Reliability And Economic Evaluation Of AC And DC Collection Systems For Offshore Wind Power

To tackle the energy crisis and environmental pollution problems around the world,wind power generation is one of the effective solutions.Compared with onshore wind power,offshore wind power has the advantages of rich wind resources,no land restriction and high annual utilization hours,so offshore wind has broad development prospects.With the wide application of high voltage direct current(HVDC)technology in large-scale,far-reaching offshore wind farms,the choices of alternating current(AC)collection-DC transmission and DC collection-DC transmission have become a research hotspot in the field of offshore wind power.Collection system is an important part of offshore wind farm,which affects planning investment and reliable operation of the whole wind farm.In this context,this thesis focuses on evaluating the reliability and economy of DC collection system,and compares it with the traditional AC collection system.The main research contents are as follows:(1)The characteristics of AC and DC collection systems of offshore wind farms are analyzed and the main electrical equipments that affect the economics of collection systems and reliability evaluation methods are summarized.Aiming at the multi-state characteristics of offshore wind power collection system,a universal generating function(UGF)method based on the analytical method for reliability evaluation is proposed.The method can realize the effective simplification of the multi-state system with multiple performance parameters,greatly reduce the number of system state spaces and significantly improved the calculation efficiency.(2)The reliability evaluation model of DC collection systems based on UGF is established.First,power model of wind turbine is determined.The Jenks Natural Breaks method is used to cluster time series power data of wind turbine into a finite number of states,and the component states are determined through the Markov process to obtain the availability of key equipment of system.Then four typical DC collection system topologies and their equivalent simplification methods are analyzed,and the reliability mathematical model based on UGF is derived.Meanwhile,the calculation methods of reliability indexes-expected energy not supplied and generation ratio availability are given based on the reliability function.(3)The economic model of DC collection system considering the initial investment cost and the loss cost during whole life cycle is proposed.Taking a largescale offshore wind farm DC collection system as an example,the reliability and economic evaluation of 12 collection schemes with different topologies and wind turbine capacities are carried out.On this basis,sensitivity analysis of wind turbine voltage,converter failure rate,wind turbine failure rate and repair time is carried out.The results show that Radial-parallel connenction-centralized boost topology with larger wind turbine capacity has better reliability and economic performance,which verifies the effectiveness of the method in this thesis.(4)Establish the reliability and economic evaluation model of AC collection system and take a 400 MW offshore wind farm power collection system as an example to compare and analyze the reliability and economy of AC and DC collection systems.The research has shown that AC collection system is still a more reliable solution compared to DC collection system,but DC collection system has cost advantage.This thesis provides some reference value for the planning and design of offshore wind farm.