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Ungrounded Wye - Delta Transformer Protection

Ungrounded Wye - Delta Transformer Protection

Ungrounded Wye - Delta Transformer Protection

(OP)
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.

RE: Ungrounded Wye - Delta Transformer Protection

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

RE: Ungrounded Wye - Delta Transformer Protection


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

RE: Ungrounded Wye - Delta Transformer Protection


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

RE: Ungrounded Wye - Delta Transformer Protection

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.

RE: Ungrounded Wye - Delta Transformer Protection

(OP)
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.

RE: Ungrounded Wye - Delta Transformer Protection

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).

RE: Ungrounded Wye - Delta Transformer Protection


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
  

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