Inductor devices – Wound core
Reexamination Certificate
2001-08-15
2003-01-14
Mai, Anh (Department: 2838)
Inductor devices
Wound core
C336S083000
Reexamination Certificate
active
06507262
ABSTRACT:
Magnetic core which is suitable for use in a current transformer, process for the production of a magnetic core, and current transformer with a magnetic core.
FIELD OF THE INVENTION
The invention concerns a magnetic core which is suitable for use in a current transformer, a process for the production of this type of magnetic core, and a current transformer with this type of magnetic core.
BACKGROUND OF THE INVENTION
To detect the energy consumption of electrical devices and facilities in industrial and household use, energy meters are used. The oldest principle in use in this regard is that of the Ferrari meter. The Ferrari meter is based on energy metering via the rotation of a disk, connected with a mechanical register, which is driven by the fields of appropriate field coils which are proportional to the current and/or the voltage. For the expansion of the functional possibilities of energy meters, such as for multi-rate operation or remote reading, energy meters are used in which the current and voltage detection is performed via inductive current and voltage transformers.
A special application, in which a particularly high exactitude is required, is the detection of energy currents in the utility company sector. In this case, the quantities of energy generated by the respective power plants and stored in the high-voltage networks must be precisely determined on one hand, and, on the other hand, the changing portions of consumption or supply in the traffic between the utility companies are of great importance for accounting. The energy meters used for this purpose are multifunction built-in devices whose input signals for current and voltage are taken from the respective high and medium high voltage installations via cascades of current and voltage transformers and whose output signals serve for digital and graphic registration and/or display as well as for control purposes in the control centers. In this regard, the first transformer on the network side serves for isolated transformation of the high current and voltage values, e.g. 1 to 100 kA and 10 to 500 kV, into values which can be handled in the control cabinets, while the second transformers transform these in the actual energy meter into the signal level necessary for the measurement electronics in the range of less than 10 to 100 mV.
FIG. 1
shows an equivalent circuit diagram of this type of current transformer and the range of technical data that can occur in various applications. A current transformer
1
is shown here. The primary winding
2
, which carries the current I
prim
to be measured, and a secondary winding
3
, which carries the measured current I
sec
are located on a magnetic core
4
, which is made from an amorphous soft-magnetic band. The secondary current I
sec
automatically establishes itself in such a way that the primary and secondary ampere turns are, in the ideal case, of equal size and aligned in opposite directions. The trace of the magnetic fields in this type of current transformer is illustrated in
FIG. 2
, with losses in the magnetic core not considered. The current in the secondary winding
3
enestablishes itself according to the law of induction in such a way that it seeks to impede the cause of its occurrence, namely the temporal change of the magnetic flux in the magnetic core
4
.
In the ideal current transformer, the secondary current is, when multiplied with the turns ratio, therefore equal to the negative of the primary current, which is illustrated by equation (1):
I
sec
ideal
=−I
prim
*(
N
prim
/N
sec
) (1)
This ideal case is never achieved, due to the losses in the burden resistance
5
, in the copper resistance
6
of the secondary winding, and in the magnetic core
4
.
Therefore, in the real current transformer, the secondary current has an amplitude error and a phase error relative to the above idealization, which is described by equation (2):
Amplitudenfehler
:
F
(
I
)
=
I
sec
real
-
I
sec
ideal
I
sec
ideal
;
Phasenfehler
:
ϕ
(
I
)
=
φ
(
I
sec
real
)
-
φ
(
-
I
prim
)
(
2
)
The output signals of this type of current transformer are digitized, multiplied, integrated, and saved. The result is an electrical value which is available for the purposes mentioned.
The electronic energy meters used for energy metering in these applications operate “indirectly,” so that only purely bipolar, zero-symmetric alternating currents must be measured in the meter itself. Current transformers which are assembled from magnetic cores made of highly permeable materials and which must be equipped with very many secondary turns, i.e., typically 2500 or more, to achieve lower measurement error via a smaller phase error &psgr;, serve for this purpose.
For the mapping of purely bipolar currents, current transformers are known whose magnetic cores consist of highly permeable crystalline alloys, particularly nickel-iron alloys, which contain approximately 80 weight-percent nickel and are known under the name “Permalloy.” These have a phase error &psgr; which is fundamentally very low. However, they also have the disadvantage that this phase error &psgr; varies strongly with the current I
prim
to be measured, which is identical with the modulation of the transformer core. For a precise current measurement with changing loads, a costly linearization in the energy meter is therefore necessary with these transformers.
Furthermore, current transformers are known which operate based on ironless air-core coils. This principle is known as the Rogowski principle. In this way, the influence of the modulation on the phase error does not apply. Because the requirements for reliability of this type of current transformer must be very high in order to allow energy metering which can be calibrated, these designs are equipped with costly shields against external fields, which requires a high outlay for materials and assembly and is therefore cost intensive.
Furthermore, solutions are known in which a ferrite pot core provided with an air gap (gapped) is used as the magnetic core. This current transformer has very good linearity, however, due to the relatively low permeability of the ferrite, a very high number of turns in connection with a very large-volume magnetic core is required in order to achieve a low phase angle in the current transformer. Furthermore, this current transformer based on ferrite pot cores also has a high sensitivity to external interfering fields, so that shielding measures must also be taken here. In addition, the magnetic values of ferrites are, as a rule, strongly temperature dependent.
SUMMARY OF THE INVENTION
The invention has as its object the specification of a magnetic core which, when used in a current transformer, allows higher measurement accuracy of a current to be measured than the prior art, while simultaneously having an economical implementation and a compact overall size. Furthermore, a process for the production of this type of magnetic core and a current transformer with this type of magnetic core are to be specified. In addition, the temperature dependency of the properties should be as small as possible.
The object is achieved by a magnetic core suitable for use in a current transformer characterized in that it consists of a wound band made of a ferromagnetic alloy in which at least 50% of the alloy is occupied by fine crystalline particles with an average particle size of 100 nm or less (nanocrystalline alloy), it has a saturation permeability which is larger than 12,000, preferably 20,000, and smaller than 300,000, preferably 350,000, it has a saturation magnetostriction whose amount is smaller than 1 ppm, it is essentially free from mechanical stress, and it has a magnetic anisotropic axis along which the magnetization of the magnetic core aligns itself particularly easily and which is orthogonal to a plane in which a center line of the band runs. The alloy has a composition which essentially consists of the formula
Fe
a
Co
b
Cu
c
Si
d
B
e
M
Otte Detlef
Petzold Jörg
Kilpatrick & Stockton LLP
Mai Anh
Russell Dean W.
Vacuumschmelze GmbH
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