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Unexpected primary fuse blown on instrument VT's

Unexpected primary fuse blown on instrument VT's

Unexpected primary fuse blown on instrument VT's

(OP)
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

RE: Unexpected primary fuse blown on instrument VT's

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?

RE: Unexpected primary fuse blown on instrument VT's


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.

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.
  

RE: Unexpected primary fuse blown on instrument VT's

(OP)
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)

RE: Unexpected primary fuse blown on instrument VT's

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.

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