Improvement Of Microprocessor-based Protection For Shunt Capacitor

As an important reactive compensation device, shunt power capacitors are applied in power system widely. But recent statistics shows that electrical harmonics would accelerate the aging of capacitor, cause damages to the electrical system and then arouse severe short circuit failure. Forty percent failures of the capacitors are caused by electrical harmonics. However, none of the protection device in our country has taken the harmonic factor into account, showing traditional power capacitor protection themselves have limitations and disadvantages.In this paper, the influences of the harmonic wave upon the operational parameters of the capacitor are analysed, and the conventional failures and abnormal operating cases of parallel capacitor are studied. Then it shows the protection theories corresponding to different electrical connections and discusses the threshold when to apply one protection function according to the national standard that operating voltage can't get over 1.1 times as rating voltage. With the hardware device which uses the TI product DSP5402 as hardcore, both C and assemble languages are adopted to program to achieve harmonic overload alarm function.. The main program includes system initialization subroutine, low-pass filtering subroutine, sampling subroutine and Fourier analysis subroutine. As for computer relaying algorithms, Difference Fourier algorithm is adopted, this algorithm eliminates the influence caused by decaying DC component.Finally, Protective relaying philosophies are analyzed and improved. Some traditional protective relaying schemes' limitations and disadvantages have been given a development in this paper. For example, zero—sequence voltage protection fails to identify the faulted phase and wipe out the influence of three—phase unbalance voltage. This paper analyses the internal over voltage and changes of the phase current, and gives corresponding equations. At the same time, it gives a developed method about eliminating the three—phase unbalance voltage and identifying the fault phase. In addition, based on the multi-branch-off capacitor, current speed off protection and over-current protection for the setting values can not be followed to make the corresponding changes in, Which is worked out properly in this paper .