Standards on providing motor differential CT's ?
Standards on providing motor differential CT's ?
(OP)
Guys,
Is there a standard for manufacturer to provide differential CT's on big motor (350HP and above) or just the custumer ask them to provide that ?
Please advise.
Thanks
Pitat
Is there a standard for manufacturer to provide differential CT's on big motor (350HP and above) or just the custumer ask them to provide that ?
Please advise.
Thanks
Pitat





RE: Standards on providing motor differential CT's ?
RE: Standards on providing motor differential CT's ?
RE: Standards on providing motor differential CT's ?
RE: Standards on providing motor differential CT's ?
Perhaps it's worth discussing briefly why the extra cost/complexity for differential protection may not be justified even for very large motors: (our plant has 8000hp motors without differential protection... uses instantaneous ground fault, instantaneous phase overcurrent, time overcurrent, current balance... all fed from the switchboard phase ct's)
- Phase to ground fault can be detected with very high sensitivity by instantaneous ground relays wired off the switchboard ct's (no extra ct's at motor required).
- Turn to turn fault cannot be detected by differential or other methods, so there's no advantage to install differential for turn-to-turn
- Phase-to-phase faults are the one type where differential protection provides a faster and more sensitive reponse than the above-mentioned protection.... but phase overcurrent will eventually protect this fault also.
Remember that ground faults are by far more common than phase-to-phase faults. And there is logical reason for this.... ground fault needs only one flaw in groundwall... phase to phase needs two flaws (one in each phase). Also phase to phase has twice the insulation but only 1.73 times the voltage.
So for the extra expense of extra set of neutral ct's and more complex relays and wiring, all you get is slightly better protection for a fairly rare fault (motor phase-to-phase).
RE: Standards on providing motor differential CT's ?
For ground-fault protection, one of the illustrations in C37.96 includes with the 6 CTs and 3 percentage-differential devices, an “87N” directional-overcurrent relay polarized by neutral current or voltage.
I think also that it’s important to note that §3.2 says: “The complete protection scheme must be chosen to achieve optimum service reliability, safety, and protection of equipment at a reasonable cost. It is essential that the operating characteristics of the chosen motor-protection system be coordinated with those of the supply and, if necessary, the process or load involved if selective operation of the protective and control devices is to be achieved.”
That seems to imply that the process loss can potentially be many multiples of the initial motor and associated protective-relay price. On the other hand, these relays must be included in the maintenance routine, so they are not “free” once installed. The driving force may be the cost of lost production for a motor outage, regardless of it’s size.
RE: Standards on providing motor differential CT's ?
It's cheaper and the relays are really ground fault not differentials.
RE: Standards on providing motor differential CT's ?
One other factor I'd like to add on to my earlier comments: although phase-to-phase fault is less frequent, they do occur from end-turn contamination and other factors... and the motor damage can be more substantial since the source path impedance is much less than for ground fault which often must go through generator or transformer neutral grounding transformer or resistor (depending on the installation).