Planning Ahead to Understand and Prevent Failures
By Gabriel Paoletti, Eaton
Switchgear is a critical part of every utility’s power distribution system; and, as such, any malfunction can be catastrophic. Utilities can take these power system components for granted, mistakenly believing that the switchgear will continue to run without any attention or maintenance.
Switchgear malfunctions can, however, spell immediate disaster in several ways. One concern is with the time required to repair or replace the switchgear following a failure. Downtime from outages can be extensive. A switchgear assembly contains the copper bus, connections and control wiring of the individual circuit breakers. So while a spare can replace an individual circuit breaker in 30 minutes, it can take weeks to repair or replace the entire switchgear assembly.
A switchgear failure also can completely prevent power flow. Should switchgear malfunction cause damage to the outgoing or incoming cables that are connected, this damage can result in additional equipment, labor and, most importantly, additional time to repair or replace.
This article explores some common causes of switchgear malfunctions and provides recommendations on the core preventative measures utilities can take to avoid unexpected setbacks from equipment failure. To avoid downtime, these basic steps must always be addressed before exploring or investing in any new maintenance automation technologies.
Attend to Environmental Conditions
Through a recent analysis of IEEE reported failure data and customer surveys, I discovered that “preventable causes account for over 50 percent of all switchgear failures.” These findings were first summarized in the paper, “Past, Present and Future Maintenance Practices: Monitoring of Electrical Equipment Failure Indicators and Alarming,” presented at the 2015 IEEE IAS Pulp and Paper Industry Committee Technical Conference in Milwaukee, Wisconsin.
The most common switchgear failures can be split into three general categories that are all environmental in nature: 1) humidity and internal arcing; 2) dust and surface contamination; and 3) mismanaged heating resulting from either overloaded equipment or deteriorated connection points or electrical contact areas. The good news is that implementing a simple maintenance program can control all these common causes.
A few additional causes, such as a lightning strike, operator error or rodent damage, might seem unavoidable. But these also can sometimes be addressed beforehand by routine maintenance. For example, monthly inspections would help ensure that no unexpected openings exist in which rodents could enter the location to destroy the switchgear.
Safety Checks, Preplanning for Switchgear Maintenance
Regular switchgear testing and inspection can go a long way. For some tasks, it’s important to ensure that factory trained or independently certified individuals complete the switchgear maintenance. Quality vendors typically provide these services and can advise on a plan specific to the site.
|For some tasks, it is important to ensure that factory trained or independently certified individuals complete the switchgear maintenance|
|Prior to beginning switchgear maintenance tasks, utilities should complete a required arc flash study and label all circuit breakers and electrical switching devices.|
Before getting started, utilities should complete a required arc flash study and label all circuit breakers and electrical switching devices. Next, the facility should have an up-to-date electrical single line diagram. Without these two items, the operation, maintenance and trouble-shooting of equipment can put individuals at significant risk. Personnel safety must always be considered and prioritized.
There have been cases where an entire substation 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 before the main circuit breaker operates.
“- Proper settings of all circuit breakers, relays (and fuse sizes) as determined by a coordination 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 system 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 equipment room is in good repair, regular maintenance tasks are addressed and critical load testing becomes less frequent.
The first tier of this plan assures that employees complete weekly inspections of the switchgear 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 sensor can be added if there are reasons to monitor for potential overhead water leaks.
The second tier is an annual opening and closing of all circuit breakers 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, corroded 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 switchgear maintenance program is a complete shutdown 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 environmental conditions.
Beyond these standard approaches are many opportunities to leverage new technologies to perform remote monitoring of internal switchgear temperatures, the build-up of dust, floor water or humidity. The application of these technologies, however, requires that utilities first consider the proper routine maintenance requirements for their electrical systems. Without that solid foundation in place, you can only hope that a failure does not occur.
Gabe Paoletti is the technical application engineering manager, Electrical Engineering Services and Systems Division at Eaton. He has over 42 years experience with switchgear maintenance.