June 1, 2010

Variable-Frequency Drives Upgrade Reactor Circulating Pumps

Mark W. Harshman

Improved System Reliability

The Siemens VFD was specifically designed for high-availability applications that require continued operation in the event of an equipment failure. This particular VFD topology has several layers of redundancy and robustness designed into the system, including N + 2 redundancy for the power converter section, fast cell bypass that will automatically bypass a failed power cell, and redundant control systems that will automatically switch over to a duplicate control system in the event of a primary control system failure. The VFD will continue to run at full power if up to two power modules fail, a primary microprocessor controller fails, or a programmable logic controller controlling the system inputs and outputs fails.

Virtually every major component is redundant, which has given the VFD the ability to operate even after every major system has a component failure. This process (reactor pump) over product (VFD) philosophy improves reliability and leads to fewer power reductions caused by circulation system problems.

Reduced Environmental Issues

Replacing an MG set with a VFD eliminates several other systems in the plant, including the oil used in the mechanical coupling between the motor and generator in the MG set. This results in an environmental improvement, as routine oil disposal is no longer required. In addition, special fire protection systems that are in place to protect against potential oil fires can be eliminated. Additional cost savings associated with eliminating these systems and maintenance processes may be achievable.

Currently, more than 50 individual VFDs are scheduled for incorporation into various nuclear plants, primarily in the U.S., in both boiling water reactors and pressurized water reactors.

—Contributed by Mark W. Harshman, a drive system consultant with Siemens LDA in New Kensington, Pa.