Estimation of Series- and Shunt-Capacitances of a Practical Core-Type Power Transformer and Establishing Mathematical Relationship Thereof

The approach of synthesising a physically realisable ladder circuit model that accurately captures the electromagnetic coupling within a power transformer by characterising its highfrequency data is widely accepted. However, the challenging part is to estimate the inductances and capacitances pertaining to a practical power transformer, as the frequency response is influenced by the core, coupling between windings, the corresponding terminal connections and frequency of excitation. In this paper, a method to estimate the series- and shunt-capacitances of the test-winding in a practical transformer is reported without disassembling the inter-connected windings. The method involves representing a multi-winding transformer (1-Φ or 3-Φ) by electromagnetically coupled ladder circuits and suggesting a way to reduce complex circuits into a single, capacitive ladder circuit for the purpose of capacitance estimation. This task essentially involves proposing novel terminal connections, utilising ontap voltage distribution, and identifying useful features of the driving-point impedance data so as to minimise the influence of the i) inductances, and interwinding capacitances, ii) nontested windings, and core. Further, it is aimed to extract exact information pertaining to series-, shunt-capacitances through systematically derived formulas. The capability of the proposed method is initially demonstrated by simulation results followed by experimental measurements on 15 MVA, 66/11.55 kV, Δ/Y, a practical 3-Φ unit of the transformer industry.