January 15, 2008

Focus on O&M (January 2008)

INSTRUMENTATION & CONTROL

 Single-window control of CHP plant’s energy converters

In February 2005, APS Energy Services and its Northwind Phoenix subsidiary together began building a gas-fired combined heat and power (CHP) plant on the campus of Arizona State University (ASU) in Tempe. Northwind markets, develops, designs, finances, constructs, and operates district-heating plants throughout Arizona.

Phase I of the ASU plant is now on-line. It has five 2,000-ton water chillers (Figure 1) from York (www.york.com), a 7.2-MW natural gas–fired combustion turbine (CT) from Solar Turbines Inc. (mysolar.cat.com), a 2-MW Murray steam turbine (www.dresser-rand.com), an 80,000-lb/hr heat-recovery steam generator (HRSG) from Rentech Boiler Systems Inc. (www.rentechboilers.com) equipped with a Coen duct burner management system (www.coen.com), and a pair of 2-MW diesel engine generators from Cummins Power Generation Inc. (www.cumminspower.com).

1. Keeping the kids cool. The combined heat and power plant on the campus of Arizona State University includes five 2,000-ton chillers like this one. The plant’s chilled-water capacity is currently being expanded to 24,000 tons. Courtesy: APS Energy Services

Subsequent construction phases will more than double the chilled water capacity and increase the power and steam outputs of the cogeneration system to levels suitable for supplying ASU’s rapidly growing research facilities. The CHP plant will ultimately generate 160,000 lb/hr of steam, 18.4 MW of electricity, and 24,000 tons of chilled water, making it perhaps the largest university central plant in the U.S.

The plant, which operates 24/7, required a state-of-the-art control system that also is easy to use. So APS and Northwind specified a Matrix Total Control unit from MTL Open System Technologies (www.mtlmost.com) that puts all operator functions within a single display window. According to Ray Tena of ASU’s facilities department, “the system’s graphics and controls are easy to navigate and provide an effective and responsive operator interface.”

The control system comprises four redundant pairs of Matrix hybrid controllers, eight remote nodes, each with a redundant pair of Ethernet bus interface and I/O modules, two double-tier operator consoles, one remote interface terminal server, and one engineering station and industrial SQL server historian, all of which communicate over a fault-tolerant Ethernet network (Figure 2). The system oversees the five chillers, the HRSG burner management system, and the two diesels and two turbines, and also operates the facility’s primary circuit breakers.

2. Tightly integrated controls. Parts of the control system communicate with each other over a fault-tolerant Ethernet network. Courtesy: APS Energy Services

Interfaces to multiple intelligent devices (including Rotork smart valves, GE smart relays, and ABB variable-speed AC drives) were essential to providing the single-window operator environment. More than 20 separate system interfaces enable more than 75 devices to talk to each other. Additionally, there are Ethernet interfaces to the programmable logic controllers of the combustion turbine and the HRSG duct burner management system.

Working with the control system, an industrial applications server and a FactorySuite Model A2 human-machine interface from Wonderware (www.wonderware.com), a subsidiary of Invensys Systems Inc., provide the single control window to all areas of the plant. The chillers and the combustion turbine are operated separately by the same operator from the facility’s only control room (Figure 3). Different-color backgrounds (blue for the chillers, green for the CT) distinguish the two systems.

3. Screening room. Color-coded backgrounds differentiate the controls of the chillers and the combustion turbine. Courtesy: APS Energy Services

Significantly, the control system enables all equipment of the CHP plant at ASU to be monitored and controlled remotely by operators at another Northwind site via the company’s wide-area network. That feature allowed operators of the ASU plant to be trained onsite without disrupting operations on campus while the system was operated from Northwind’s downtown Phoenix headquarters.

The Tempe plant’s control system also shares information with ASU’s energy information system (EIS) through a Modbus interface. The EIS, which is maintained by APS Energy Services, provides detailed information about individual buildings’ energy usage. The university uses that data to develop building-specific invoices for electricity, gas, chilled and hot water, and steam usage.

—Contributed by Ed Antone (edmund.antone@apses.com), senior project manager of APS Energy Services and Tim Schweitzer (tim.schweitzer@mtlmost.com), project manager for the control system vendor.