Ferroresonant Overvoltage Caused by a Transformer Interacting with Underground Cables

T.P. Tsao and C.C. Ning (Taiwan)


Ferroresonance, Overvoltage, Power Transformer, Hysteresis Loop, Magnetization Curve


2. Some FR Experiences Currently power transformers are made of highly permeable materials, so their hysteresis loss is lower than that of the low-voltage transformers in a distribution system. On ferroresonance simulation, the hysteresis losses are usually ignored for a power transformer, and the magnetization curve is used to analyze nonlinear characteristics of a power transformer. This paper considers the hysteresis loss by analyzing the ferrorsonance problem and compares it with having no hysteresis losses by a practical example. The difference between the two simulation results is clear. Consequently, the hysteresis loss should be included in the analysis as an important factor for solving the ferroresonance problem. Due to economic consideration, the operating point of a power transformer is closed to the knee point of the magnetization curve. Hence, the transformer could possibly enter a saturation region while system outages occur. Due to transformer nonlinear characteristics, inductance decreases when the transformer enters a saturation region. With an increase of resonance current, the reactance of the transformer will decrease quickly. This becomes a vicious circle, leading to FR and over voltage, which may cause serious damage. Generally speaking, FR occurs frequently in distribution system, especially for potential transformers (PT). Since transmission systems increase higher voltage grade to reduce the losses and lines adapt composite conductors to avoid corona effect, the reactance of transmission lines is quite low. Furthermore, line distance may be quite long, resulting in more line capacitance between lines to ground. Finally the network frequency would be decreased and close to the resonant frequency, possibly leading to FR in the transmission system.

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