Fluke 43B Application Note 2103535_HarmonicResonance

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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

Fluke.Keeping your world

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Fluke Corporation

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Eindhoven, The Netherlands

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©2003 Fluke Corporation. All rights reserved.

Printed in U.S.A. 8/2003 2103535 A-ENG-N Rev A

Fluke 43B displayed 5th

harmonic distortion rising to

80 % of the fundamental prior

to the stop signal.

Theory and analysis

The ideal displacement power

factor (DPF) is 1.0. This occurs

when the current and voltage

are “in-phase.” Inductive motor

loads cause the current to lag,

thereby lowering the DPF. This

usually results in a penalty

charge from the utility, so many

users install a capacitor to

provide DPF correction.

However, the combination of

inductance and capacitance will

form a resonant circuit that may

result in high circulating current

at the resonant frequency. It’s

standard practice to size a

capacitor at a value large

enough to correct the DPF to

at least 0.9, but not so large

that it makes the voltage lag the

current. The resulting configura-

tion yields a resonant frequency

between the 5th and 7th

harmonic.

Problems can occur if the

circuit has a source of harmon-

ics near the resonant frequency.

In the case of the backwash

pump, the source of harmonics

was the semiconductor switch-

ing operation of the soft start.

Why did the circuit work fine

when powered by the utility,

yet not work when powered by

the generator? The difference is

the source impedance. The low

impedance of the utility means

it can absorb harmonic currents

without causing severe voltage

distortion. The impedance of

the generator is much higher

than that of the utility.

Harmonic currents flowing into

the generator caused enough

voltage distortion to trigger the

negative sequence protection

circuit to produce a stop signal.

Solution

The city engineer solved the

problem by moving the capaci-

tor to the utility side of the

transfer switch. With this

configuration, the generator

never sees the capacitor and

the resonance condition does

not exist when the generator

powers the circuit.

System testing under gener-

ator power revealed a problem.

The backwash pump would

suddenly stop part way through

the start sequence. And it was

stopping because the negative

sequence detector was putting

out a stop signal to the motor

controls. What electrical anom-

aly was the negative sequence

detector protecting the motor

from? Was it a phase loss?

Excessive voltage distortion?

What were the characteristics

and source of this anomaly?

Unfortunately, the city engineer

was unable to answer these

questions with his existing

test equipment. After some

research to identify a device

that would allow him to see

what was going on, he

purchased a Fluke 43B.

Measurements

The engineer suspected voltage

distortion, so he connected the

Fluke 43B phase-to-phase

across the input to the soft

start and selected “Harmonics.”

During the start sequence, the

2 Fluke Corporation Power Quality Case Study: 5th Harmonic resonance