Application Note

Power Quality Troubleshooting Fluke Corporation 29
A. System Diagram
B. Equivalent Circuit
Harmonic
source
X
C
X
T
X
T
X
S
X
S
X
C
kvar
(nonwork producing)
VA
kW
(work producing)
Harmonics
(nonwork
producing)
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 correc-
tion cap form a parallel resonant
circuit: X
L
= X
C
at a resonant fre-
quency 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
Figure 7.8 Resonant circuit when X
C
= (X
T
+ X
S
)
Figure 7.7 Total Power Factor increases with harmonics.
which are many times greater
than the exciting current. This
so-called “tank circuit” can
severely damage equipment,
and it will also cause a drop
in power factor. Perversely, this
resonant condition often ap-
pears 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. (Fig. 7.8)
Imagine coming to work on
a Monday and seeing the insu-
lation 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 been 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.