Dual Channel Control With DC Fault Ride Through for MMC-Based, Isolated DC/DC Converter

This paper presents the two-channel controller with inner current loops for dual bridge, DC/DC converter, based on modular multilevel converter (MMC) technology. The DC/DC control strategy is based on two inner fast current control loops in dq rotating frame at each of the two MMC bridges. These current controls facilitate operation through dc faults at either dc bus. The active power control is shared by two MMCs, at the slower outer control level. The second outer control loop minimizes losses, which is achieved by feedback control of magnitude of both modulation indices at maximal value of 0.95 at all loading levels. The controller is symmetrical, provides bidirectional power flow and responds equally to faults on either dc bus. Under dc fault conditions, one MMC actively controls the inner ac current, while temporary blocking of MMC on faulted side is required to prevent cell capacitor discharge. The validity of the proposed control is verified on PSCAD using a 600 MW, 500-kV/640-kV test dc/dc model. The controller can be used for control/stability studies with large dc grids that contain dc/dc converters.