POWER June 2011 - 20

plant's target leakage goal is 6% to 8%
and, to achieve this, the plant is now also
considering a complete set of circumferential
brush seals.
-Contributed by Patrick T. Fitzgerald
(pfitzgerald@sealeze.com) business development
manager, Power Generation at Sealeze.
Twin Pac Controls
Upgraded
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).
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.
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
20
www.powermag.com
POWER | June 2011
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
7. A typical 1970s-vintage FT4 governor and sequencer panel. Courtesy: HPI
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
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
http://www.powermag.com

POWER June 2011

Table of Contents for the Digital Edition of POWER June 2011

Contents
POWER June 2011 - Cover1
POWER June 2011 - Cover2
POWER June 2011 - Contents
POWER June 2011 - 2
POWER June 2011 - 3
POWER June 2011 - 4
POWER June 2011 - 5
POWER June 2011 - 6
POWER June 2011 - 7
POWER June 2011 - 8
POWER June 2011 - 9
POWER June 2011 - 10
POWER June 2011 - 11
POWER June 2011 - 12
POWER June 2011 - 13
POWER June 2011 - 14
POWER June 2011 - 15
POWER June 2011 - 16
POWER June 2011 - 17
POWER June 2011 - 18
POWER June 2011 - 19
POWER June 2011 - 20
POWER June 2011 - 21
POWER June 2011 - 22
POWER June 2011 - 23
POWER June 2011 - 24
POWER June 2011 - 25
POWER June 2011 - 26
POWER June 2011 - 27
POWER June 2011 - 28
POWER June 2011 - 29
POWER June 2011 - 30
POWER June 2011 - 31
POWER June 2011 - 32
POWER June 2011 - 33
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POWER June 2011 - Cover3
POWER June 2011 - Cover4
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