Triplett Mitigator Loop Coil – 3232 - Manual (Page 12)

Harmonic currents can also come from a lightly loaded transformer. Transformers “reflect” their sec-
ondary load current into their primary winding. This means that if a heavy load is placed on the second-
ary of a transformer, the load will reflect into the primary of the transformer, causing it to be heavily
loaded. This heavy primary load will cause the primary voltage to sag (be lower than normal) due to
resistance in the power lines feeding the primary winding. If, under this loaded condition, the primary
voltage is at its nominal rating (100% rating), and if the secondary load decreases, the primary voltage
will rise above its nominal current generation. As previously mentioned, a small amount of overvoltage
can cause a significant rise in harmonic current generation. Consequently, lightly loaded transformers
can be just as guilty as heavily loaded transformers when it comes to harmonic current generation. It is
actually more appropriate to consider the effect of the primary voltage (or overvoltage), but this depends
to some degree on the loads on the transformer.
It is also necessary to consider the possibility of a bad substation transformer. Some transformers
contain “tertiary” (third) windings. These windings are incorporated into the transformer specifically for
the purpose of suppressing harmonic current generation. They typically consist of a set of delta con-
nected windings in the transformer. These windings sometimes supply power to loads in the substation.
Other times, they are only connected to each other inside of the transformer and are not connected to
any external loads. Consequently, if one or more of the tertiary windings fail, it may not be immediately
obvious to power company personnel. No customers call up to complain about their power going off.
Instead, if other conditions warrant it, harmonics suddenly appear on the power line.
Not all transformers contain tertiary windings. They are typically only necessary in wye-wye connected
transformers.
As in the telephone company, some power line installations function for many years requiring a mini-
mum of maintenance. In these installations, transformers that are 20 or 30 years old are not uncommon.
The craftsperson should also be aware that the power company reuses a lot of their old gear. In general,
transformers have a long life if properly cared for. Consequently, as power lines are rerouted, the used
equipment is often sent to a refurbishing / warehousing depot where old transformers are refurbished
and, after some time in the warehouse, are eventually reinstalled to repair a blown transformer, or in-
stalled in a new power line run. The depot seldom checks the transformers for their susceptibility to
produce harmonic currents.
As the country has gone through some periods of prosperity and decline, the transformers made in
those eras show signs of the national financial health. A typical method of reducing the generation of
harmonic currents in a transformer is simply by using a large magnetic core. This core is made out of
silicon steel, typically called “transformer steel”. It is possible to get the transformer to work reasonably
well, within limits, with less steel. When this is done, however, the transformer has very little “head-
room” and often becomes a significant producer of harmonics when the primary voltage is increased by
just a few percentage points. Be particularly wary of transformers made during World War II. A lot of
products skimped on basic materials in order to satisfy the appetite of the war effort. Transformers are
usually marked with their date of manufacture.
The situation just presented (over-excited transformer) is only the simplest of the possible scenarios.
The example was purposely simplified to illustrate the basic causes of the problem. In the real world,
there are usually several in-line distribution transformers as well as capacitor banks between the cus-
tomer and the substation. We will examine how these elements affect the generation and propagation
of harmonic currents.
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