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 certainly is, and a starting place to
understanding this puzzle lies in the distinction between Displacement
PF (DPF) and Total Power Factor (PF). The penalty for not
understanding 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 Power 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 fundamental 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). It follows that with the presence of harmonics,
PF is always 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, measured 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 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
correction capacitors are installed. Upstream system capacity, both in
the plant and at the utility level, is released and available for
other uses. 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 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 correction 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.
The cap acts like a sink for harmonic currents.

Power System Resonance
In a worst-case scenario, the inductive reactance (XL) of the
transformer and the capacitive reactance (XC) of the PF correction cap
form a parallel resonant circuit: XL= XC 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 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 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.

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 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. (One useful tool for measuring
harmonics and capacitance is the Fluke 43B Power Quality Analyzer,
http://tinyurl.com/ykot9lj.) 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.