Improvements in connection with Electrical Measuring Instruments Employed in Combination with Current and Potential Transformers for Measuring Electrical Energy.
Year: 1899

page_text: Date of Application, 25th Aug., 1899-Accepted, 28th Oct., 1899
COMPLETE SPECIFICATION.
Improvements in connection with Electrical Measuring Instruments
Employed in Combination with Current and Potential Transformers
for Measuring Electrical Energy.
We, SIEMENS BROTHERS & Co. LIMITED, of 12, Queen Anne's Gate, West-
minster, S.W., Electrical Engineers, do hereby declare the nature of this invention
(as communicated to us from abroad by Siemens & Halske Aktien Gesellschaft,
of Markgrafen Strasse 94, Berlin, S.W., Germany, Electrical Engineers), and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement: -
When measuring electric current of great strength it is of advantage to use
shunts to the measuring instruments, so that only a known part of the current
passes through the instrument. For alternating currents it is usual to use so
called current transformers, instead of these shunts, which transform the current
to be measured, into one of less strength. Such a current transformer is
essentially built as follows:
-
Referring to the accompanying drawings, in Fig. 1 c represents an iron body
composed of insulated iron laminae, the one limb a of which carries the secondary
fine wire winding s. Over this fine wire, is wound the primary winding p, of
copper of greater or less section according to circumstances.
Fig. 2 shews the connection of a current transformer T with a measuring
instrument A. For the sake of clearness the primary and secondary windings
are here shewn on separate limbs of the iron core.
Fig. 3 represents the diagram of this arrangement. The primary current J¹
consists of two components, the magnetising current J for the transformer
field M, and the secondary current J flowing through the instrument A, Fig. 2.
The instrument A has some self-induction. and consequently the current .J* is
shifted in phase relatively to the E.M.F. E of the transformer by an angle q.
The ratio J J¹ does not strictly correspond to the ratio of the transformer;
it will however remain practically constant in cases where the magnetic resistance
It would therefore
of the air gaps is large, as compared with that of the iron.
only be necessary to multiply the readings of the instrument A by a certain con-
stant for obtaining the primary current J¹.
This arrangement is suitable for measuring the strength of the current, but
not for the measurement of electric power, since the current J2 flowing through
the measuring instrument A is shifted in phase relatively to the primary cur-
rent J by the angler and if this were not taken into consideration, the power
measurements will be wrong.
The present invention relates to an arrangement which permits the shifting of
the secondary current J2 of current transformers through exactly 180°.
If the instrument A Fig. 4 be considered as a power measuring instrument
and a non-inductive resistance be connected in parallel with the current coil,
we obtain with a suitable resistance r the diagram Fig. 5.

page_text: The secondary current Ja is now composed of two parts, namely, of the cur
rent J' in the resistanger and of the current J^ in the measuring instrument A,
which current is now shifted exactly 180° relatively to the primary current J.
By means of this arrangement the readings on the power measuring or register-
ing instrument become independent of the phase shift in the current transformer.
Generally the angle q through which the phase is shifted, owing to the self-
induction of the power measuring or registering instrument, is sufficiently large,
otherwise it can be raised by inserting a suitable impedance coil.
Instead of connecting the non-inductive resistance in parallel with the
meter, or counter A a second secondary winding of a few turns may be added to
power
the current transformer at at Fig. 6, and which is closed by a suitable resistance
for instance a piece of wire r.
The effect is in this case the same, as with the
arrangement shewn in Fig. 4.
Similar conditions to those existing in connection with current transformers
exist in connection with potential transformers, which serve in one case to reduce
the high potential of the network, or of the generators to one suitable for the
measuring instrument, and secondly to render the instrument safe for handling.
Fig. 7 shews such an arrangement. Diagram Fig. 8 shews that the current
flowing through the measuring instrument B is shifted by an angle y as regards
the primary potential D. Whereas this would influence ordinary measurements
of potentials only to a small extent, considerable errors would be caused in
measurement of power.
By connecting an impedance coil S parallel with the current coil of the power
measuring or registering instrument, such coil S being of a greater self-induction,
than that of the current coil of the instrument, as at Fig. 9, then assuming
that the main current J¹ has no phase shift relatively to the primary potential D,
it is possible, by judicious selection, to shift the phase of the current J³ of the
current coil of the power measuring or registering instrument in respect to the
secondary current of the potential transformer by 180°, as at Fig. 10, so that
also in this case the work is measured independently of the shifting of phase in
the potential transformer. J³ in conjunction with current J' of the impedance
coil now form the main current J¹. By inserting non-inductive resistances in
the circuit of J3 and J' the shifting of phase between J³ and J* can be varied within
wide limits.
It is further possible to employ a similar arrangement to that described in
connection with current transformers in Figs. 4 to 6. Since the current in the
potential winding of the measuring or registering instrument generally does not
show a great phase difference relatively to the potential at the terminals of the
instrument, it is possible by inserting an impedance coil S¹, Fig. 11, to shift it
through an angle 7 when, by connecting a non-inductive resistance in parallel
with the impedance coil S¹ and the potential coil of the power measuring instru-
ment or counter, the current will be shifted relatively to the primary
potential D, by 180°, as shewn in diagram Fig. 12. In this case likewise a second
secondary winding may be added to the potential transformer in place of the
non-inductive resistance r. In cases where a power measuring or registering
instrument is to be connected to a current transformer T' and a potential trans-
former Te Fig. 13, the total correction of the shifting of phase can only be effected
on the current transformer T. In this case the methods described in Figs. 4
and 6 may be used with the difference that the selection of resistance must be
made so that J and have the same phase when I and J₁ are likewise of one
and the same phase, Fig. 14.
The advantage of the arrangement described in Fig. 13, whereby the current
transformer may have the ratio one, consists in the fact that the handling of
the measuring instruments will not be dangerous when measuring power in con-
nection with high potential systems.
It should be stated that for this invention only measuring instruments of the
electro-dynamometer type are employed, where as in the American Patent

page_text: No. 423210 and the English 2285297 measuring instruments based on Ferraris'
principle are employed, in which a phase shift by 90° is essential.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed, we declare that what we claim
18:―
1. For measuring power in electrical circuits, the combination of a current
transformer or potential transformer, or of both of these, with a measuring instru-
ment acting on the dynamometer principle, in such manner that an induction-
free resistance is connected parallel to the windings fed by the transformers, or
an induction resistance is connected parallel to the directly fed windings, for
the purpose of so adjusting the phases of the magnetic fields that with an
induction-free load on the working circuit, the said phases are not shifted
relatively to each other, substantially as described.
2. In the arrangement of measuring apparatus referred to in the first claim,
the employment in place of the said resistances of a second secondary winding on
the transformers closed by an induction-free resistance, substantially as described.
Dated this 25th day of August 1899.
ABEL & IMRAY,
Agents for the Applicants.