GB2334339A

GB2334339A - Measuring current

Info

Publication number: GB2334339A
Application number: GB9812961A
Authority: GB
Inventors: Mihai Bucsa
Original assignee: Ampy Automotion Digilog Ltd
Current assignee: Landis and Gyr Ltd
Priority date: 1998-02-17
Filing date: 1998-06-17
Publication date: 1999-08-18
Also published as: GB9803210D0; GB9812961D0

Abstract

An alternating current meter has a shunt resister (20) through which current to be measured is forced to pass. An isolation transformer has a primary (22) connected to opposite ends of the shunt and a secondary (26) for supplying a current, related to the current flowing in the primary, to processing circuitry (28) for providing an output signal indicative of the current induced in the secondary of the transformer.

Description

Title: Improvements in and relating to electricity meters Field of the Invention This invention concerns electricity meters and in particular meters which are designed to measure alternating electric current. The invention is applicable to meters which are also designed to measure voltage and thereby measure power.

Backaround to the Invention It is well known to provide a low resistance shunt in series with a load supplied with alternating current and to measure the voltage developed across the resistance in order to determine the current flowing through the load. The value of the shunt resistance has to be very small in relation to the load otherwise appreciable amounts of power are developed across the shunt, and therefore unwanted heat, and if of appreciable resistant, the shunt could alter the current flowing through the load.

When measuring the current supplied from the national grid it is a reauirement that the current is measured in the live supply line rather than in the neutral line. This means that the associated circuitry will be connected directly to the live side of the supply and this can give rise to problems associated with electrical installation and safety.

A particular problem exists where the current measuring circuitry forms part of a pre-payment meter where isolation from the supply to the customer interface is required to satisfy safety rules.

The second reason for isolation is that in certain test methods (where many meters are tested together) isolation is required between the voltage measuring circuits and the current measuring circuits.

The third reason for isolation is where the meter is designed to measure the current in a polyphase supply. Here isolation is required between individual phases but the signal processing circuitry is preferably common.

Existing techniques for isolation involve the use of photocouplers. However these can be expensive depending on the number of isolation links required. Furthermore these are active devices which require power supplies and associated circuitry in order to process the signal. This tends to add cost to the end product. Additionally photocouplers can normally only transfer binary signals therefore preventing their use in analogue situations.

It is an object of the present invention to provide an alternative current measuring device which overcomes the above problem.

Summary of the Invention According to the present invention in an alternating current measuring meter which includes a shunt through which current to be measured is forced to pass, the primary of an isolation transformer is connected to opposite ends of the shunt and the current induced in the secondary of the transformer is supplied to a current measuring circuit for providing an output signal indicative of the magnitude of the current induced in the secondary of the transformer.

Preferably the transformer includes a ferrite core.

In a preferred embodiment a toroidal ferrite transformer is employed.

Typically high voltages will be involved, and preferably appropriate steps are taken to isolate the windings from the core, and from each other, so as to provide the required level of electrical insulation.

The use of a toroidal ferrite transformer provides high mutual inductance and high linearity, and additionally it can be physically very small.

According to a preferred aspect of the invention, the secondary may have more turns than the primary so as to provide voltage gain. Whilst this will reduce the value of the induced current in the secondary winding, the higher voltage will reduce signal to noise ratio.

Where the frequency of the alternating current is fixed and known, the secondary may be tuned or a filter may be included so as to render the following circuitry less sensitive to frequencies which are not equal to the fundamental of the alternating current that apply.

Since a transformer is a passive device, no external power source is required.

Where the signal processing circuitry is intended also to measure the power, a high value resistor is conveniently connected between one end of the shunt, and voltage measuring circuits, which are also connected to the neutral of the supply. In known manner the processing circuitry typically derives a voltage equivalent to the current passing through the shunt, and a voltage proportional to that between the live and neutral lines of the supply, and a product of the two values of voltage is obtained, using an appropriate processor, so as to derive a power value from the two measured parameters.

Where the processing circuitry includes a phase sensing circuit, any phase shift between the voltage and current signals can be determined and the power computation corrected to take account of any phase shift (power factor).

The turns ratio of the isolation transformer may of course be reversed so that a relatively small current flow in the primary winding is magnified to give a larger current but lower voltage in the secondary. The choice of turns ratio is a matter of choice and will depend upon the shunt and the characteristics of the processing circuitry to which the secondary is connected.

Brief Description of Drawings The invention will be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic circuit showing a power meter embodying the invention; and Figure 2 is a more detailed schematic block diagram showing signal processing circuitry and a display which are part of the meter of Figure 1.

Detailed Description In the drawing the live and neutral in and out terminals 10, 12, 14 and 16 of a power meter 18, are shown connected through a shunt 20 in the case of the live terminals 10 and 12, and through a short circuit link in the case of the neutral terminals 14 and 16.

Two tapping points on the shunt provide current to a primary winding 22 of a toroidal ferrite core isolation transformer 4.

The secondary winding 26 provides an input current to processing circuitry 28 which generates an output signal along line 30 for controlling a display 32 to indicate in the display a value either by a scale or by a digital display, attributable to the current flowing through the shunt 20.

The meter additionally includes a high resistance connection 34 between the live terminal 10 and another input to the signal processing circuitry 28, and the latter is also connected as appropriate to the neutral terminal 14. The circuitry 28 processes the two inputs to produce two output signals whose values are proportional to the current flowing in the shunt 20, and the voltage between the live and neutral lines of the supply. Another output signal can be generated for supply along line 36 also to the display device 32 corresponding to the power.

Although shown as being supplied to the same display device, the two output signals along lines 30 and 36 may of course be supplied to different display devices, if more appropriate.

With reference to Figure 2, the processing circuitry 28 comprises two Digital to Analogue converters 38 and 40. The converter 38 has inputs which are connected to the high resistance connection 34 and to the neutral terminal 14, and thus produces a digital output signal representative of the voltage between the live and neutral terminals. The inputs of the converter 40 are connected to the secondary winding of the 26 transformer 24, so that the output of the converter 40 is representative of the current flowing in the shunt 20.

The output signals from the converters 38 and 40 are fed to a microprocessor 42 which drives the display (in this case a digital LCD or LED display) 32.

The signals produced by the converters 38 and 40 are at any time representative of instantaneous value of the alternating voltage and current, and the microprocessor is operable to calculate from those values the rms current and voltage in the circuit to which the meter is connected. The microprocessor 40 is also operable to compare the times of occurrence of peaks or zero crossing points of the voltage and current values represented by the converter output so as to determine the phase difference between the voltage and current measured by the meter, and hence to calculate the power factor (the cosine of the phase difference). The rms voltage is then multiplied by the rms current and the power factor, in order to determine the amount of power being consumed by the circuit.

Claims

Claims 1. An alternating current measuring meter having a shunt through which current to be measured is forced to pass, and an isolation transformer the primary windings of which are connected to opposite ends of the shunt, the transformer having a secondary winding connected to processing circuitry for providing an output signal indicative of the magnitude of the current induced in the secondary of the transformer.
2. A meter according to claim 1, in which the transformer includes a ferrite core.
3. A meter according to claim 2, in which the isolation transformer is a toroidal ferrite transformer.
4. A meter according to any of the preceding claims, in which the windings are electrically insulated from the core, and from each other.
5. A meter according to any of the preceding claims, in which the secondary windings have more turns than the primary windings so as to provide voltage gain.
6. A meter according to any of the preceding claims, in which the meter includes a high value resistor connected between one end of the shunt and voltage measuring circuits which are connected to the neutral of the supply, the arrangement being such that the meter is operable to measure power consumed in the circuit to which it is connected.
7. A meter according to any of the preceding claims, in which the processing circuitry includes a phase sensing circuit, for determining any phase shift between the voltage and current signals can be determined and the power computation corrected to take account of any consequent change in power factor.
8. A meter substantially as described herein with reference to, and as illustrated in, the accompanying drawings.