TVGS Best Practice: Real-time response with automatic generator control

Challenge. The facility was required to respond in real time to generation dispatches that changed every five minutes or less. The plant needed to develop rate limiters for the setpoint changes that vary at the desired rate depending on how many GTs are in operation and if duct burners are firing. Any solution we chose had to function within Automatic Generator Control (AGC) throughout all power ranges and configurations of equipment within the facility.

To be successful, we had to overcome three challenges:

  • Determine how much load each GT should produce at a given setpoint.

  • Pass that value through to the turbine control system for remote load control.

  • Develop logic for automatically starting and stopping duct burners.

Solution:

  • Teamwork among departments identified clear and measurable goals. Engineering identified allowable maximum rate-of-change and output limits set by contract with the customer. These limits were programmed into the DCS.

  • Automation of GT controls was simplified by an EPC-provided input to the turbine controls that allowed an analog setpoint to be passed from the DCS to the GT controller via a 4-20-milliamp hardwired input. We developed a new method that allowed a GT to switch out of automatic load control while allowing the remaining GTs to function in automatic.

  • Duct-burner operation used empirical data to determine megawatt output versus duct- burner control-valve position. Operating curves were developed to determine maximum plant output versus GT inlet temperature. The curves were then used to develop a signal identifying the megawatts needed from duct-burner operation. This “needed-megawatt” signal is divided by the number of duct burners operating in automatic and then the programmed valve position sent to the duct-burner control-valve logic.

  • If the signal identifying needed duct-burner megawatts was greater than zero, the first duct burner would light. If greater than 10, the second duct burner would light. And if greater than 20, the third duct burner would light. The signals were blocked until the GTs reached the load required for duct-burner operation, and a process was implemented to light the next burner if the earlier burner failed.

  • When the GT load declined below the minimum required for duct-burner operation, a trip signal was developed. This logic would not allow the burner to light unless the operator reset the burner. An automatic reset was developed when the GT passed through 85% load.

  • We analyzed automatic duct-burner control when implementing steam augmentation. These conditions included steam flow to warm up the lines, initiation of steam augmentation, and the augmentation system reaching high flow.

Results. The customer benefits by receiving greater speed and flexibility in plant maneuvering. This flexibility provides the customer with an increased ability to take advantage of real-time market opportunities and increase its profit margin. The plant benefits by avoiding the need for additional operators in the control room. Operators need only monitor and ensure that all equipment performs correctly as the plant follows the dispatch signal. Off-take contract delivery-tolerance requirements are also assured as the signal varies every five minutes or less.

Key project participants:

Sam Graham, maintenance manager
Ed Puskaric, I&E technician
Alan Stewart, I&E technician
Mike Bolton, I&E technician
Rick McWhorter, plant engineer
Donnie Scott, operations manager
Brian O’Neill, LCRO
Steve Mattson, LCRO