Research On Typical Overvoltage And Suppression Measures Of A Wind Farm

The secure operation of wind turbines has become a concern with the growth in the scale of wind farms.Transient overvoltage,i.e.,lightning transients and switching transients,is a major threat to the secure operation of wind farms.The lightning current flows through the blade,nacelle,tower,and grounding system into the ground,thus forming a circuit.However,the tower and signal cable are located where the electric and magnetic fields change rapidly when dozens or even hundreds of kiloampere of lightning current flows into the ground.The tower and signal cable has coupling capacitances and mutual inductances,thus high transient voltage on the signal cable will be induced.This phenomenon threatens and even damages the control system.Moreover,when vacuum circuit breakers(VCBs)are applied in a collecting system of a wind farm,under certain system conditions may cause multi-prestrike and multi-reignition.The transient process often comes with high-frequency and high-amplitude overvoltage surges in the wind farm,and that would be a serious threat to the safe operation of electric equipments in the wind farm.In order to research the typical overvoltage mechanism in a wind farm,a comprehensive surge impedance model of a wind turbine under lightning and a switching transients simulation model of wind farm which took the multi-prestrike and multi-reignition of VCBs into account were built by Electro Magnetic Transient Program/Alternative Transient Program(EMTP/ATP).Then electrical characteristics of the overvoltage in the wind farm and key factors influencing the transients were analyzed according to simulation calculations.Finally,suppression measures were proposed and calculated quantitatively.The main contents of this paper are as follows:A comprehensive surge impedance model including the blade,the nacelle,the tower,the signal cable,the power cable and the grounding system using ? networks of a wind turbine struck by lightning was established.It is worth mentioning that the capacitive coupled and electromagnetic-induced overvoltage among the tower,the shielding layer and the core of the signal cable has been taken into account simultaneously.Based on the comprehensive surge impedance model of a wind turbine under lightning,influences of the cable shielding layer,soil resistivity and lightning current waveform on the overvoltage were also analyzed.Then,two overvoltage suppression measures,i.e.,grounding at two ends of the outer shielding layer and installation of a surge protective device(SPD),were tested.Combined with the actual parameters of the VCBs in the wind farm,the multi-prestrike model and multi-reignition model of the VCBs were invented by using Fortran language in the MODELS module in EMTP/ATP.In addition,a suitable cable model was selected by comparing different models.Because of the change of electrical characteristics of transformers under the condition of high frequency,an improved model of the transformer under high frequency was established.Based on the model of the major electrical equipments,a switching transients simulation model of a typical wind farm was set up.The VCB model was successfully verified in the transient process.Then three factors,i.e.,closing angles,length of feeder and topology of a wind farm,influencing the overvoltage in the wind farm,were discussed.Finally,a resistance capacitance absorber used as a suppression measure was tested.In a word,on the lightning overvoltage of the wind turbine: results show that a signal cable with double shielding layers reduced the overvoltage in the signal cable.And it was more vulnerable to the insulation at the top or bottom of the signal cable,which would be not conducive to the reliable operation of the wind turbine.Higher soil resistivity resulted in increased voltage on the tower base.In addition,the peak and the front time of the lightning current significantly influenced the overvoltage on the tower and the cable.The effectiveness of the two suppression measures was also verified.On the switching transients of the wind farm: the relationship between the overvoltage amplitude and the closing angle was sinusoidal approximately,and the maximum value appeared when the phase angle was near the peak of the power supply.In addition,the overvoltage at the end of feeder terminal was the largest among all transformers of the same feeder.The overvoltage decreased slowly with the increase of the feeder length when closing the VCB,while the overvoltage which was larger than that under the former situation increased with the increase of the feeder length when opening the VCB.And the star topology was the most superior to cope with the overvoltage caused by switching transients.Finally a resistance capacitance absorber can effectively limit the amplitude of overvoltage in a reasonable range.The results obtained in this paper will provide guidance for the protection of the overvoltage and the design of wind farms.