Medium voltage motors - differential protection (87)

vici,

I do not understand over distance to mean without cables. I believe it just means that the relay and the breaker are in two separate locations and there is no immediate limitation on that separation or the length of the connecting cables. Practical matters will start to intrude if you make it hundreds of metres but as most trip signals are either Under-voltage or Shunt-trip it is not a signal transmission issue.

As Mike said, this may not be the best forum, and I would have suggested "Ask the Community", but as you are based in the Home Counties I doubt if you are after a US-Centric answer. I have used the various ANSI relay numbering convention as it is fairly international and if you are dealing with the big international boys (e.g. Siemens, ABB, etc.) they will be using this numbering. The real reason for not having it on this forum is that it is not really covered by BS 7671 (as far as I know - no doubt someone reading this will chip in and say "But in clause....").

My feeling here is that you need to look at the purpose of the protection and think about what fault(s) you are trying to protect against. One of the main faults you are considering with differential protection (if my understanding is correct) is a fault - generally short circuit - on the connecting cable. The protection is therefore trying to shut off the energy feeding in to the fault. You therefore need the CB protecting the cable and can't dispense with it. However the CB at the motor is not actually protecting the motor but rather isolating the motor so that it doesn't feed power into the cable fault. Large MV motors are often synchronous machines with subtransient reactances of around 10-15% and with significant inertia, so can feed quite a lot of energy into the fault. Therefore there needs to be a means to prevent this, which in the situation you describe is a breaker at the motor. With synchronous machines another possible means is immediately shutting off the excitation, which would do away with the extra breaker, but many machines now are permanent magnet machines which means you can't kill the field, and leaves the CB as the most practical solution.

Hope this helps.

Alasdair

vici,

I do not understand over distance to mean without cables. I believe it just means that the relay and the breaker are in two separate locations and there is no immediate limitation on that separation or the length of the connecting cables. Practical matters will start to intrude if you make it hundreds of metres but as most trip signals are either Under-voltage or Shunt-trip it is not a signal transmission issue.

As Mike said, this may not be the best forum, and I would have suggested "Ask the Community", but as you are based in the Home Counties I doubt if you are after a US-Centric answer. I have used the various ANSI relay numbering convention as it is fairly international and if you are dealing with the big international boys (e.g. Siemens, ABB, etc.) they will be using this numbering. The real reason for not having it on this forum is that it is not really covered by BS 7671 (as far as I know - no doubt someone reading this will chip in and say "But in clause....").

My feeling here is that you need to look at the purpose of the protection and think about what fault(s) you are trying to protect against. One of the main faults you are considering with differential protection (if my understanding is correct) is a fault - generally short circuit - on the connecting cable. The protection is therefore trying to shut off the energy feeding in to the fault. You therefore need the CB protecting the cable and can't dispense with it. However the CB at the motor is not actually protecting the motor but rather isolating the motor so that it doesn't feed power into the cable fault. Large MV motors are often synchronous machines with subtransient reactances of around 10-15% and with significant inertia, so can feed quite a lot of energy into the fault. Therefore there needs to be a means to prevent this, which in the situation you describe is a breaker at the motor. With synchronous machines another possible means is immediately shutting off the excitation, which would do away with the extra breaker, but many machines now are permanent magnet machines which means you can't kill the field, and leaves the CB as the most practical solution.

Hope this helps.

Alasdair