wilhendrix wrote:
It seems that relying on the primary to clear a secondary short is not a good practice.
The secondary side of a transformer is always troublesome when it comes to incident energy. These are the viable solutions to it that I've found and actually put into service:
1. Keep transformers small enough. For instance with ANSI standard impedance with a 480 V transformer stay at or below about 1500 kVA and use more, smaller transformers. Alternatively raise the impedance although again we're trading off time/current and it is not always beneficial to do this. I haven't investigated the reverse case (lower impedance to trip faster on the primary side).
2. Use the primary side disconnect only if the equipment arrangement is such that there is no possibility of confusion. Where there is a single transformer and/or the disconnect is arranged right beside it, this really can't be screwed up as long as backfeeding is also arranged so that it can't occur, although protective grounding might also be able to mitigate this (never bothered).
3. Use circuit breakers or fuses built into the transformer housing. As an example on an excavator I worked on recently we purposely installed molded case breakers directly on the secondary lugs feeding MCC's. At the MCC end we used main breakers. Thus for disconnect/service purposes only the MCC main breakers are used. The transformer secondary breakers are set so high that they only trip for arcing or short circuit faults on the feeders to the MCC's. This was also done with a panelboard in another application. The breakers on the transformer can only be serviced by using the primary side disonnection means.
4. Another alternative is that at least one vendor makes elbow connectors with a fuse built into the connector. I haven't used this method personally.
5. Put bushing CT's on the transformer secondary bushings and wire these to a relay that shunt trips a circuit breaker mounted on the primary side. This arrangement fixes both issues. It reduces the size of the "high incident energy" zone to existing only inside the transformer tank itself and provides good, fast secondary protection that can be easily adjusted to protect against arcing faults.
6. Put fuses or circuit breakers outside the enclosure but upstream of existing equipment. This is not ideal because it doesn't eliminate the hazard but when retrofitting existing applications quite often the options are limited. As there is really no need for a disconnect here (incident energy is very high) it removes temptation to operate a handle if there isn't one there so a simple enclosure with fuses only (no disconnect) is ideal for the application.
7. Raise the secondary voltage. As voltage increases incident energy decreases in this particular situation. Obviously this is again another limited application situation but if you are already faced with a large above NEMA (500+ HP) motor it may make sense overall to switch to a 4160 V application instead of attempting to run lots of 350+ MCM cables, huge breakers, large motor housing for heat dissipation, etc., just to avoid the dreaded "medium voltage". Even though there is still a significant cost advantage to avoiding medium voltage VFD's up to around 1000 HP, this disadvantage is slowly coming down.