jdsmith wrote:
What if we had a transformer differential relay with CTs on the transformer primary and CTs on the load side of the LV main breaker? The entire fire pump controller would be in the zone of protection. The differential pickup could be set above the locked rotor amps of the fire pump.
Not quite. To begin with the motor current doesn't matter. The differential relay only looks between the CT's (in - out < threshold value) so the zone only extends from CT to CT. As the motor current no matter what it is is outside of the zone, it does not affect the response of the differential relay. You could put the second set in the motor branch wiring which would be silly but would achieve the intended goal, if installed as secondary protection in conjunction with the existing primary side protection. I know it seems like the second set of CT's should be doing SOMETHING useful, even just basic 50/51 relaying, but 87 relays don't traditionally do that. A simpler solution if you are going this far is to simply install the system as described but trip based on simple 50/51 protection on the primary and secondary sides of the transformer and bushing CT's could be mounted inside the transformer compartment directly on the bushings eliminating arc flash except in the tiny zone on the bushing itself, although some creativity could also use this for 87 (differential) relaying over the transformer itself eliminating this zone, too.
Another strike against 87 relaying is that for some reason it tends to be very expensive and the price is over and above the cost of the extra set of CT's and the wiring associated with it. The 87 relays I've priced out in the past have been very expensive which is the primary driver for not recommending 87 relaying on transformers under 10 MVA. Although with sudden pressure rise valves, that's the threshold that most insurance carriers begin requesting more exotic transformer protection.
Not that it can't be done or that cost is necessarily an issue but most of the fire pumps I've seen, especially those that are fed from a pole mounted transformer with primary protection only, run around 25-100 HP and that's it. The customer often listens to the local fire marshal and even questions something as simple as a fused lockable disconnect switch ahead of the starter. despite the fact that the secondary fuses are Code required to achieve the required level of short circuit protection without tripping out on transformer inrush and the fact that the disconnect is OSHA mandatory for a lockout point. That sort of customer probably isn't interested in springing for the kind of protection that you typically see in 15 kV+ double ended substations with redundant 10+ MVA transformers, where spending another $10K on instrumentation is buried in a line item in the budget.