Digital protective relays improve power stability and enhance control and safety.
When a natural gas plant loses electric power, the entire production run can go up in flames - literally, via flaring - to the tune of daily revenue losses in the hundreds of thousands of dollars. That's why most large oil and gas facilities have onsite power generation, with a local grid connection serving primarily as insurance. So protecting and monitoring onsite generation is crucial.
This is especially true at Marathon Oil's Yates field gas plant at Iraan, Texas. It's a premier asset of the company's Southern Business Unit, so avoiding shutdowns is paramount.
That's where Charlie Adams comes in. Adams, a 20-year industry veteran, is one of a handful of senior Marathon engineers who support power systems that drive the company's upstream operations. His work on a diverse range of projects includes designing and installing SCADA measurement systems, instrumentation, controls and power systems.
The Yates plant serves as a large compressor station to recycle gas for reservoir injection at the venerable Yates oil field. The plant's onsite generator sets are self-contained, with the utility connection as backup. However, if the utility trips offline, it eliminates the backup, risking bigger problems should the onsite power system malfunction. Marathon assigned Adams to minimize such trip-offs.
Even a brief power shutdown raises the risk of losing control of the plant's processes, where both chemical and cryogenic operations go "off spec," said Adams. Restarting can cost as much as $15,000 in fuel, which must be purchased commercially, he added. An even longer power outage could make a significant dent in Yates' 20,000-b/d average oil production.
Initially, Adams struggled with developing a way to measure the results of any changes made to firm up reliability. But one of the metrics he chose was the frequency of breaker trips in the utility feeders that tie into the bus. Adams calls this frequency the "trip index." With paper maintenance records showing a thousand trips on the utility tiebreaker during a 3-year period - translating to about one a day - Adams concentrated on lowering the trip index. Deeper 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.
But there would be no quick fix. The existing hardware was pure analog, said Adams, with no historical data-capture capabilities. In effect, pinning down problems had been a series of educated guesses, since most electrical power events are quick and leave little to trace.
However, the project's scope eventually resulted in replacing unreliable relays on the utility feeders with SEL 351 digital relays manufactured by Schweitzer Engineering Laboratories (SEL) of Pullman, Wash., a designer and developer of power relay and monitoring technology. Marathon also installed two of SEL's 300G generator relays at Yates and swapped 34 aging electromechanical relays with new SEL hardware.
But switching to the SEL equipment was no slam dunk, either. With another manufacturer's relays already in place, Adams needed compelling data to justify a change.
Calls to companies that had used both relays included the very utility that supplies backup power to the Yates plant. The utility had recently purchased a large number of breakers for its substations and the same relays as those installed at Yates. Their experience was not positive, Adams said, and included service and firmware problems.
Due to the exposure of public utilities to close scrutiny, reliability is a big issue, Adams said, so when the utility replaced the relays with SEL-351 feeder management relays, it was all the testimony Adams needed to justify to management the switch to SEL relays as well.
The retrofit of the 12.5kv switchgear at Yates was significant, and had to be done inside a 1-week scheduled maintenance shutdown window.
But it took 6 months of planning and prep for the relay integration, working closely with SEL and colleagues at Marathon, said Adams. In addition to removing the old relays and installing and programming the new hardware, they also installed new power capacitors to stabilize generators. That enabled the plant to run more efficiently and much closer to unity.
The result was a fourfold improvement in the trip index. When the power system was brought back up, said Adams, the index dropped from a factor of nearly 1.0 to about 0.25.
Marathon also started collecting digital data from the SEL equipment that exposed the root causes of many problems, including whether there was trouble on the supply side or demand side, allowing Marathon to focus on "smart" fixes. The newly captured information also helped the utility company solve problems on its side of the system.
The success at Yates resulted in Adams' later use of SEL-351 feeder management relays on the utility feeder and bus at Marathon's Indian Basin gas plant at Lakewood, N.M. The resulting plant reliability improvements led Adams to propose retrofitting the plant's generator relays with SEL equipment.
Marathon is typical of many companies that don't maintain a broad power engineering base, Adams said. Supporting widely spread power systems is a challenge, he said, so having the ability to manage devices remotely is a distinct advantage. Since this is possible with the SEL equipment, the result has been discovery of possible problems, followed by local engineers taking a closer look.
As an example, Adams said a recent problem occurred with direct current batteries that power the critical trip circuits for the breakers at Yates.
The battery systems are 125-vDC. He noticed that twice a day, the batteries would cycle through an intense recharge that was well in excess of 140v before cycling back to normal levels.
Keeping an eye on the pattern for a few weeks, Adams was able to capture hard data demonstrating the events. this was passed on to the yates electrician, who located and fixed the charging connections.
To find out more about SEL technology, contact the company at 1-615-507-2184 or visit www.selindustrial.com.

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