Application Note

Application Note
Problem description
A small city gets its water from
a mountain lake 30 miles away.
A pumping system at the lake
brings water up a short incline
and into a long gravity pipeline
that feeds the city’s water
distribution system.
Before entering the pipeline,
the water passes through a
filter that removes debris. When
the differential pressure across
the filter gets too high, a large
backwash pump reverses the
flow momentarily to clear the
filter. The backwash flow
carries debris to a settling pond.
Powering this backwash
pump is a 650 hp, 3-phase
motor equipped with a soft
start. A “negative sequence
detector” helps protect this
important motor by shutting it
off upon the loss of a phase, or
when voltage distortion exceeds
a predetermined level.
5th Harmonic
resonance
From the Fluke Digital Library @ www.fluke.com/library
The power to this motor
comes via a 30-mile utility line
originating in the city. A power
factor correction capacitor near
the backwash pump improves
power factor and reduces voltage
loss at the end of that 30-mile
line. For a long time, this
configuration worked with no
problems.
After a small earthquake in
the nearby mountains, questions
arose regarding the continuation
of the water supply in the event
of a power interruption. How
would the city get its water if
the utility connection were
lost due to an earthquake?
The answer was a generator.
Subsequently, the city added
a diesel-powered engine
generator and transfer switch
to the system (see Fig. 1).
Shop
Transfer Switch
Main
Pump
Generator
Utility
Office
Backwash
Pump
P.F. Correction
Capacitors
Fig. 1 One-line diagram of the water plant
Operator: Water plant electrical
engineer
Measuring tools: Fluke 43B Power
Quality Analyzer
Features used: Voltage, harmonic
spectrum
Power
Quality
Case
Study