Keeping it on tap

This Michigan water plant keeps taps flowing and lowers costs with a 5.4-MW standby power system from Cummins Power Generation

Water treatment plants are part of any community’s critical infrastructure. Residents expect water to be available every time they turn the tap, even during local utility power outages. A large regional water treatment plant near the shores of Lake Michigan wanted to ensure that it could provide water for more than 200,000 residents, even if the electric utility failed, so it installed a standby power system to prevent service interruptions.

Going a step further, the standby system also saves the plant money on its electric bills by generating its own lower-cost power during the electric utility’s summer peak demand periods when lawn watering and other activities push demand to as much as 80 million gallons per day.

The standby power system includes two PowerCommand 2.7-MW diesel generator sets from Cummins Power Generation Inc. for a capacity of 5.4 MW. In addition to the standby generators, the water treatment plant had two separate utility feeds brought in from different substations for additional reliability.

As water demand and the plant’s electrical loads grow over time, there is room to add two more generator sets. Cummins Bridgeway, the local distributor for Cummins Power Generation in Michigan, supplied the system.

The plant’s maintenance supervisor says the generators provide dependable standby power for continuous flow of water to community customers.

“Basically, we’re using the generators as an uninterruptible power source so that if we have a power outage — or have our power curtailed by the utility for load-shedding during the summer peak — we can run these two generators and supply enough power to keep the water flowing,” he says. In the rate agreement with the local utility, it receives an incentive for allowing the utility to interrupt electrical service and help reduce the utility’s peak demand. This saves the plant about 15 percent off its electric bill — up to $300,000 per year.

Power system’s dual role
There are two circumstances in which the generators need to produce power for the plant. One is when both utility feeds fail due to a widespread power outage. The other is when the utility asks the plant to remove all or part of its load from the grid during days with high peak demand.

In the case of a utility outage, the generators are programmed to start after a 10-minute delay. This allows time for the pump valves to close before the pumps are restarted. During the 10-minute switchover, water storage tanks throughout the distribution system prevent any noticeable decrease in water flow to consumers.

Whenever power to the plant is to be interrupted because of summer peak demand, the electric utility calls the plant 24 hours in advance. These power curtailments may happen only six or seven times, but they occur during the lawn-watering season when water demand is the highest. However, the generators have enough power capacity to operate the plant even when the demand for water is high.

Under the rate agreement with the utility, the plant also is required to limit the amount of power it consumes between the hours of 11:00 a.m. and 7:00 p.m., Monday through Friday. If the plant needs more power during that timeframe, it had the ability to generate power in parallel with the utility and just shed some of the plant’s load.

Transfer system avoids breaks in power
When the utility requests the plant to switch to self-generated power, the plant starts the generator sets and makes the transition using a paralleling load transfer system. This style of transfer equipment parallels the generator and the utility together for a brief period during the transition, and then gradually ramps the load onto the generators and off the utility. The process is reversed when utility notifies the plant that they can resume the use of utility power. This system reduces stress on components and avoids a break in power flow when the plant is transitioning from generator to utility power and back again.

Since the Cummins Power Generation standby power system was installed, it has not only kept the water flowing to more than 200,000 customers during utility outages, it has helped reduce plant’s electric bills and the cost of water.

For more information, contact Madeline Foss at 763-574-5942; or visit www.cumminspower.com.

This article appeared in the August/September 2008 issue of MRO Today magazine. Copyright 2008.

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