Full resolution (JPEG) - On this page / på denna sida - A Survey of the Possible Effects upon A. C. Networks Supplying Television Receivers in Residential Districts, by Sture Colding
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
A Survey of the Possible Effects upon
A. C. Networks Supplying Television
Receivers in Residential Districts
Sture Colding,
The City of Stockholm Electricity Authority
Med anledning av att televisionsapparaternas antal
kraftigt ökat, har man befarat att matningsströmmen
till dessa mottagare skulle kunna åstadkomma en
olämplig likströmsmagnetisering av
distributions-transformatorerna med härav följande förlustökning,
spänningssänkning och uppkomst av övertoner i
spänningen.
I nedanstående artikel framlägges en kortfattad
teori för det fysikaliska händelseförloppet. Detta
klarlägger storleksordningen av förlustökningen
m.m. Teorin har verifierats genom praktiska prov,
vilka pekar mot en förlustökning av 2 % av
transformatorns totala förlusteffekt, exceptionella fall
undantagna.
Andra biverkningar synes ej bli störande i normala
nät, trots att övertonshalten t.o.m. väsentligt
överskrider den för gasurladdningslampor
representativa.
Current supply to television receivers
Some television receivers, for example the French
and the American ones, are mounted with
transformer input to the rectifier, others generally produce
the d.c. directly via a half-wave rectifier.
Only the "transformerless" sets are dealt with in
this article. They are characterized by a typical
capacitor input to the filter after the rectifier. As a
result a large d.c. component and a bad waveform
occur.
An analysis of the supply current of a medium
receiver has given the values
Harmonics No Hz Percentages of the fundamental wave
0 0 40
1 50 100
2 100 50
3 150 49
4 200 40
5 250 28
6 300 17
7 350 10
8 400 6
9 450 3
10 500 3
11 550 2
12 600 2
RMS value (total): 130 % or 1.2 A (220 V).
621.316.13 : 621.397.62
It is possible to feed the set either from the
positive half-wave or from the negative half-wave of the
supplying network.
Influence on the supply network
Distribution transformers
With reserve to possible modifications in coming
techniques the demand pro consumer in a residential
district where few electrical cookers are installed is
1.2 kW, calculated at the distribution transformer
point. If the transformer is rated at 250 kVA
probably 200 television sets may be supplied
simultaneously.
Each phase of the transformer will have a load of
24 A d.c. if the distribution of the sets of the phase
is most unfavourable, i.e. rectification only from
the positive or only from the negative half-waves
of the supply tension. However according to the
distribution of chance we may calculate with a
representative case when 20 % of the receivers of
one phase are not compensated by an equal
number of sets supplied from a wave of the opposite
polarity. Thus it remains a d.c. excitation of 5 A
arising from the l.v. side. The same conditions are
thought to be prevailing in another phase, but the
rest excitation there is assumed to have the
opposite sign.
A modern transformer with the core manufactured
by cold rolled steel sheets has a magnetizing current
of about 1 % if the load impedance is linear. This
current, referred to the secondary side of the before
mentioned transformer is of the order of 4 A. With
respect to economical reasons and in order to reduce
the magnitude of the transformer size the flux
density of the core is relatively high, 1.5 or 1.6 Wb/m2
(peak value). At a cursory glance it might be
assumed that the mean value of the working flux of the
transformer core would be displaced with a value
corresponding to a separate d.c. excitation of 5 A.
As a result from that it would be needed that the
a.c. flux variations have to vary, following a sinus
curve around this new mean level, lying straight
under the saturation level.
Doubts have been pronounced concerning the
fitness of the new core material in modern power
supply. It is thought that the increased flux
density may cause an unallowed enlargement of the
transformer losses and the violent excitation current
ELTEKNIK 1959 1 145
<< prev. page << föreg. sida << >> nästa sida >> next page >>
