Negative Phase Sequence

This is not my speciality but does this help? What is negative sequence current and how does it affect generator work | EEP (electrical-engineering-portal.com)


The heating effects need to be kept low to prevent generator damage. Perhaps the 20 per cent or less is a rule of thumb thing.


Balanced loads ensure better efficiency.


Z.

I dont think an unbalanced load can exist for long in 3 phase , one story I heard was that on motors a phase drop out is about the most dangerous thing and perhaps spectacular thing you might be lucky enough to live to tell the tale about ,in electrical work on large motors/pumps. The 20% figure perhaps suggests a partial failure , but my understanding is when a phase drops out , time is pretty short

I have spoken to a few people over the past few days and I have found out a little more info, for an alternator with 12 poles, L1/L2/L3 there would be a 30 degree spacing between each pole, SIN(30) equals 1/2 in part of the equation and COS(30) equals SQRT(3)/2 so each pole is separated by 30 degrees. the unbalanced load gives rise to negative phase current which is an opposing magnetic flux that causes a heating effect that can damage the alternator and it also increases the load current drawn, the 20% can be an additional load to the maximum load on a phase and trip the breaker for overcurrent. I think the value is also divided by 3 for each phase.

Seems to me things should be 120o apart on three phase , or you get the problems you outline.

I think Cameron is talking about the angular physical separation of the individual pole winding groups, not the vector angular separation of the phases of the supply circuit. For a two-pole alternator the separation would be 180° and such a generator would rotate at 50 revolutions per second or 3000 rpm at 50 Hz. A 12-pole alternator would rotate at 500 rpm.

This has been an interesting topic, reminding me of my earlier days working in the electricity supply industry. Thank you all contributors.


I once heard some electricians saying that three phase induction motors could keep running on loss of one phase but not start under these conditions. Maybe so but I don't think they would run for long, based on the analysis we have had.


More recently, working as an IT contractor on field service, I experienced a situation where a building lost phases. The small team arrived at a bank, late afternoon, to update IT equipment, including a server, after business hours. The weather had been exceptionally bad that day, with widespread flooding. When we arrived, the bank had closed early for business, because of a power cut we were informed. There were just a few lights on in places, which I initially presumed were on an emergency lighting circuit. We could do little more than unpack the equipment. We could not start work because of the risk of not being able to finish, for the bank to be ready for next morning's business.


A team member subsequently reported that he had discovered that in fact only one of the three phases supplying the building was live - hence just a few lights on. We sat and waited. In due course a second phase became live and eventually the third. In spite of the delay we were able to complete work that evening.


I presume that the loss of two of the phases was cased by a fault between the substation and the building. Its impact on the national grid would be quite small and would not in itself cause a problem on a generator.


When I worked for the CEGB I did not actually experience a situation where a generator lost a phase of its load, though I was well aware that negative phase sequence protection was in place to protect against this possibility.

Hi All,


Thanks again for the info.


I have been reading through the regs and Annex A722 (Informative) - 722.1 opened up in front of my eyes and I couldn't believe it.


The same equation is written but slightly transposed.


I hope this might help someone looking for info on this in the future.