Wednesday, August 3, 2011

Single-phasing is the worst case of unbalance and can either be due to an open phase in the primary or secondary side of a transformer. However, this post will only tackle single-phasing on transformer primary, which exposes electrical equipment such as three-phase motors to unbalance voltages. Consequently, excessive unbalance currents will exist with one line to have at least 230% of normal current drawn by the motor.



There are several circumstances that will result to primary single-phasing. Basically, this is any open circuit in any phase anywhere between the primary of the distribution transformer and the utility substation transformer. These include, but not limited to the following:

·         Primary fuse open.
·         Open winding in one transformer.
·         Primary line burned off by fault due to birds, other animals and objects.
·         Defective contacts on fuses, switches or primary circuit breaker.
·         Primary wire broken by weather, trees and other objects.
·         Open pole on three-phase automatic voltage tap changer.
·         Failure of three-pole reclosers in closing all three lines.

Harmful Effects to Motors and Protection

Single-phasing on transformer primary will cause the current in one of the phases to increase to 230% of normal motor current. This destructive primary single-phasing condition happens with WYE-delta or DELTA-wye transformer configuration. In such cases, the motor temperature may rise at a rate greater than the increase in current. Generally, the overload relays will protect the motor. However, time-delay dual-element fuses (e.g. Cooper Bussman’s Low-Peak and Fusetron) that are properly sized for back-up overload protection could take the motor off from its power supply should the overload relays fail to do so.
Single-Phasing on Transformer Primary
Single-Phasing on Transformer Primary (Note: IFL = 10 A)
It is practically impossible to eliminate single-phasing. Thus, a protective device should sense and operate in the appropriate length of time to protect the motor from the destructive overcurrent conditions due to single-phasing.

Cooper Bussmann


SG said...

Really useful info..
Thanks a lot administrators....

yepyep said...

You're very much welcome..!

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About Me

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.