Optimized Design of Collector System for Offshore Wind Farms and Development of A Hybrid Controller for Single VSC-HVDC and Multi-Terminal VSC-HVDC System

Abstract

This thesis deals with the optimal design of the electrical collector system of offshore wind farms (OSWFs) and the design of a robust controller for the grid-integrated OSWF with voltage source converter (VSC)- high voltage direct current (HVDC) transmission system. The worldwide installation of offshore wind farms consists of hundreds of higher rated wind turbines, which have been significantly increased in number due to their economic benefits. First part of the work in this report describes an efficient approach for improving the wind farm power production by appropriate placement of wind turbines in OSWF using the larsen and jensen wake models. A new optimization approach based on (a) elitist ant colony optimization for travelling salesman problem and multiple travelling salesmen problem and (b) firefly algorithm for travelling salesman problem and multiple travelling salesmen problem are applied to design an optimal electrical collector system for OSWF with the objective of minimizing inter-array cable length and there by reducing the cost of power production. The objective function of the electrical collector system design is expressed based on the levelized production cost and aims to minimize the levelized production cost, minimize the length of the inter-array cable between the wind turbines, achieve wake loss reduction, and optimize the power production of OSWF. The proposed approach is tested using North Hoyle and Horns Rev OSWFs with 30 and 80 wind turbines, respectively and the results obtained is observed as a valid optimal electrical collector system design. The thesis further proposes a new hybrid controller for AC grid integrated offshore wind farm with VSC-HVDC transmission system and AC grid integrated offshore wind farms with multi-terminal VSC-HVDC transmission system. It is combination of proportional{integral (PI) based inner and sliding mode control based outer controller. With the hybrid controller, the VSCs of the HVDC transmission system are connected for control of the AC voltage, DC-link voltage, reactive power and effective power transfer between the OSWFs and an onshore AC grid. An evolutionary algorithm and proportional{integral{derivative (PID) tool are iiiutilized to realize the tuned gain parameters for hybrid and conventional controllers. The FRT capability, small signal analysis, and controller stability of the VSC-HVDC systems are analyzed. To check the stability of the system, small signal stability analysis is carried out with the hybrid controller and performance is compared with conventional PI controller. To examine the fault ride through (FRT) capability, a symmetrical fault and unsymmetrical fault are applied at an onshore AC grid side and the performances of the system based on the hybrid and PI controllers are analyzed. Dynamic model and linerized state-space model of the VSC-HVDC systems with hybrid and conventional controllers are developed. The analysis of the VSC-HVDC systems with hybrid and conventional controllers is conducted in the software environment of the MATLAB/Simulink. The simulation results show that the proposed control scheme provides effective active power transmission, AC voltage control, minimum reactive power transfer among the VSCs, and DC-link voltage regulation in the presence of system uncertainties and faulty condition. The controller stability is observed with the help of the Nyquist plot and eigenvalue analysis. The effect of parameter uncertainty on total system stability is examined with the help of eigenmatrix of the VSC-HVDC system.