Triplett Mitigator Loop Coil – 3232 - Manual (Page 15)

unless there is some source of harmonic current. For our example, we’ll accept that the transformer is
“over-excited” (has too much voltage applied to it) and is generating harmonic currents.
Because the capacitor bank is acting as a load for these harmonics, these currents will flow from the
transformer to the capacitor bank. A small portion of the harmonic currents will flow through the induc-
tion motors, but due to their inductive reactance, the currents will be small compared to those flowing
through the capacitor. Consequently, in the section of power line between the transformer and the
capacitor bank, harmonic currents will be flowing, which means that harmonic magnetic fields will be
created, and the potential to induce noise into an exposed telephone cable is very real.
To complicate matters even more, the over-excited transformer could cause harmonic currents to flow
on the power line that goes back to the substation. It all depends on whether there is a load for the
harmonic currents to flow to. This load doesn’t even have to be in-line with the power line. It could be
at a customer site being fed from the power line.
Industrial customers often have their own power factor correction capacitors on their premises. Since
the power company charges a premium for supplying power to customers with poor power factor, it is
often to the customer’s advantage to correct his power factor himself, with his own capacitor bank, so
that his facility approaches unity power factor.
Consequently, a customer could have a capacitor bank acting as a resonant circuit or a sink on his
premises. This means that a Loop Coil survey of a power line going to a customer could indicate that
the customer is either generating harmonics or is acting as a load for harmonics. As stated before, it is
not possible to tell which is occurring with just the Loop Coil readings.
This is the problem with Loop Coil readings. Although they indicate that magnetic fields are present,
they are unable to indicate which way the current is flowing in the power lines. In fact, this is difficult to
do with the large majority of test equipment, even equipment attached directly to the power lines.
I hope the reader has recognized by now that the harmonic currents do not flow in the same way as do
the 60Hz [50Hz] fundamental currents. While the 60Hz [50Hz] currents will always flow from the power
line to the customer, harmonic currents can flow from the customer back into the power lines. In fact,
harmonic currents can flow between several customers without flowing back to the substation. Con-
sider a situation in which a customer has an on-site power distribution transformer which is over-ex-
cited, and an adjacent customer has his own power factor correction capacitor bank. Harmonic current
could flow between the transformer and the capacitor, “noising up” any exposed telephone lines in
between.
Because of the variety of loads that customers put on a power line and the transformers and capacitors
placed on the line by the power company, the combinations that could result in harmonic current flow
are numerous. In most cases, the possibility of causing harmonics in a new installation is not easily
predictable. It is not even easy to predict how changing an existing installation may affect harmonics.
So many variables are involved that a “cut and try” approach is often used. Unfortunately, this approach
seldom solves the problem unless those involved have a clear understanding of the nature and causes
of harmonic propagation on the power lines. A lot of this “understanding” is just plain old common
sense and some experience. And if it isn’t already obvious, the craftsperson is going to have to learn
more about how power lines work. He (or she) is also going to need to have a knowledge of power line
paths as well as phone cable paths in the area in which he works.
SECTION 10: POTENTIAL VERSUS ACTUAL
Up to this time, we have been talking about the potential noise that could be induced onto a telephone
line by a magnetic field. The word “potential” is very important. The presence of a magnetic field does
not guarantee that noise will be induced onto a phone line immersed in the field.
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