Research On Internal Switching Overvoltage Of Offshore Wind Farms Considering The High Frequency Transient Characteristics Of Vacuum Circuit Breaker

Vacuum circuit breakers(VCB)are widely used in various locations in the collector system of an offshore wind farm(OWF).The problem of severe damage to equipment insulation caused by overvoltage due to frequent switching operations is becoming more and more serious.At the same time,due to the complex marine environment and higher maintenance costs compared to those of an onshore wind farm,the requirements for equipment insulation protection in an OWF are also higher.Therefore,it is very essential to build a simulation model that can accurately reproduce the internal transient overvoltage caused by VCB switching operations,and analyze the generation mechanism and suppression scheme of such kinds of overvoltages based on this model.This dissertation has conducted the research on this subject in-depth,and the main work contain the following aspects:(1)This dissertation proposes a high-frequency(HF)modeling method for VCB based on the experimental statistical analysis in which the data are gathered from a simulation test platform of an offshore wind farm.First,on the basis of the micro-process analysis of the dielectric strength(DS)recovery after the arc is quenched in the vacuum gap during the re-ignition of the VCB switching off process,a piecewise linear calculation method for the DS of the VCB based on the statistical analysis of the breakdown voltage distribution is proposed.Compared with the linear or polynomial fitting method,it reflects the actual change law of the dielectric strength during the switching off process,and more precisely reproduces the measured over-voltage during the re-ignition process.In addition,the modeling method also proposes a parameter setting method of high frequency quenching capability(HFQA)based on statistical distribution analysis.The HFQA of the circuit breaker is fitted linearly by analyzing the distribution law of the change of rate of the current at arc extinction points during the reignition.The current fitting method is proven to be able to reproduce the measured reignition arc currents accurately.(2)This dissertation builds a set of simulation test platform for reproducing the switching overvoltage of single wind turbine system in an OWF.Its topology,voltage level,cable length,parameters of the key equipment,etc.are consistent with the actual parameters,and the platform overcomes the problems of difficulty of measuring and the lack of test flexibility of the on-site measurement in an actual OWF.Also,the calculation accuracy of our test platform is very close to the measurements in an actual OWF.On the basis of this test platform,simulation experiments of switching operations under different scenarios were carried out,and it was found that when the transformer close to the wind turbine generator(WTG)was fully loaded,the opening operation of the VCB would cause serious HF overvoltage.Thereinto,the amplitude of single-phase overvoltage reaches 130 k V and the maximum overvoltage steepness can reach 135.6 k V/?s,which is 1.5 times and 13.5 times of the rated withstand voltage standard,respectively.(3)This dissertation analyzes the generation mechanism of the most severe reignition overvoltage detected in the scenario that the WTG transformer is switched off with full load.It is concluded that the cause of overvoltage is that the VCB itself has a certain arc quenching capability and the forced cut-off of the HF coupling currents at non-zero crossing points,which is originated by inter-phase HF coupling effect in three phase submarine cables.Based on this,three key factors affecting the overvoltage are obtained,and then their quantitative analysis of the trend of their influence on the magnitude and steepness of overvoltage and specific measures to suppress the overvoltage caused by such ”virtual current chopping” are given: including increasing the phase-to-phase capacitance of the cables in the WTG tower,reducing the operating power of WTG or performing the opening switch after the WTG is shut down,and appropriately increasing the equivalent ground capacitance in front of the WTG transformer.Finally,above conclusion is applied to the analysis of an overvoltage insulation breakdown accident caused by VCB operations in an actual OWF.It is proposed to cut the actual power of the WTG transformer by half and add a resistor-capacitor snubber,and the method is verified to effectively suppress the HF and high amplitude overvoltages.(4)This dissertation proposes a combined protection scheme of ”arrester + resistancecapacitor snubber” for the mitigation of the severe HF overvoltage caused by the switching off operation of VCBs in an OWF.In the simulation,the comparisons for the overvoltage suppression effect of different protection schemes in multiple scenarios(including switching off a single WTG,switching off the entire feeder,and switching off the parallel reactor)were conducted.The simulation results show that the addition of the customized resistance-capacitance snubber on the load side of the VCB can effectively suppress the low-frequency oscillation generated during the feeder switching off and the severe HF overvoltages generated when the WTG transformer or the reactor is switched off.