This thesis presents a comprehensive study on the operation, principles and theory of wind farms consisting of doubly-fed induction (DFIG) and fully-rated converter (FRC) wind generators working together in a hybrid arrangement. The main objective of this study is to develop control strategies for the hybrid arrangement of wind turbines in order to improve the grid code compliance of the overall wind farm by exploiting the unique characteristics of both technologies to support each other in case of a fault happening. As a result, this thesis presents a novel control strategy for fully rated converter wind generators to improve the fault ride through capabilities of DFIG wind turbines during an AC voltage sag. The developed controller enables the creation of an environment where the FRC and DFIG technologies coexist for the purpose of compliance of grid code requirements. This type of environment can be created in both new wind farm developments and in already deployed DFIG-based wind farms where the addition of FRCs results in the overall improvement of the grid code compliance of the wind farm.The novel controller developed in this thesis uses the FRC to supply reactive power to the hybrid wind farm network during faults with the objective of reducing the magnitude of the voltage dip. This has positive effects in DFIG rotor speed and DC voltage variables during and after the fault period. These variables, as explained in the thesis, play a crucial role in stabilising the dynamic fault ride through capabilities of the turbine. Additionally, the novel controller developed in this thesis does not compromise the integrity of the FRC system.This thesis also contributes to the investigation of hybrid wind farms connected to the main grid using voltage source converter high-voltage direct current (VSC-HVDC) as a mean to increase renewable energy penetration and transmission capacity without affecting voltage stability or power quality of the specific case of the Great Britain's (GB) network. One main control approach was investigated on the sending-end converter to integrate offshore wind power from hybrid network where all generated power is injected to the point-to-point DC link with a stiff AC bus at the wind farm network. Additionally, two possible connection topologies of future VSC-HVDC were investigated for steady state and transient conditions. The operation of a multi terminal DC network (MTDC) for hybrid offshore wind farms is analysed with power flow studies of a 5-terminal MTDC model regulated by droop control.Finally, this thesis investigates three VSC-HVDC connections schemes designed to transfer 2.4GW of power from two separate Dogger Bank wind farms to the GB grid, in terms of the investment costs, controllability and reliability against expected scenarios. The benefits and drawbacks of all three scenarios are highlighted. These include the benefits of auxiliary cables on AC and DC site of the multi-terminal connections.
|Date of Award
|20 Sept 2017
- University Of Strathclyde
|University of Strathclyde
|Olimpo Anaya-Lara (Supervisor) & Bill Leithead (Supervisor)