Wye-Delta Overhead Bank Overvoltages

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 pole-top distribution transformers connected in wye-delta. One phase was previously closed, the second primary phase was closed at 21 ms on the plot shown above.

Wye-delta bank overvoltages are the result of one of the following phenomena:

  • Neutral Shift - Neutral shift is primarily a 60 Hz phenomenon which occurs when there is unequal load on the secondary phases and only one primary phase is disconnected from the source. The open-phase condition and the unbalanced secondary load, combined with the transformer’s delta secondary connection, cause a large shift of the banks neutral-to-ground potential. A large voltage, as high as 2.65 times the normal phase-ground voltage, can appear between the bank’s open phase and ground (feeder neutral) if the load imbalance is sufficiently severe.

  • Ferroresonance - Ferroresonance is a complex nonlinear interaction between the saturation characteristics of the transformers and a capacitance. Most often, the capacitance involved is the internal winding-ground capacitances of the transformers. If there is overhead line or cables connected to the transformer bank, downstream of the open point, the line or cable capacitance can also contribute to the interaction. For ferroresonance to occur, there must be little or no load on the transformer bank. This is in contrast to the neutral shift overvoltage for which a significant load must be present.

  • Saturation-Driven Neutral Instability - Saturation-driven neutral instability is characterized by bursts of high frequency voltage oscillations which occur when the second phase of a bank is closed during energization. It is caused by abrupt saturation of the transformers during inrush, in turn causing the neutral voltage to oscillate. These neutral voltage oscillations are magnified on the phase which has not yet been energized. Extreme over-voltages, possibly exceeding the transformer BIL, can occur.

The project was based on both full-scale field testing and computer simulations. Testing included silicon-steel and amorphous-metal core transformers ranging in size from 10 kVA to 100 kVA. Phase-by-phase switching was performed with a wide range of loads applied to the wye-delta banks.

Explicit guidelines for design, operation, and protection of wye-delta banks are provided in the final report which can be used to minimize risks of transformer failure and safety concerns due to possible eventful failure of arresters.

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This study was conducted in: P5-2