Internal Energy Distribution Control Based Fault Ride-Through and Postfault Recovery Strategy for Offshore Wind Farms Connected to DR-MMC HVDC Under Onshore AC Grid Faults

Abstract

Offshore wind farms (OWF) connected to diode rectifier (DR) and modular multilevel converter (MMC)-based HVDC confront challenges of surplus power induced by onshore AC faults. This paper proposes an internal energy distribution control (IEDC) strategy, which utilizes the rotor kinetic energy (KE) of wind turbines (WT) and the capacitor energy of MMC submodules to achieve fault ride-through (FRT) and postfault recovery (PFR). Firstly, the mechanism of OWF is analyzed, and an onshore AC fault detection method based on local measurements is proposed. Then, a two-stage FRT control strategy is proposed. Three preset power reduction and energy absorption curves are designed to utilize the internal energy to actively absorb excess power, and flexibly distribute surplus power to KE and MMC energy. An additional pitch angle control (APAC) is devised, which can reduce captured wind power and eliminate surplus power when the internal energy reaches its maximum value. Thirdly, a two-stage PFR control strategy is proposed. The preset power and energy recovery curves are designed to achieve fast active power recovery and release of stored excess internal energy after fault clearance. Case studies are performed on 2-terminal and 4-terminal test systems to validate the performance and effectiveness of the proposed strategy.