Criteria for reliable dual voltage power transformers

Power transmission systems often have more than one voltage level. They also may have several subtransmission levels or be in the process of converting from an existing voltage level to a new one. In such cases economic pressures dictate the purchase of power transformers with primary or secondary windings (or both) which can be easily reconnected from one operating voltage to another. Transformers can then be relocated as conditions change or emergencies occur with a minimum requirement for spare transformers. Designing and building transformers with reconnectable windings requires special consideration to maintain electrical symmetry at all connections. Impedances and short circuit forces can vary widely on the different connections depending on winding arrangements, tapped out spaces, and other design details. These must all be considered in order to produce a dual voltage transformer with acceptable reliability and performance at all connections. The purpose of this paper is to acquaint transformer users with design techniques that produce such dual voltage transformers. Users may then evaluate their dual voltage transformer reliability and also appreciate that some dual voltage combinations and designs are not compatible with acceptable reliability, particularly in regard to short circuit strength.<
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