Application Note

4 Fluke Corporation How adjustable speed drives affect power distribution
Is it possible to install “Power
Factor Correction Capacitors” and
have PF get worse? It certainly is,
and a starting place to under-
standing this puzzle lies in the
distinction between Displacement
PF (DPF) and Total Power Factor
(PF). The penalty for not under-
standing the difference 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 P
ower
to Apparent Power, or Watts to
VA. DPF is the classic concept of
power factor. It can be considered
as the power factor at the funda-
mental frequency. 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, Figure 7). It follows
that with the presence of har-
monics, PF is always lower than
DPF and is also a more accurate
description of total system effi-
ciency 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.
Y
ou ha
ve to be clear which one
you
’re talk
ing 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, meas-
ured in Volt-Amps (VA) to supply
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
b
elow 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 correc-
tion capacitors are installed.
Upstream system capacity, both
in the plant and at the utility
level, is released and available
for other uses. (Figure 6)
Historically, this has been the
gist of the PF story: a relatively
well-known problem with a rela-
tively straightforward solution.
Harmonics and
capacitors
Harmonics have had a dramatic
impact on our approach to Power
Factor correction. The motor and
capacitor loads described above
are all linear and for all practical
purposes generate no harmonics.
Non-linear loads such as ASDS,
on the other hand, do generate
harmonic currents.
Take a plant which is step-by-
step putting adjustable speed
drives on its motor loads. ASDs
generate significant harmonic
currents (5th and 7th on six-
pulse converter drives). Suddenly
the fuses on existing PF correc-
tion caps start blowing. Since
these are three-phase caps, only
one of the three fuses might
blow. Now you’ve got unbalanced
currents, possibly unbalanced
voltages. The electrician replaces
the fuses. They blow again. He
puts in larger fuses. Now the
fuses survive, but the capacitor
blows. He replaces the capacitor.
Same thing happens. What’s
going on? Harmonics are higher
frequency currents. The higher
the frequency, the lower the
impedance of a cap (X
C
= 1/2πfC).
The cap acts like a sink for
harmonic currents.
Power system resonance
Hot vibes can result when harmonics and capacitors get together
After: PF = 100 %Before: PF = 42 %
1/6 HP Motor
Active
165 Watts
Reactive
360 VAR
3.3 A
1/6 HP Motor
Active
165 Watts
Reactive
360 VAR
1.4 A
Capacitor
60 µF
Figure 6. Capacitor corrects Displacement Power Factor (DPF).