Construction of the SHF Test Apparatus to Measure the Partial Discharge Current Waveforms in the Practical Operation Conditions of Power Transformer and Application to Influence of BTA Addition

Insulation diagnosis and partial discharge (PD) test have been performed from the viewpoint of operational reliability and quality evaluation of high-voltage electrical power apparatus. The Ultra high frequency (UHF) method that measures the PD-emitted electromagnetic (EM) waves with UHF band (0.3-3 GHz) components is known as one of powerful methods of insulation diagnosis based on PD detection for highvoltage electrical power apparatus. To improve the UHF method, accurate measurement of PD current pulse is quite important because the PD current is a radiation source of the EM wave. Therefore, we have developed the SHF_PDPW (super high frequency wideband PD current pulse waveform measurement) system that enables us to obtain accurately waveforms up to SHF band (3-30 GHz). Using this system, we have investigated PD characteristics such as PD current pulse waveforms in mineral oil used as insulation oil for transformer. As a result, this system could observe the PD phenomena which were a few tens picoseconds order rise time, i.e. undetectable ones in the conventional measurement band. It is necessary to investigate the insulation oil considering practical operation conditions such as degassing, oil temperature change, generation of the combustible gas by discharge and heating oil, addition of additives like 1,2,3-benzotriazole (BTA) and so on, in order to improve the operation method and the reliability of it. In this study, we constructed the new experimental system which could investigate PD properties under the practical conditions of the power transformer. Then, as a first step, we investigated influence of BTA additive on PD characteristics of insulation oil measured by the SHF-PDPW system. BTA has been widely used in transformer oil in Japan. The BTA addition was specified in the Japanese standard (JIS C 2320) in 1988 in order to decrease electrostatic charging. As a result, it could be confirmed that the usefulness of developed experimental system and the influence of BTA additive on PD characteristics.