High-Frequency Transient Modeling Of Power Collection System And Occurrence Mechanism Investigation Of Internal Overvoltage In Offshore Wind Farms

With the shortage of energy and the deterioration of environment,offshore wind energy is not only a clean,pollution-free and renewable resource,but also has some advantages over onshore wind resources,such as abundance,stability and the bigger generating equipment availability hours.Therefore,the global countries pay attention to the development and utilization of the offshore wind energy,which also results in the focus of global researchers.The technology is relatively mature,which can provide references for the relevant researches of offshore wind farms(OWFs).However,the topology of an OWF power collection grid of OWF is fundamentally different from that of an onshore wind farm,thus transient characteristics,phenomena and occurrence mechanism of switching overvoltages(SOVs)caused by vacuum circuit breakers(VCBs)are also different between the two systems.Currently,researches about SOVs in an OWF power collection system are less,while the aggregated high frequency(HF)transient overvoltages may damage the insulation of main components in OWFs and influence the normal running,even bring economic losses.As a result,it is necessary to analyze the transient characteristics,phenomena,occurrence process of SOVs occurred in an OWF power collection system with various typical conditions,reveal the occurrence mechanism of SOVs,further study the affecting factors of SOVs and its influences to OWF networks,and develop effective mitigation measures of SOVs,witch are important for the safety and reliability of OWFs and even the entire power grid.(1)Due to the unique design of OWFs,VCBs switch on/off wind turbines frequently,which cause high-magnitude,high-steepness and HF SOVs.Thereby when studying internal overvoltages in OWFs,it is necessary to consider HF characteristics of main components in the power collector system of OWF,such as VCBs,transformers and submarine cables.However,models embedded in the component library of the existing electromagnetic transient softwares are only suitable for power frequency analysis,which are not capable of simulating HF characteristics.Consequently,in order to improve the accuracy of transient simulation results of OWFs,this thesis proposes a HF transient modeling methodology of main components for accurately simulating HF transient behaviors generated in a real world.Finally,a test circuit is built to verify the effectiveness of user-defined models.Analysis results demonstrate that the user-defined models are capable of simulating HF transient characteristics occurred in the real operations accurately.(2)Based on the proposed HF transient models of main components,this thesis builds a complete model of an OWF.Then the amplitude,front time and steepness of prestrike overvoltages are comprehensively analyzed with typical energization conditions,the occurrence mechanism of internal prestrike overvoltages in OWFs is revealed.Moreover,the effectiveness of several overvoltage mitigation methods when they are applied into the power collection system in OWFs,mainly including arrester,surge capacitor,resistor-capacitor filters and smart choke.The results can provide valuable references for the operation,maintenance,the design and application of overvoltage protection devices of OWFs.(3)As it is difficult and limited to conduct measurements from an actual OWF,at the same time there is a lack of an experiment platform to conduct researches for analyzing SOVs in OWFs,which is consistent with the real voltage level of an OWF power collection system.To address this issue,this thesis proposes a design scheme of a laboratory experiment setup for accurately replicating HF transient characteristics of a power collection system in OWFs.Based on the proposed design scheme,a laboratory experiment setup is built.Then according to the recorded experiment data,transient phenomena,characteristics and its occurrence mechanism during the operation process of VCB in OWFs are further studied.(4)Due to the difficulty in carrying out many laboratory tests and the cost of manpower,economics and time,based on the topology,nominal parameters of main components and the statistical characteristics of experiment data of the laboratory experiment platform,this thesis proposes a HF transient modeling methodology of the power collection system in OWFs.By utilizing the proposed modeling methodology,an experiment-based OWF model covering main OWF components is built in PSCAD/EMTDC.Comparison results between simulation results and experiment data verify the accuracy of the proposed modeling methodology.In addition,the proposed HF transient OWF model can also provide a solid foundation for the deep-in investigation of transient characteristics and occurrence mechanism of reignition overvoltages during the opening process of VCBs in the power collection system in OWFs.(5)In order to reinforce the understanding on the occurrence mechanism of highmagnitude,high-steepness and HF reignition overvoltages(ROVs),this thesis proposes a sensitivity analysis research methodology framework for internal ROVs of the power collection grid in OWFs,which is used for recognizing the important affecting factors and causes of ROVs.The statistical result verifies the effectiveness of the sensitivity analysis methodology.Study results can provide references for the construction and insulation deign of main components,overvoltage protection and insulation coordination during the project development of OWFs.