Research On The Mechanism Analysis And Countermeasures Of The Corona Phenomenon In The Stator Windings Of Large Hydro-generators

With the increasing of single generator capacity and rated voltage,such as Baihetan generator which will be put into operation soon has reached 1000 MW and24k V,the stator corona problem of large hydroelectric generators,especially the problem of end corona,is increasingly prominent.The generator is an important equipment of the hydropower station,the stator winding is the core component of the generator,and its winding insulation is the key component to ensure the reliable operation of the generator and determine the life of the generator.The corona of the stator winding of the generator will accelerate the aging of the insulation,thereby endangering the safe operation of the generator and reducing its service life.Based on the introduction of generator insulation system,this paper summarizes the thermal effect,mechanical damage,chemical damage and power loss of generator partial discharge.This paper theoretically analyzes three types of partial discharge in generator main insulation,Slot Corona and end corona,and expounds various influencing factors of partial discharge in generator from internal and external aspects.This paper discusses various measures to prevent the corona of the slot and end of the stator bar of the large hydroelectric generator.The main measures for the anti-corona of the slot part include the low-resistance anti-corona treatment of the winding slot surface and the elastic fixing process of the semiconductor slot lining.The main measures for the end anti-corona include the resistance voltage division method,the capacitor voltage equalization method and the addition of additional insulation.With the continuous increase of the rated voltage and capacity of the hydroelectric generator,the end anti-corona structure and the optimal configuration of its parameters have attracted much attention.The end anti-corona structure generally adopts a nonlinear high-resistance two-level or multi-level external screen anti-corona structure,at the same time some manufacturers add the inner-screen anti-corona structure.For large hydro-generators of 24 k V and above voltage class,a "end-all anti-corona structure" has proposed.At present,the stator end corona prevention of large-capacity and high-voltage hydro-generators is a hot issue.The relevant anti-corona technology has matured,but for a large generator with a certain rated voltage,how to optimize the stator winding connection to reduce the end corona is insufficiently considered.Taking Xiaowan generator as an example,this paper analyzes the above problems and discusses the applicable scope of the method,in combination with other literatures,two general methods,exchange method and sandwich method,are proposed to reduce the corona at the end of stator by optimizing the connection of stator windings.In order to evaluate the feasibility of this method,this paper analyzes the influence of using "exchange method" to optimize the winding connection of Xiaowan generator from four aspects,including Generator stator end connection,electromagnetic parameters,main protection configuration scheme and stator winding single-phase grounding protection,the main protection configuration scheme is compared with the quantitative design method based on the comprehensive internal short circuit analysisand calculation.The results show that the optimization method is feasible and can minimize the amount of work and related effects.It can be extended to other large hydro-generators.Considering the development trend of “large capacity and high voltage” for hydro-generators,the potential difference between the phase bars at the ends of the stator winding will increase synchronously with the rated voltage of the generator.Therefore,it is more and more necessary to reduce the end corona by optimizing the connection of generator stator winding,and its effect will become more and more obvious.It is suggested that in the generator design stage,the optimization of stator winding connection should be carried out while considering the optimization of the stator bar end anti-corona structure and parameter design.In this way,the problem of generator end corona caused by design can be reduced to the greatest extent,not only the operation reliability of generator can be improved,but also the maintenance cost of power plant can be reduced.