On-Site Diagnosis of Transformer Winding by Frequency Response Analysis

Abstract

Displacement and/or deformation of a coil in oil-immersed transformers are hazardous. Frequency Response Analysis (FRA) has the possibility to detect these abnormalities with high sensitivity. However, a guideline for diagnosis of transformers by FRA is not yet firmly established. In this paper, FRA is applied to seven oil-immersed transformers installed in substations. Diagnosis methods when initial data do not exist are evaluated. When the initial data do not exist, it may be possible to diagnose by comparing transfer functions of three phases (phase-to-phase comparison) or by comparing transfer functions to those of sister units (sister-unit comparison). The phase-to phase comparison cannot be applied to some kinds of vector groups. The sister-unit comparison is fully applicable and the performance of the comparison can be improved by additionally comparing the transfer functions phase-to-phase. Furthermore, applicability of measurement of the transfer functions without disconnecting the bus from the transformer is evaluated. The bus should be disconnected from the transformer under tests because the measured transfer functions are influenced by bus.