Discrimination between inrush and fault currents in an unloaded three phase power transformer based on pre-fluxing and harmonic analysis techniques

[Truncated abstract] This thesis is devoted to the application of suitable techniques to mitigate inrush phenomenon and discriminate inrush from faults in a three phase unloaded power transformer. Further, this thesis applies appropriate technique to estimate and eliminate DC component embedded in the inrush and fault wave during the power transformer energization. The technique used for mitigating inrush applying pre-fluxing is based on setting the power transformers residual flux to a known polarity after the transformer has been de-energized and controlling the incomer circuit breaker closing time. The device used in the pre-fluxing technique is simple to use and easy to construct and doesn't require prior flux knowledge of the transformer core. The key driver of this device is a pre-fluxing device which can operate at a lower voltage level as compared to the overall voltage of the transformer. Using this model a specific flux pattern is established in the power transformer, prior to its energization in an unloaded condition. In the second part of the pre-fluxing technique application, the circuit breaker associated with the power transformer is energized at a positive or negative polarity. With the application of this technique, inrush is greatly reduced and is very close to prospective flux during transformer energization. The motivation to apply pre-fluxing technique is validated using software wherein the inrush is reduced considerably with respect to the normal current as opposed to 10 times inrush without pre-fluxing device. Having established the pre-fluxing technique method of mitigating inrush in the fourth chapter, a method to estimate and eliminate unwanted DC component embedded within inrush and fault is taken up in the fifth chapter, based on harmonic analysis. The practical problems associated with DC component on the protection system has been elaborated in this thesis including, the fundamental concept and initial appearance of DC component. Due to presence of the DC component the protection system response is slow and usually delays the discrimination ability of the relay. One of the significant contributions in this thesis is the estimation and elimination of DC component appearing as a noise and identifying inrush from fault condition based on second harmonic ratio to fundamental, of an unloaded power transformer. In order to eliminate the DC component, a compensating signal is generated based on Taylor series expansion and Least Square Method (LCM) for the inrush and fault current. As the second harmonic ratio (SHR) is a dominant feature in the inrush, discrimination has been carried out by comparing it with a preset value. Details on SHR are found in the review chapters of this thesis. The algorithm developed to discriminate inrush from fault current based on harmonic analysis is validated using MATLAB...