Dsp-based Low-voltage Short-circuit Protection Of The Research

Relay protection is one of device to respond the fault and abnormal circulate situation of electric power system, to make the breaker action and to send out the signal. With the development of the electric power industry, the structure of low-voltage distribution network and the customer properties change greatly. The traditional stage type current relay protection what is widely used in low-voltage short transmission line, is setting difficulty and easy to action without sequence. It is very difficult to satisfy the demand of the modern electric power system development.This topic analyzes the characteristics of low-voltage short transmission line fault and the methods of relay protection used in this field on the foundation of the research present situation, and present a new protection scheme based on interlocking between impedance relay and over-current relay to the scope extension problem. Because the short-circuit type doesn't influence the sensitivity of over current protection responding the phase differential current, and the impedance relay possess well standing resistance ability, this protection scheme express a good characteristic in low-voltage short transmission line.At the same time, this paper analyzes the hardware of existing computer relay protection, summarize the advantage and disadvantage of those, then puts forward the computer relay protection scheme of low-voltage short transmission line, design the software and the hardware of data acquisition system. Because DSP has prolific hardware resource and eminent function, and its exploitive platform are advanced, and it enhances the performance of protection products greatly, the TMS320LF2407 chip produced by TI Company is used to the main CPU of computer relay protection, to realize thefunction.Because build-in 16 channels 10-bit A/D conversion of the TMS320LF2407 can be started by double event manager, this paper presents a design method which enhances the precision of build-in A/D conversion, and analyzes the error of the new A/D conversion. With this method, one and the same analog signal is entered into the EVA and EVB through two converters of different variation rates .The CPU judges which sequence should be started according to the breadth of the input analog signal, chooses the converting channels, and then outputs the result after A/D conversion.The data acquisition designed on this foundation adopts an improved half-cycle Fourier algorithm which can filter decaying DC component. Without digital filter and A/D chip, the precision of this data acquisition is high. Because the system saves the equipments investment, this distance protection can be used to low-voltage short transmission. This system has already verified at laboratory. The results show that it is accurate enough to meet the engineering requirement and capable of convenient hardware implementation.