harmonic elimination (SHE) mode or by using a multiplexed configuration, and simulation analysis and validation of DC-fault-proof capability.