Accurate readings needed to assess power factor
Editor's note: The Energy
Services Bulletin features real answers to real questions
posed to our staff at the Energy Services Power Line. We hope
you find it useful.
Question:
How will power factor affect metering in a distribution system?
Will a poor power factor increase line loss?
Answer:
The answer to your first question is slightly different depending
whether you are referring to the displacement power factor or
the true power factor. The displacement power factor equals
the cosine of the phase angle between the voltage wave and the
current wave. The true power factor is the ratio of the true
RMS real power to the true RMS apparent power, i.e. kW/kVA.
Both methods give exactly the same power factor
if no harmonics are present. But if significant harmonics occur
in either the voltage or current, the true power factor is lower.
Fluorescent lights, electronic adjustable speed drives on motors,
and/or lots of computers or other electronic devices may cause
high-current harmonics in a circuit.
All AC electric meters account for power factor when they register
and record real power. Most utility revenue meters only recognize
displacement power factor, so they will be accurate only when
the real power factor is close to the true power factor. The
presence of significant harmonics will tend to cause the power
factor to read higher than true power.
To find out if your displacement power factor
is different from your true power factor, you can measure both
with a high-end power analyzers, such as a Fluke 43B. These
analyzers recognize displacement power factor and some will
display both true and displacement power factor. If the readings
are very different, it is likely that the true power factor
is lower than is what your meter is measuring.
Line Losses
The quick answer to your second question is "yes,"
poor power factor will increase line losses significantly. The
table below shows how a poor power factor affects line losses
compared to the same real power delivered at a good (i.e. high)
power factor.
| PF |
Increase in line losses |
| 100% |
0% |
95% |
10.8% |
| 90% |
23.5% |
| 85% |
38.4% |
| 80% |
56.2% |
75% |
77.8% |
| 70% |
104.1% |
Clearly, the increased line
loss is dramatic. The table is calculated based on a constant
resistance of the conductors. Since temperature increases conductor
resistance, the actual increase is even greater than shown.
The increase in line losses is related to true power factor.
If harmonics are present, the metered power factor may understate
the true power, so line losses will be higher than your calculation
using the power factor the meter measured.
However, to put this in perspective,
only losses on the line going from a power panel to a motor
or other low power factor device will be affected. Also, line
losses tend to be quite a small percentage of total plant energy
use—typically in the range of 1 to 2 percent. Thus, increasing
line losses by 104 percent means that your losses are going
from 1 percent to 2 percent, for instance.
According to the Industrial
Power Factor Analysis Guidebook, published by the Bonneville
Power Administration, a typical plant can save between 0.5
and 1.5 percent of its total energy use with reduced line losses
by correcting power factor. Note that line losses are only reduced
when the power factor correction (capacitors) is applied at
or near the load.
Usually, the savings in line
losses is not enough to justify the cost of correcting power
factor, but is an added benefit on top of other reasons for
correcting power factor, such as reducing or eliminating a power
factor penalty and increasing the load-carrying capacity of
the circuit.
