Unexpected primary fuse blown on instrument VT's

[jabarriosc]

Repetitive and aleatory faults due to primary fuse blown without apparent cause on instrument voltage transformers, open delta connection. No secondary faults has been reported. Usually, events occur in A and C phases and rarely in B phase. VT's are 4200/120 V, 60 Hz, protected by current limiting fuses E rated. Some voltage sags and distortioned voltage waveforms has been registered.
Thanks to anyone for comments.
Jose Barrios

 

[scottf]

Do they fail under steady-state conditions, or when the VT's are energized?

If they are failing under perceived steady-state conditions, you may have some ferroresonance on your system. Do you have any secondary voltage traces from the VT's at the time of the fuse failure?

 

[busbar]

Long Shot — See pages 10-14 on VT-primary fusing. For open-delta PTs, there is some discussion of fusing “…those connections in which each fuse carries the exciting current of only one transformer.” The text referes to this as a “Class-1” connection.

http://www.geindustrial.com/products/buyersguide/GEP-9186-6.pdf

Although the “four-fuse” recommendation seems a bit unusual, it may be applicable to the subject installation for lack of other solutions.

 

[jabarriosc]

Thanks for your comments.
Fuses failed only under steady-state conditions.
Secondary traces from the VT´s at the time of the fuse failure shown a distorted waveform and undervoltage (only one phase)

 

[scottf]

You may have some ferroresonance issues, which is a tough topic to diagnose over a forum. You may want to bring in an experience consultant to take a look at your system.