High voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance offshore wind power transmission. However, compared with conventional onshore networks, the dynamic behaviour and operation of power electronic based offshore network is significantly different, especially during offshore grid disturbances. Thus, to ensure a secure and reliable power transmission, this thesis investigates the different fault characteristics of HVDC connected offshore windfarm systems and proposes several fault rides through control and system recovery schemes.;The first topic discussed in this thesis is the offshore AC fault ride through operation. When modular multilevel converter (MMC) based HVDC connection is used for offshore windfarm system, the responses of both the offshore MMC station and wind turbine (WT) converters need to be carefully designed to ensure their safety operation during offshore AC faults. Maintain balanced and controlled current contribution to offshore AC grid during asymmetrical AC fault is possible, but it has several drawbacks such as increased risk of protection failure due to the absence of sufficient fault currents, and the inability of post-fault AC voltage recovery. Therefore, based on a detailed sequence network analysis, an enhanced control strategy is proposed during offshore asymmetrical faults to exploit the induced negative sequence and zero sequence voltages to facilitate controlled injection of negative sequence current while avoiding excessive overvoltage in the healthy phases. By adopting the proposed control, the AC fault current can be well regulated and the voltage restoration after fault clearance can be achieved as demonstrated by detailed simulation studies.;After the evaluation of offshore AC faults, the second research topic of this thesis moves to the offshore DC fault ride through operation. In a multi-terminal DC (MTDC) grid that connects multiple offshore windfarms, continued operation in an effort to retain large proportion of power transfer during a DC fault is very important for critical power corridors. Partially selective protection which only installs fast acting DC circuit breakers (DCCBs) in limited cable locations while the main protection uses cheap DC disconnectors and AC circuit breakers (ACCBs), is a cost-effective solution. However, such protection scheme requires significant modifications to WTs control in order to retain the offshore AC network to ensure fast system recovery after fault isolation. Detailed analysis reveals that the sudden MMC blocking or opening of ACCBs due to the DC fault clearance can cause significant over-voltage and over-frequency in the offshore AC grid, which could necessitate immediate shutdown of the wind farm and damage the offshore infrastructure. To tackle these issues, an enhanced control for wind turbine (WT) converters is proposed to facilitate seamless transition of the WT converters between grid following and forming modes to maintain the offshore AC grid stable when the control from the offshore MMC is lost. The viability of the proposed control is demonstrated in wider context of partially selective DC fault protection in a meshed DC grid. The proposed control method ensures the continuous control of the offshore AC networks and enables the fast power transfer restoration.;Finally, a black start service aiming to support the onshore power networks restoration provided by the diode rectifier (DR) based HVDC connected windfarm is studied. A new frequency-AC voltage (f-V) droop control of WT converters is proposed to dynamically regulate the offshore AC voltage to ensure the DC voltage of the DR-HVDC link remains in the safe range when the active power consumption by the onshore network varies during black start. The detailed sequential black start is demonstrated, including DR-HVDC link energization, onshore AC voltage build-up and load pick up. Comprehensive simulation results confirm the validity of the proposed black start scheme using DR-HVDC connected offshore wind farms.
|Date of Award
|2 Feb 2021
- University Of Strathclyde
|University of Strathclyde
|Lie Xu (Supervisor) & Olimpo Anaya-Lara (Supervisor)