Optimizing energy usage is an essential measure for long-term viability of the sugar industry. Sugar mills use waste energy to produce power by burning the fibrous residue of sugar-cane production called bagasse in boilers. The steam powers large turbines, which drive the choppers, mills, and pumps and generate power for the rest of the factory.

Like many sugar mills, La Union SA—in Santa Lucia Cotzumalguapa, Escuintla, Guatemala—has expanded its generation capabilities to sell excess energy to the local utility.

Always searching for higher efficiency, the plant required a control system that could handle block loading and heavy load starting associated with sugar cane processing, plus speed variation from 65% to 100% of maximum. This meant replacing low-efficiency steam turbines that drove the mechanical equipment with more efficient electrical motors, and using variable-speed ac drives in boiler fans and pumps.

Since La Union's large machinery requires ratings of 1,000 hp and above, a medium-voltage (MV) motor and drive is the preferred choice. The solution engineered by Guatemala distributor Intek and Rockwell Automation consists of a 2.3 kV, heavy-duty PowerFlex 7000 pulse-width modulated MV ac drive and a 1,000 hp, 1,200 rpm, 2.3 kV Reliance G50 TEFC (totally enclosed, fan-cooled) motor.

PowerFlex 7000 drive allows a maximum of 150% rated torque for one minute in every 10 minutes of operation at rated load, which accommodates the mill's block loading and load-starting requirements. La Union selected this drive, in part, because it provides a simpler design and requires no isolation transformer. Low harmonics generation was another important consideration. PowerFlex 7000 achieves low line harmonics of less than 5%, compared to a dc solution with total harmonic distortion (THD) typically greater than 15%.

Since there is no process by-pass possibility, downtime halts production. PowerFlex 7000 drive addresses the need for high reliability through use of the latest power-switching technology, namely symmetrical gate-commutated thyristors (SGCTs), and a low component count with only six inverter SGCTs needed.

"Our medium-voltage solution has lower installation costs, less space requirements and best efficiency," compared to another ac drive solution, says Intek general manager Jorge Guzman. "The main competitor had a rating limitation of 2.3 kV, maximum 1,250 hp—with no overload capacity—which forced them to propose a mixed solution using a 4.16 kV step-up transformer and a 4.16 kV drive and motor."

At La Union, space limitations around the motor made it necessary to install the drive approximately 200 meters (656 ft) from the motor. The MV drive's current-source inverter structure permits long drive-to-motor distances.

Installed in November 2002, the medium-voltage drive and motor set and ran 24 hours a day until the end of harvest, totaling 138 problem-free days of operation. In April 2003, La Union's in-depth analysis proved the MV technology fulfilled reliability requirements and showed a significant improvement in power generation.

The new drive and motor set uses 66% less steam to create the equivalent power. The previous turbine used 48 pounds of steam for every kW of power while the new, efficient electrical motor and drive system uses only 16.2 pounds of steam per kW to provide 1,420 kW of electrical power per hour.

Energy consumption of the electrically driven mill is approximately 550 kW/hr, leaving roughly 870 kW/hr for sale to the utility. At US$0.055/kW, this meant additional revenue of $158,480 in six months of harvest time, or $1,148 per day. Payback on the MV drive and motor was realized in one season of sugar mill operation.

La Union took a forward-looking step with MV ac drives in the tough environment of a sugar mill—one of the first applications of its kind.

For more, visit www.rockwellautomation.com

Edited by Frank J. Bartos, fbartos@reedbusiness.com