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

TL;DR

This paper tackles protection challenges for offshore wind farm collector cables connected via MMC-HVDC by examining how inverter-based resources alter fault currents and investigating the limitations of conventional differential protection. Using PSCAD/EMTDC simulations, it compares standard phase-current differential protection with sequence-based schemes 87Q and 87G under various internal and external fault types and fault resistances, incorporating two OMMC control modes that either suppress or permit negative-sequence currents. The results show that conventional 87L can miss internal faults when negative-sequence suppression is active, while sequence-based protections can improve sensitivity in several scenarios but are still sensitive to system configuration and control strategies; external faults generally do not trigger misoperations. The findings underscore the need for robust, converter-aware protection approaches for future grids and provide guidance for designing offshore MMC-HVDC protection schemes and standards.

Abstract

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.

Figures (13)

• Figure 1: Test system.
• Figure 2: GFM voltage and current controls.
• Figure 3: Responses of $87L_{a}$, $87L_{b}$, and $87L_{c}$ elements for AG faults, considering (a) C1 and (b) C2 controls.
• Figure 4: Responses of $87Q$ and $87G$ elements for AG faults, considering (a) C1 and (b) C2 controls.
• Figure 5: Sequence network for PG fault.