Fluke 27 Ii Application Note

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Troubleshooting

power harmonics

Basic troubleshooting using

multimeters and current clamps

A mystery is occurring in today’s office buildings and

manufacturing plants. Transformers supplying seemingly

average loads are overheating. Neutral conductors in

balanced circuits are overheating from excessive loads.

Circuit breakers are tripping for no apparent reason.

Yet the standard troubleshooting procedures show

everything to be normal. So what’s the problem?

In one word—harmonics.

New technology,

new challenges

Harmonics are the byproducts

of modern electronics. They

are especially prevalent wher-

ever there are large numbers of

personal computers, adjustable

speed drives, and other types of

equipment that draw current in

short pulses.

This equipment is designed

to draw current only during a

controlled portion of the incom-

ing voltage waveform. While

this dramatically improves

efficiency, it causes harmon-

ics in the load current. And that

causes overheated transformers

and neutrals, as well as tripped

circuit breakers.

If you were to listen to an ordi-

nary 60-cycle power line, you’d

hear a monotone hum. When

harmonics are present, you hear

a different tune, rich with high

Figure 1. Near sine wave.

Figure 2A. Distorted current waveform.

Figure 2B. Distorted voltage waveform.

notes. The problem is even more

evident when you look at the

waveform. A normal 60-cycle

power line voltage appears on

the oscilloscope as a near sine

wave (Figure 1). When harmon-

ics are present, the waveform

is distorted (Figure 2A and 2B).

These waves are described as

non-sinusoidal. The voltage and

current waveforms are no longer

simply related-hence the term

“non-linear.”

Getting to the root of the

problem

Finding the problem is relatively

easy once you know what to look

for and where to look. Harmonics

symptoms are usually anything

but subtle. This application note

provides some basic pointers on

how to find harmonics and some

suggestions of ways to address

the problems they create.

From the Fluke Digital Library @ www.fluke.com/library

Application Note

2 Fluke Corporation Troubleshooting power harmonics

Sources of harmonics

Defining the problem

Harmonics are currents or volt-

ages with frequencies that are

integer multiples of the funda-

mental power frequency. For

example, if the fundamental fre-

quency is 60 Hz, then the second

harmonic is 120 Hz, the third is

180 Hz, etc.

Harmonics are created by non-

linear loads that draw current

in abrupt pulses rather than in

a smooth sinusoidal manner.

These pulses cause distorted

current wave shapes which in

turn cause harmonic currents to

flow back into other parts of the

power system.

charge a large capacitor. After

a few cycles, the capacitor is

charged to the peak voltage of

the sine wave (e.g., 170 V for a

120 V ac line). The electronic

equipment then draws current

from this high dc voltage to

power the rest of the circuit.

The equipment can draw the

current down to a regulated

lower limit. Typically, before

reaching that limit, the capacitor

is recharged to the peak in the

next half cycle of the sine wave.

This process is repeated over

and over. The capacitor basically

draws a pulse of current only

during the peak of the wave.

During the rest of the wave,

when the voltage is below the

capacitor residual, the capacitor

draws no current.

The diode/capacitor power

supplies found in office equip-

ment are typically single-phase,

non-linear loads (Figure 3A).

In industrial plants, the most

common causes of harmonic cur-

rents are three-phase, non-linear

loads which include electronic

motor drives, and uninterruptible

power supplies (UPS) (Figure 3B).

Figure 3B. Three-phase, non-linear load

current waveform.

Figure 3A. Single-phase, non-linear load

current waveform.

The inside story

This phenomenon is especially

prevalent with equipment that

has diode-capacitor input power

supplies; i.e., personal comput-

ers, printers and medical test

equipment.

Electrically what happens is

the incoming ac voltage is diode

rectified and is then used to

Voltage harmonics

The power line itself can be an indirect

source of voltage harmonics.

The harmonic current drawn by non-linear

loads acts in an Ohm’s law relationship

with the source impedance of the supplying

transformer to produce voltage harmonics.

Source impedance includes the supplying

transformer and branch circuit components.

For example, a 10 A harmonic current being

drawn from a source impedance of 0.1 W will

generate a harmonic voltage of 1.0 V.

Any loads sharing a transformer or a

branch circuit with a heavy harmonic load

can be affected by the voltage harmonics

generated.

The personal computer can be affected by

voltage harmonics. The performance of the

diode/capacitor power supply is critically

dependent on the magnitude of the peak

voltage. Voltage harmonics can cause “flat

topping” of the voltage waveform lowering

the peak voltage (see Figure 2B). In severe

cases, the computer may reset due to insuf-

ficient peak voltage.

In the industrial environment, the induc-

tion motor and power factor correction

capacitors can also be seriously affected by

voltage harmonics.

Power correction capacitors can form a

resonant circuit with the inductive parts

of a power distribution system. If the reso-

nant frequency is near that of the harmonic

voltage, the resultant harmonic current

can increase substantially, overloading the

capacitors and blowing the capacitor fuses.

Fortunately, the capacitor failure detunes the

circuit and the resonance disappears.

3 Fluke Corporation Troubleshooting power harmonics

Symptoms of harmonics usually

show up in the power distribu-

tion equipment that supports the

non-linear loads. There are two

basic types of non-linear loads:

single-phase and three-phase.

Single-phase, non-linear loads

are prevalent in offices, while

three-phase loads are wide-

spread in industrial plants.

Each component of the power

distribution system manifests

the effects of harmonics a little

differently, yet all are subject to

damage and inefficient perfor-

mance if not designed to handle

electronic loads.

Neutral conductors

In a three-phase, four-wire

system, neutral conductors can

be severely affected by non-

linear loads connected to the 120

V branch circuits. Under normal

conditions for a balanced linear

load, the fundamental 60 Hz

portion of the phase currents will

cancel in the neutral conductor.

In a four-wire system with

single-phase, non-linear loads,

certain odd-numbered harmon-

ics called triplens—odd multiples

of the third harmonic: 3rd, 9th,

15th, etc—do not cancel, but

rather add together in the neutral

conductor. In systems with many

single-phase, non-linear loads,

the neutral current can actually

exceed the phase current. The

danger here is excessive over-

heating because, unlike phase

conductors, there are no circuit

breakers in the neutral conductor

to limit the current.

Excessive current in the neu-

tral conductor can also cause

higher-than-normal voltage

drops between the neutral con-

ductor and ground at the 120 V

outlet.

Circuit breakers

Common thermal-magnetic

circuit breakers use a bi-metallic

trip mechanism that responds to

the heating effect of the circuit

current. They are designed to

respond to the true-rms value of

the current waveform and will

trip when the trip mechanism

gets too hot. This type of breaker

has a good chance of protect-

ing against harmonic current

overloads.

A peak-sensing, electronic trip

circuit breaker responds to the

peak of current waveform. As a

result, it won’t always respond

properly to harmonic currents.

Since the peak of the harmonic

current is usually higher than

normal, this type of circuit

breaker may trip prematurely at a

low current. If the peak is lower

than normal, the breaker may

fail to trip when it should.

Bus bars and

connecting lugs

Neutral bus bars and connecting

lugs are sized to carry the full

value of the rated phase current.

They can become overloaded

when the neutral conductors are

overloaded with the additional

sum of the triplen harmonics.

Electrical panels

Panels that are designed to carry

60 Hz currents can become

mechanically resonant to the

magnetic fields generated by

higher frequency harmonic

currents. When this happens,

the panel vibrates and emits a

buzzing sound at the harmonic

frequencies.

Telecommunications

Telecommunications systems

often give you the first clue to

a harmonics problem because

the cable can be run right next

to power cables. To minimize

the inductive interference from

phase currents, telecommunica-

tions cables are run closer to the

neutral wire.

Triplens in the neutral conduc-

tor commonly cause inductive

interference, which can be heard

on a phone line. This is often the

first indication of a harmonics

problem and gives you a head

start in detecting the problem

before it causes major damage.

Transformer

Commercial buildings commonly

have a 208/120 V transformer

in a delta-wye configuration.

These transformers commonly

feed receptacles in a commer-

cial building. Single-phase,

non-linear loads connected to

the receptacles produce triplen

harmonics, which add up in the

neutral. When this neutral cur-

rent reaches the transformer, it is

reflected into the delta primary

winding where it causes over-

heating and transformer failures.

Another transformer problem

results from core loss and copper

loss. Transformers are normally

rated for a 60 Hz phase current

load only. Higher frequency har-

monic currents cause increased

core loss due to eddy currents

and hysteresis, resulting in more

heating than would occur at the

same 60 Hz current.

These heating effects demand

that transformers be derated for

harmonic loads or replaced with

specially designed transformers.

Effects of harmonic currents

01

02

03

120 V Branch Circuits

208/480 Volt Transformer

Secondary

Primary

Neutral

A

B

C

4 Fluke Corporation Troubleshooting power harmonics

Finding harmonics

Generators

Standby generators are subject

to the same kind of overheat-

ing problems as transformers.

Because they provide emergency

backup for harmonic producing

loads such as data processing

equipment, they are often even

more vulnerable. In addition to

overheating, certain types of

harmonics produce distortion at

the zero crossing of the current

waveform, which causes inter-

ference and instability for the

generator’s control circuits.

Classification of harmonics

Each harmonic has a name, frequency and sequence. The sequence refers

to phasor rotation with respect to the fundamental (F), i.e., in an induction

motor, a positive sequence harmonic would generate a magnetic field that

rotated in the same direction as the fundamental. A negative sequence har-

monic would rotate in the reverse direction. The first nine harmonics along

with their effects are listed below:

Name F 2nd* 3rd 4th* 5th 6th* 7th 8th* 9th

Frequency 60 120 180 240 300 360 420 480 540

Sequence + — 0 + — 0 + — 0

*Even harmonics disappear when waves are symmetrical (typical for electrical circuits)

Sequence Rotation Effects (from skin effect, eddy currents, etc.)

Positive Forward Heating of conductors, circuit breakers, etc.

Negative Reverse Heating as above plus motor problems

Zero** None Heating, plus add in neutral of 3-phase, 4-wire system

**Zero sequence harmonics (odd multiples of the 3rd) are called “Triplens” (3rd, 9th, 15th, 21st, etc.)

A harmonic survey will give you

a good idea if you have a prob-

lem and where it is located. Here

are a few guidelines to follow.

1. Load inventory. Make a

walking tour of the facility

and take a look at the types of

equipment in use. If you have

a lot of personal computers

and printers, adjustable speed

motors, solid-state heater

controls, and certain types of

fluorescent lighting, there’s a

good chance that harmonics

are present.

2. Transformer heat check.

Locate the transformers feed-

ing those non-linear loads and

check for excessive heating.

Also make sure the cooling

vents are unobstructed.

3. Transformer secondary

current. Use a three-phase

true-rms power quality

analyzer to check transformer

currents.

• Verify that the voltage rat-

ings for the clamp meter are

adequate for the transformer

being tested.

• Measure and record the

transformer secondary cur-

rents in each phase and in

the neutral (if used).

• Compare the kVA deliv-

ered to the load against the

nameplate rating. (If har-

monic currents are present,

the transformer can overheat

even if the kVA delivered

is less than the nameplate

rating.)

• Use the k-factor measure-

ment from a three-phase

analyzer to determine

de-rating or transformer

replacement.

• Measure the frequency of

the neutral current. 180 Hz

would be a typical reading

for a neutral current consist-

ing of mostly third harmonic.

4. Sub-Panel neutral current

check. Survey the sub-

panels that feed harmonic

loads. Measure the current

in each branch neutral and

compare the measured value

to the rated capacity for the

wire size used. Check the

neutral bus bar and feeder

connections for heating or

discoloration. A non-contact

infrared thermometer, like the

Fluke 61, is useful for detect-

ing excessive overheating on

bus bars and connections.

5. Receptacle neutral-to-

ground voltage check.

Neutral overloading in

receptacle branch circuits

can sometimes be detected

by measuring the neutral-

to-ground voltage at the

receptacle.

• Measure the voltage when

the loads are on. Two volts

or less is about normal.

Higher voltages can indicate

trouble depending on the

length of the run, quality of

connections, etc. Measure

the frequency. A frequency

of 180 Hz would suggest

a strong presence of har-

monics, while 60 Hz would

suggest that the phases are

out of balance. Pay special

attention to under carpet

wiring and mod-ular office

panels with integrated

wiring that uses a neutral

shared by three-phase

conductors.

• Because the typical loads in

these two areas are com-

puter and office machines,

they are often trouble spots

for overloaded neutrals.

5 Fluke Corporation Troubleshooting power harmonics

To determine whether you have

a harmonics problem you need to

measure the true-rms value and

the instantaneous peak value

of the wave shape. For single

applications, you need either a

clamp meter like the Fluke 335,

336 or 337 or a multimeter like

those in the Fluke 80, 170 and

180 Series that makes true-rms

measurements. For three-phase

applications, you’ll need a power

quality analyzer like the Fluke

430 Series.

“True-rms” refers to the

root-mean-square, or equiva-

lent heating value of a current

or voltage wave shape. “True”

distinguishes the measurement

from those taken by “average

responding” meters. The vast

majority of low-cost, portable

meters are average responding.

These instruments give correct

readings for pure sine waves

only and will typically read low

by as much as 50 percent when

confronted with a distorted

current waveform. True-rms

meters give correct readings

for any wave shape within the

instrument’s crest factor and

bandwidth specifications.

Crest factor

The crest factor of a waveform is

the ratio of the peak value to the

rms value. For a sine wave, the

crest factor is 1.414. A true-rms

meter will have a crest factor

specification. This spec relates to

the level of peaking that can be

measured without errors.

A quality true-rms handheld

digital multimeter has a crest

factor of 3.0 at full scale. This

is more than adequate for most

power distribution measure-

ments. At half scale the crest

factor is double. For example, the

Fluke 187 DMM has a crest factor

spec of up to 3.0 when measur-

ing 400 V ac and a crest factor

of up to 6.0 when measuring

200 V ac.

Using a true-rms meter with a

“Peak” function—like the Fluke

187—the crest factor can be

easily calculated. A crest factor

other than 1.414 indicates the

presence of harmonics. In typical

single-phase cases, the greater

the difference from 1.414, the

higher the harmonic content. For

voltage harmonics, the typi-

cal crest factor is below 1.414;

i.e., a “flat top” waveform. For

single-phase current harmonics,

the typical crest factor is much

higher than 1.414.

Three-phase current wave-

forms often exhibit a “double

hump” waveform, therefore the

crest factor comparison method

should not be applied to three-

phase load current.

After you’ve determined that

harmonics are present, you can

make a more in-depth analysis

of the situation with a harmonic

analyzer such as the Fluke 43B

Power Quality Analyzer.

Meter Type

Measuring

Circuit

Sine Wave

Response*

Square Wave

Response*

Distorted Wave

Response*

Average

Responding

Rectified

Average x 1.1 Correct 10 % high Up to 50 % low

True-rms RMS Calculating

converter. Cal-

culates heating

value.

Correct Correct Correct

*Within multimeter’s bandwidth and crest factor specifications.

Multimeter performance comparison average

responding vs. true-rms

Troubleshooting tools

6 Fluke Corporation Troubleshooting power harmonics

The following are suggestions

of ways to address some typi-

cal harmonics problems. Before

taking any such measures you

should call a power quality

expert to analyze the problem

and design a plan tailored to

your specific situation.

In overloaded neutrals

In a three-phase, four-wire

system, the 60 Hz portion of the

neutral current can be minimized

by balancing the loads in each

phase. The triplen harmonic

neutral current can be reduced

by adding harmonic filters at the

load. If neither of these solutions

is practical, you can pull in extra

neutrals —ideally one neutral for

each phase—or you can install

an oversized neutral shared by

three phase conductors.

In new construction, under

carpet wiring and modular office

partitions wiring should be

specified with individual neutrals

and possibly an isolated ground

separate from the safety ground.

Derating transformers

One way to protect a trans-

former from harmonics is to limit

the amount of load placed on

it. This is called “derating” the

transformer. The most rigorous

derating method is described

in ANSI/IEEE standard C57.110-

1986. It is somewhat impractical

because it requires extensive

loss data from the transformer

manufacturer plus a complete

harmonic spectrum of the load

current.

The Computer & Business

Equipment Manufacturers Associ-

ation has recommended a second

method that involves several

straightforward measurements

that you can get with com-

monly available test equipment.

It appears to give reasonable

results for 208/120 V receptacle

transformers that supply low

frequency odd harmonics (third,

fifth, seventh) commonly gener-

ated by computers and office

machines operating from single-

phase branch circuits.

Derating factor

To determine the derating factor for the transformer, take the peak and true-

rms current measurements for the three phase conductors. If the phases are not

balanced, average the three measurements and plug that value into the follow-

ing formula:

HDF = Harmonic derating factor

= (1.414)(true-rms phase current)

(Instantaneous peak phase current)

This formula generates a value between 0 and 1.0, typically between 0.5

and 0.9. If the phase currents are purely sinusoidal (undistorted) the instanta-

neous peaks are 1.414 times the true-rms value and the derating factor is 1.0.

If that is the case no derating is required.

However, with harmonics present the transformer rating is the product of the

nameplate kVA rating times the HDF.

kVA derated = (HDF) x (kVA nameplate)

For example: 208/120 Y transformer rated at 225 kVA:

Conductor

name

True-rms

current amps

Instantaneous

peak current

Load currents were measured with

a Fluke Model 87 and an 80i-600 ac

current probe to produce the follow-

ing results:

01 410 A 804 A

02 445 A 892 A

03 435 A 828 A

I phase avg. = 410 + 445 + 435 = 430 A

3

I pk avg. = 804 + 892 + 828 = 841 A

3

HDF = (1.414) (430) = 72.3 %

841

The results indicate that with the level of harmonics present the transformer

should be derated to 72.3 % of its rating to prevent overheating.

Solving the problem

7 Fluke Corporation Troubleshooting power harmonics

The high voltage and current

present in electrical power

systems can cause serious

injury or death by electrocu-

tion. Consequently, only trained,

experienced electricians who

have knowledge of electri-

cal systems in general and the

equipment under test should

perform testing and modification

of electrical systems.

Fluke cannot anticipate all

possible precautions that you

must take when performing the

measurements described here.

At a minimum, however, you

should:

• Use appropriate safety equip-

ment such as safety glasses,

insulating gloves, insulating

mats, etc.

• Be sure that all power has

been turned off, locked out,

and tagged in any situation

where you will be in direct

contact with circuit compo-

nents. Be certain that the

power can’t be turned on by

anyone but you.

• Read and understand all of

the applicable manuals before

using the application informa-

tion in this application note.

Take special note of all safety

precautions and warnings in

the instruction manuals.

This article is a general guide to

understanding harmonics. It is

not intended to substitute for the

services of a professional electri-

cal systems consultant. Before

you take any measures to diag-

nose or address your potential

harmonics problems, you should

have your operation thoroughly

analyzed by a professional elec-

trical engineer.

Situation

A modern office building dedi-

cated primarily to computer

software development contained

a large number of personal com-

puters and other electronic office

equipment. These electronic

loads were fed by a 120/208 V

transformer configured with a

delta primary and a wye second-

ary. The PCs were fairly well

distributed throughout the build-

ing, except for one large room

that contained several machines.

The PCs in this room, used exclu-

sively for testing, were served by

several branch circuits.

The transformer and main

switch gear were located in a

ground floor electrical room.

Inspection of this room immedi-

ately revealed two symptoms of

high harmonic currents:

• The transformer was generat-

ing a substantial amount of

heat.

• The main panel emitted an

audible buzzing sound. The

sound was not the chatter

commonly associated with

a faulty circuit breaker, but

rather a deep resonant buzz

that indicated the mechanical

parts of the panel itself were

vibrating.

Ductwork installed directly over

the transformer to carry off some

of the excess heat kept the room

temperature within reasonable

limits.

Defining the problem

Transformer – Current mea-

surements (see Table 1) were

taken on the neutral and on

each phase of the transformer

secondary using both a true-rms

multimeter and an average-

responding unit. A 600 A

clamp-on current transformer

accessory was connected to each

meter to allow them to make

high current readings. The cur-

rent waveshapes are shown in

Figures 4 and 5.

Conductor name

True-rms multimeter

(amps)

Average responding

multimeter (amps)

Instantaneous peak

current (amps)

Phase 1 410 328 804

Phase 2 445 346 892

Phase 3 435 355 828

Neutral 548 537 762

Work safely

Case study

8 Fluke Corporation Troubleshooting power harmonics

The presence of harmonics

was obvious by comparison of

phase current and neutral cur-

rent measurements. As Table 1

shows, the neutral current was

substantially higher than any of

the phase currents, even though

the phase currents were relatively

well balanced. The average-

responding meter consistently

took readings approximately 20

percent low on all the phases.

Its neutral current readings were

only 2 percent low.

The waveforms explain the

discrepancy. The phase currents

were badly distorted by large

amounts of third harmonic cur-

rent, while the neutral current

was nearly a pure sinewave at

the third harmonic frequency.

The phase current readings

listed in Table 1 demonstrate

clearly why true-rms measure-

ment capability is required to

accurately determine the value of

harmonic currents.

The next step was to calculate

the “harmonic derating factor” or

HDF (Refer to “Derating trans-

formers” section on page 6.)

The results indicated that,

with the level of harmonics

present, the transformer should

be derated to 72.3 percent of

its nameplate rating to prevent

overheating. In this case the

transformer should be derated

to 72.3 percent of its 225 kVA

rating, or derated to 162.7 kVA.

The actual load was calcu-

lated to be 151.3 kVA. Although

that figure was far less than the

nameplate rating, the transformer

was operating close to its derated

capacity.

Subpanel—Next a subpanel

which supplied branch circuits

for the 120 V receptacles was

examined. The current in each

neutral was measured and

recorded (see Table 2).

When a marginal or over-

loaded conductor was identified,

the associated phase currents

and the neutral-to-ground

voltage at the receptacle were

also measured. When a check

of neutral #6 revealed 15 A in

a conductor rated for 16 A, the

phase currents of the circuits

Figure 4. Phase current.

Figure 5. Neutral current.

Circuit

number

Phase

current

(amps)

Neutral-to-ground voltage

drop at receptacle

25 7.8 3.75 V

27 9.7 4.00 V

29 13.5 8.05 V

Table 3. Phase currents and neutral-to-ground voltage for neutral #06.

Neutral

conductor

number

Current

(amps)

01 5.0

02 11.3

03 5.0

04 13.1

05 12.4

06 15.0*

07 1.8

08 11.7

09 4.5

10 11.8

11 9.6

12 11.5

13 11.3

14 6.7

15 7.0

16 2.3

17 2.6

Table 2. Subpanel branch circuit neutral

currents.

(#25, #27, and #29) that shared

that neutral were also measured

(Table 3). Note that each of the

phase currents of these three

branch circuits was substan-

tially less than 15 A, and also

the same phase conductors had

significant neutral-to-ground

voltage drops.

In the branch circuits which

had high neutral current, the

relationship between the neutral

and the phase currents was

similar to that of the transformer

secondary. The neutral cur-

rent was higher than any of the

associated phase currents. The

danger here is that the neutral

conductors could become over-

loaded and not offer the warning

signs of tripped circuit breakers.

Recommendations

1. Refrain from adding addi-

tional loads to the receptacle

transformer unless steps are

taken to reduce the level of

harmonics.

2. Pull in extra neutrals to the

branch circuits that are heav-

ily loaded.

3. Monitor the load currents on a

regular basis using true-rms

measuring test equipment.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

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In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2003-2009 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 6/2009 1260362K A-EN-N

Modification of this document is not permitted

without written permission from Fluke Corporation.

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