Apparatus for Measuring Alternating Current Flowing in an Electric Conductor
The present invention relates to apparatus for measuring alternating current flowing in an electric conductor and more particularly but not exclusively to apparatus when used for monitoring main supplies.
It has previously been proposed to use current transformers provided with a core of magnetic material for monitoring and measuring alternating currents particularly in main supplies. However, such current transformers are easily saturated by high current or externally applied high magnetic fields. This leads to inaccuracies and possible deliberate manipulation of measurements. While these disadvantages can be overcome, at least to a certain extent, this is only possible by increasing the cost and complexity of the current transformers.
Air cored current transformers are already known and while they do not suffer from the saturation problems of the magnetic material-cored transformers, the flux linkage from the conductor being monitored to the transformer is less and the linearity of the device is also not as good as compared with a magnetic material-cored transformer, particularly at lower current loads.
It is an object of the present invention to utilise an air cored current transformer suitably modified in order to measure alternating current flowing in an electric conductor.
The present invention provides a current transformer assembly having a current transformer device comprising means for receiving a portion of a conductor, which is preferably U-shaped, the current being in which is to be monitored, a coil with a nonmagnetic core, which is preferably an air-cored coil arranged to be disposed with the U-shaped portion, and a compensation coil arrangement connected in series with the air-cored coit but having a winding direction opposite to that of the air-cored coil.

The assembly is provided with a first housing of magnetic material with fairly linear magnetic properties to reduce the leakage flux from the conductor and improve flux linkage with the air-cored coil.
Increased immunity to strong external influencing magnetic fields can be further achieved by either using a second outer shield made of material of higher saturation flux density as compared to the first housing or by utilising the same material as used for the first-mentioned housing but utilising thicker material. We prefer to utilise deep drawn or mild steel for the outer housing and silicon steel for the inner housing.
Preferably a shield is placed between the main current carrying conductor and the air cored coil. This shield prevents electric field coupling between the two, thereby preventing spurious noise pick off by the coil due to voltage on the current carrying coil.
In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example only with reference to the accompanying drawings in which :-
Fig.l shows a plan view of a diagrammatic representation of an air-cored current
transformer according to the present invention;
Fig. 2 shows a side view of the arrangement shown in Fig. 1; and
Fig. 3 shows an exploded perspective view of an air cored current transformer
according to the present invention.
Fig. 4 shows a perspective view of an air cored current transformer according to a
preferred embodiment of the present invention.
Referring now to Fig. 1, this shows a main conductor 10 the current flowing in which it is required to monitor and measure. The measurement is achieved by means of a device including an air-cored sensing coil 11. In order to improve the flux linkage, the conductor 10 is formed into a U-shape and the coil is located midway between the two legs of the U and close to the arcuate portion, or inside, of the U. The magnetic field

direction within the U-shape conductor portion is represented in the conventional manner in Fig. 1 and will be seen to be going into the plane of the paper within the confines of the U-shape while coming out of the plane of the paper outside the U-shape. This serves to give maximum flux coupling to the sensor coil 11 and consequently yield maximum voltage output from the coil. The sensor coil outer and inner diameter are optimised for maximum output signal and the coil is disposed so that its axis is parallel to the axis about which the U-shaped is formed.
A compensation coil 12 is provided and as will be seen from Fig. 2, the compensation coil is in the form of two coil portions 12A and 12B located on opposite ends of the sensor coil. The coil winding direction of each of the compensation coils is the same but the winding direction of the sensor coil 1 1 is in the opposite direction to that of the coil portions. The sensor coil and coil portions are connected in series and preferably all three coils are wound on a single bobbin. The location of the compensation coil 12A,12B with respect to the current carrying conductor 10 is such that the total flux linkage to the compensation coil produced by the current carrying conductor 10 is minimised. The net effect of the compensation coil on the output voltage generated by the main sensing coil 1 1 is substantially zero but it does reduce the effect of external homogeneous magnetic fields on the device.
It is preferred that the compensation coil 12A,12B has a larger cross-sectional area than the main sensing coil 1 1 and consequently it needs fewer turns in total than the number of turns in the sensing coil. The geometry of the sensing and compensation coils depends upon the actual device requirements and consequently the shape and orientation of the various coils and coil portions can be changed.
To express the compensation feature in mathematical terms, the output signal from the arrangement is such that
B = BM Sin cot
A = area of the coil
N = number of turns in the coil
B is the peak saturation flux density
co = 2 TI f, where f is the frequency of the supply.
E can be calculated both for the sensing coil 11 and the compensation coil 12. The number of turns, gauge and wire material depends upon the output signal requirement and magnetic field immunity requirement. In practice the sensing coil may vary from 1,000 to 5,000 turns and the compensation coil may vary from 40 to 200 turns. The combination of the two depends upon overall requirements for the device, that is current range to be measured, magnetic field immunity level and output signal strength. The actual loop of conductor in the device has a cross section and shape which depends upon the end use of the device.
The preferred embodiment also includes shielding to prevent capacitive coupling between the current carrying member 10 and the coil portions 12A,12B. This is achieved in the preferred embodiment by means of electric shield plates 15A and 15B which are earthed and disposed between the ends of the measuring coil 11 and the coil portions 12A and 12B. Preferably, the members ISA and 15B are identical and are both constructed from flat sheets of copper, or the like.
With the above construction, it is possible to construct an air-cored current transformer device which has much improved linearity and sensitivity while not suffering from the saturation problems which are exhibited by magnetic material-cored current transformers. A practical example of such a device is shown in Fig. 3 where the same reference numerals are utilised for the same parts as described above.
Fig. 3 also shows a magnetic shield 18 which is used to house the above described device to improve the flux linkage of the main air-cored coil 12. It effectively acts as a core therefore improves the flux linkage. This is generally made of a material of fairly
linear magnetic properties, for example silicon steel. The shield 18 also contributes in shielding from influences of potential neighbouring current carrying conductors. Such conductors will be present in a 3-phase meter.
The shield 18 is formed of a looped metal casing with open ends such that it surrounds the air coiled core 12 and the shield plates 15A, 15B. The plane of the loop of the shield 18 is perpendicular to the plane of the U-shape portion of the conductor 10.
Preferably, as shown in Fig. 4, there is an additional magnetic shield 20 (not shown in Fig, 3) to protect the current transformer from strong directed magnetic fields of external origin. However, the saturation flux density capacity of the additional magnetic shield has to be higher than the flux density of the inner shield 18. This can be achieved either by utilising a different magnetic material for the outer shield as compared with the inner shield and/or altering the thickness of the material so that the outer shield is made of thicker material than the inner shield. The reason for this additional shield is apparent when one considers that when two or more current measuring devices are disposed in close proximity to each other, the magnetic field generated by the conductor in one, affects the measurement of the others. The height, length, width and thickness of material utilised for the external shield are optimised for best immunity with minimal effect on accuracy of the current measurement.
Although not shown, the device can be provided with a voltage tapping arrangement contacting the conductor 10 so that both current and voltage measurements can be taken from the device and utilised for measurement purposes. Further a calculating and display module (not shown) can be provided to form an electronic meter.
The U-shape conductor 10 may be a part of the device, as shown, but it is equally possible to construct the device such that an existing conductor can be formed into the U-shape and the rest of the device assembled around it. This avoids the need to cut and terminate the existing conductor.
It will be appreciated that although the above description is directed to a current transformer with an air-cored coil, the same effect of the present invention can be achieved by using a coil with any type of core which is non magnetic such as plastic.

WE CLAIM:
1. A current transformer device comprising means for receiving a portion of a
conductor (10) containing current which is to be monitored, a sense coil
(ll)with a non-magnetic core arranged to be disposed with the portion of the
conductor, and a compensation coil arrangement (12) connected in series with
the coil but having a winding direction opposite to that of the coil, wherein the
conductor receiving means, the sense coil with the non-magnetic core, and the
compensation coil arrangement are contained within a first housing (18) of
magnetic material.
2. The current transformer device as claimed in claim 1, wherein between the said
sense coil (11) and the said conductor (10) a shield member (15) disposed.
3. The current transformer device as claimed in claim 1, wherein the said
compensation coil arrangement (12) is in the form of at least two coil portions
(12A,12B), such disposed at a respective end of the said sense coil (11), each
displaced at a respective end of the sense coil.
4. The current transformer device as claimed in claim 3, wherein two shield
members (15A,15B) are displaced between the ends of the sense coil (11) and
the compensation coil portions(12A,12B).
5. The current transformer device as claimed in any of the preceding claims,
wherein the first housing (18) is made of silicon steel.
6. The current transformer device as claimed in any of the preceding claims,
wherein the said conductor (10) is a U-shaped conductor.
7. The current transformer device as claimed in any of the preceding claims,
wherein a second housing (20) of a magnetic material is provided external to
the first housing (18).

8. The current transformer device as claimed in claim 7, wherein the saturation
flux density of the material of the second housing (20) is greater than that of
the metal framing the first housing (18).
9. The current transformer device substantially as herein described with reference
to the foregoing examples and the accompanying drawings.