Vfd's on a Utility Cooling Tower
Petersburg Cooling Tower Auxiliary Power Usage with VFD’s/ Control Strategy
Michael J Smith – IPL Engineering & Project Management
Abstract--The temperature control strategy for the cooling tower fans involves two automatic modes of operation as well as provisions for manual operation. Both modes will provide sequencing for starting/stopping each of the 14-250hp, 480vac fans that have been made available for automatic control. Auxiliary power savings on the 3MVA, 4160vac tower load can be between 0.5 – 1.0MVA 6 months a year in the Midwest with the variable frequency drives (VFD’s). The first, automatic mode, HEAT RATE, will be aimed at operating the fans at or near the circulating water temperature which results in the optimal unit heat rate for the given load. The goal of the second mode, AUX POWER, will be to minimize the auxiliary power consumption by operating at or near the highest back pressure possible without reaching 7.0 inHgA. This mode is only intended for use during normal operation at full/ partial load, and not during start-up/shutdown. During both modes, the minimum circulating water temperature will not be allowed to go below 70o F at low ambient temperatures to prevent tower cell icing. Each fan may be operated manually in either bypass or through the VFD via the DCS or locally at the MCC. When all fans in AUTO have been running continuously at 1 00% speed for 15 minutes in either automatic control mode, a sequential transfer to BYPASS shall begin using fan hysteresis. In HEAT RATE mode, if none of the fans in AUTO are running, the first fan in the sequence shall be started and held at 25% minimum speed. The remaining available fans shall be sequentially started until all are at 25%. The speed set point shall then be raised to 100% for all fans. When AUX POWER mode is selected, the DCS PID will receive HP hot well temperature as its input. The set point shall be the saturation temperature corresponding to 6.25 in HgA (3.07 psi), with
Michael J Smith – IPL Engineering & Project Management
Abstract--The temperature control strategy for the cooling tower fans involves two automatic modes of operation as well as provisions for manual operation. Both modes will provide sequencing for starting/stopping each of the 14-250hp, 480vac fans that have been made available for automatic control. Auxiliary power savings on the 3MVA, 4160vac tower load can be between 0.5 – 1.0MVA 6 months a year in the Midwest with the variable frequency drives (VFD’s). The first, automatic mode, HEAT RATE, will be aimed at operating the fans at or near the circulating water temperature which results in the optimal unit heat rate for the given load. The goal of the second mode, AUX POWER, will be to minimize the auxiliary power consumption by operating at or near the highest back pressure possible without reaching 7.0 inHgA. This mode is only intended for use during normal operation at full/ partial load, and not during start-up/shutdown. During both modes, the minimum circulating water temperature will not be allowed to go below 70o F at low ambient temperatures to prevent tower cell icing. Each fan may be operated manually in either bypass or through the VFD via the DCS or locally at the MCC. When all fans in AUTO have been running continuously at 1 00% speed for 15 minutes in either automatic control mode, a sequential transfer to BYPASS shall begin using fan hysteresis. In HEAT RATE mode, if none of the fans in AUTO are running, the first fan in the sequence shall be started and held at 25% minimum speed. The remaining available fans shall be sequentially started until all are at 25%. The speed set point shall then be raised to 100% for all fans. When AUX POWER mode is selected, the DCS PID will receive HP hot well temperature as its input. The set point shall be the saturation temperature corresponding to 6.25 in HgA (3.07 psi), with
References: Technical Papers: D. Oldham, Black and Veatch (May 4, 2005), “Cooling Tower Performance Specification Recommendations” Black and Veatch (October 14, 2005), “Indianapolis Power and Light- Petersburg Unit 3- Cooling Tower- Programmable Logic Controller- 140863.64.3600- General Specifications” Black and Veatch (March 10, 2006), “IPL Petersburg Unit 3- Cooling Tower Fan Analog Control- BV Project 140863” C. Barron, M. Wildeman, B. Horstman, Project Associates Inc. (December 20, 2010), “IPL Petersburg Generating Station- System Description Document- Unit 4 Cooling Tower Control” Cooling Tower Test Associates, Inc (March 21, 2012), “Summary of Test Results- Indianapolis Power & Light- Unit 4 in Petersburg, IN- Thermal Acceptance Test on the SPX Cooling Technologies 13 Cell, Mechanical Draft, Cross-Flow Cooling Tower” Michael J. Smith IPL (May 31, 2013) “U4 Cooling Tower Economic Analysis” Microsoft Excel 2010 Financial Spreadsheet Functions Author, Bios and photos Michael J. Smith is the Unit Manager, Engineering and Project Management for the Power Supply Division of Indianapolis Power & Light Co, Inc. He holds a BSEE from the University of Evansville in Evansville, IN, is certified as a Six Sigma Black Belt, and is currently pursuing an MBA to be completed November 2013. Smith formerly worked in automation consulting, heavy industry, and manufacturing with Amax, ALCOA, Whirlpool, and Hershey before joining the power industry in 2005.