Study Of Microprocessor Based Middle-low Voltage Busbar Protection

At present the special protective devices for busbars in middle and low voltage distribution networks are still not common. Only appended to original busbar protections, so the faults occurred in middle and low voltage busbars have to be cleared by back-up relaying with long time delay. Thus, some kind of faults may spread which originally could be cleared and tremendous economy loss may appear. The necessity of appending special protective device for middle and low voltage busbars in important consumers' substation and that in electrical auxiliary system of power plant is disserted.Microprocessor based high voltage busbar protection has rapidly expanded. On the basis of analysis grouping-principle based middle and low voltage protection are put forward. The special busbar protection has adopted effective measures so that large improvement such as simpleness, reliability, economy, sensitivity and some appending additional functions has been obtaimed.The bus differential protection based on instantaneous values of currents is the most useful scheme nowadays. But it is easy to be influenced by current transformer saturation and to make misoperation. A new method is proposed to detect and distinguish current transformer saturation and linear sections, and the wavelet transform theory is used to detect the singular points of signals. Because the current waveforms got from current transformer have singularity at the points of fault occurrence, the start of saturation and the end of saturation, we can distinguish these points according to their characters on line. Thus, to open or block it in current transformer linear or saturation sections, the bus differential protection can be realized. Even if the fault spread from external fault zone to internal fault zone, the differential protection can avoid misoperation.Main criterion of protection is composite-percentage current differential algorithm according to every sampled value, which can accelerate the protection's trip, reduce large amounts of calculation and have strong stability to TA saturation.