| Due to the increasingly serious energy crisis and environmental issues, wind resources as anew type of clean energy has a broad prospect. The research focus of the domestic and foreignresearchers has gradually turned to the offshore wind field. Meanwhile, the study of the prob-lems in wind farms is relatively mature, so it is possible to study the related issues for offshorewind farm using the technical achievement of onshore wind farm. However, the transmissionnetwork of the offshore wind farm is fundamentally different from that of the onshore windfarm, therefore, the mechanism of overvoltage transients is different under the two system, andthe characteristics of transient overvoltage is various. Recently, the research about the transientovervoltage in offshore wind farm is less, especially due to China started late on offshore wind,the domestic researchers have not discussed the related issues deeply.Based on a typical offshore wind farm system, this article studies the characteristics oftransient overvoltage caused by energising cable feeder or fault in offshore wind farm usingthe PSCAD/EMTDC simulation software to build a system simulation model. And during theprocess of building the system model, the electrical component models are obtained from thebuilt-in model library of PSCAD.Due to the unique design of offshore wind farms, the phenomena of high frequency tran-sient overvoltage is a main concern of system operators caused by various device operations andfaults, and the overvoltage has the characteristics of high frequency, high gradient, uncertain ex-istence time, etc.. When studying the internal overvoltage in offshore wind farm, it is necessaryto consider the high frequency characteristics of the electrical component in collector network.However, the component model in the standard software package of the existing power systemtransient simulation software is suitable to the power frequency analysis, with regard to studyingthe system exists high frequency voltage and current, these models based on the power frequen-cy are no longer effective. In order to improve the accuracy of the transient simulation analysisof offshore wind farms, this thesis develops a detailed circuit breaker model reflecting the actualoperation of vacuum circuit breaker combining with the PSCAD software. By comparing thefrequency response characteristics of different cable models, a frequency-dependent model ischosen for the high frequency transient analysis in offshore wind farms. What’s more, basedon the actual physical parameters provided by the cable manufacturers, the parameters of thecable model has been calculated and corrected. Consequently, the simulation analysis using thedeveloped cable model is able to reflect the actual physical behaviour of cables. Finally, theUMEC model in PSCAD has been improved in some ways and broadened its frequency range.Based on the modified electrical component model, the simulation system model of off-shore wind farms has been rebuilt. And some cases have been simulated under the same systemmodel. Firstly, the impact of the characteristics of the vacuum circuit breaker on the transientovervoltage has been studied. Then, Combined with different collector system topologies, thephenomenon for the transient overvoltage has been compared. Finally, an investigation on theeffectiveness of the overvoltage protection devices in offshore wind farms. The results obtainedfrom this research can be used as a reference for the insulation level designation of electricalcomponents in offshore wind farms and the planning and design of offshore wind farms. |
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