Operations & Maintenance – Balance of Plant – McClain Power Plant

VFDs for cooling-tower fans boost performance, improve safety

McClain Power Plant
Owned by OGE Energy Corp and Oklahoma Municipal Power Authority
Operated by OGE Energy Corp 

Challenge.Fans regulate air flow to compensate for changes in ambient air and load conditions. In the past, this was achieved by cycling fans on and off, manipulating fan capacity, or by varying the pitch of fan blades. These methods can have considerable drawbacks and do not leave much room for error.

Solution. Variable frequency drives (VFDs) not only save on energy and increase system efficiency, they perform numerous other functions that eliminate the need for additional equipment. For example, VFDs can sense fan rotation. If a fan is rotating in reverse because of windmilling, a VFD can catch it, slow it down, and ramp it back up in the correct rotation.   

   However, the primary driver/justification for the plant is to gain the ability to run the fans in reverse and control fan speeds. The ability to reverse fan rotation and control speed makes them beneficial in cold weather. Reversing the fans causes warm air to be blown back into a tower, melting any ice that has formed. These functions are performed using mechanical brakes or reversing contactors.

   Since the original design did not have any variable operating capabilities, the cooling tower suffered from excessive icing in sub-freezing temperatures. Operating personnel traditionally have removed the ice manually, but doing so exposes them to injury from falling ice and therefore creates added safety risk.

Across-the-line motor operation would have been efficient had the system been designed for the fans to operate at full speed all the time. However, that rarely is the case at this site. As conditions change (cycling, ambient, etc), air flow needs to change as well. As we cycled our fans on and off, leaving-water temperature fluctuated, thereby contributing to inefficiency, control difficulties, higher motor/gearbox maintenance costs, etc. VFDs are more economical in terms of energy and mechanical cost savings (that is, less wear on the motors, gearboxes, and fans).

Results. The plant successfully retrofitted its nine cooling-tower fans in fall 2011. The scope of the project included addition of the following:

  • 200-hp Powerflex 755 series drives
  • 24-V Powerflex I/O modules
  • MCC buckets with modified doors to mount the replacement drives (Fig 18)
  • Load-side line reactors sized for application to each drive in a NEMA 1 enclosure (Fig 19)
  • Ethernet I/P tap in each drive cell

   This project has dramatically improved the cooling-tower motor control performance, motor efficiency, and icing control. A significant risk to plant operators has been eliminated by not having to manually remove ice formations.