Modeling of MMCs With Controlled DC-Side Fault-Blocking Capability for DC Protection Studies

Fault current characteristics in dc systems depend largely on the response, and hence also the topology, of the ac-dc converters. The presently used ac-dc converter topologies may be categorized into those with controlled or uncontrolled fault-blocking capability and those lacking such capability. For topologies of the former category, generic models of the dc-side fault response have not yet been developed, and a characterization of the influence of control and sensor delays is a notable omission. Therefore, to support accurate and comprehensive dc system protection studies, this article presents three reduced converter models and analyzes the impact of key parameters on the dc-side fault response. The models retain an accurate representation of the dc-side current control, but differ in the representation of the ac-side and internal current control dynamics, and arm voltage limits. The models have been verified against a detailed (full-switching) simulation model for the cases of a full-bridge and a hybrid modular multilevel converter and validated against experimental data from a laboratory-scale prototype. The models behave similarly in the absence of arm voltage limits, but only the most detailed of the three retains a high degree of accuracy when these limits are reached.