Tee Tang

Optimisation of a sensor for onsite detection of partial discharges in power transformers by the UHF method

Among the different solutions which allow onsite partial discharge measurement in energized power transformers, the UHF technique is gaining general interest. In order to apply this method in existing transformers, it is considered advantageous to design a UHF sensor to be fitted inside the transformer tank. This paper describes the constraints affecting the development of such sensor and the process followed to optimize its design. During this process, different solutions for broadband UHF antennas were analysed. The selection was based on computer simulation and experimental results. Computer simulation was used to evaluate differences in radiation pattern, antenna impedance, gain and effective area. Measurements of the antenna response to different partial discharge sources in oil were performed using an oil dielectric breakdown test set in an electromagnetic shielded laboratory. A group of selected antennas were then tested in a power transformer simulating the broadband signals generated by partial discharges by injecting controlled voltage pulses in the transformer. Finally, a sensor based on a conical monopole antenna structure was built and attached to a specially designed housing to be fitted inside the transformer tank through the drain valve. This final prototype was benchmarked during a high voltage factory test of a power transformer which showed a significant partial discharge activity.

Design and testing of UHF sensors for partial discharge detection in transformers

Fifteen years since the first application of the UHF method for partial discharge in gas insulated switchgear, this has become widely used and the preferred method for PD diagnostics in GIS. Same technique is currently starting to be applied for onsite PD testing of Power Transformers. Significant advantages of this UHF method include the reduction of background noise and the possibility of on-line testing. It does provide a relief of background noise by selecting a working frequency band where interference is minimal or absent . In order to apply this technique to power transformers in service, a specific UHF sensor which can be fitted in a transformer without the need of a power outage has been developed. The process followed to design several suitable sensors and the experimental tests chosen to select and validate the best of these designs are presented in the paper.

EHV Circuit breaker condition monitoring using antenna-based coupling methods

Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of destruction of the interrupter nozzle by cumulative re-strikes is found. Monitoring of voltage waveforms during switching using established method would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, waveform measurements at a moderately high frequency require plant outages to connect equipment. A non-intrusive measurement technique has been developed to measure the high and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measuring technique are discussed and an example of its application in both shunt reactor and capacitor switching is presented.

A Noninvasive Method for Detecting Restriking: Application to the Switching of HV Shunt Reactors

Modern circuit breakers (CBS) may be susceptible to catastrophic failure possibly because they are designed to be more highly stressed than earlier CB designs with multiple interrupters per pole. Improved performance is obtained with dielectrically well-designed interrupters by taking advantage of advanced controlled switching to achieve a reignition-free window during opening to minimize the risk of nozzle puncture. On occasion, CB owners may wish to check whether equipment is performing satisfactorily and whether controllers are producing reliable and repeatable stress control. The monitoring of voltage waveforms during switching using established methods will provide information about the magnitude and frequency of small reignitions and restrikes. However, waveform measurements at a moderately high signal frequency require plant outages to connect specialized equipment. A nonintrusive measurement technique has been developed to measure the high- and low-frequency voltage waveforms occurring during reactor switching operations without the need for an outage. Results of tests performed in the laboratory and in a 275-kV substation demonstrating the capability of this new method to detect reignitions are presented.