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Title: Control and operation of offshore wind farms connected with diode rectifier HVDC systems
Author: Yu, Lujie
ISNI:       0000 0004 7972 2849
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2019
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Due to the advantage of abundant space and more consistent wind speed, interest in offshore wind energy has significantly increased. To reduce the cost related to offshore wind power integration, this thesis investigates the control, modelling and operation of offshore wind farms connected with diode rectifier HVDC (DR-HVDC) systems, where a diode rectifier is used offshore and a modular multilevel converter (MMC) is used onshore. Compared to MMC based HVDC systems, the main benefits of DR-HVDC are lower investment, lower space requirement, higher efficiency and improved robustness. However, as the diode rectifier is unable to control offshore frequency and voltage as the MMC counterpart does, permanent magnet synchronous generator based wind turbines (WTs) have to perform more control functions including the establishment of the offshore AC network. In order to ensure that each WT converter autonomously contributes to the regulation of the overall offshore voltage and frequency, a distributed phase locked loop-based control for WT converters connected with DR-HVDC is proposed. A small-signal state-space model of the DR-HVDC system is developed to justify the use of active power and voltage (P-V) control, and reactive power and frequency (Q-f) control. The WT level analysis is implemented and to reveal the coupling between WT active power and reactive power with such control scheme. An angle compensation control is further proposed to reduce the coupling during WT active power change. Small-signal analysis is also carried out to investigate the impact of the angle compensation control parameters, active power control parameters and reactive power control parameters on system stability. To ride-through onshore faults, an active MMC DC voltage control combined with a WT overvoltage limiting control is proposed. With this control scheme, active power re-balance between the offshore and onshore side is achieved faster and thus, the MMC submodule capacitor overvoltage is alleviated. During offshore AC faults, a current limiting method is proposed to ensure the safe operation of WTs and an effective offshore overcurrent protection solution is proposed for fault detection and isolation. In addition, the system response during permanent DC pole-to-pole faults is analysed. Finally, the operation of offshore wind farms connected with two parallel transmission links, i.e. a DR-HVDC and a HVAC link, is investigated. A hierarchical control structure which contains primary, secondary and tertiary controls, is proposed to ensure reliable operation and smooth transition between DR-HVDC mode, HVAC mode and parallel mode.
Supervisor: Xu, Lie Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral