Is this ferroresonance ?

[reactive]

Your opinions would be appreciated.

We have a client who is experiencing VT failures. The circuit is as follows :
- 88/11kV 75 MVA star/star supply transformer; HV & LV earthed via NER.
- Approx. 80m cable to sub. Furnace breaker (furnace fed via 3 x 1 phase trfrs).
- Surge arrestors are downstream of the furnace breaker.
- VTs are upstream of the furnace breaker on the 11kV sub busbars. VTs are 2 single phase units connected R-Y and Y-B. It is reported that both VTs are failed simultaneously.

Various other VT failures have occurred on the same circuit over the last 2 years. Haven't had chance to get up to site to verify all info.

Seems to me that with the furnace breaker open a resonant circuit may exist, although simultaneous failure of the VTs has got me going (this may not be correct, as many of you know reporting from site can be like the old bush telegraph).

Alternatively, could this be a transient due to opening of the furnace breaker, bearing in mind that the surge arresters are downstream of the furnace breaker.

 

[busbar]

From what I’ve read, for ferroresonance, single-pole switching is usually involved. One good overall reference is IEEE Std C57.105-1978[R1999] Guide for Application of Transformer Connections in Three-Phase Distribution Systems §7 Ferroresonance It lists an open-delta primary and secondary configuration as one susceptible to ferroresonance.

 

[Shortstub]

Two scenarios you may wish to explore:

1) The NER may not be effectively-earthed!

2) Assuming the VTs are open-delta connected, the secondary voltage burden circuit(s) may be causing a sufficient overcurrent forcing the VT(s) into saturation. Check the their voltage kneepoint!