Application Note

5 Fluke Corporation How adjustable speed drives affect power distribution
many times greater than the
exciting current. This so-called
“tank circuit” can severely dam-
age equipment, and it will also
cause a drop in power factor.
Perversely, this resonant condi-
tion often appears only when the
system is lightly loaded, because
the damping effect of resistive
loads is removed. In other words,
we have what the audio buffs
call a “high Q” circuit. (Figure 8)
Imagine coming to work on a
Monday and seeing the insulation
on your cables melted off. How
can this happen over a weekend
when there was hardly any load
on the system? Has Ohm’s Law
b
een overruled? Not quite. Your
power system just spent the
weekend tanked out on the
Harmonics. It was quite a party,
but now comes the clean-up.
Start with harmonics
mitigation
The correct solution path starts
with measuring and mitigating
the harmonics generated by the
drives. Harmonic trap filters
would generally be called for.
These trap filters are installed
locally on the line side of the
drive. Their effect is very much
like the traditional PF correction
cap, in two senses: they reduce
DPF as well as PF, and also they
localize the circulation of the
problem harmonics (generally the
5th). Harmonics mitigation and
traditional DPF correction should
be addressed as one systems
issue. In other words, manage
Total PF, not just DPF.
Power system resonance
In a worst-case scenario, the
inductive reactance (X
L
) of the
transformer and the capacitive
reactance (X
C
) of the PF correction
cap form a parallel resonant
circuit: X
L
= X
C
at a resonant
frequency which is the same as
or close to a harmonic frequency.
The harmonic current generated
by the load excites the circuit into
oscillation. Currents then circulate
within this circuit which are
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O Box 9
0
9
0, Everett, W
A USA 98206
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PO Box 1186, 5602 BD
Eindhoven, The N
etherlands
For more information call:
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ax (425) 446-5
1
1
6
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-
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ax (9
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From other countries [phone] or
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Web access: http://www.fluke.com
©2004 Fluke Corporation
. All rights reserved.
Printed in U.S.A. 10/2004 2403028 A-US-N Rev A
Fluke. K
e
eping your world
up and running.
A. System Diagram
B. Equivalent Circuit
Harmonic
source
X
C
X
T
X
T
X
S
X
S
X
C
Figure 8. Resonant circuit when XC = (XT + XS)
kvar
(nonwork producing)
VA
kW
(work producing)
Harmonics
(nonwork
producing)
Figure 7. Total Power Factor increases with harmonics.