Ungrounded Wye - Delta Transformer Protection

[ePOWEReng]

Does anybody know what causes the voltage spike when energizing a three phase ungrouned wye - delta transformer using three single phase switches. Upon energization of the last phase a MOV was blown as well as fuses.

 

[rbulsara]

Ferroresonance, if you are single phasing it and MOV are blowing.

 

[busbar]

A reasonable overview may be §7 in ANSI/IEEE Std C57.105-1978 …Transformer Connections in Three Phase Distribution Systems standards.ieee.org

 

[busbar]

The ungrounded-wye—delta configuration is in the referenced “Connections Susceptible to Ferroresonance” table.

 

[jbartos]

Comment: It is not clear from the original posting whether or not the transformer is loaded and how much. Open secondary transformer can be turned on by using three single phase switches.
Also, it is not clear which voltage level is involved, and whether or not the transformer consists of three single phase transformers or one three phase transformer.
E.g. 500kV line three single phase transformers can be turned on via three single phase switches, when unloaded.

 

[ePOWEReng]

This transformer had some single phase and some three phase load on it. The transformer is a 13.2kV isolated wye to 240V delta. After finding the paper "Self-Generated Overvoltages Due to Open-Phasing of Ungrounded-Wye Delta Transfomer Banks" IEEE Transactions on Power Delivery, Vol. 10, No. 1, Jan. 1995, it looks like the overvoltage was due to neutral shift, not ferroresonance.

 

[jbartos]

Comment on the previous posting: Reference:
R.A. Walling, R.K. Hartana, W.J. Ros "Self-Generated Overvoltages due to Open-Phasing or Ungrounded-Wye Delta Transformer Banks," IEEE Transactions, Volume 10, Number 1, pp 526-531, January 1995 covers "Disconnection of one phase of an ungrounded-wye delta transformer bank from the source that can result in severe overvoltages due to neutral shift and ferroresonance (more exactly self-ferroresonance of the transformer).

 

[busbar]

Should extra ‘ammunition’ be desired, two papers are

Bohmann, L.J.; McDaniel, J.; Stanek, E.K.; Lightning arrester failure and ferroresonance on a distribution system; IEEE IAS Transactions, v29n6, Nov-Dec 1993

Rusch, R.J.; Good, M.L.; Wyes and wye nots of three-phase distribution transformer connection;, IEEE IAS Transactions, v26n4, Jul-Aug 1990