Monitoring-point selection for phase-monitoring relays in legacy contactor-based motor starters: evidence from Iraqi refineries

Abstract

Legacy contactor-based induction motor starters in Iraqi refineries continue to rely on phase-monitoring relays to protect motors against single-phasing and severe voltage unbalance. In many aging installations, these relays are wired on the line side of the starter, which can prevent detection of downstream phase loss caused by welded or contaminated contactor poles. This paper proposes a practical monitoring-point selection framework that guides engineers in deciding whether phase-monitoring relays should sense the line side, the load side, or both.

The framework is assessed by using a field-based case study in an aging motor control centre in Samawah Refinery. The four contactor-based starters with feeding three-phase induction motors of between 7.5 kW and 22 kW were chosen because of reported history of downstream single-phasing despite a phase-monitoring protection. Relays were moved to the load-side and not the line-side as per the proposed structure, and 3 months before the adjustment and three months after, downstream phase-loss events were documented.

The four circuits had a total average of fourteen downstream single-phasing events per month before the implementation of the framework. Following the transfer of the relays to the load side, post-implementation period did not show any downstream single phasing events and there were no nuisance tripping reports. These results are explained in a simplified manner using a reliability interpretation, and are used to demonstrate that load-side sensing greatly decreases the likelihood of undetected downstream phase loss. The research proves that when selecting monitoring-points carefully, it is possible to achieve significant protection performance improvement in the legacy motor starters, wherein the limited observation window is one of the primary limitations of the evidence.