When a three-phase three-limb transformer, whose primary windings are Y-connected with isolated neutral, is energized or reclosed by asynchronous closure of the circuit breaker poles, the transient voltage at the still disconnected transformer terminal and the transient current flowing from the connected windings may reach considerably high values. This can cause serious damage to a transformer and equipment connected to the transformer. The transient phenomenon is closely related to the switching sequence of the circuit breaker poles, switching-on angles, remanent flux, and the magnetic saturation characteristics of the transformer's iron core. This paper presents a simple and effective method to predict the transient phenomenon of transformers. The proposed method formulates a simplified model to represent the transient phenomenon under various operating conditions. Simulations and experiments both are demonstrated to support the proposed method.
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