Research Of Medium And Low Voltage Capacitor Group Protection Based On DSP

Power capacitor is usually used as var compensator in various sub-stations and factories. For a long time, monitoring the fundamental wave value of running voltage of power capacitor has been considered the main subject in capacitor protection. But recent statistics shows that electrical harmony would accelerate the aging of capacitor, cause damages to the electrical system, and then arouse severe short circuit failure. 40 percent failures of the capacitors are caused by electrical harmony. However, none of the protection device in our country has taken the harmonic factor into account.This paper analyses conventional failures and abnormal operating cases of parallel capacitor, studies 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.This paper studies the influences of the harmonic wave upon the operational parameters of the capacitor, and then sets up a life-span model for capacitors within 10 percent of capacitor loss. Finally, a method which can be used to forecast the life span of capacitors under the accumulation of harmonic wave has been worked out.With the hardware device which uses the TI product DSP2407 as hardcore, such functions have been implemented: interphase short-circuit failure protection, over-voltage protection, over-current protection, over-load protection and etc. we have analyzed the harmonic wave of power line using the 128-dot FFT arithmetic, and got the Voltage Wave Coefficient. After this we can forecast the capacitor life-span online.Using the DSP chip as controlling unit in the capacitor protection device to achieve various functions, the user can cast or cancel one function according to the practical connections of capacitors. The capability of forecasting the life-span of capacitor which can avoid disabled capacitors being put into the power system again and causing subsequent accidents, has a significance for the stabilities of the working capacitors.