Performance of Differential Protection Applied to Collector Cables of Offshore Wind Farms with MMC-HVDC Transmission

The ongoing global transition towards low-carbon energy has propelled the integration of offshore wind farms, which, when combined with Modular Multilevel Converter-based High-Voltage Direct Current (MMC-HVDC) transmission, present unique challenges for power system protection. In collector cables connecting wind turbines to offshore MMC, both ends are supplied by Inverter-Based Resources (IBRs), which modify the magnitude and characteristics of fault currents. In this context, this paper investigates the limitations of conventional differential protection schemes under such conditions and compares them with enhanced strategies that account for sequence components. Using electromagnetic transient simulations of a representative offshore wind farm modeled in PSCAD/EMTDC software, internal and external fault scenarios are assessed, varying fault types and resistances. The comparative evaluation provides insights into the sensitivity and selectivity of differential protection and guides a deeper conceptual understanding of the evolving protection challenges inherent to future converter-dominated grids.

💡 Research Summary

The paper investigates the performance of differential protection applied to collector cables that interconnect offshore wind turbines with an offshore modular multilevel converter (OMMC) in an MMC‑HVDC transmission system. Because both ends of the cable are fed by inverter‑based resources (IBRs), the magnitude and waveform of fault currents differ markedly from those in conventional AC systems. The authors model a realistic 450 MVA‑per‑cluster offshore wind farm (two clusters, each connected to the OMMC by a 20 km, 230 kV cable) in PSCAD/EMTDC and simulate a comprehensive set of fault conditions: internal faults at five locations along the cable, external faults on either side of the cable, four fault types (AG, AB, ABG, ABC), and four fault‑resistance levels for each type.

Two OMMC control strategies are examined. Control C1 actively suppresses negative‑sequence current during faults, while Control C2 does not impose such suppression. The wind‑turbine converters are assumed to prioritize positive‑sequence current injection, but the paper also discusses the impact of alternative strategies that provide negative‑sequence reactive current.

Three differential protection schemes are evaluated: conventional phase‑current differential protection (87L a, b, c), negative‑sequence differential protection (87Q), and zero‑sequence differential protection (87G). All schemes use a percentage‑differential logic with an operating current I_OP, a restraint current I_RST, a slope K = 0.5, and a minimum pickup K₀ = 0.3 p.u.

Key findings:

1. Phase‑to‑Ground (PG) faults – With C1, the negative‑sequence circuit is effectively open, resulting in near‑zero fault current; none of the three protections operate. Only by lowering K₀ or K can 87Q/87G be made sensitive, but this compromises selectivity for external faults. With C2, sufficient negative‑sequence current flows and all protections trip correctly.

2. Phase‑to‑Phase (PP) faults – The positive‑ and negative‑sequence networks are in parallel. Under C1 the negative‑sequence suppression again eliminates fault current, causing all protections to miss the fault. Under C2, the protections work as expected.

3. Phase‑to‑Phase‑to‑Ground (PPG) faults – The zero‑sequence path provides a fault current even with C1, but high fault resistance can reduce the current below the pickup of 87L and 87Q. 87G remains the most reliable element, especially for low resistance values. With C2, all protections operate across the full resistance range.

4. Three‑Phase (PPP) faults – Current limiting in the MMC reduces fault current magnitude, but the presence or absence of negative‑sequence suppression again determines whether the differential relays see enough current to operate.

The study also highlights the influence of transformer grounding (grounded‑wye connections create a zero‑sequence path) and cable impedance on the zero‑sequence protection’s effectiveness.

Overall, the paper demonstrates that conventional phase‑differential protection, designed for symmetrical fault currents, loses reliability in converter‑dominated offshore wind systems. Sequence‑based differential protections can recover sensitivity, but only when the control strategy permits non‑zero negative‑ or zero‑sequence currents, and even then a trade‑off exists between sensitivity and selectivity. The authors recommend re‑tuning protection settings, incorporating sequence‑based relays, and coordinating transformer grounding and converter control to achieve robust protection for future converter‑dominated grids.