Utilities Rely On Automatic Insulation Testing Systems
On January 1, 2008, the Green Bay Metropolitan Sewerage District announced the acquisition and consolidation with the DePere, WI, Wastewater Treatment Plant.
On January 1, 2008, the Green Bay Metropolitan Sewerage District announced the acquisition and consolidation with the DePere, WI, Wastewater Treatment Plant. These two formerly independent utilities are located along the shore of the Fox River, approximately five miles apart. With the consolidation, both wastewater treatment facilities will combine operations, serving a total population of over 219,000 residents in the metropolitan Green Bay area.
Prior to this acquisition, both utilities operated separately but both faced similar challenges. Consequently, both utilities independently sought solutions to specific motor operation problems through automatic insulation resistance testers from MEG-alert of Minocqua, WI.
"You may assume critical-duty motors are in good shape until you see or hear evidence to the contrary. But that assumption leaves you open to problems," said Mike Kersten, Operations Manager at the DePere plant.
Power-line spikes, environmental conditions such as moisture and dirt, and aging can reduce the quality of the motor winding insulation. Taking manual meg-ohm readings gives only a momentary snapshot of insulation quality – and that assumes you are doing any manual testing on a regular basis.
The DePere plant installed MEG-alert automatic insulation resistance testers on two critical operational elements:
One "Multi" machine insulation resistance tester was installed in the motor control cabinet that controls four 150 hp, 480 volt motors on Worthington centrifugal pumps. These pumps are at the core of plant operations and must be operational at all times. Normally, two of these motors are operated for 24 hours while the other two are offline, after which the offline pairs are rotated. The MEG-alert unit cycles between the off-duty motors on a 30-second interval, applying a 500 vDC test voltage automatically between the motor windings. The test gives an exact (and continuous) reading of the insulation resistance. The tester automatically bypasses the online motors during the cycle.
Three separate testers also monitor the readiness of three 250 hp, 480 volt motors on Roots centrifugal blowers, each delivering 4000 cfm for biomass aeration. These single MEG-alert units omit the scanning feature of the "Multi" model as each is assigned to its own motor, monitoring insulation resistance levels when the motors are off-line.
A flashing green LED indicates that testing is occurring. The units are preset to sound a "pre-alarm" if insulation resistance drops below 20 meg-ohms, and this is also indicated by a flashing yellow LED and a meg-ohm meter mounted on the door of the MCC.
If the insulation resistance falls below 3.5 meg-ohms, a red LED indicates a fault and the testers lock out the motor starting circuit, completely eliminating the chance of motor burnout upon startup. If the motor is in the fault mode, attempting to start the motor could result in catastrophic damage.
Meanwhile, downstream at the Green Bay plant, the automatic insulation testers have been employed in a different application. The Green Bay plant is unique in that it is one of only a handful of wastewater utilities licensed to incinerate waste. Part of this licensing is based on the quality of the stack scrubbers that eliminate particulate discharge. These scrubbers (and countless other water systems in the huge plant) are supplied by recycled effluent water.
The service pumps that supply the effluent water are powered by a series of four 150 hp, 480 volt motors. These motors are located in a damp environment and, combined with rotations between online and offline use, were subject to failures. Insulation resistance testers were installed in the motor control cabinets.
According to Marty Pyke, Electrical Foreman at the Green Bay plant, the effluent pumps provide over 1 mgd to supply the stack scrubbers and various water needs within the plant. Operations would need to be drastically curtailed if this water supply was interrupted.
One consequence of the consolidation of these two utilities is that the DePere plant will soon become an unmanned, "upstream" branch of the Green Bay plant. As such, this eliminates the opportunity for manual meg-ohm testing. Manual testing has always been time consuming and dangerous. Safety is a concern when electricians are forced to access the high voltage controls of any motor. The automatic insulation testing units eliminate the need for manual testing.
With the consolidation, meg-ohm testing results can be transmitted directly to the Green Bay plant control room, along with the pre-alarm and fault alarms. This also makes it possible to remotely track meg-ohm readings for trend analysis. Proactive testing allows for motors to be reconditioned by re-dipping the windings versus much more costly rewinding or replacement of burned out motors.
The automatic and continuous insulation resistance testing gives both plants a solid base of information on the status of the connected motors. As preventive and predictive maintenance becomes increasingly important it is also true that utilities (and all businesses) are forced to try to accomplish more with tighter budgets. By automating the motor testing and protection functions, MEG-alert testers have enabled these two plants to maximize efficiency while avoiding the expenses of emergency repairs and costly downtime.