Triplett Mitigator Loop Coil – 3232 - Manual (Page 11)

reverse the polarity of the magnetic field radiating from the line following the transformer. This does not
cause any cancellation of magnetic fields, but an interesting phenomenon takes place in the phone line.
On one side of the transformer, the noise induced into the phone line is of a particular polarity (or phase).
On the other side of the transformer, the induced phone line noise polarity is not opposite to the original
polarity. If the overall strength of the noise signals on either side of the phase reversal transformer is the
same, the noise itself will cancel out.
Although it is best to “solve” the problem of excessive influence (i.e. power line magnetic fields) at its
source, this is not always possible or reasonable. In these cases, devices can be added to the tele-
phone line to mitigate the effects of the excessive influence. These effects may be excessive 60Hz
[50Hz], excessive harmonics, or both.
Some of the available devices treat individual pairs, some treat groups of pairs, and some treat entire
cables.
These devices are often called “noise chokes”, “drainage coils”, “choke and drains”, Harmonic Drainage
Reactors (HDR’s), Induction Neutralizing Transformers (INT’s), etc. Each of these devices has a particu-
lar function, and, consequently, their use is dependent upon the specific situation.
9.3: SUBSTATION HARMONIC GENERATION
In general, the most common malfunctions at a power substation that could lead to the generation of
excessive harmonic currents are over-excited or defective power transformers. Over-excited transform-
ers are transformers that have too much primary voltage applied to them. Independent tests have
shown that as little as 1 or 2% of primary “overvoltage” can cause a significant increase in the harmonic
currents produced by the transformer. At specific harmonic frequencies, current can rise by 10dB
(approximately 3 times as much, or 200% more) due to a 3% increase in primary voltage. Although the
overall THD (Total Harmonic Distortion) may increase only slightly in this situation, the harmonic currents
rise dramatically. This increase in harmonic current can translate into a rise in induced noise on the
telephone line.
Why would the primary voltage on the substation transformer be too high? Often, the power company
does this on purpose! They are required to deliver at least 120 volts [240 volts] (usually) to any customer
on the line. Sometimes, in order to satisfy the end-of-line customer, they simply crank up the voltage at
the substation. This is similar to the telephone company practice of cranking up the C.O. battery in
order to get more loop current on long lines.
When the primary voltage is raised, however, a significant rise in harmonic currents in the power line can
occur, which in turn causes noise on exposed telephone lines. Since the power company seldom
checks for harmonic current every time they adjust a voltage, they don’t know that they’ve just created
the potential for some very noisy phone lines in the surrounding area.
To make matters worse, the power company sometimes installs “voltage regulators” (also called “auto-
matic tap changers”) at the substation (and in the power lines). A voltage regulator automatically adjusts
the voltage applied to a transformer or power line. Regulators are usually installed on power lines that
experience large changes in load during the day. When the line is heavily loaded (by customers using a
lot of power), the line voltage has a tendency to sag. A voltage regulator automatically cranks up the
voltage to make up for the sag. Alternately, when the line is lightly loaded, the regulator turns down the
voltage. This could lead to a situation in which, under heavily loaded conditions, the regulator cranks up
and causes harmonic current generation in a transformer, which then sends the harmonic current down
the power line past telephone cable exposures. This means that the telephone line only “noises up”
when the loads on the power line are heavy. If the craftsperson is unaware of this “connection” between
the power line load and telephone noise, it may appear to the craftsperson that the noise comes and
goes randomly. More often than not, however, the noise will be discovered to be cyclic in nature,
varying with the time of day, as the power line loads vary with the time of day.
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