Application Note

ManualsBrandsFluke ManualsPower Quality AnalyzersFluke 1745

3 Fluke Corporation Applying power quality measurements to predictive maintenance

oltage sag count

Taking a single voltage reading

tells only part of the story. How is

the voltage changing during an

hour? During a day? Sags, swells

and transients are short-term

variations in voltage. The voltage

sag (or dip) is the most common

and troublesome variety.

Sags indicate that a system is

having trouble responding to load

requirements and significant sags

can interrupt production. Voltage

sags can cause spurious resets on

electronic equipment such as

computers or controllers, and a

sag on one phase can cause the

other two to overc

ompensate,

potentially tripping the circuit.

Sags ha

ve several dimensions:

depth, duration, and time of day.

Utilities use a special index to

track the number of sags that

occur over a period of time. To

gauge the depth of the sags, they

count how often voltage drops

below various thresholds.

The longer and larger the

voltage variations, the more likely

equipment is to malfunction.

For example, the Information

Technology Industry Council

(ITIC) curve specifies that 120 V

computer equipment should be

able to run as long as voltage

does not drop below 96 V for

more than 10 seconds or below

84 V for more than 0.5 seconds.

Current

Current measurements that trend

upward are a key indicator of a

problem or deg

radation in your

load. While equipment is run-

ning, monitor phase, neutral and

round current over time. Make

sure none of the currents are

increasing sig

nificantly, verif

that they’re less than the name

plate rating, and keep an eye out

for high neutral current, which

can indicate harmonics and

unbalance.

oltage unbalance

In a three-phase system, sig

nifi

cant differences in phase voltage

indicate a problem w

ith the sys

tem or a defect in a load

•

High voltage unbalance causes

three-phase loads to draw

exc

essive current and causes

motors to deliver lower torque.

•

Unbalance is tracked in per-

centages (see Figure 3). The

negative sequence voltage

(Vneg) and zero sequence

voltage (Vzero) together iden-

tify any voltage asymmetry

between phases.

•

Using a power quality analyzer

to do the math, high percent-

ages indicate high unbalance.

EN 50160 requires Vneg to be

less than two percent.

Voltage harmonic distortion

Harmonic distortion is a normal

consequence of a power system

supplying electronic loads such

as computers, business machines,

electronic lighting ballasts, and

control systems. Adding or

removing loads from the system

changes the amount of distortion,

so it’s a good idea to regularly

check harmonics.

Harmonics cause heating and

reduced life in motor windings

and transformers, excessive neu-

tral current, increased suscepti-

bility to voltage sags, and

reduced transformer efficiency.

As current harmonics interact

with impedance, they’re con-

verted into voltage harmonics.

Total Harmonic Distortion (THD) is

a sum of the contributions of all

harmonics. By tracking Voltage

THD over time you can determine

if distortion is changing. For volt-

age harmonics, IEEE 519 recom-

mends less than 5 % T

Figure 3. This unbalance display shows the voltage unbalance

parameters (Vneg and Vzero) as well as current unbalanc

Figur

e 4.

This Harmonics table show

s the voltage T

D for each

phase. N

ote that it

’s normal for the voltage T

D on the neutral to run

close to 100 %.

Fluke Corporation

O Box 9

0, Everett, W

A USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The N

etherlands

For more information call:

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

ax (425) 446-5

In Europe/

East/

Africa (3

1 40) 2 6

5 200 or

Fax (31 40) 2 675 222

In Canada (800) 36-FLUKE or

ax (9

05) 89

0-6866

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

Fax +1 (425) 446-5116

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

Printed in U.S.A. 3/2005 2435544 A-US-N Rev A

Fluke. K

eping your world

up and running.

Over

voltage Summary Description