Patent Description:
By the expression "high voltage distribution line" should be understood an air insulated power line for distribution of electric power. Preferably such power line carries electric power in the range of <NUM> to <NUM> kilo Volts (kV).

A commonly used apparatus for current measurement of a high voltage electric conductor is a current transformer having a primary winding, a circular magnetisable core and a secondary winding. The alternating current in the primary winding, which is the power line to be measured, produces an alternating magnetic field in the core, which then induces an alternating current in the secondary winding. An essential objective of current transformer design is to ensure the primary and secondary circuits are efficiently coupled, so the secondary current is linearly proportional to the primary current. For high voltage applications a common current transformer is costly to produce and for installing the conductor line needs to be cut and the system de-energized.

A current transformer with an openable core such as a crimp ampere meter is commonly used for low voltage applications. The core of such apparatus comprises a pair of openable jaws of a magnetisable material to be encircling the conductor. The conductor needs not to be cut nor de-energized. However the crimp ampere meter comprises an opening and a fulcrum, both of which disturbing the magnetic field.

For high voltage applications however there is a need for a sufficient insulation between the conductor and the secondary coil. Especially if the signal from the secondary coil is to be measured at ground level there must be provided sufficient protection against hazardous voltage levels in the signal conductors. Thus the secondary winding and the conductors carrying the signal must have sufficient insulation and be protected against flashover. The current transformer may be regarded as a three-part system comprising the primary winding, the coil and the secondary winding. For high voltage use there must be sufficient insulation between either the primary winding and the core or between the core and the secondary winding.

It is known the use of a crimp ampere meter hung on the high voltage line where the signal from the secondary coil is sent to an instrument on ground potential by an optic fibre or an airborne signal. Still the core of the crimp ampere meter is isolated from the high voltage conductor. Also the core of the crimp ampere meter comprises an opening and a fulcrum, both of which disturbing the magnetic field. The known solution necessitates the presence of an electronic circuit and an electric power source such as a battery on the high potential level. This is a severe complication since such power source is increasing the production cost. An electronic circuit has limited service life time and increases the risk of malfunction. Besides such a power source needs maintenance and sometimes replacement.

<CIT> discloses a measurement transformer for detecting faults on electrical cables. The measurement transformer comprises an openable ring core which has a centrally placed bearing surface that can be attached to the cable exterior by means of an elastic tie. The ring core runs through a secondary winding which is seated in a plastic material casing. There is no provision in this document of an insulation construed to protect the secondary winding from partial discharge as would be required if used in connection with high voltage power conductors.

<CIT> discloses a measurement transformer for detecting faults on electrical cables. The measurement transformer comprises an openable ring core that runs through a secondary winding that encloses a section of the ring core. The disclosure contains no suggestion for protecting the secondary winding against partial discharge as would be required if used in connection with high voltage power conductors.

From <CIT>) a current sensor assembly is previously known. The current sensor comprises a clamp meter with two moveable jaws and a corona structure. The corona structure has an outer boundary surrounding the electronics assembly and the conductor mountable device. The corona structure may shield the electronic assembly and conductor mountable device from a corona producible with the power conductor. The current sensor assembly may be a split-core design that includes multiple transformer cores. The electronic assembly and the conductor mountable device may be powered by a line voltage supply on the power conductor. Data may be wirelessly transmitted and received with the sensor apparatus.

From <CIT>) a current transformer for lines is previously known. The current transformer is able to be mounted with a switching bar on a conductor having a high voltage. The current transformer has a coil surrounding a U-shaped iron yoke. A U-shaped ground iron is swivel-mounted on one leg of the iron yoke and is supported at a break point. Springs are anchored on a housing of the transformer. When the current transformer is mounted, the conductor moves the ground iron in a closing position and is maintained in such position by the springs. The measurement signal is transferred by an optical fibre.

From <CIT>) a magnetic core structure is previously known. The object of the structure is to provide an openable core without pivotal connections for embracing an alternating current carrying conductor. The magnetic core is formed of a plurality of strips of laminations of oriented silicon steel. After forming the core a secondary winding is applied. In order to avoid corrosion the core is covered with a hardenable plastic neoprene rubber. In an embodiment the core is self closing in butt contact and in yet another embodiment the core contains coupling members.

However not mentioned the known core and secondary winding as shown in the figures is only applicable on low voltage connectors. Although there is mentioned a coating of insulating material the thickness of such coating (Fig. <NUM> to <NUM>) does not indicate the use in high voltage environment. The secondary winding and its connection are covered only with tape. Hence there is not discussed the presence of an electric field originating from the core in contact with a high voltage conductor and how to control such electric field in order to avoid partial discharges.

A primary object of the present invention is to seek ways to improve a current transformer capable of sensing a current on a high voltage power line and producing a non-hazardous signal at ground potential level.

This object is achieved according to the invention by a current transformer characterized by the features in the independent claim <NUM> or by a method characterized by the steps in the independent claim <NUM>.

The invention also relates to the use of a current transformer according to claim <NUM>.

According to the invention the current transformer comprises a power line encircling core at high voltage potential. The core comprises an openable homogeneous ring of a magnetisable material. By the expression homogeneous ring should be understood a ring comprising the same material and cross section all around. Hence the core contains no fulcrum, joint or other discontinuities. By the expression high voltage potential should be understood a voltage potential deviating within <NUM> % of the power line potential. In an embodiment the core is in electric contact with the power line. Thus the core receives the same potential as the power line. The core is openable to be hung onto the power line.

According to an embodiment of the present invention, the tubular insulating body comprises an inner conductive layer, an insulating layer and an outer conductive layer. According to the invention, the tubular insulator body comprises stress grading means at each end portion of the tubular insulator body. Such means comprises for instance a field control stress cone or a field control compound, which is used for high voltage cable terminations. By receiving essentially the same voltage as the power line conductor, the core surfaces which close the magnetic circuit need no high voltage insulation. This makes it possible to obtain a minimum air gap. Hence the current transformer according to the invention is completely passive and needs no extra power to produce a measureable signal at ground level. Analyzing instruments and means for wirelessly transmitting the signal may be easily used and powered on ground level.

According to an embodiment of the invention, the core comprises a plurality of iron strips. The plurality of iron strips forms in an embodiment a square cross section of the core.

In a first aspect of the invention the object is achieved by a current transformer for measurement of an air insulated high voltage power line according to claim <NUM>,.

thereby providing a non-hazardous measurable current at ground level.

In a second aspect of the invention the object is achieved by a method for current measurement of an air insulated high voltage power line according to claim <NUM>.

According to an embodiment of the invention, the method may further comprise forming an inner conductive layer around part of the ring core, providing on the inner conductive layer a tubular insulating layer, providing on the tubular insulating layer an outer conductive layer, and providing at each end of the outer conductive layer a stress grading means.

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which:.

The current transformer according to <FIG> comprises an iron core <NUM> encircling a high voltage conductor <NUM>, and a tubular insulating body <NUM> comprising the secondary winding. The tubular insulating body provides high voltage insulation between the core and the secondary winding. In the embodiment shown the insulating body is covering part of the core leaving a part of the core to be in contact with the conductor. Thus the core receives the same potential as the high voltage conductor. In an embodiment the core is embedded in a thin protective coating and thus receiving a voltage potential somewhat lower than the high voltage conductor. The core comprises an openable homogeneous ring of a magnetisable material. By the expression homogeneous should be understood that the ring comprises the same material and preferably the same cross section all along. The ring may have any geometrical form but is preferably circular.

The insulating body <NUM> comprises a mid section <NUM> carrying a secondary winding, a first electric field grading end portion <NUM> and a second electric field grading end portion <NUM>. The tubular insulating body comprises a weather protection <NUM> with a plurality of circular sheds <NUM> to lengthen the creepage distance. The secondary winding is thus fully insulated from the iron core. The tubular isolating body may be regarded as a short tube insulating an inner conductor, which in this case is the iron core. In the embodiment shown the tubular insulating body comprises a cable <NUM> connected to the secondary winding to carry the signal from the secondary winding to an apparatus at ground level.

Referring to <FIG> the tubular insulating body <NUM> comprises an inner conductive layer <NUM>, an insulation layer <NUM>, and an outer conductive layer <NUM>. Conductive in this context means sufficient conductivity to generate an almost equipotential layer, but not such high conductivity that any significant current is induced by the magnetic flux. The secondary winding <NUM> is wound around the conductive layer <NUM>. The tubular insulating body comprises a stress relief pad <NUM> in each direction from the secondary winding. The stress relief pad is overlapping the conductive layer <NUM>. Each stress relief pad is made of a high permittivity stress grading compound and is gradually distributing the electric field along the surface over a distance long enough to avoid partial discharges. This can be seen as two mirrored conventional cable terminations. Alternatively other cable terminations, such as stress cones, may be applied to control the electric field. Instead of a current carrying high voltage inner conductor the function of the iron core is to conduct a magnetic field.

According to an embodiment of the invention shown in <FIG> the iron core <NUM> comprises axially oriented overlapping ends 17a, 17b. The overlap area <NUM> in the preferred embodiment in <FIG> is larger than the cross section of the iron core <NUM>. This overlap reduces the magnetic resistance in the core. The magnetic flux is shown by dotted lines. The overlap also makes it possible to apply a pressing force F between the air gap surfaces. The magnetic flux is gradually passing from one side to the other in the overlap area. By making the overlap area greater than the dimension of the iron core cross section, the magnetic resistance or equivalent air gap is reduced. In the embodiment shown the ring core comprises a plurality of iron sheets. The overlap arrangement as shown in <FIG> makes it possible to orient the ring core almost in parallel with the power line. This is advantageous since the distance between power lines can be kept small. The cross section of the iron core is in the embodiment shown about <NUM> in square. In the embodiment shown the diameter of the core is greater than <NUM>.

In an embodiment of the invention the core comprises a locking mechanism according to <FIG>. A first spring element <NUM> and a second spring element <NUM> are used to install the current transformer. In the left hand embodiment the spring mechanism is charged to an open position. When mounting, the second spring element <NUM> will be pressed downwards by the high voltage conductor <NUM>. When further pressed by the conductor <NUM> the spring element <NUM> flips over (buckle) into the position shown in the right hand sketch. The first spring element <NUM> maintains a pressing force between the iron core endings and ensures that the air gap is kept at minimum. The click on spring device contains means to keep the iron core almost parallel to the overhead power line and increase thereby the distance to adjacent phase line. The air gap is protected by a foil which is removed by the overhead power line when it enters into the iron ring. This prevents particles from being trapped in the air gap during installation. In an embodiment the air gap surfaces comprises a glue compound which when the core is mounted on a power line fixes the overlapping ends for a long time.

Measuring electric current of a high power electrical conductor at ground level demands that the secondary winding which produces the measuring signal to a measuring instrument at ground potential exhibits sufficient insulation. However an overhead power line comprises a naked metal wire. If a signal from a current transformer should be at ground potential to serve low voltage electronic devices, the insulation has to be such that flashover or raised voltages are prevented. The overhead lines which carry the primary current contain high voltage of <NUM>-<NUM> kilo Volts (kV).

The ring formed iron core is preferably made by winding a plurality of layers of an iron strip or lamination of oriented silicon steel to form a ring. When fully winded the ring is cut thus resulting in a stack of strips of different lengths having first and second end portions. The strips are oriented in parallel and all end portions at one side are welded together. The prefabricated tubular insulating body <NUM> is threaded onto the now flat iron core. The collection of strips is then together with the tubular body bent to form a ring core with overlapping ends 17a, 17b. In this position all end portions of the other side of the core are welded together.

Claim 1:
Current transformer for measurement of an air insulated, high voltage power line (<NUM>), the current transformer comprising an open magnetisable ring core (<NUM>) encircling the power line and a secondary winding (<NUM>) for sensing a current in relation to the current of the high voltage power line, wherein the open ring core (<NUM>) has a square cross section, side surfaces and two square end faces wherein the two end faces do not face each other and where two of the side surfaces have ends (17a, 17b) which overlap as seen in direction of a centre axis through the ring core (<NUM>), the current transformer further comprising a tubular insulating body (<NUM>) containing the secondary winding and providing high voltage insulation between the ring core (<NUM>) and the secondary winding (<NUM>), and the tubular insulator body (<NUM>) comprising stress grading means (<NUM>) at each end portion (<NUM>, <NUM>) of the tubular insulator body (<NUM>).