engleski

Common winding of autotransformer and loading of tertiary

Besides YNa-connected high-voltage and medium-voltage windings, autotransformers in high-voltage networks usually also have a tertiary winding. It is a delta-connected low-voltage winding intended for loading and sized mostly for one third of autotransformer rated power. The paper describes winding loads imposed on an autotransformer with tertiary by loads at autotransformer's medium-voltage and tertiary terminals, and also by loads at its high-voltage and tertiary terminals. It is shown that, if in the autotransformer design the tertiary load is ignored, the common winding can become overloaded. The solution is a control of the common winding current by an over-current relay. The relay current is obtained by current transformers at high-voltage and medium-voltage terminals connected at their secondaries through an intermediate current transformer, and it corresponds to the common winding current.

autotransformer; tertiary; overload

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