Triplett Mitigator Loop Coil – 3232 - Manual (Page 10)

substation and sent to the customer site. In either case, the harmonic current flow along the power line
looks the same (as far as the Loop Coil is concerned). All we know for sure, at the moment, is that a
particular power line is radiating a lot of magnetic fields along its length.
How can we determine what is really happening? Unfortunately, there is no easy way to tell where the
harmonic currents are originating from. The best method to use, with the resources available to the
craftsperson, is to examine the installations at either end of the power line for signs of possible harmonic
current generation (such as customer loads known to produce harmonic currents).
In the example of Figure 7, the spectral signature of the harmonic currents in the power lines provides an
additional clue. The rectifier loads at the customer site (factory) will produce a signature indicative of
rectifier loads. This type of signature is unlike most signatures produced by any common substation
problems. Hence, the craftsperson can surmise that rectifier loads at the customer site are the most
likely source of the harmonic currents.
9.2: LOAD BALANCING
When dealing with 3 phase lines, the most direct method of controlling the radiated magnetic fields is to
make sure the loads on the lines are balanced. (This is not the same balance as telephone line balance.)
It may be found that the magnetic fields responsible for noise on a telephone line are coming from an
exposure where 3 phase power lines are poorly balanced. It is quite possible for the same power lines to
be well balanced in the rest of the exposure. Balance depends upon the customer loads on the power
line.
Balance simply means there is about the same amount of current flowing in each phase wire. When this
is the case, magnetic field cancellation will take place. The MITIGATOR Applications Manual gives more
detail on this topic.
Load balancing is generally most helpful for suppressing 60Hz [50Hz] and the “odd non-triple harmon-
ics”. Some cancellation of odd triple harmonics may occur, depending upon the amount of Ground
Return Current flowing in the earth.
If a Loop Coil measurement of a 3 phase exposure shows a large amount of 60Hz [50Hz] and odd non-
triple harmonics, and a Probe wire test confirms that Ground Return I*T is high, it would be advisable to
have the power company measure the load balance in the suspected section of the power line. They can
do this with a clamp-on ammeter at the site. Measuring load balance of the power line at the substation
WILL NOT necessarily indicate the load balance at the Exposure site.
If they (the power company) decide to correct a poor load balance problem at the exposure site, be
aware that they may end up making the load balance worse at the different location. They will, of course,
try to avoid doing this, but sometimes they may just “cut and try.” The results may be somewhat ran-
dom. Translation: When the power company re-balances the power line at the exposure site, you may
end up with more noise on previously quiet phone lines. It may then be necessary to find the “new”
poorly balanced power line section and have it re-balanced, etc. If the unbalance can be forced into an
area where there is no telephone exposure, the problem may be solved.
Two phase power lines cannot be balanced to completely cancel the radiated magnetic fields. Even so,
SOME magnetic field cancellation will take place. So, it is beneficial to balance the line as well as
possible, even though the results will not be as good as those obtainable with 3 phase lines.
Single phase lines cannot be balanced. A phase reversal transformer, however, may prove effective in
reducing noise in the paralleling phone line. If the exposure is fairly long, at the approximate midpoint of
the exposure, the polarity of the power line can be reversed with a 1:1 (one-to-one) transformer. This will
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