Extending analyzed frequency range in interpretation of frequency responses measured on a distribution transformer

Khoi Dinh Anh Pham,Thai Thi Minh Pham

doi:10.32508/stdj.v18i3.883

Abstract

In the field of diagnosis of mechanical failures in power transformer’s active part, i.e., windings, leads and the core, the technique of Frequency Response Analysis (FRA) has been recently approved as the main application tool. Mechanical failures in transformer windings reflect changes on measured terminal frequency responses normally in medium frequency range, from several to hundreds of kHz, which is in fact not easy to interpret for diagnosis. The authors proposed a new method based on simulation of a lumped three-phase equivalent circuit of power transformers to interpret frequency responses effectively, but only within low frequency range. This limitation is due to the fact that, the circuit cannot reflect well physical phenomena at medium and higher frequencies. To improve the FRA performance of the proposed method at medium frequencies for transformer failure diagnosis purpose, the paper introduces an investigation on a distributed three-phase equivalent circuit of a 200 kVA 10.4/0.46 kV Yy6 distribution transformer. Result of the investigation is a simplified procedure in determination of electrical parameters associated with the distributed circuit for better simulation based FRA interpretation at medium frequencies.

Highlights

To understand what happens in transformer’s active part after a suspected through fault or during transportation for diagnostic purpose, measurement of terminal frequency responses of voltage ratios in broad frequency range, e.g., from 20 Hz to 2 MHz, are often made and compared with those performed when transformers were in good condition
There is no guide from current relevant CIGRE and IEC standards [1, 2] to identify type and level of fault based on the comparison since there are so many factors influencing measured frequency responses such as transformer type, winding type, winding number, winding connection, winding’s terminal condition, Trang 39
The paper investigated a simulation approach in extending the analyzed frequency range for frequency response interpretation based on a distributed circuit of a distribution transformer

Summary

INTRODUCTION

To understand what happens in transformer’s active part after a suspected through fault or during transportation for diagnostic purpose, measurement of terminal frequency responses of voltage ratios (end-to-end, inductive and capacitive interwinding) in broad frequency range, e.g., from 20 Hz to 2 MHz, are often made and compared with those performed when transformers were in good condition. In order to determine electrical parameters in the distributed circuit based on analytical calculation, complete geometrical data and magnetic-electric properties of transformer components (core, windings and insulation system) must be available [8, 9]. Are analytical formulas for calculating self and mutual inductances in the distributed circuit based on geometrical data and magnetic-electrical properties of the core. The two parameters rel and eff can be determined if one has measurements of self/mutual impedances of/between winding segments as investigated in [10]. The procedure of identifying value of rel and eff is based on the fact that, rel influences much analytical inductances at low frequencies whereas eff shows strong effect at medium frequencies, as illustrated in Figures 6 and 7, respectively

Per segment resistances and conductances

RESULTS

CONCLUSION