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.
(CLICK HERE TO READ ABOUT SECONDARY SINGLE-PHASING)
(CLICK HERE TO READ ABOUT SECONDARY SINGLE-PHASING)
Causes
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.
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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.
Reference:
Cooper Bussmann
2 comments:
Really useful info..
Thanks a lot administrators....
You're very much welcome..!
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