Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: How efficient are transformers?
Summary: Power line regulation...
Message-ID: <3094@kitty.UUCP>
Date: 15 Apr 89 20:10:48 GMT
References: <731@hutto.UUCP> <880004@hpmtlx.HP.COM>
Organization: Recognition Research Corp., Clarence, NY
Lines: 45

In article <880004@hpmtlx.HP.COM>, dc@hpmtlx.HP.COM ($Dave Crook) writes:
> In many cases, especially in residential lines, the losses in the power 
> lines themselves are far greater than the losses in the transformers.

	This is true.  Efficient power distribution has many trade-offs.
There is a significant trend toward higher distribution voltages to
minimize the effects of I^2R losses.  Many electric utilities have switched
from 4160 volts to 13.2 kV for distribution in residential areas.  For
commercial distribution 34.5 kV has also become common.

	However, a negative effect with higher voltages is increased losses
due to capacitance, corona and - especially - insulator leakage losses
during wet weather.

> If the transformer does not have large heat radiating tubes or fins, the
> chances are that it is quite efficient.  Since all losses are converted
> to heat,  the heat dissipating sinks are a directly measure of the losses
> the transformer was designed to dissipate at maximum load.

	While you have a point, this is not necessarily true.  With advances
in high-voltage insulating materials, many "intermediate"-sized transformers
contain no oil or gas (like SF6), but are air-cooled through convection,
and consequently have no fins as part of their design.  Air-cooled
transformers still get hot, though.

>    Many "transformers" are actually transformer regulators.  Inside the 
> "transformer" is an electronic voltage detection circuit which controls
> the voltage ratio.  Under heavy loads the ratio increases which compensates
> for significant voltage and power loss in the power lines feeding the 
> transformer.

	There are high-voltage power line voltage regulators of two types:
(1) buck-boost autotransformer; and (2) a true transformer with a tap
changer.  However, these regulating devices are used more infrequently
than you suggest.  Large transformers of the multi-MVA variety that
distribute voltages of 34.5 kV and above usually have taps that are
changed either manually (through a switch on the transformer), or
through a remote telemetering circuit by a human operator.  Large
transformers that employ automatic tap changing to achieve automatic
regulation are not very common.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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