EP3371817A1

EP3371817A1 - Current sensor

Info

Publication number: EP3371817A1
Application number: EP16804898.1A
Authority: EP
Inventors: Bertil Moritz
Original assignee: HM Power AB
Current assignee: HM Power AB
Priority date: 2015-11-05
Filing date: 2016-11-04
Publication date: 2018-09-12
Also published as: CN108352249A; CN108352249B; WO2017077379A1

Abstract

A current transformer for measurement of a high voltage power line (12) comprising an openable magnetisable ring core (13) comprising a plurality of iron strips encircling the power line and a secondary winding (9) for sensing a current in relation to the current of the high voltage power line. The core (13) comprises overlapping ends (17a, 17b) having an overlapping area (14), and the core receives high voltage potential.

Description

Current sensor

TECHNICAL FIELD

The present invention concerns measurement of electric current in an electric power line. More precisely the invention concerns an apparatus and a method for measurement of electric current in a high power distribution line. Especially the invention concerns a sensor for current measurement. In particular the invention concerns a current ' transformer for high voltage.

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 10 to 40 kilo Volts (kV).

BACKGROUND OF THE INVENTION

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-enerqized. 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 flashove,r 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. From US 7557563 (Gunn et al) 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 US 5483215 (Mies) 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 US 3725832 (Edmund at al) 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. 8 to 11) 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.

SUMMARY OF THE INVENTION

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 1 or by a method characterized by the steps in the independent claim 7. Preferred embodiments are defined by the dependent claims.

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. In an embodiment the core material comprises a plurality of stacked metal sheets. By the

expression high voltage potential should be understood a voltage potential deviating within 10 % 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. In an embodiment the core is embedded in a thin protective coating which would result in a voltage potential somewhat lower than the power line. The core is openable to be hung onto the power line. In an embodiment the core comprises a flexible structure with one opening only. In an embodiment the core comprises openable overlapping ends which when closed offers a large overlapping surface which strengthen the magnetic field in the core.

The current transformer comprises high voltage insulation between the core and the secondary winding. In an embodiment the high voltage insulation comprises a tubular insulation body covering part of a ring formed core, and a secondary winding wound on the insulating body. The tubular insulating body comprises an inner conductive layer, an insulating layer and an outer conductive layer. The tubular insulating body comprises at both ends electric field stress reducing means. 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 airgap. In an embodiment the airgap in the magnetic circuit becomes almost zero. By having the secondary winding fully insulated from the core and the power line the signal may, be measured at ground level without any risks of hazardous incidences. Hen,ce 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. In an embodiment 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 an embodiment the openable ring core comprises

overlapping ends providing a big contact area for the magnetic field. By the overlapping design the magnetic resistance is reduced and

consequently the magnetic flux increased. The design is equivalent to the use of a magnetic material with higher permeability. By designing the overlap area to be larger than the cross section of the core a bigger airgap may be accepted without jeopardizing the function. Thus the material surfaces in the overlap area need not be in perfect contact. Hence the requirement of smooth surfaces is decreased.

The current transformer according to the invention provides an easy to install current transformer on a live overhead line conductor, which requires the current transformer to open and close the magnetic circuit. Both sufficient insulation of the secondary winding and sufficient magnetic properties of an openable core are fulfilled. The iron core is a magnetic circuit which should have high permeability all 360 degrees around. If the openable section contains insulation required to

withstand high voltage, the magnetizing current would be far too high to fulfil the requirement of a current transformer. Therefore the iron core is positioned at high voltage potential. In an embodiment the iron core receives the same voltage as the overhead line conductor. All non openable iron cores are traditionally at low voltage or ground potential. By the expression "iron core" is understood as synonymous to high magnetic permeability material formed in an enclosing loop.

In an embodiment the core comprises a mechanism for keeping the overlapping ends of the core in an open position. This enables an operator to push the core onto the power line with a long insulated rod. In an embodiment the mechanism comprises locking means for pressing the overlapping ends towards each other after the core has been mounted onto the line. The presence of the overlapping ends of the core provides the core to be positioned almost in parallel with the power line. The demand of a current transformer is only that the ring core encircles the power line. By keeping the core in almost parallel position with the power line the distance to adjacent lines may be kept smaller without any risk of flashover.

In a first aspect of the invention the object is achieved by a current transformer for measurement of a high voltage power line comprising an openable magnetisable ring core encircling the power line and a secondary winding for sensing a current in relation to the current of the high voltage power line, wherein the core comprises overlapping ends having an overlapping area, extending the cross section of the core, and that the core receives high voltage potential. The current transformer comprises a tubular insulating body enclosing part of the ring core and the tubular insulating body insulates the secondary winding from the ring core thereby providing a non-hazardous measurable current at ground level.

In further embodiment the openable ring core comprises overlapping ends. The overlapping distance is greater than the cross section of the core. The tubular insulating body comprises an inner conductive layer, an insulating layer and an outer conductive layer. The tubular insulator body comprises stress grading means at each end portion. The

magnetisable core comprises a plurality of iron sheets. The tubular insulating body comprises a weather protection with a plurality of protruding circular sheds for increasing the creeping distance. In a second aspect of the invention the object is achieved by a method for current measurement of a high voltage power line comprising an openable magnetisable core encircling the power line and a secondary winding for sensing a current in relation to the current of the high voltage power line, wherein the method contain providing a stack of iron strips having a first and second end portion, welding the first end portions of all strips, threading a prefabricated tubular insulating body onto the stack of strips, bending the stack of strips together with the tubular insulating body to form a ring core with overlapping ends, threading the openable ring core onto the power line, and reading the current at ground level.

The method further comprises 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

Fig 1 is a view of a current transformer according to the

invention hung on a high voltage power line,

Fig 2 is a cross section of a longitudinal cut of a tubular

insulation body on the ring core according the invention,

Fig 3 are principal sketches of openable ring cores, and

Fig 4 are two principal sketches of the opening and locking arrangement of the openable core. DESCRIPTION OF PREFERRED EMBODIMENTS

The current transformer according to Fig 1 comprises an iron core 13 encircling a high voltage conductor 12, and a tubular insulating body 1 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 1 comprises a mid section 2 carrying a secondary winding, a first electric field grading end portion 3 and second electric field grading end portion 4. The tubular insulating body comprises a weather protection 10 with a plurality of circular sheds 11 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 16 connected to the secondary winding to carry the signal from the secondary winding to an apparatus at ground level.

Referring to Fig 2 the tubular insulating body 1 comprises an inner conductive layer 6, an insulation layer 8, and an outer conductive layer 7. 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 9 is wound around the conductive layer 7. The tubular insulating body comprises a stress relief pad 5 in each direction from the secondary winding. The stress relief pad is overlapping the

conductive layer 7. 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 the left hand embodiment of the invention shown in Fig 3 the iron core 13 comprises overlapping ends 17. The overlap area 14 in the preferred embodiment in Fig 3 is larger than the cross section of the iron core 15. 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 airgap 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 airgap is reduced. In the embodiment shown the ring core comprises a plurality of iron sheets. The overlap arrangement as shown in the lower left of Fig 3 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 12 mm in square. In the embodiment shown the diameter of the core is greater than 200 mm. In an embodiment of the invention the core comprises a locking mechanism according to Fig 4. A first spring element 20 and a second spring element 21 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 21 will be pressed

downwards by the high voltage conductor 12. When further pressed by the conductor 12 the spring element 21 flips over (buckle) into the position shown in the right hand sketch. The first spring element 20 maintains a pressing force between the iron core endings and ensures that the airgap is kept at minimum. The click on spring device contains means to keeps 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 airgap surfaces comprises a glue compound which when the core is mounted on a power line fix 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 exhibit 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 10-40 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 1 is treaded 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.

Although favourable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art. For instance the ring core must not be circular but may comprise any geometric form encircling the line such for instance an oval shape.

Claims

Current transformer for measurement of a high voltage power line (12) comprising an ppenable magnetisable ring core (13) encircling the power line and a secondary winding (9) for sensing a current in relation to the current of the high voltage power line,

c h a r a c t e r i z e d i n that the core (13) comprises overlapping ends (17a, 17b), and that the current transformer comprises a tubular insulating body (1) containing the secondary winding and providing high voltage insulation between the core (13) and the secondary winding (9).

Current transformer according to claim 1, wherein the overlapping ends (17) comprises an overlapping area (14) extending the cross section (15) of the core.

Current transformer according to claim 1 or 2, wherein the tubular insulating body (1) enfolds part of the ring core (13), and that the tubular insulating body insulates the secondary winding (9) from the ring core (13) to provide a non-hazardous measurable current at ground level.

Current transformer according to any of the preceding claims, wherein the ring core (13) comprises a plurality of iron strips.

Current transformer according to any of the preceding claims, wherein the tubular insulating body (1) comprises an inner conductive layer (6), an insulating layer (8) and an outer

conductive layer (7).

6. Current transformer according to any of the preceding claims, wherein the tubular insulator body (1) comprises stress grading means (5) at each end portions (3, 4).

7. Current transformer according to any of the preceding claims,

wherein the tubular insulating body (1) comprises a weather protection (10) with a plurality of protruding circular sheds (11) for increasing the creeping distance.

8. Method for current measurement of a high voltage power line (12) comprising an openable magnetisable core (13) encircling the power line and a secondary winding (9) for sensing a current in relation to the current of the high voltage power line,

c h a r a c t e r i z e d b y providing a stack of iron strips having a first and second end portion, welding the first end portions of all strips, threading a prefabricated tubular insulating body (1) comprising the secondary winding (9) onto the stack of strips, bending the stack of strips together with the tubular insulating body to form a ring core with overlapping ends (17a, 17b), threading the openable ring core onto the power line, and reading the current at ground level.

9. Method according to claim 7, wherein the tubular insulating body (1) is formed by providing an inner conductive layer (6) around part of the ring core (13), providing on the inner conductive layer (6) a tubular insulating layer (8), providing on the tubular insulating layer an outer conductive layer (7), and providing at each end of the outer conductive layer (7) a stress grading means (5).

10. Use of a current transformer according to any of the claims 1 to 7 or a method according to any of the claims 8 or 9 for measurement on an electrical distribution power line.