PSNetters,
To repeat an old quotation: Those that don't know their history are
bound to repeat it.
The introduction of SMPS into electronic equipment in large numbers
led to unexpected problems in electrical installations.
The key technical problem initially identified was the development
of harmonics which were not properly accounted for in the installations.
One issue was triplen harmonics (especially 3rd, but includes 9th,
15th etc) which were fed back into the delta primary of distribution
transformers where they circulated and produced heat. This caused large
transformers to overheat and burn up unexpectedly, to the chagrin of
utilities and industrial & commercial customers who had not understood what
they were getting into when they purchased large numbers of computer
terminals and, later, computers and other electronic equipment.
In the early '90s I participated in an ongoing UofWisc/CBEMA
sponsored seminar on 'Effects of Non-linear Loads on the Power Distribution
System' presented a number of times in the US. We presented some clipped AC
voltage waveforms commonly found in our industrial buildings as part of this
discussion. The purpose of the day-long 2 dozen paper presentation was to
get facilities engineers apprised as to this developing problem and provide
some guidance in sizing their system to properly handle this non-linear
load. A common experience was that only about 1/3 of the expected load
could be connected in the facility before these problems developed.
From my recollection, the Euro drive for a standards based fix was
driven by the British experience. From what we heard, at the beginning of
the Falkland's war, when it was announced that Queen Elizabeth would speak
during the dinner hour that the troops were going to protect the peace
everyone had hurried home and turned on their Sony TVs to get the
announcement. The large transformers that serve 600 or more residences each
in the apartments in London developed the primary triplen harmonics
overheating mentioned above and a number of them burned out leaving large
groups of folks without power until the transformer could be replaced.
Americans, as John Woodgate continues to remind us, are smarter than
that and have left the solution to the private market.
From my point of view another unexpected consequence from this
technology has been the development of non-sinusoidal Touch Current from the
use of these SMPS. The addition of Power Factor Correction has only served
to make the problem worse and now the further addition of another switcher
to get to the high efficiency and low power sleep modes will only make this
worse.
To see some TC waveforms go to Art Michaels site: www.safetylink.com
and search on my name Perkins and you will find a couple of dozen TC
waveforms along with their respective input current waveforms. These have
been taken from many different products as a part of my safety & regulatory
consulting business over the last 12 -15 years. From this I have given a
number of papers at the IEEE PSES on TC issues showing the need to move to
peak TC measurements of these waveforms because of this unexpected
complexity that has developed from all of this.
The increasing use of switching electronics in consumer products
(both electronic and motor driven) will further acerbate this problem.
Another American quote (attributed to Patrick Henry, a founding
father): Eternal vigilance is the price of freedom.
So, be alert, the world needs more lerts.
br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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