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Wye-Delta
Overhead Banks
Switching overhead wye-delta three-phase
transformer banks phase-by-phase can create overvoltage problems and fail arresters or
transformers. This topic was investigated by DSTAR research, yielding guidelines on
switching practices, overvoltage protection, and the capability of arresters to survive
these overvoltages. This research has even discovered a previously unreported overvoltage
phenomenon, creating extreme overvoltages magnitudes, which is the apparent cause for
numerous transformer failures reported in the field. An example of this newly
discovered overvoltage phenomenon is shown below.
Phase to ground voltage on a bank of three 25 kVA low-loss poltop distribution
transformers connected in wye-delta. One phase was previously closed, the second
primar
phase was closed at 21 ms on the plot shown below
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