Application Note

28 Fluke Corporation Power Quality Troubleshooting
Power system resonance
Hot vibes can result when harmonics and capacitors get together
Is it possible to install “Power
Factor Correction Capacitors”
and have PF get worse? It cer-
tainly is, and a starting place to
understanding this puzzle lies
in the distinction between Dis-
placement PF (DPF) and Total
Power Factor (PF). The penalty
for not understanding the differ-
ence can be blown capacitors
and wasted investment.
Total PF and Displacement PF
are the same in one basic
sense: they are the ratio of Real
Power to Apparent Power, or
Watts to VA. DPF is the classic
concept of power factor. It can
be considered as the power fac-
tor at the fundamental fre-
quency. Total Power Factor,
abbreviated to Power Factor
(PF), now includes the effects
of fundamental and of harmonic
currents (it is also referred to as
True PF or Distortion PF,
Fig. 7.7). It follows that with the
presence of harmonics, PF is al-
ways lower than DPF and is
also a more accurate description
of total system efficiency than
DPF alone.
Strictly speaking, the term
“Power Factor” refers to Total PF,
but in practice can also be used
to refer to DPF. Needless to say,
this introduces some confusion
into discussions of power factor.
You have to be clear which one
you’re talking about.
Displacement Power
Factor
Lower DPF is caused by motor
loads which introduce the need
for Reactive Power (Volt-Amp
Reactive or VARs). The system
has to have the capacity, mea-
sured in Volt-Amps (VA) to sup-
ply both VARs and Watts. The
more VARs needed, the larger
the VA requirement and the
smaller the DPF. The cost of
VARs is accounted for in a
power factor penalty charge.
Utilities often levy additional
charges for DPF below a certain
level; the actual number varies
widely, but typical numbers are
0.90 to 0.95.
To reduce VARs caused by
motor loads, power factor cor-
rection capacitors are installed.
Upstream system capacity, both
in the plant and at the utility
level, is released and available
for other uses. (Fig. 7.6)
Historically, this has been the
gist of the PF story: a relatively
well-known problem with a
relatively straightforward
solution.
Harmonics and Capacitors
Harmonics have had a dramatic
impact on our approach to
Power Factor correction. The
motor and capacitor loads de-
scribed above are all linear and
for all practical purposes gener-
ate no harmonics. Non-linear
loads such as ASDS, on the
other hand, do generate har-
monic currents.
Take a plant which is step-
by-step putting adjustable
speed drives on its motor loads.
ASDs generate significant har-
monic currents (5th and 7th on
six-pulse converter drives). Sud-
denly the fuses on existing PF
correction caps start blowing.
Since these are three-phase
caps, only one of the three fuses
might blow. Now you’ve got un-
balanced currents, possibly un-
balanced voltages. The
electrician replaces the fuses.
They blow again. He puts in
larger fuses. Now the fuses sur-
vive, but the capacitor blows.
He replaces the capacitor. Same
thing happens. What’s going
on? Harmonics are higher fre-
quency currents. The higher the
frequency, the lower the imped-
ance of a cap (X
C
= 1/ 2πfC).
The cap acts like a sink for
harmonic currents.
After: PF = 100%Before: PF = 42%
1/6 HP Motor
Active
165 Watts
Reactive
360 VAR
3.3A
1/6 HP Motor
Active
165 Watts
Reactive
360 VAR
1.4A
Capacitor
60 µF
Figure 7.6 Capacitor corrects Displacement Power Factor (DPF).