Electricity + Control November 2018

DRIVES + MOTORS + SWITCHGEAR

for potential overhead water leaks. The second tier is an annual opening and closing of all circuit break- ers as well as inspection of all cubicles. The failure of a circuit breaker to properly interrupt a fault can lead to a complete switchgear failure. Some circuit breakers are not exercised (opened and closed) for many years during typical operations, which could result in deteriorated lubrication of moving parts and possible wedding together of overheated contacts. These inspection would most probably occur during a weekend maintenance period. Simply operating a circuit breaker is a form of maintenance. In preparation of this, an infrared (or heat-sensing survey) should be completed. In this instance, a trained person observes the electrical equipment front circuit breakers and rear live-bus with an infrared camera to detect any loose, cor- roded or deteriorated equipment. To properly complete these inspections, rear doors or covers must be removed, which is risky. To enhance safety, it is often recommended that infrared windows be installed during a planned maintenance outage to make sure that these critical inspections can continue to be completed without opening any doors or covers. The third and critical aspect of a robust switch- gear maintenance program is a complete shut- down of equipment to conduct a detailed cleaning of the entire switchgear assembly. Simulated load testing of circuit breakers or relays or both, which control the trip circuit of a circuit breaker, should also occur at this time. This type of maintenance is typically performed every three to five years, but timing depends on equipment age, usage and en- vironmental conditions. Beyond these standard approaches are many op- portunities to leverage new technologies to perform remote monitoring of internal switchgear tempera- tures, the build-up of dust, floor water or humidity. The application of these technologies, however, requires that utilities first consider the proper rou- tine maintenance requirements for their electrical systems.Without that solid foundation in place, you can only hope that a failure does not occur.

trouble-shooting of equipment can put individuals at significant risk. Personnel safety must always be considered and prioritised. There have been cases where an entire sub- station shut down because the internal circuit breakers did not interrupt a downstream fault in the proper time period. To avoid this scenario, a routine switchgear maintenance program should always include the following: • Simulated load testing to help ensure that the circuit breakers will operate properly under a fault condition (versus failing to interrupt the fault, which creates the potential for complete switchgear failure and shutdown.) • Testing of circuit breakers and relays for proper tripping-times to support a properly coordinated electrical distribution system. If a feeder circuit breaker is feeding a fault, it should operate be- fore the main circuit breaker operates. • Proper settings of all circuit breakers, relays (and fuse sizes) as determined by a coordi- nation study. Such a study provides updated equipment settings. In addition, it provides arc flash findings and their impact on safety. The study, therefore, should be completed prior to each maintenance period. An update of the sys- tem short-circuit study, coordination study and arc flash study can be completed as one task. Three-tier approach to preventative maintenance Utilities should practice a three-tier approach to switchgear maintenance that assures the equip- ment room is in good repair, regular maintenance tasks are addressed and critical load testing be- comes less frequent. The first tier of this plan assures that employ- ees complete weekly inspections of the switch- gear rooms to help ensure that no water leaks exist; that the gear is not exposed to the outside atmospheric conditions or humidity; and that the room is generally clean, with easy, open access to all equipment and exit doors. A floor water sen- sor can be added if there are reasons to monitor

Gabriel Paoletti is the technical application engineering manager, Electrical Engineering Services and Systems Division at Eaton.

Electricity + Control

NOVEMBER 2018

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