Digital relays improve power stability, control, and safety for Marathon Oil

To improve power stability, enhance control and safety, eliminate costly shutdowns, and increase operational knowledge, Marathon Oil recently implemented SEL-351 digital protective relays from Schweitzer Engineering Laboratories Inc. (Franklin, Tenn.) at its Indian Basin (Lakewood, N.M.) and Yates Field (Iraan, Tex.) natural gas plants.

Charlie Adams, Marathon’s senior engineer, says the Yates plant is a support facility for the Yates oil field, serving as a large compressor station that recycles gas for injection into the reservoir. Self-contained sets of generators prevent offline trips from stopping the plant, but even a short power shutdown poses the risk of losing control of plant processes in which chemical and cryogenic processes within the plant go “off spec.” Mr. Adams says it could cost $15,000 in commercially purchased fuel alone to restart the Yates facility, and an extended power outage could dent Yates’ daily 20,000-barrel production.

Similarly, with a daily production of 259 million ft3of natural gas, Indian Basin could lose $30,000 per hour and face fixed startup costs of $100,000 if a severe power shutdown occurred.

‘Trip index’ aids reliability

To increase reliability at the Yates plant, Mr. Adams focused on the breaker trip frequency of the utility feeders that tie into the bus, where they had a utility tie and two generator sets feeding power into the bus. “We refer to this frequency as the trip index,” he says.

Previous records showed 1,000 trips on the utility tiebreaker in just three years, or about one per day, a figure that Mr. Adams says had to be brought down. Yates also had 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.

Following an evaluation process that included input from Yates’ local power supplier, Mr. Adams and his team opted to replace unreliable relays on the plant’s utility feeders with SEL-351 digital relays; install two SEL-300G generator relays; and replace 34 aging electromechanical relays with other new SEL hardware. Retrofit of the 12.5-KV switchgear at Yates had to occur within a rigid one-week shutdown window.

The ultimate measure of the project’s success was an immediate four-fold improvement in the trip index. When the power system was brought back up, the trip index factor decreased from nearly 1.0 to about 0.25. Mr. Adams says the plant’s new SEL-351 relays were the biggest contributors to this victory.

The team also started collecting data that allowed them to find the root causes of plant problems, and find out if trouble was occurring on the supply side or demand side. “Finally having legitimate data collection and reporting lets us focus on smart fixes. We’ve even brought data to the utility that helped us solve problems on their side of the system,” he says. Mr. Adams also uses SEL’s remote access and monitoring capabilities to configure devices and access surveillance reports, typically calling the relays twice a day to check status and look at the triggers and histories for “smoking guns”.

Following the Yates project, Mr. Adams was called to the Indian Basin plant, which has a more complicated power system consisting of a 480-V bus, three turbine generators, a utility connection, and a standby feeder. The new challenge was to integrate two additional distributed generators and a 900-hp motor, which both feed into and draw startup power from the bus. Indian Basin also had many of the same problems as Yates, including phantom trips and fluttering generator controls.

Following another evaluation, an SEL-351 was installed on Indian Basin’s utility feeder, though the generator relays and other aging hardware was not replaced. Later, an additional SEL-351 relay was added to Indian Basin’s bus. Mr. Adams adds that he is currently pitching a retrofit of Indian Basin’s generator relays.

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Author Information

Jim Montague, news editor jmontague@reedbusiness.com

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