Water treatment plant reducesmaintenance costs, energycharges and ensures reliabilitywith medium voltage drive solution

BACKGROUNDThe City of Lethbridge WaterTreatment Plant provides potablewater to a population ofapproximately 80,000 residentsand about 2,000 commercialand industrial customers in andaround the City of Lethbridge,Alberta. Over the past severalyears it has expanded waterservices to communities in the

surrounding county. The watertreatment plant produces anaverage of 55 megaliters perday (MLD) and has a maximumdesign capacity of 150 MLD.

Water is pumped from theOldman River into the treatmentplant where it is treated andfiltered, and then stored in aclear well. High lift pumps lift the

WATER/WASTEWATER INDUSTRY

The City of Lethbridge Water Treatment Plant in Lethbridge, Alberta, uses PowerFlex® 7000medium voltage variable speed drives for reliable, consistent water supply and cost savingsin spare parts, maintenance and energy use.

SOLUTIONS • Solution providers:

- Rockwell Automation- Westburne Electric Supply Ltd.- Associated Engineering Ltd.

• Medium voltage variablefrequency drives- 4160V, 1000hp PowerFlex® 7000 MV variable frequency drive

- 4160V, 2000hp PowerFlex® 7000 MV variable frequency drive

- Active front end (AFE) rectifiercontributes to low lineharmonics, high power factorand commonality of parts

- Outdoor isolation transformerresolves limited space issues

RESULTS • Operations impact

- Reliable, consistent water supply and no downtime in its first year of operation

- Speed regulation and optimumcontrol of the system pressuresand flows

• Power system impact- Increased energy efficiency(system uses 230 kW less thanbefore MV drive installation)

• Financial impact- Savings of $20,000 CDNannually in maintenance costs

- No slip ring replacement costs

- No spare parts stocking

treated water about 90 meters fromthe river valley to four storagereservoirs distributed throughout the city.

The City currently uses seven pumpswith a configurable piping system.The City has two large pumpscapable of running on variablespeed control (1000 & 2000 hp),another two pumps (1000 & 2000hp) available in acrossthe- line mode,as well as three smaller 300 hppumps with motors operating inacross-the-line mode. Thisarrangement assures that maximumflow can be delivered with the largestunit out of service.

CHALLENGEReliability, downtime, andmaintenance costs were issues theCity needed to address due to theage of its drives and motors. The Cityhad been using two drives purchasedin 1981 to control the two 1000 hpprimary pumps that lift treated waterfrom the river valley. These drivesemployed wound rotor inductionmotors (WRIMs) with a slip recovery

system to allow for limited speedcontrol for fluctuations in waterdemand. There were reliability issueswith these particular motors anddrives early in their service life. Thisissue was compounded with only one2000 hp fixed speed unit availablefor very high flow and as a backupwithin the original configuration.

Ensuring consistent supply tocustomers was key, particularly duringthe high-demand summer months.City of Lethbridge Water PlantElectrician Jim McCann says, "Duringthe winter months water consumptionaverages 35 MLD, however demandpeaks during the summer can be upto 120 MLD. The three to four monthsof heavy pumping during the summermonths was a concern, becausethere was no more available backuponce a drive or motor failed."

When a drive did fail, mean time torepair was only one to two days ifthe spare parts needed were on site.However, if a motor failed, downtime had been up to 9 months.While water had always beenavailable to customers during these

situations, the concern was that therewasn't adequate back-up to providefull service in the event of a problemwith the remaining pumps. Further,most of the back-up motors had to berun in the across-the-line mode, andthe constant speed made operationalresponses to demand fluctuationsmore difficult and wasted energy.

The WRIMs required continuousmaintenance to the brush systemsbased upon the high range in currentdensities experienced. Excessive ringwear meant that the motors had to besent out to have the slip ringsreplaced at a cost of about$25,000 every couple years.Maintenance to repair these failuresalso added to costs. Further, becausethe application of this drivetechnology was relatively rare andparts often had to be specialtyordered, the City chose to stockabout $30,000 in spare parts for thedrives and motors to reduce the riskof down time.

SOLUTIONThe City knew that variable frequencydrives were the answer to controllingthe fluctuating water needs, and thatnew motors were also needed. TheCity went to tender for a mediumvoltage variable frequency drivepackage and medium voltage motorsseparately. Allen-Bradley distributor,Westburne Electric, in Lethbridge,worked with consulting engineerAssociated Engineering (Alberta) Ltd.and Rockwell Automation in Calgary,to find the best solution for the City of Lethbridge.

The City had already been usingAllen-Bradley 1336Plus and PlusII lowvoltage drives on its water distributionpumps, and were impressed with thereliability of the products and the high

The City of Lethbridge Water Treatment Plant uses outdoor transformers to resolve issues of limitedspace within the building.

level of local support from WestburneElectric in Lethbridge, and theRockwell Automation office in Calgary.

The Allen-Bradley PowerFlex 7000medium voltage drive had the benefitsthey needed. PowerFlex 7000 MVdrives provide energy savings,reduced maintenance costs andincreased life of mechanicalequipment, as well as reducedharmonics.

They purchased one, 4160VAC,1000hp, PowerFlex 7000 MV VFDand one, 4160VAC, 2000hpPowerFlex 7000 MV VFD, with twoTeco-Westinghouse MV motors fromWestburne Electric. Both of thePowerFlex 7000 MV drives wereequipped with outdoor isolationtransformers and active front end(AFE), also known as pulse widthmodulated (PWM) rectifiers. Byinstalling the transformers outdoors,they resolved the issues of limitedspace within the control room and ofcooling the isolation transformer. ThePowerFlex 7000 MV drive gives theCity near unity power factor andlower harmonics, thanks to the drive'sAFE. Low component count and thepatented PowerCage™ make it easyto install, run and maintain.

The City of Lethbridge's purchasingdepartment favored the WestburneElectric bid due to a lower capitalcost. Operations and maintenancepersonnel appreciated the localsupport, ease of use and diagnosticcapabilities.

RESULTSThe Allen-Bradley 2000 hp VFD andinduction motor was commissioned inMay 2003, and the 1000 hp VFDand induction motor in October

2003. The City of Lethbridge reportsit has had virtually no down time inthe first year of operation due to thedrive reliability and enhanced drivediagnostics.

The City of Lethbridge has incurred nomaintenance costs to date, and itscustomers have enjoyed a reliable,consistent water supply. The City isconfident with local support and issaving up to $20,000 annually inmaintenance costs.

The water treatment plant operatorswho run the water treatment processand change the pumping rates arepleased with the speed regulation ofthe drives, which provides optimumcontrol of their system pressures and flows.

"The operators love these new drivesand couldn't be happier because theymake their lives easier," says Jim."They don't have to worry aboutjuggling the pumps. They punch in asetpoint to get the desired flow and itsends a signal to the drive. It's allautomated, so there's less chance for error."

The diagnostic capabilities of thePowerFlex 7000 also take out theguesswork for the maintenance staff."Now they don't have to docomponent-by-component testing. Ittells you the problem and you fix it,"says Jim. This will reduce downtimeand maintenance costs andsubsequent overtime pay.

In addition, plant maintenance staffwas extremely impressed with theRockwell Automation GMS fieldservice engineers, Karl Chan andSteve Goodwin, who commissionedthe drives. The field service engineersperformed thorough commissioningprocedures and provided customertraining throughout the process andextensive training on the operation ofthe drives.

The total retrofit time for both drives,including removal of the old drives,installation of the new drives,rewiring, and commissioning, wasless than two weeks per drive. Thedrives were installed at night tominimize disruption to the watersystem. It took less than four hours tocomplete the electrical tie-ins from the

The PowerFlex® 7000’s patented PowerCageTM with SGCT technology.

existing control system for each new drive.

The PowerFlex 7000 “B” Framemedium voltage AC drive. The City ofLethbridge is now confident that waterwill be supplied even in peakdemand times, and in the process willsave thousands of dollars in spareparts, maintenance and energy costswith the new variable speed drivesand motors.

"We like the new PowerFlex 7000drives, they've saved us time, moneyand future headaches, " says Jim."And I just can't say enough about the

service from the local WestburneElectric and the Rockwell AutomationGMS engineers. If you have aquestion or need some help withsomething, they are there to help you."

The results mentioned above arespecific to the City of Lethbridge’suseof Rockwell Automation products in conjunction with other products.Specific results may vary for other customers

Publication WATER- AP003B-EN-E – May 2006 — Supersedes Publication WATER- AP003A-EN-E – October 2004 Copyright ©2004 Rockwell Automation, Inc. All Rights Reserved. Printed in USA.

The City of Lethbridge Water Treatment Plant uses outdoor transformers to resolve issues of limitedspace within the building.

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