Research And Simulation On Overvoltages Of Vacuum Circuit Breaker Switching Transformer In Offshore Wind Parks

Offshore wind farm is a huge cable system.It consists of large numbers of cables,unconventional turbines,dry-type transformers and frequent operated breakers in the switchgear.High frequency transients propagation and the multiple reflections in collection grids may create overvoltages with high magnitude and high steepness,which are suspected to damage the insulation of transformers.Therefore,in this thesis work,the no-load energization of step-up transformers for wind farms are modeled and simulated in ATP-EMTP software.Issues concerning different wind farm topologies,different operation modes,different numbers of running cables and distance between two generators,for the impact on the magnitude and the time derivative of TO Vs are discussed.Also,some overvoltage protection schemes are analyzed and suppression effects of different mitigation methods are studied.This thesis consists of three parts.The first part is the reason analysis and qualitative calculation at high-frequency transient overvoltages based on wave reflection and refraction theory.The second part includes correct modeling of high frequency components in WPs with the software ATP-EMTP.Firstly,the paper establishes circuit breakers by MODELS module in ATP-EMTP,which can simulate current chopping,re-ignition and pre-strike,and the influence of breaker characteristics on overvoltages are studied.Secondly,frequency-dependent cable is modeled by support program ATP_LCC,and capacitive coupling effects of cable are also simulated.Thirdly,high frequency transformer is modeled by BCTRAN transformer model shunted with type-96 pseudo-nonlinear hysteretic inductor and winding equivalent capacitance.The last part of the thesis is modeling of on-load switching transformer for wind farm collection grid.Issues concerning different wind farm topologies,different operation modes,different numbers of running cables and distance between two generators,for the impact on TOVs are simulated and analyzed.The paper also analyses overvoltage suppression mechanism of RC protection,ZNO protection and ferrite rings and simulates overvoltage suppression effects using ATP-EMTP.Simulation results show that the combination of vacuum circuit breaker and cable networks has been recognized as main cause of problems with transient overvoltages on offshore wind farm.Grid connection topologies,operating modes,numbers of running feeders and cable lengths between two generators have a strong influence on the TOVs caused by circuit breaker operations.The overvoltage suppression effect of magnetic ring shunted with damping resistances is superior to surge capacitor protection and RC protection.