Detection of Inter-turn Winding Fault in Single-phase Transformers Using a Terminal Measurement Based Modeling Technique
Date
2013-12-12
Authors
Bhowmick, Shantanav
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Abstract
Transformers form a very important part of the power transmissions and distribution network; as they are responsible for the transfer of electrical energy from the power generation sites onto the transmission lines and finally to the distribution stage. Dry-type and oil-filled single-phase transformers, either alone or as a part of three-phase banks, are used extensively in the power distribution network, ultimately providing power to the domestic consumers. Any faults in the single-phase transformers leading to power outages or catastrophic power systems failures cause huge loss of capital, property and in some cases even human casualties. Gradual deterioration of the electrical winding insulation ultimately leads to inter-turn winding short circuit faults; which account for a significant proportion of all transformer failures. Incipient stages of inter-turn winding faults have negligible impact on the terminal voltages and currents of transformers; thus these faults often go undetected by the traditional differential relay based protection mechanisms. By the time, the faults manifest themselves into severe winding short-circuit faults consequently forcing the differential relays to operate for tripping the circuit breakers; a significant part of the transformer windings and core may get extensively damaged. Over the years, various techniques have been developed for detecting and studying inter-turn winding faults; however their practical implementation involves quite a few challenges such as high cost, lack of reliability, low accuracy and need for mounting additional equipment inside the transformer casing. Additionally, none of the existing techniques are suitable for online and real-time condition monitoring of the transformers. This absence of any proven technique to detect incipient levels of inter-turn winding faults in single-phase transformers has motivated the research of this thesis.
In the thesis, firstly, a non-invasive technique for modeling single-phase transformers has been developed which is based solely on the terminal measurements of voltages and currents. The effects of transformer core saturation, non-linearity, hysteresis are incorporated in the model by considering a time-varying magnetizing inductance comprising of any desired number of harmonic components. The coefficients of the magnetizing inductance are computed from the instantaneous values of flux linkage and magnetizing current over one complete cycle. The model is found to replicate the behaviour of the single-phase transformer with an extremely high level of accuracy, under any load conditions for healthy as well as faulty operations. Detailed simulation and experiment based studies have been performed for corroborating the effectiveness of the proposed terminal measurement based modeling technique not only in detecting incipient stages of inter-turn winding faults (involving less than 1% of the turns) but also in estimating fault severity.
Also, a non-invasive, online and real-time implementation of the proposed inter-turn winding fault detection technique for continuous monitoring of the transformer health has been suggested. Firstly, with the experimentally acquired primary line voltage and line current data of the healthy transformer, a healthy no-load model of the transformer is generated. Next, a healthy estimated indicator value, computed from this model under the given input voltage condition, is compared with the actual indicator value for detecting the presence of an inter-turn winding fault. It involves minimum hardware (only two current sensors and one voltage sensor), low memory requirements and low computational complexity and thus holds a good promise for practical applications. Further discussion is made on the possible challenges for realizing the proposed fault diagnostic technique in the industry and suitable recommendations have been made for further improvement.
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Keywords
Condition monitoring, Electric machines, Transformers, Single-phase, Terminal measurement, Fault detection