SINGLE-PHASING ON TRANSFORMER PRIMARY

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)

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.

Single-Phasing on Transformer Primary (Note: I_FL = 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