In November 2008, a central Texas utility commissioned HPI, a full-service turbomachinery design and construction firm based in Houston, to perform a major upgrade of its plant’s power distribution and turbine control systems. The scope of the project included upgrades to four gas-fired Pratt & Whitney FT4 Twin Pac gas turbine generator packages, new programmable logic controller (PLC)–based control panels, generator protection panels, new fuel valves, and emergency batteries and chargers. A single Twin Pac package uses two combustion turbines to drive a single double-ended generator (Figure 6).
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| 6. Upgrading a Twin Pac. A central Texas utility recently upgraded its four Pratt & Whitney Twin Pacs with new wiring, controls, safety systems, and an updated dual-fuel system. Courtesy: HPI |
The 926-MW plant, originally constructed in 1967–68, had original gas turbines that were designed for dual-fuel operation; however, only the natural gas fuel system was in use, and the liquid fuel system had been partially dismantled. The project also upgraded and recommissioned the liquid fuel system.
Tough Scope of Work
A front-end engineering and design study determined the technical requirements and scope of work for the project. In general, it required a major refurbishment of the entire station’s electrical systems. Specifically, all the cross-site cables for the four Twin Pacs were replaced in new overhead cable trays. Much of the original cabling was installed in underground cable runs, which were often flooded by heavy rain. New control subpanels mounted inside existing control cabinets (Figures 7 and 8) were also added, as were new fuel valves.
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| 7. A typical 1970s-vintage FT4 governor and sequencer panel. Courtesy: HPI |
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| 8. A modern replacement control panel. In addition to the modern digital controls, the Class I, Div II design of the panel enclosures was retained, ensuring that positive air pressure is maintained inside the enclosure at all times. Courtesy: HPI |
The project required much demolition work, including removing complete panels and junction boxes. The original cabling within the turbine enclosures was installed at the factory as part of the original package. When the demolition began, workers quickly discovered control wiring mixed in with power cabling that terminated at the motor control center—no longer an acceptable wiring practice.
Instead, all of the control and power cabling was run in separate trays from the marshalling cabinets to the field to eliminate the possibility of “cross talk” between adjacent wires, an HPI best practice. Digital and analog signals were separated into individual cables for the same reason.
HPI also aggregated the cabling between cabinets, such that all control or power conductors running between points are most efficiently run in a common, multi-conductor cable, where practical. If this practice is not followed, the number of cables increases dramatically, along with the associated terminal blocks, analog cards in the PLCs, and so on. The cost of the cables and the labor to install the cables also increases—a situation to be avoided.
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