Relay Retrofit Boosts Reliability

Most large oil and gas facilities maintain on-site power generation, with grid connection serving as an insurance policy. This is exactly the case at Marathon Oil’s Yates Field in Iraan, Tex., according to senior engineer Charlie Adams, whose work includes the design and installation of SCADA measurement systems, instrumentation, controls and power systems.

“The generator sets are self-contained. When the utility trips offline it doesn’t necessarily mean the plant stops, but it does mean we’ve lost our insurance policy, and it’s only a matter of time before that bites you,” says Adams.

A short power shutdown presents the very real risk of seeing chemical and cryogenic processes within the plant go “off spec.” It may cost $15,000 in fuel alone to restart the Yates facility. An extended power outage can also make a significant dent in production at Yates, which generates 20,000 barrels a day, a daily revenue stream of around $400,000.

“The Yates Gas Plant had a history of unreliable power service,” Adams says. Maintenance records showed 1,000 trips on the utility tie-breaker in a period of just three years—an average close to one per day. The primary job, Adams knew, was to reduce what he calls “the trip index.”

Investigation uncovered a messy, overlapping combination of generator-control and governor-control problems, reverse power trips, significant transient issues and unreliable electromechanical relays with questionable maintenance histories.

“Existing hardware was all pure analog devices with no capabilities to capture historical data. So, pinning down problems was really a series of educated guesses, since most electrical power events are so quick and there is very little evidence left behind as to what really happened,” Adams says.

Eventually, Adams settled on replacing unreliable relays on the utility feeders with digital relays, installing two generator relays and replacing 34 aging electromechanical relays with new hardware. He sought recommendations from industry contacts and his power supplier—a utility.

“Reliability is a big issue due to the exposure a public utility has, and they made the decision to change out the other relays and go exclusively with a specific supplier of relays. When I heard that, it was all the testimony I needed. When I showed my people what our local power supplier had gone through, that was all it took. My recommendation wasn’t even challenged,” he emphasizes.

Retrofit of the 12.5-kV switchgear at Yates had to be orchestrated within a rigid one-week shutdown window. Six months of planning and preparation for the relay integration were involved, with Adams and his team anticipating every aspect of the project. In addition to removing the old relays and installing and programming the new hardware, they also installed new power capacitors to stabilize the generator sets, enabling the gensets to run more efficiently and much closer to unity.

A measure of the project’s success was a four-fold improvement in the trip index. When the power system was brought back up, the trip index factor had gone from nearly 1.0 to about 0.25. Further, the new relays enabled Marathon to collect data and to find root causes of problems.

“Finally having legitimate data collection and reporting let us focus on smart fixes, and there have been times when we’ve gone to the power company and said, ‘We have information here that we’ve captured, and we think you should see it.’ The data helped us solve problems on their side of the system,” explains Adams.

From Pure Power, Summer 2002

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