I have a situation with a customer that has had issues with loss of 1 phase on primary side of the 500 KVA POCO owned transformer that serves only them. This has happened 4 times in 3 years due to storms. The service is 208/120Y, 4000 amp protected by a SQD Powerpact breaker. I'm wondering about the wisdom of adding a phase loss kit to the breaker to open it on phase loss. Or would it be better to add shunt trip and use a stand alone phase loss detector. Does anyone have have any opinions about the reliability of phase loss detectors on the secondary of a transformer to detect a phase loss on the primary side?
Personally I wold go with a system voltage monitor that would monitor not only the voltage but for loss of phase. I remember that Westinghouse used to have what they called an SVM which would do the job.
But then how would you reliably trip the breaker if it so happens that the phase that is lost provides the power to trip the shunt trip?
You could use a capacitive trip device (CTD) where a capacitor stores enough energy in order to trip a suitably matched shunt trip installed in the breaker or use an undervoltage release in the breaker that would trip when the UVR looses power. You would connect the UVR in series with a N/C contact in a loss of phase device which upon a loss of phase would openthe N/C contact dreopping out the power to the UVR tripping the breaker.
Loss of phase devices are often used with a N/C contact in series with the motor control coil circuit droppinout the contactor when there is a loss of phase.
