AU2001100615A4 - Terminal arrangement for electricity meter - Google Patents

Description

I

AUSTRALIA

Patent Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Terminal Arrangement for Electricity Meter BACKGROUND OF INVENTION This invention relates to polyphase electricity meters and in particular to the terminal arrangements of such meters.

Prior art terminal blocks of the kind shown in Fig 1 typically include a casing 10 with a first set of entrance apertures 12 that receive a set of current carriers and a second set of entrance apertures 13 that receive a set of voltage carriers and auxiliary connections. Current and voltage sensors disposed within the casing 10 measure the current and voltage of the carriers connected through the respective sets of apertures 12, 13.

Typically the current sensor is connected to the current carrying conductor through a copper/brass connection including a series of contact strips and grub screws. Due to the presence of these connections there is an unwanted heat rise at the interface between the typically copper wiring and the contact strips. If aluminium wiring is used, as is common in many countries, a galvanic interface is created that can result in corrosion problems.

SUMMARY OF INVENTION In one form as described herein the invention resides in a terminal arrangement for a polyphase electricity meter including means for receiving a plurality of voltage carriers, means for receiving a plurality of current carriers, voltage sensing means associated with each said voltage carrier receiving means and in conductive contact with each voltage carrier, and current sensing means associated with each said current carrier receiving means for sensing current in each said current carrier without conductive contact therewith.

Preferably the formation includes a casing, wherein each said current sensing means includes a formation on the casing for receiving each current carrier, and an inductive current sensor is associated with each such formation.

Preferably also each formation includes a through aperture in the casing, and each current sensing means includes a plurality of planar coils arranged radially about a respective aperture.

Preferably also the coils are provided as printed circuits on a radial array of substrates.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are now described by way of example only with reference to the accompanying drawings, in which: Fig. 1 shows a perspective view of a prior art terminal block; Fig. 2 shows a perspective view of a terminal block embodying the present invention; Fig. 3 is a side cross-section of the terminal block of Fig. 2; and Fig. 4 is a perspective view of an inductive current sensor.

DESCRIPTION OF PREFERRED EMBODIMENTS Terminal block illustrated in Fig. 2 includes a casing 20 having a set of apertures 22 that receive wiring to be connected to voltage sensing means located within the casing The apertures 22 are one-way apertures, that is, there is no corresponding exit aperture with the connecting wire terminating at the voltage carrier contact means.

The casing 20 also includes a set of openings 21 passing entirely through the casing that receive current carriers. As an alternative, the openings 21 may be formed as slots in the casing or as other formations that securely maintain the current carrying wires external to the casing.

Fig 3 is a side cross-section of the casing. The cross-section shows voltage sensing means 32 that connect through contacts 33 with cables and wiring disposed in entrance apertures 22. The contacts 33 are electrically conductive contacts or any conventional or other suitable connection for connecting external cabling to the voltage sensors.

As also seen in Fig 3, a plurality of non-contact current measuring sensors 31 are located within the casing but are situated near to the openings 21 through the casing Many types of non-contact current measurement sensors are known and may be used for measuring the current flowing through a current carrying conductor. One example of such sensors is a sensor having a magnetic core that is adapted for encircling a current carrying conductor. The current in the current carrying conductor acts to magnetically adjust the magnetic core by way of coupling the magnetic field generated by the current. As a result, a magnetic flux is produced. This magnetic flux induces an output voltage on the output terminals. The detected value is proportional to the amplitude of the current in the current carrying conductor. Where DC current is present on the conductor being measured or if the primary current is very high, the ferrite material used in the core can become magnetically saturated. Therefore sensors exhibiting hysteresis must be demagnetised to remove any residual field before they can be used again. In addition, such sensors significantly load the circuit under test and suffer from non-linearity problems.

In the practice of the present invention it is therefore preferred that the current measuring sensors 31 are inductive sensors using non-magnetic materials.

Fig. 4 illustrates a novel form of sensor which may be constructed using printed circuit techniques.

Fig. 4 shows a printed circuit board 41 carrying printed coil bodies 42. Said inductive coils are placed at a fixed distance from the current carrying conductor and disposed in a polygonal contour, so as to surround the current carrying conductor. Each coil body 42 consists of a pair of coils 43 mounted on each side of a substrate 44, with contact elements 45 and interconnecting tracks 47. The substrates 44 are mounted on the face of the printed circuit board 41, which has a central recess 46 through which the current carrying conductor may pass. Such sensors Can be made by standard printed circuit methods using glass epoxy base material with the planar coils produced by etching copper tracks, or by conductive traces deposited or otherwise applied to non-conductive substrates of polymeric, ceramic or other material.

The voltage induced in the coils 43 is a function linearly proportional to the derivative of the current flowing through the current carrying conductor. It is therefore necessary to integrate the coil voltage to produce voltage proportional to the current.

Typically, the inductive sensor is connected to an integrator, which may be passive or active or a combination of both.

The above-described planar sensors provide very accurate measurement. For example, the accuracy of measurement may be as high as 0.25% of reading.

For simplicity in this specification the current sensor of Fig. 4 is shown as a discrete unit. Preferably, however, each of the sensors is mounted directly on the circuit board which carries the remainder of the meter circuitry by which measurements from the voltage and current sensors are employed to calculate polyphase power consumption.

Apart from the entrance apertures 22 that allow passage of external cabling to the voltage sensing means 32, the casing is substantially sealed. In particular the casing is closed in the region of the openings 21 through the casing that receive the second set of conductors in order to protect the current sensors 31 from environmental factors.

The advantage of a terminal arrangement as described is that the heat rise that is prominent in prior art terminal blocks can be reduced or eliminated because there is no physical electrically conductive contacts between current carrying conductors and the current sensors. This non-contact measuring system also removes the possibility of galvanic corrosion. It is envisaged that terminal arrangements of the present invention can be made cheaper and more compact than prior art systems due to the elimination of bulky and expensive terminal connections for the current sensors including contact strips and grub screws.

While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be within the competence of those skilled in the art are therefore intended to be embraced therein.

Claims (4)

1. A terminal arrangement for a polyphase electricity meter including means for receiving a plurality of voltage carriers means for receiving a plurality of current carriers voltage sensing means associated with each said voltage carrier receiving means and in conductive contact with each voltage carricr, and current sensing means associated with each said current carrier receiving means for sensing current in each said current carrier without conductive contact therewith.

2. A terminal arrangement according to claim 1 further including a casing, wherein each said current sensing means includes a formation on said casing for receiving each current carrier, and an inductive current sensor associated with each such formation.

3. A terminal arrangement according to claim 2 wherein said formation includes a through aperture in said casing, and each said current sensing means includes a plurality of planar coils arranged radially about a respective aperture.

4. A terminal arrangement according to claim 3 wherein said coils are provided as circuit patterns on a radial array of planar substrates. A terminal arrangement substantially as described herein with reference to the accompanying drawings.