부산대학교 도서관

This paper describes the specific requirements on measurement and analysis of the dielectric response of oil-paper-insulated, inductive instrument transformers and presents various case studies. Dielectric response testing has been developed decades ago for condition assessment of power transformers. Measurement and analysis of the dielectric response in a wide frequency range provides information about the insulation condition of oil-paper-insulations; that is water content in the solid insulation and oil conductivity. The adaption of this method for instrument transformers requires knowledge about the insulation construction and influence of specific effects. This paper describes the effect of different instrument transformer designs on the dielectric response. It has been found that for voltage transformers with their specific HV insulation design, the measurement and analysis of the dielectric response is of almost no benefit. In contrast to this, for current transformers a model similar to power transformers can be utilized for estimating the insulations aging condition. This is similar for combined voltage and current transformers. Case studies with scrapped instrument transformers and a large number of field measurements underline the findings. The dielectric response (capacitance, losses and dissipation factor) has been measured across a wide frequency range of typically 1000 Hz to about 1 mHz. Lower frequencies are in most cases not necessary due to the absence of large oil gaps. With on-site measurements, a major challenge comes from noise and disturbances of the surrounding substation. Various attempts are discussed to limit the effect of noise. These are special connection techniques, proper guarding, the evaluation of specific criteria in the dielectric response and the application of high measurement voltages. In summary, dielectric response analysis is found valuable for current transformers with oil-paper insulation, but not for voltage transformers. Measurement and analysis still requires the knowledge of an experienced user.