Case Studies

Power factor is a measurement that compares
the real power (watts) being consumed to the
apparent power (Volts-Amps) of the load. A
purely resistive load would have a power factor
of 1.0.
What does it do?
The power a
vailable to perform work is called
real power (kW). Inductive loads such as motors,
transformers, and high-intensity lighting intro-
duce reactive power (kVARs) into a power
system
. The system capacity is rated by the
apparent power (kVA) which must be large
enough to ac
c
ommodate both the real power
(kW) and reactive power (kVAR). Since reactive
power requires system capacity, but performs no
work, utilities charge more for it and plants try
to keep net kV
A
Rs low
. High reactive power
translates to low power factor
.
Power factor facts:
•
Utilities ma
y charge higher rates or penalties
for low power factor or high VARs.
•
System capac
ity restrictions cause voltage
drops and overheating
.
•
Inductive VARs can be corrected by applying
capac
itors or active c
onditioners
.
Power correction
The power correction systems
were being installed primarily
to correct power factor. The
same c
onductors were used to
feed both adjustable-speed
motor drives (ASDs) and across
the line motor starters. The
ASDs are non-linear loads and
draw harmonic currents, espe-
cially at the fifth harmonic.
Power factor capacitors present
a lower impedance to higher
frequencies, than to the 60 Hz
fundamental. So a system with
harmonic currents can cause
overcurrent in correction capac-
itors. Maintenance staff at the
plant realized this and decided
to use a semiconductor-based
device instead.
2 Fluke Corporation Power Quality Case Study: Clean power delivers clean water
Motor
Motor
Adjustable
speed motor
drive
00
012
4
Power factor
correction unit
Motor control center
Fluke 434
Figure 1. System connections.
How much does poor PF cost?
Sample calculation
Assume the utility adds 1 % of demand charge
for each 0.01 below PF of 0.97. Assume your PF
averages 0.86 each month and your demand
charge is $7000. (0.97 – 0.86) * 100 % = 11 %
(11 % x $7000) x 12 months = $9,240
avoidable annual cost
What to check?
•
See if your utility rate plan imposes a charge
for reactive demand or power factor
.
•
Find out how the utility measures power factor
or VARs. For example, are they looking at peak
inter
vals or a
verages?
•
Identify loads that are causing lagging reactive
power and develop a strategy for power factor
c
orrection
.
How much is OK?
To avoid paying higher utility fees, power factor
should b
e higher than .9
7. Capacitors or condi-
tioners may be applied on individual loads, at a
confluence of several, or at the service entrance
to improve power factor.
Note: This varies. Some utilities charge a rate for every percentage
point between .85 and .97. Some charge a rate based on the number
of VARs you use. Some don’t charge at all.
What’s power factor and why does it matter?
The dev
ice they connected to
each MCC continuously moni-
tors a circuit’s power factor. It
generates leading reactive
power (VAR’s) to counteract the
inductive, lagging VAR’s from
the motors. It can respond
instantly to load changes. In
addition, it looks for any
harmonic current and generates
an inverted signal to cancel it.
So, to any upstream devices it
appears as if the MCC is a
resistive load.