An AC Fault Ride Through Method for MMC-HVDC System in Offshore Applications Including DC Current-Limiting Inductors

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results. HVDC high voltage direct current PCC point of common coupling FRT fault ride-through CLI current-limiting inductor RS rectifier station IS inverter station 3P-G three-phase-to-ground P-G single-phase-to-ground u D z YD11 triangle side voltage u z YD11 star side voltage i D zp YD11 triangle side current izp YD11 star side current i zpd, i zpq d-and q-axis of i zp m zp AC modulation Mzp the amplitude of mzp izpu upper arm current i zpl lower arm current u zpu upper arm voltage uzpl lower arm voltage m zpu upper arm modulation m zpl lower arm modulation uzpsm submodule capacitor voltage uzpc capacitor voltage summation u dc zp unbalance voltage i dc zp unbalance current m dc zp circulating control modulation m dc zpz zero sequence of m dc zp u zpdc RS DC voltage i zpdc RS DC current ubpdc IS DC voltage Csm submodule capacitance L arm arm inductance N number of submodules upccn rated PCC voltage ω s rated synchronous angular velocity i bb output current of RES ibbd, ibbq d-and q-axis of ibb Lac equivalent inductance of the collection line P tot RESs rated power R f grounding resistance at fault point uza A phase voltage of uz Pzp active power measured at AC side of RS positive pole P zpdc active power measured at DC side of RS positive pole Ye et al. : AC FRT method for MMC-HVDC Including DC Current-Limiting Inductors Rdc, Ldc inductance and resistance of DC cable L dctot, R dctot total inductance and resistance at DC side u dcn rated DC voltage uzref references of uz izpdcref reference of izpdc k zpzp, k zpzi zero-sequence control parameters δϵ disturbance step ϵ disturbance value of current reference ndis inserted group of energy dissipation device n disn total resistor groups in energy dissipation device R dis resistance in energy dissipation device uset threshold voltage of energy dissipation device PS positive sequence NS negative sequence