Peaking plant speeds turbines with parallel programming

By Paul Studebaker

Nov 15, 2018

When is a peaking plant not a peaking plant? When it can’t start fast enough to fulfill its purchase power agreement. Such was the case in 2016 at the recently purchased Utah Municipal Power Agency (UMPA) gas turbine power plant in West Valley, Utah. The 217 MW plant has five simple-cycle GE LM6000 natural gas turbines. “It’s a simple-cycle peaking asset, but they were having technical challenges getting it running quickly on the original control system,” said Andy Dieball, vice president of sales, EthosEnergy, in the Power and Energy Industry Forum at Automation Fair this week in Philadelphia.

UMPA is a consumer-owned corporation established in 1980 to supply reliable and economical power to its member municipalities. It sells about 1,225 MWh per year for an annual revenue of about $68 million, using a fuel and resource mix of 33% contract, 38% thermal, 29% hydro, 0.04% wind and 0.07% gas.

The West Valley plant uses GE LM6000 turbines, which have been produced since 1994 and are based on the CF6-80C/E flight engine. Power output is 47-55 MW, and they were originally shipped with GE or Woodward turbine control systems. They use standard combustion with water injection for NOx reduction. “These are the race cars of the gas turbine world, able to make power very quickly and be on the grid in 10 minutes,” Dieball said.

But not the ones at West Valley, so UMPA hired EthosEnergy to bring them up to speed and modernize the control system. EthosEnergy is an independent turbomachinery service provider with more than 4,500 employees and $1 billion in annual sales. “We help customers keep their equipment up and running, and extend equipment life,” Dieball said. A Rockwell Automation recognized system integrator, EthosEnergy also operates plants under contract and supports those plants with engineering services.

“Turbines last 20 to 30 years, but their control systems only last about 10,” Dieball said. “We’ve completed more than 1,000 turbine control upgrades, and Rockwell Automation is our platform of choice.”

Duck curves and purge credits

The plant’s existing control infrastructure included GE Mark VI fuel control and GE Fanuc 90/70 PLC sequencing on the turbines, Brush Micro automatic voltage regulator (AVR) and Satec power meters on the generators, a Bently Nevada 3500 vibration monitoring system, and redundant Rockwell Automation PLC-5 controllers for the balance of plant and an OSIsoft PI historian.

“The plant needs to follow the ‘duck curve’ commonly created by renewables, and reliable, fast starts are critical for its purchase power agreement,” Dieball said. The electric power demand duck curve is named for its shape, which arises when renewable energy supplies that taper off in the evening meet the increased demand from consumers arriving home from work.

“UMPA wanted to be able to reliably start the turbines in 10 minutes and thought they needed a purge credit system to do that,” Dieball said. “They also wanted high-speed data capture to diagnose trips, redundancy, elimination of obsolete hardware, and a single, integrated control system for the entire plant.”

As part of the conventional startup sequence, a fuel-fired turbine must be purged with air to ventilate fuel vapors and remove the risk of explosion on ignition. Since the approval of NFPA 85 in 2011, that purge can be removed by installing a “purge credit” system, which maintains airflow through inactive turbines. Removing the startup purge can reduce startup time.

A single, plantwide system

The solution designed by EthosEnergy for the West Valley plant included converting the entire facility to Rockwell Automation controls, with the separate turbine governor and sequencing control moved to a single controller. The Brush Micro AVR moved to a combination generator control module (CGCM); PLC-5s migrated to ControlLogix control systems; the plant HMI was upgraded to FactoryTalk View with its server/thin client architecture; and the valve/actuator drivers were upgraded.

“The resulting system allowed us to provide the high-speed data capture for troubleshooting turbine trips,” Dieball said. “It also eliminated the failure-prone UV flame detectors on the combustion chambers by inferring flames with a combination of turbine speed and temperature measurements.

“We implemented EthosEnergy cold-climate ventilation, which uses variable-speed drives on the cooling fans to reduce differential expansion and eliminate fan blade contact with the housings, and a closed-loop emission control system to improve control over water usage.”

Above all, applying EthosEnergy Swift Start Upgrade reduced the overall start time to 10 minutes by running starting sequences in parallel instead of series. “This avoided a significant investment in a purge credit system,” Dieball said.

Along with improved performance, the plant is now a much better place to work, Dieball concluded. “What was six control systems now run on a single integrated architecture—one platform to learn.”

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