Note: Descriptions are shown in the official language in which they were submitted.
1 The present invention relates in general to
high-voltage transformers, and in particular to -trans-
formers in which the transmission error is compensated
by means of an amplifier.
~ n unloaded voltage transformer produces at
the terminals of its secondary winding a voltage corre-
sponding to the so-called open-circuit transformation
ratio. As soon as the secondary winding of the trans-
former is connected to a load, a voltage drop will re-
sult which changes the open-circuit secondary voltage
and introduces the aforementioned transmission error.
In order to keep the load-dependent deviations o:E the
secondary voltage within the desired limits, the active
parts of the transformer, such as coils and the iron core,
are designed with corresponding dimensions. These measures,
however, particularly in high-voltage transformers, are
costly and, moreover, the insulation problems are increased,
inasmuch as the insulation is directly proportional to the
size and weight of these active parts. From prior art,
transformers circuits are known in which the transmission
errors in the secondary winding are more or less compen-
sated by various circuits elements.
For example, a device is known in which the sec-
ondary circuit of a voltage transformer is connected in
series with an impedance Zl corresponding to the short-
circuit impedance of the secondary circuit, this impedance
Zl being connected in series with a coupling impedance Z2
in the input of an amplifier; the coupling impedance is
again connected in series with an impedance of the output
circuit of the ampliEier, the output impedance being similar
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1 to the input impedance Zl The amplifier delivers at its
output an electromotive force which compensates the voltage
drop across the impedance Z] and simultaneously the voltaye
drop caused by the transformer itself, so that the secondary
voltage transformed from the primary voltage exhibits prac-
tically no error due to the load.
This known device utilizes a difference measuring
process which, due to its sensitivity, is susceptible to
disturbances and unreliable operation. The measuring imped-
ance of such differential measuring circuits is too largeand consequently, in case of failure in the amplifier, the
error voltage in the transformer may increase to such a level
that the transformer is made unusable in a practical appli-
cation.
Also known is a compensation circui-t for reducing
the load dependence of a voltage transformer. In this prior-
art circuit, the load-dependent voltage is coupled back into
the primary circuit with such a phase as to reduce the error.
This kind of compensation is effective when the load alter-
nates between zero and a maximum value while the nominalvoltage remains constant. Nevertheless, this prior-art
compensation circuits is effective also during the upward
and downward devia-tion of the primary voltage~ and thus
introduces a measuring error into the secondary circuit
when the load remains constant.
Also known are automatic devices for reducing
transmission errors in voltage transformers, in which trans-
ducers and amplifiers are connected in the secondary current
circuit. The elimination of transmission errors caused by
the load in such devices is effected by means of a correction
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1 voltage dependent both upon the load current and on the
secondary voltage. The correction voltage is employed as
an input signal for a transducer connected in the secondary
circuit. Since the transducer employed is connected im-
mediately in the secondary circuit, the full secondary
current flows therethrough and consequently it must be
designed to withstand a considerable power. The switching
conditions of the transducers are influenced by the trans-
mission errors to be compensated, and consequently this
solution provides only a reduction of the transmission
error but not its complete elimination.
Furthermore, a device is known which employs two
auxiliary voltage transformers arranged in the primary cir-
cuit of the hi~h-voltage transformer. The secondary voltage
of one of the auxiliary transformers is applied to the input
of an amplifier, and the output of the amplifier is connect-
ed to the secondary winding of the out auxiliary transformer.
The first auxiliary transformer serves for the generation
of an open-circuit reference voltage which is compared with
the secondary voltage of the other, loaded auxiliary trans-
fer, to produce a difference signal which as to its magni-
tude and phase corresponds to the transmission error and is
employed for the compensation of such error. In this prior-
art compensation device, however, it is practically impossible
to drive one of the auxiliary transformers in a no-1oad con-
dition, and consequently in practical applications the afore-
mentioned operational principle is not completely successfulO
In addition, this known arrangement is very expensive.
Devices are also known which amplify low signals
on the secondary winding of the high-voltage transformer to
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1 a desired value. The total power, however, is delivered by
the amplifier, and the transformer serves only as a control
element. Consequently, such power amplifiers have to meet
high operational requirements as to their stability with
respect to time and temperature and with respect to the
phase error between their input and output circuits. The
last-mentioned parameters for the amplified power must
always be kept within the tolerance range of the desired
class of accuracy.
It is therefore a general object of the present
invention to overcome the aforementioned disadvantages.
More particularly, it is an object of the inven-
tion to provide an improved error-compensated high-voltage
transformer which eliminates the transformation errors and
which is effective both in the open-circuit or no-load con-
dition of the secondary circuit and in the loaded condition
of the same.
In keeping with these objects, and with others
which will become apparent hereafter, one feature of -the
invention resides, in an error-compensated high-voltage
transformer having a secondary circuit, in a combination
which comprises a measuring impedance connected in series
with the secondary circuit to produce a small voltage drop
from the secondary current in the same vector direction,
an amplifier having its input connected across the measur-
ing impedance and an output connected in series wlth the
secondary winding. With advantage, the ampoifier is
power-supplied by means of a transformer from the secondary
circuit.
~30 In principle, the accurancy of the compensating
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1 circuit of the voltage transformer must be kept within a
predetermuned operational range, such as within 0.8 to 0.2
multiple of the phase voltage in the case of single-phase
transformers (minimum and maximum measuring voltage). The
transformer itself is during operation subject to a substan-
tially increased accuracy range. For instance, the last-
mentioned transformer operates up to 1.9 times the phase
voltage. The maximum operational voltage together with the
nominal load of the transformer determines the required load-
ing capacity of the amplifier. For instance, if the inputof the amplifier is connected to a transformer which is
designed such that the saturation of its iron core is at-
tained slightly above the current corresponding to the max-
imum measuring voltage at a nominal load, then the ampli-
fied voltage increases only slightly with currents or volt-
ages exceeding the maximum measuring voltage. It is suf-
ficient to design the amplifier for a loading capacity
which only slightly exceeds the value of the maximum measur-
ing voltage at a nominal load. As a consequence, there re-
sults a substantial saving in the design of the amplifier.This saving is apparent from the following example:
Single-phase transformer for nominal
power 60 VA
Operational voltage between 0.22 to 1.9 Un
Measuring voltage in the range of 0.8 to
1.2 Un (Un = phase voltage)
If the transformer or converter produces during its rated
operation an error of for instance 6%, then in order to
achieve an accurancy class of 0.5, it is necessary to pro-
duce a compensation power of about 5~, corresponding to 3 VA.
-
5~
1 Without the aforementioned adjustment of the transducer tothe input of the amplifier, the maximum loading capacity of
the amplifier would have to be about 1.92 3, that is
approximately 11 VA.
By using impedance transformers at the input and
output sides of the amplifier, the measurement at a reduced
accuracy can take place even if the amplifier becomes dis-
abled. It is of advantage that each secondary winding of
respective impedance transducers is bridged by a measuring
impedance. These measuring impedances are dimensioned such
that, in case of failure of the amplifier, a sufficient de-
crease of the transmission impedance is produced. As a re-
sult, the transmission error typically remains within ac-
ceptable limits. This feature is unattainable in all con-
ventional error-compensating systems.
Due to the additional impedances connected paral-
lel to the windings of the transformers in the secondary cir-
cuit, the vector position of the compensating voltage trans-
mitted by the measuring transformers can be regulated. This
adjustability of the vectors of the compensating voltage in
turn makes it possible that ~he error compensa-tion be carried
out in the ampllfier itself. It is of advantage when ~he
compensation voltage delivered by the amplifier is combined
with an additional compensation voltage from a series-
connected capacitor. In this manner, the orientation of -the
vector of the compensation voltage at the output of the
amplifier is adjustable within a broad range, because this
voltage counteracts the phase shift of the compensation volt-
age. Consequently, only low requirements are placed on the
arnplifier for the transmission of phase of signals and the
5;:~2
1 same holds true for the vector conditions of the vol-tage
applied to the input of the amplifier.
The aforementioned capacitor can be connected
either directly or via a transformer in series with the
secondary circuit of the high--voltage transformer. The
error-compensation method of this invention enables also
a partial compensation of the errors, so that a residual
error of an arbitrary magnitude and vectorial direction
is preserved.
The amplifier can be power-supplied from the
secondary winding of the high-voltage transformer. This
power-supply connection causes an additional load on the
high-voltage transformer. The resulting additional trans-
formation error, however, is compensated in the manner de-
scribed above.
The novel features which are considered charac-
teristic for the invention are set forth in particular in
the appended claims. The invention itself, however, both
as to its construction and its method of operation, together
with additional objects and advantages thereof, will be
best understood from the following description o~ specific
embodiments when read in connection with the accompanying
drawingO
FIG. 1 is a schematic circuit diagram of one
embodiment of the error-compensated transformer of this
invention; and
FIG. 2 and 3 show, respectively, another two
embodiments of this invention.
In the Figures, reference numeral 1 indicates
the secondary winding of a high-voltage trans~ormer or
3~
transducer connected to terminals 2' of à load 2, An amplifier 3
has an input 4 which is connected across a measuring impedance 5.
The impendance 5 is connected in series with the secondary winding
1 and the output 6 of -the amplifier is connected in series with the
impedance 5 and the load 2. The impedance 5 produces only a small
voltage drop from the load current, and the amplifier is designed
so that its output 6 compensates automatically all voltage losses
resulting from the application of load 2. The amplifier is power-
supplied from a circuit 7.
A preferred embodiment of this invention is shown in
FIGS. 2 and 3, where measuring transformers 8 and 9 have primary
windings connected in series with the secondary winding l and the
load ~.. The secondary windings of measuring transformers 8 and 9
are bridged by resistances 10 and 11 and connected respectively to
the input 4 and the output 6 of amplifier 3. The resistances 10 and
11 adjust the phase conditions of the compensation voltage. An
additional transformer 12 is connected in series with the secondary
winding l and its secondary circuit is bridged by a capacitor 13. The
additional tr.ansformer 12 serves for the generation of the additional
compensation voltage.
The operation of the error compensated circuit will be
explained by way of an example with reference to FIG. 2.
Assuming a load 2 of 60 VA at a rated voltage lO0/ ~ 3,
the secondary winding 1 of a non-compensated high voltage transformer
would show 3% voltage drop due to ohmic component and a 3% voltage drop
to a reactive component of the transmission path.
According to this invention, -these voltage drops are
compensated in such a manner as to allow a voltage accuracy of 0.2
at the nominal output of 60 VA. The value of capacitor ~ is 0.082
microfarad; the winding ratio of transformer 12 is 24/3648; the value
of resistor 10 is l ohm; the winding ratio.of transformer 8 is 2l/5;
the wlnding ratio of transformer 9 is 111/24; the frequency i.s 50 Hz;
the resistance ll in this example is 117 ohms; and the amplifi.er 3
is designed for an input voltage of 0.25 vol-ts and for an outpu-t
voltage f 8 volts.
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The linearity and angular accuracy of the amplifier
are set to be 1.25 times the nominal value of the input voltage,
as far as in this range the error of the amplification factor of
the amplifier is not higher than ~2,5% and its phase displacement
l1'~5
does not exceed -~60 minutes. From 0~ 5 up to 1,9 times the rated
,, _
value of the input voltage, the voltage error may rise up to -~10%
without any limitation to the phase displacement. The alternating
current at 50 Hz, which at a load of 60 VA corresponds to 1.03 A,
causes a voltage drop of 3% across the capacitor 13. This voltage
drop compensates the reactive voltage drop in the transformer 12.
The resistance 10 (value 1 ohm) which is connected to
the input terminals of the amplifier 3 and via the transformer 8
with the turn ratio 21/5 to the load 2, produces at the current of
1.03 ~ a voltage of 0.25 V is produced across the resistance 10 at
the input of amplifier 3. This voltage is amplified to 8 volts.
The output voltage at the amplifier 3 is introduced via the transformer
9 (with the turn ratio 111/24) into the load circuit 2. Since the
transformed voltage is in opposite phase to the ohmic voltage drop of
1.73 volts corresponding to the aforementioned 3% drop of the rated
voltage in the load circuit, the latter voltage drop of 3% in the
transformer is thus compensated. As a result, the amplifier is loaded
with an active power of
1.03 (~) X 24/111 X 8 (V) = 1.78 (W).
The resistance 11 across the secondary winding of
transformer 9 is preferably constituted by the impedance of a capacitor
of 27 microfarads which at 50 Hz has an impedance of 117 ohms. At
the primary winding of transformer 9, this impedance amounts to 5.36
ohms and as a result a voltage drop due to the load current of 1.03
amperes is applied to the load to compensate reactive voltage drop in
case of a failure of the amplifier 3~ The capacitance 11 causes a
supplementary Load on the amplifier of
82/117 (ohm) = 0.547 YA.
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FIG. 3 illustrates a circuit similar to that of
FIG. 2, in which a voltage regulator 14 is connected across the
secondary winding 1 and the regulated voltage is used for feediny
the amplifier 3.
It will be understood that each of the elements
described above, or two or more together, may also find a useful
application in other types of constructions differing from the
types described above.
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1 While the invention has been illustrated and
described as embodied in a high-voltage error-compensated
transformer, i.t is not intended to be limited to the details
shown, since various modifications and structural changes
may be made without departing in any way from the spirit
of the present invention.
Without further analysis, the foregoing will
so fully reveal the gist of the present invention that
others can, by applying current knowledge, readily adapt
it for various applications without omitting features that,
from the standpoint of prior art, fairly constitute essential
characteristics of the generic or specific aspects of this
invention.
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