Negative Phase Sequence

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.

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.