Triplett Mitigator Loop Coil – 3232 - Manual (Page 13)

9.4: TRANSFORMERS
There are several types of transformers that are often seen on the power lines that parallel telephone
lines. The transformers at the customer locations that step down the power line voltage to the voltages
used by the customer (often 120/240 volts) are called distribution transformers. In rural areas, there is
often one transformer per customer, and in urban and suburban areas, several customers are often
connected to the same transformer. Customers that use large amounts of power may have several
transformers or a small substation dedicated exclusively for their usage.
An observer will also note that some transformers are inserted in-line in the power line. These are
generally found in rural areas where it is necessary to boost the line voltage slightly to make up for losses
in the power line. Sometimes these are fixed “boosters”, and sometimes they are voltage regulators (as
previously described).
All of these transformers found on the power line have the potential to generate harmonic currents.
These currents may travel down the power lines in either direction, or both directions, from a power
transformer. Just as previously described, the generation of harmonic currents is usually due to el-
evated line voltage or defective transformers.
A common test procedure used by noise mitigators who suspect that elevated power line voltages are
causing harmonic currents is simply to measure the voltage coming out of the typical AC wall outlet.
Measurements of the wall outlet voltage along a suspected power line can indicate if the harmonic
currents may be due to elevated line voltage. The nominal outlet voltage is 120 [240] volts, and it has
been stated that small increase in the percentage of excess voltage can cause dramatic increases in
harmonic current. Therefore, if the voltage is 3 [6] or more volts above 120 [240], an excess voltage
condition may be present. Low wall outlet voltage is usually not indicative of a problem (as far as noise
on the telephone line is concerned).
It must be remembered that in order for the harmonic currents to flow in the power line, there must be
a load on the line. If harmonic currents are generated but there is NO LOAD for them to flow to, THEY
WILL NOT FLOW down the power line. If there is no current flow, there are no magnetic fields pro-
duced, and no noise induced into nearby ‘phone lines. While this is a well-understood basic principle,
it gets more complicated where harmonics are concerned.
This is because all of the loads on the power line are designed to be loads at 60Hz [50Hz], the funda-
mental current that the power line was designed to transmit. Since the loads were only intended to be
loads at 60Hz [50Hz], their load characteristics at harmonics of 60Hz [50Hz] are uncertain. Engineers
often talk about the way loads behave at different frequencies (such as the fundamental and the har-
monic frequencies) by noting how much “reactance” the loads have at different frequencies. If a load
has less reactance, it means that more current will flow through it, and conversely, if a load has more
reactance, less current will flow through it. It gets complicated though, because different types of loads
have different reactances at different frequencies. Fortunately, there is a general trend to the way reac-
tance varies in different loads, so we can make some reasonably valid assumptions that suit our situa-
tion.
9.5: INDUCTIVE REACTANCE
Most devices that have coils of wire in them have inductive reactance. Actually, even a straight piece of
wire has a small amount of inductive reactance. These devices are often called “inductors”, “chokes”,
transformers, coils, etc. A common industrial source of inductive reactance is induction motors. Most
AC motors that don’t have brushes are induction motors. In these inductive devices, the reactance is
lowest at low frequencies, and it increases at high frequencies. This means that at higher frequencies,
like at the frequency of harmonic currents, current will not flow through these devices very well. As with
every rule or principle, there are exceptions. But the general behavior of current through an inductor is
that it diminishes as the frequency of the current increases.
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