The pulp and paper making process requires the continuous operation of critical machines. Disruptions in power (i.e. voltage sags) can have serious adverse affects to any continuous process, not just paper making. Engineers and plant operators face a daily challenge: ensuring consistent power quality to their processes.

The Challenge:
Voltage Sags Disrupt Drives on Continuous-Operation Paper Machine

Voltage sags at Caledonian Paper, a paper mill in Scotland, were causing tripping of 525-VAC adjustable-speed motor drives, creating major production problems. In this continuous-process plant—which is intended to operate for weeks or months without interruption—pulp slurry is passed between felt pads that are set very close together to produce a single layer of paper. If even one motor drive trips, a clump of paper can be forced through the gap, causing significant damage. It can take many attempts to restart the machinery after such an event.

The Solution
Modifications were made to the motor drives to desensitize them to the voltage sags. But these changes resulted in an unacceptable reduction in productivity. A more cost-effective solution was sought. An S&C PureWave DVR Dynamic Voltage Restorer provided that solution.
Of the 50-MVA total load of the paper mill, 8.3 MVA was identified as critical. This critical load was segregated from the remainder of the feeders and connected to a critical-load bus. A 4-MVA PureWave DVR was installed in series with the 11-kV utility source serving this critical-load bus.
(see attached diagram – page 2)

The PureWave DVR maintains the voltage applied to the critical-load bus during sags by injecting a voltage of compensating amplitude and phase angle into the line.

The Results
The PureWave DVR installation has been a success. The system is capable of injecting approximately 46% of nominal line-to-ground voltage on all three phases for 14 cycles. An analysis of previously recorded voltage sags has confirmed that the PureWave DVR would have maintained the voltage to the critical load through 84% of these events.

S&C Electric Company, headquartered in Chicago, IL, specializes in the development and manufacture of innovative products and services for the electric power industry. S&C's family of PureWave® Power Quality Systems have been installed at critical-process facilities worldwide to deliver reliable protection from the power quality problems that can disrupt operation. Information about S&C is available at www.sandc.com/PQ/PR

Application Diagram – PureWave DVR

Single-line diagram of 11kV distribution system (top). Modified to provide DVR protection for the plant critical load (bottom).