Research On Several Problems In Relay Protetaion Of Large Hydro-generator

In order to satisfy the need of sustainable development strategy and the policy ofadjusting energy structure, China has greatly developed hydro-power. Many large and gianthydro-generator are being in operation, becoming the important power support of powersystem. For the safety of generator operation, complete protection should be configured. Atpresent there are some existing problems to be solved during the operation of largehydro-generator. This article is about in-depth studies and discussions on these issues.Zero sequence transverse differential protection is an main protection of largehydro-generator, it can react the turn to turn and phase to phase faults. And its sensitivity isinfluenced by unbalanced current. After analyzing the causes and influencing factors of theunbalanced current, this paper points out that present single threshold over current criterionand over current criterion taking of generator terminal as restraint quantity have someshortages. A new restraint quantity-gap induced electromotive force-is introduced, and itcan compensate the defect of the phase current restraint criterion. So we can combine thistwo criterions as the main criterion, assisted with blocking criterion taking positivesequence fault component and third harmonic component, it can distinguish betweeninternal and external faults and improve the sensitivity of zero sequence transversedifferential protection greatly.Excitation system is an important component of large hydro-generator. According tothe analysis of the structure characteristic and protection configuration of excitation system,and an accident case occurred in a large power plant, this paper find that existing protectionconfiguration can，t react the short fault of slip ring. This paper puts forward a protectionscheme of adding an over current protection in the excitation transformer low voltage side.The current value and the delay time are recommends on the base of simulating.Large hydropower plant generally have multiple generators operating in parallel. Ifseveral generators are out-of-step and trip, it will cause great impact on system and is notbeneficial to system synchronization. This paper indicates the reasons of out-of-step by afour-machines infinite-bus system simulation model and analyzes the principles ofout-of-step protection and out-of-step prediction protection. On the bases of analyzing theoperation characteristic of multi-machine out of step, this paper gives an new operationscheme. That is, under the premise of multi-machine information exchange and multi-machine out-of-step prediction, we can choose the serious generators to trip,thenthe other generators is easy to synchronize with the system. It is meaningful for thestability and safety of power system.For large hydro-generator, complete and reasonable protection configuration andsetting calculation is key to the accurate reaction of relay protection. This paper takes largehydroelectric generator in a large hydropower plant as the research project. During theprocess of the research, we find several problems about protection configuration and settingcalculation of the generators, and these problems are detailed analyzed in this paper, and itcan provide guidance for the setting calculation of other large hydro-generator.