Evaluating the Inter-Resonance Characteristics of Various Power Transformer Winding Designs

Resonance is a very familiar phenomenon in electrical circuits that occurs when a frequency of incoming signal is similar to the natural frequency of the electrical circuit. When resonance occurs, excessive over-voltage may appear. Resonance over-voltage is one of the factors suspected as the cause of the power transformer failure, especially when the transformers are in good condition and also protected by arresters. In this study, inter-resonance phenomenon, namely the resonance that occurs inside the transformer windings, were investigated. The small-scale transformers were designed with four different type windings: continuous disc winding, shield disc winding, closing disc winding and layer winding with additional measurement points. The phenomenon of resonance was characterized by sweep frequency response using current SFRA technology. The resonance frequency in the inside winding shift several kHz from one in the full winding. This may lead to inter-resonance indicated by the voltage at a location cross over to the other location. Based on SFRA characteristics, it was found that the highly potential occurrence for inter-resonance is above 100 kHz. This may strongly influence the voltage distribution along the winding. Layer winding is less risk of inter-resonance than disc winding. In a homogeneous winding, the inductance curve can be used as a reference to predict the resonance frequency in the real transformer by extrapolating.