Create a living program that’s monitored regularly.
There’s a gap in most maintenance and reliability plans: the electrical power system. Yes, maintenance is done and technicians recognize that electrical systems are critical, but important questions need to be asked:
• What powers the most critical plant assets?
• Where is budgeting directed for electrical maintenance?
• Do you fully understand the electrical system?
• Are you overloading the system?
• Is the electrical equipment properly rated?
• What changes have been done since commissioning?
The answers are vital because power failures can cripple a plant at enormous cost.
Proper and effective power system maintenance was addressed at the MainTrain maintenance conference, convened by the Plant Engineering and Maintenance Association of Canada (PEMAC). Paul Baker, president, and Kerry Heid, manager of technical services, both with Shermco Industries Canada Inc. in Regina, delivered a paper on the basics of electrical systems and their maintenance.
Shermco provides engineering and technical services for electric power systems, maintenance and commissioning for utilities, industrial plants, mining and pipelines.
“Remember a fully functioning electrical maintenance plan is not as simple as setting up some testing tasks and intervals, and then walking away. Things change, electric fault current changes as the utility becomes more robust. A maintenance plan needs to be a living program that’s regularly monitored,” says Baker, who is also president of PEMAC’s Saskatoon Chapter and co-chair of the MainTrain committee.
Major components of electrical power systems include: transformers, circuit breakers, switches, cables, motor control centres, capacitors, reactors, battery systems and emergency generators. A catastrophic failure in any of these major components usually results in cascading damage. An arc flash blast, for example, will severely damage a single piece of equipment that has failed, but also many of the components around it. This means major downtime and worker safety is compromised, so begin with an arc flash and protection coordination study.
Baker emphasizes the importance of understanding the P-F curve. It illustrates how equipment fails and how early detection saves time and money. The P identifies a potential failure. For instance, vibration or heat indicates a functional failure is imminent. F is the point where the asset fails to perform one or more of its key functions.
If you can find a P and determine the time between the P and the F, the P-F curve will help asset owners understand the right inspection method to detect a potential failure early. This allows for planning and scheduling of the proper corrective action.
Some of the common tests that are recommended for major components include:
Transformers: test insulation resistance.
Circuit breakers, relays and trip units: test relays, insulation and contact resistance to determine levels of degradation.
Switches: test insulation and contact resistance to determine levels of degradation.
Cables: Try to make a cable fail. The newest and most accurate test is partial discharge.
Baker also draws attention to hidden failures. They’ve already occurred and under normal operating circumstances go unnoticed until another failure occurs.
For example, company XYZ had the protective relays on its 13.8 kV power system calibrated each year, but didn’t maintain or test the circuit breakers because they were new and never operated. But an underground feeder cable failed several months after a maintenance shutdown and the fault cascaded through six circuit breakers before it was cleared. But it was thought the fault lay in the protective relays not being properly calibrated, but upon inspection their indicators showed they had operated. It was determined that because the operating mechanisms were so dry from lack of lubricant, the opening coils burned up and destroyed their enclosures. The intense heat burned the ceramic in the arc extinguishers; all cable terminations and insulations were destroyed; metal inside the circuit breaker was vaporized or melted; and the switchgear had to be replaced. Costs in addition to equipment replacement included those related to production downtime and lost revenue.
There are industry standards and guidelines available to help with a maintenance plan. CSA Z463 deals with electrical systems, and ANSI/NETA MTS are specifications for maintenance and testing. In addition, there are some new technologies available that will help identify potential problems.
What’s the takeaway for maintenance pros? Treat electrical systems with the same care as mechanical systems.
Steve Gahbauer is an engineer, a Toronto-based business writer and a regular contributing editor.
This article appeared in the November-December 2018 print issue of PLANT.