Research On Current Limiting And Breaking System Based On Current Commutation Driven By High Arc Voltage

The construction and development of the power system in china has promoted larger capacity generators being connected to the grid and regional power grids interconnections.Therefore,the short-circuit current of the power system gradually increases and are even beyond the interrupting capacities of existing circuit breakers,which prevent safe and reliable operations of the power system.In order to tackle down the growing short-circuit current problem,the short-circuit current must be effectively limited,and fast removal of the fault is required.Fault current limiters and fast circuit breakers,as effective methods to limit and cut off the short-circuit current,needs significant research attentions.Based on the analysis of fault current limiter and fast circuit breaker,a current limiting and breaking system based on high arc voltage and current commutation is proposed in this dissertation.It enables fast and efficient limits to the first half-wave peak value of the short-circuit current and cut off the current quickly.The system consists of a current limiting unit and a breaking unit.Under normal operation,current flows through the metal contacts.When short-circuit fault occurs,the current limiting unit is activated.The arc quickly moves to the plate under the action of air blowing,so that the arc voltage increases rapidly,such that the current commutation completes quickly.Then the breaking unit quickly cut off the short-circuit current at the nearest zero point.It has been proved that the connecting multiple current limiting units in series and high-voltage fast circuit breakers are effective to improve the operation voltage level of the system.A mathematical model of current commutation and current limiting process is established.The relevant factors affecting the effective function of the current limiting and breaking system are analyzed.It is pointed out that the rapid increase of the arc voltage in the plate arc chamber is the key point for the rapid and effective current limiting.And the reasonable design of the fast circuit breaker is the key technology to reliably cut off the short-circuit current.The research in this paper focuses on these two key factors.In order to increase the arc voltage,based on the principle of magnetohydrodynamics(MHD),a three-dimensional mathematical model of the arc in the plate arc chamber is established in this paper.An arc simulation and solution platform of air-blowing plate arc chamber is established using fluid simulation software.The arc development and arc voltage characteristics under the influence of arc blowing pressure,plate material,plate structure and plate spacing are calculated.The influence of different factors are analyzed in detail.It reveals the internal mechanism that the arc is cut and elongated by the plate,which causes the change of arc voltage.General design guidelines of the air-blowing plate arc chamber are obtained.Considering that the development of the arc and the external circuit affect each other in the current commutation process,a co-simulation method of the arc simulation platform and the power system simulation platform is proposed.The entire commutation process is clearly and accurately reflected.A simulation analysis of a three-phase grounding short-circuit in an actual 110kV/10kV substation is carried out.The effectiveness of the current limiting scheme and the co-simulation method is proved.Based on the simulation,an experimental platform for the current limiting unit is prototyped,which mainly includes the experimental circuit,air-blowing device,plate arc chamber,and timing controller.Experimental analysis of different influencing factors such as air blowing pressure,plate material,plate spacing,plate arrangement,and combined with the arc image,the correctness of the arc development and the co-simulation method is proved.The internal principle of arc voltage increase in the plate arc chamber under the action of air blowing is revealed.It provides important theoretical and technical support for the optimization design of the current limiting unit.Finally,the fast circuit breakers of different voltage levels in the current limiting and breaking system are studied.The permanent magnet-repulsive hybrid fast mechanism is designed for medium and low voltage levels.And 12kV and 40.5kV fast vacuum circuit breakers are fabricated.According to the national standard GB19 84-2014,type tests including basic short circuit test,endurance test and other test contents are carried out.The two hybrid fast vacuum circuit breakers can cut off the short-circuit current of AC component 31.5kA(effective value),DC component 75%and AC component 25kA(effective value),DC component 50%within 6-7ms respectively.The short-circuit current can be quickly disconnected at the first zero-crossing point after the short-circuit current is limited.In order to meet the requirements of the current limiting and breaking system for the development of higher voltage levels such as 126kV,a fast magnetic operating mechanism suitable for high-voltage circuit breakers is proposed.It has large stroke,high reliability,high speed and high controllability.Aiming at the voltage level of 126kV,a two-dimensional(2D)finite element simulation model of the magnetic mechanism is established.A 126kV single-break fast vacuum circuit breaker based on a magnetic operating mechanism is developed.According to the international standard IEC62271 and the national standard GB 1984-2014,the type tests are passed.The experimental results show that for the AC component of 40kA(effective value),the short-circuit current of 45%of the DC component can be reliably interrupted within 30ms.Compared to the existing 126kV single-break vacuum circuit breaker,the breaking speed has been greatly improved,which fully meets the requirements of the development of current limiting and breaking system of 126kV level.