Characterizing high-frequency behaviour of transformer by reduced-order circuit model and assessing the severity of mechanical deformations

Stability and reliability of the power system is highly affected by the state of apparatus connected to it. High voltage power transformer is one of the crucial elements associated with transmission and distribution of electrical energy and hence its uninterrupted functioning is of utmost importance. Abnormal forces generated during short-circuit and transportation would cause permanent me- chanical deformations in transformer. Such incipient fault grows and eventually would lead to catastrophic failure. Therefore, identifying mechanical deformation and assessing its severity is paramount for smooth functioning of transformer and power system. Mechanical deformation of windings gets reflected as changes in the high-frequency behaviour of transformer. Hence, characterizing its high-frequency behaviour is essential. For this purpose, physically realizable ladder circuit corresponding to the high-frequency behaviour of the transformer winding can be built. On this line, ladder circuit incorporating electrical and magnetic couplings has been widely accepted as suitable representation. In order to develop circuit model, certain terminal quantities are required. In this work, a generalized procedure is proposed to estimate various quantities such as effective shunt-capacitance, equivalent inductance and capacitance from the terminal data. It is found in literature that estimating effective series capacitance is not straightforward. To this end, an algorithm is presented. The algorithm is validated considering case studies on circuit model, single-winding and two-winding transformer. To assess the status of transformer winding with regard to mechanical deforma-tion, an approach is proposed. Utilizing the above mentioned terminal quantities and by performing sequential iterations, high-frequency behaviour is represented by an equivalent circuit model. The circuit model is synthesized such that its natural frequencies, terminal inductance and capacitance are nearly same as that obtained through measurement. This methodology involves comparing the cir-cuit model corresponding to its present state with that of its counterpart when it was healthy. The location of deformation is identified by the changed parameter in the circuit. Further, the amount of change reveals the severity of introduced deformation. In practice, the transformer has many windings. In such scenario, extending the proposed approach for fault diagnostics becomes very challenging. As there is a need to model the high-frequency behaviour of winding by a ladder circuit, multiple winding unit corresponds to multiple ladder circuits. All these ladder circuits are electrically and magnetically coupled. Using such complex multi-ladder network for the purpose of diagnostics is really cumbersome. To this end, an approach is presented for simplifying this complex network. In this work, multi-winding transformer unit is realized by a reduced-order circuit model, that is, single ladder circuit. Once such ladder circuit is available, the same principle of comparing two circuits is followed. The algorithm is demonstrated with a two-winding transformer unit. Further, a novel method is proposed to minimize resonance effects in transformer. This method involves designing transformer winding corresponding to power frequency excitation. Utilizing such geometrical information, estimation of its constants such as inductances and capacitances is achieved. Then, these parameters are utilized for constructing physically realizable ladder circuit model. From the circuit model, natural frequencies are estimated. If any of the natural frequencies coincides with the dominant frequency of the incoming surge then winding geometry is modified. The new set of natural frequencies can be obtained such that the resonance phenomenon is avoided.by Krupa Rajendra ShahPh.D