The Design And Realization Of A New Electric Trip Unit

The MCCB(Moulded Case Circuit Breaker)is usually used for the protection of distribution feeder in the low-voltage distribution system.Following the mission promotion of Smart Grid construction in our country,MCCB as the basic component of the power grid,has become an important constituent part of the Smart Grid.It is an inexorable trend for the MCCB to developing in the direction of intellectualization,systematization and networked.The electric trip unit as the control core of the MCCB,is developing along with the development of the MCCB.For the problem of working unreliability,poor measuring accuracy,single protecting function and incompatible communication on the existing traditional electric trip unit,at the same time,consider of both the actual demand and the development trend in the future for the electric trip unit,a new type of electric trip unit have been designed in this paper.It has the functions of four over current protection(include overload long delay protection,short-circuit short delay protection,short-circuit instantaneous protection,ground fault protection),self-diagnosis,ZSI(Zone-Selection Interlock),pre-alarm and Modbus communication.It can complete real time monitoring and protection for the current,and has an advantage of intellectualization,systematization and networked.This paper mainly research in the following aspect.Firstly,the self-generated power supply control of the new electric trip unit was designed and realized.It acquires the power supply energy,and modulates the obtained voltage signals through the power supply circuit,as the power supply of the microprocessor and other modules.Secondly,the current signals sampling processing of the new electric trip unit was realized.It realize the real time sampling of the input signals by means of sampling 96 points of each cyclic wave and extract 32 efficient point adopted median filtering.It uses the algorithm of fast Fourier transform(FFT)for the current sampling compute processing to realize the four over current protection.Further,according to the sufficient research and explore of the electric trip unit functions,the self-status perception,analytical judgment,decision-making,implementation,overload fault prediction and fault preprocessing of this new electric trip unit were accomplished,make the new electric trip unit developed towards the intelligent direction.It design and realized the function of ZSI,solve the problem of selective overstep operation;make the new electric trip unit has the ability of systematical analysis.Afterwards,according to the National Standard of Communication specification for low-voltage electrical apparatus,the Modbus communication of this new electric trip unit have been designed,make the new electric trip unit can direct access the Modbus fieldbus network.Lastly,through the experiment and test,the new electric trip unit functions were proved correctly.It index conform to the requirement of our National Standard.The new electric trip unit has a higher reliability,better real time performance,electromagnetic compatibility,and achieves the design objective of anticipation.