A novel method to on-line measuring of power factor (tan δ) and capacitance of transformers´ bushings

Power transformers are one of the most important and expensive components in electric energy networks. The majority of them have been in service for many years under different environmental, electrical, and mechanical conditions. To extend transformers life as long as possible and prevent failures, predicting diagnostic methods and condition monitoring systems are used. Statistical studies have shown that failures of bushings are one of the main causes for long duration outages of transformers. The development of an instrument for supervising the conditions of transformers´ bushings is, therefore, of special interest. Values of power factor (tan δ) and capacitance (PF&C) of bushings are the best indicators of the bushing´s condition. Using two sensors that are installed around the bushing´s surface, it is possible to obtain two on line signals. The first signal shows the bushing´s condition and the second signal can be used as a reference voltage. A bushing monitoring and diagnostic system is introduced. The system consists of hardware and software. The hardware performs four functions: filtering, amplifying, converting obtained signal to digital, and storing the result in computer. The software compares the amplitude and phase difference of two signals to determine the probable difference in PF&C of each bushing.