Identification of Inrush and Fault Conditions in Power Transformer using Harmonic Distortion Computation

Signal distortion is mainly caused due to harmonics and non linear core characteristics of transformer. Despite of having extensive methods to mitigate the effects of harmonics, they are always remains present in the current waveform due to inherent characteristic of load and transformer core. In this article, authors have taken advantage of these harmonics and utilized it to identify inrush condition and fault condition. Total Harmonic Distortion (THD) of the current waveform for one cycle, will be measured by performing Fast Fourier Transformation (FFT) analysis. It is observed that the percentage THD will be high in the case of inrush condition due to asymmetry in the waveform. Contrary, during fault condition, % THD will be low due to symmetric waveform. The magnitude of %THD below predefined range will be considered as normal condition and excess THD will be considered as inrush condition, on the other hand, if THD remains within the defined range then it is considered as fault condition. In this article, authenticity of the presented scheme is proved by performing various test cases like initial inrush, internal fault, switching on of considered transformer in faulty condition and CT saturation during fault condition. The scheme presented here is simple and capable to solve complex problem of classification of inrush condition and fault condition.

I. Introduction

Sensitivity of protective scheme against fault with reliable operation is the state of art in digital protection of power transformer. Low to high frequency phenomena takes place due to harmonics, magnetic field, transients and system parameter variations. Waveform distortion is generally caused by harmonics. Due to inherent non linear characteristics of core, transformer itself is a major source of harmonics in power system. Philosophies of Total Harmonic Distortion (THD) based transformer protection is due to algebraic relations of power system parameters and transformer core nonlinearity. Leading parameters, which are utilized to obtain transformer harmonic model are, winding resistance, leakage reactance, core characteristic and core-loss component [1]. Presence of DC component in AC system increases heat and decrease lifespan of the equipment. Effects of harmonics in power system will results in mal-operation of control devices, additional losses/noise, telephonic interference, resonance etc. Effect of current harmonics includes overloading, overheating, mal-operation of relay, skin effect and also capacitor overstressing. Similar consequences due to voltage harmonics are, rise in voltage distortion, noise, induction motor operation and protections.