WO1999009421A1

WO1999009421A1 - Current monitoring device

Info

Publication number: WO1999009421A1
Application number: PCT/GB1998/002480
Authority: WO
Inventors: Roger Francis Golder; Christopher Davies
Original assignee: Cambridge Consultants Limited
Priority date: 1997-08-20
Filing date: 1998-08-19
Publication date: 1999-02-25
Also published as: GB9717667D0

Abstract

A current monitoring device comprising an electrically conductive element (3) through which a current to be monitored passes in use. A voltage sensor (10) determines the potential difference between two points on the electrically conductive element. A temperature sensor (19) monitors the temperature of the electrically conductive element. A current processor (10) determines a value, related to the current flowing through the electrically conductive element (3), in accordance with a predetermined algorithm based on the determined potential difference and the monitored temperature so as to compensate for variations in the determined potential difference with temperature.

Description

CURRENT MONITORING DEVICE

The invention relates to a current monitoring device, for example for inclusion in an electricity meter. Typical devices for monitoring the magnitude of a flowing electric current include the use of a low value resistor to create a voltage proportional to line current. The problem with this approach is the sensitivity of such low value resistors to temperature with the result that they undergo significant changes in their resistive characteristics with temperature changes brought about by changes in ambient temperature and self-heating effects. To deal with this, therefore, it is known to use materials which have a zero temperature coefficient, such as Nicrome or Manganin. However, these materials are expensive and not ideally suited for use in commercial, mass produced products such as electricity meters.

In accordance with one aspect of the present invention, a current monitoring device comprises an electrically conductive element through which a current to be monitored passes in use; a voltage sensor for determining the potential difference between two points on the electrically conductive element; a temperature sensor for monitoring the temperature of the electrically conductive element; and a current processor for determining a value, related to the current flowing through the electrically conductive element, in accordance with a predetermined algorithm based on the determined potential difference and the monitored temperature so as to compensate for variations in the determined potential difference with temperature.

The invention accepts that there will be a variation in electrical performance of the electrically conductive element with temperature and deals with this by compensating for that variation by monitoring the temperature of the element. Although the temperature of the element could be monitored at a number of positions along its length, it has been found sufficient to monitor the element at just one location along the element, typically centrally between the two points between which the potential difference is determined. An important aspect of the invention is the manner in which the temperature sensor is provided. Although such a sensor could be provided in a variety of ways, in the preferred approach, the temperature sensor is provided on a semiconductor chip secured in heat conducting contact with the electrically conductive element. This approach is convenient since the position of the temperature sensor on the semiconductor chip is not important. The whole chip will take the temperature of the electrically conductive element . Conveniently, the semiconductor chip also includes the voltage sensor. This reduces the complexity of the device and results in a compact construction.

The electrically conductive element can comprise any convenient material but in the preferred approach comprises copper, for example in the form of a copper bar.

In the preferred arrangement where the temperature of the electrically conductive element is monitored at a single position along its length, preferably the electrically conductive element is substantially symmetrical about the temperature sensing position and between said two points. A source of error in the performance of some electrically conductive elements made of pure metals (for example copper) because of their lower resistance, is the effect of voltages produced in response to varying currents due to the relatively larger inductive component of the impedance. This error can be reduced by symmetrical design of the electrically conductive element and the connections to the two voltage sensing points.

A further problem which can arise are inductive effects of the electrically conductive element on leads extending from the voltage sensing point to the voltage sensor. This could be dealt with by downstream processing to compensate for the inductive effect but preferably and in accordance with a second aspect of the present invention, a current monitoring device comprises an electrically conductive element through which a current to be monitored flows in use; a voltage sensor for determining the potential difference between two points on the electrically conductive element; and a current processor for determining a value, relating to the current flowing through the electrically conductive element, in accordance with a predetermined algorithm based on the determined potential difference, wherein the voltage sensor is coupled to the two points on the electrically conductive element via leads which extend at least partly within sections of the electrically conductive element so as to reduce inductive effects on the leads.

By locating the leads at least partly within sections of the electrically conductive element, it is possible to eliminate or at least reduce inductive effects thus avoiding the need for subsequent compensation. In some cases, the electrically conductive element may be tubular, for example cylindrical, with the leads extending through the tubular element while in others the electrically conductive element may comprise a pair of spaced bars between which the leads extend. A further problem that can arise with these devices is long term stability. The conductivity of metals is dependent on the crystal size which may in turn be affected by its state of "working" or annealing. Preferably, therefore, the electrically conductive element has been pre-worked so as to have an anneal state substantially matching its end of life anneal state. Thus, the metal is pre-worked to the anneal state to which the temperature and working which it is expected to experience over life would drive it. The current monitoring device can be used simply to monitor current but the device is particularly useful in an electricity power meter for monitoring the use of electrical power supplied along dual lines, the meter further comprising a voltage monitor for monitoring the potential difference between the two lines; and a power processor for computing power from the monitored current and potential difference.

Conveniently, where the temperature sensor and voltage sensor are provided on a semiconductor chip, the voltage monitor and/or the power processor are also mounted on the same chip. Particularly conveniently, the current and power processors are implemented by the same processor.

An example of an electronic power meter according to the invention will now be described with reference to the accompanying drawings, in which: -

Figure 1 is a schematic, perspective view of part of the meter;

Figure 2 is an exploded view of the components shown in Figure 1 ;

Figure 3 is a section taken along the line 3-3 in Figure 1 ; Figure 4 is a section taken along the line 4-4 in Figure 1 ;

Figure 5 is a block circuit diagram of the meter; and,

Figure 6 shows temperature profiles for a lOOμΩ Cu bar carrying 100A. The electronic power meter shown in the drawings is suitable as a Class 2 meter and comprises a printed circuit board (PCB) 1 which carries an integrated circuit, semiconductor chip 2 on one surface and adjacent an edge.

The PCB 1 has typical dimensions 80mm x 45mm. The PCB 1 and chip 2 are sandwiched between a pair of legs 4A,4B of a U-shaped stamped copper bar 3, the bar being riveted at each end to respective terminals 5,6. Each terminal 5,6 has a pair of screws 7 to enable the terminal to be connected to a respective part of a live line 8 carrying, along with neutral line 9, an AC current (Figure 5) . The components connecting the chip 2 to the neutral line 9 are not shown in Figures 1 and 2. The components carried on the chip 2 can be seen in Figure 5. These components comprise a central processor 10 connected to memory 11 for storing programme instructions and also to an EEPROM 12 mounted on the PCB 1 which stores calibration information. The central processor 10 is connected via an A to D convertor 13 to a line current conditioning amplifier 14 coupled via leads 25,26 on the PCB to two contact points 27,28 on the copper bar 3. The amplifier 14 amplifies the monitored voltage drop by for example 16 to 32 times and has a temperature stable gain to at least 30ppm/°C. The central processor 10 monitors the amplified voltage drop (V_bar) between the contact points 27,28 in order to determine the current flowing along the line 8, as will be described in more detail below. The central processor 10 is also connected via an A to D convertor 15 to a line voltage conditioning amplifier 16 via respective resistor and capacitor pairs 17A, 17B and 18A,18B. This enables the central processor 10 to determine the voltage drop (V_line) across the lines 8,9. A temperature sensitive device 19 is mounted on the chip 2 to monitor the temperature of the chip, the output from the device being fed via an A to D convertor 20 to the central processor 10. The device 19 may have any conventional form including any type of Si junction such as a diode or transistor, a polysilicon material whose resistance varies with temperature or an FET.

To temperature compensate the copper bar or shunt 3 with a single point temperature measurement in the centre of the shunt we must be able to deduce sufficiently accurately the temperature distribution and thus resistance variation of the shunt, under any valid operating conditions of the meter.

It is assumed that the worst case practical conditions are that cooling may occur is only through the terminal blocks 5,6 and that a terminal may generate heat, for example, as a result of poor electrical connection or connection to a device (e.g. solenoid unit) which generates heat .

Figure 6 shows the temperature profiles, above ambient, to be expected from a simple copper bar or shunt of lOOμΩ resistance connected between the live terminal blocks 5,6. The length of 60mm is determined in this example by the distance between the live terminals of a standard UK Class 2 domestic electricity meter. The cross-

2 section of 10.32 mm is determined by the lOOμΩ resistance requirement and the resistivity of copper at 20°C. Temperature compensation against ambient variations requires that the measured temperature is accurate over the working temperature range . The expected maximum error of 0.5°C corresponds to about 0.2% variation (error) in meter sensitivity.

Correct compensation under conditions where one terminal is heated (e.g. by a contactor solenoid) and the other cooled, is achieved by temperature sensing at the centre of the shunt if the temperature coefficient of resistance is constant over the temperature range of the shunt. Interpolation from known data indicates that this condition (constant temperature coefficient) is satisfied for a lOOμΩ shunt dissipating 1 Watt (Figure 6a) .

Compensation for self-heating can only be achieved over a limited temperature rise. The parabolic temperature profile caused by the self-heating of 1 Watt, which would be produced by an I_maχ of 100A, is shown to cause a temperature rise of 1.8125°C at the measurement point

(Figure 6b) . The average temperature rise over the parabolic distribution is in fact only three quarters of this. In order to ensure that the error due to this parabolic term is sufficiently small, the resistance of the shunt must be limited. The lOOμΩ copper shunt results in an error of - 0.228% at I_maγ = 100A rms . Other cooling situations (e.g. Figure 6c) may be regarded as linear combinations of the above two conditions and thus fall between the two extremes: Figure 6a where the average temperature can be determined by the single, central temperature measurement without error, and Figure 6b where the error is sufficiently small to be ignored. Consequently, this shows that the single temperature measurement is acceptable.

It will also be noted in Figures 1 and 2 that the copper bar 3 has a substantially symmetrical form on either side of the temperature sensing point defined by the chip

2 between the voltage monitoring points 27,28. This limits the effect of magnetic fields which would otherwise degrade accuracy and performance range .

To control operation of the processor 10, the processor is connected to a real time clock circuit 21 and a system clock 22 while power, derived from the lines 8,9 is fed to conventional power generation circuitry 23.

Information generated by the central processor 10 is fed to an LCD display 24 or the like.

The meter components can all be housed in a casing, with suitable windows to allow the display 24 to be visible, having dimensions in the order 125mm x 100mm x

40mm.

Since the chip 2 is in intimate contact with the copper bar 3, the material of the chip takes up the temperature of the adjacent portion of the bar 3. Thus, the transistor 19 which monitors the temperature of the chip, will effectively monitor the temperature of the bar

3 enabling the central processor 10 to determine the temperature of the bar 3.

In operation, the processor 10 determines line current as a function of the temperature (T) of the copper bar 3 at time t, the temperature (Teal) at the time t_cal at which calibration measurement was made, and the resistance of the copper bar 3 (R_cal) determined at time t_cal . That is, line current (I) is defined as: I = f_n(T,T_cal,R_cal) . This function f_n may be implemented as a look-up table or as a mathematical formula. Such a formula would typically be

¹ "= ^Vbar/ ( Rcal ^{+ R}cal < T- T_eal ) p ) where p = an appropriate temperature coefficient of resistance .

In order to determine power usage, the processor then determines electrical energy passed through the meter (E) as follows:

where K is a constant.

Energy usage can then be displayed via the display 24 and stored in the EEPROM 12.

It will be noted in Figures 1 and 2 that the leads 24,25 which will be in the form of printed tracks on the PCB 1, extend substantially completely between the legs 4A,4B of the copper bar 3. As explained above, the reason for this is to minimise or at least reduce inductive effects on currents passing through the leads 24,25. In another example, not shown, the copper bar 3 may be in the form of a tube such as a cylinder in which case the leads again would pass wholly within the cylinder.

In the preferred example, the copper bar 3 is pre- worked so as to take up an anneal state which corresponds to that to be expected at the end of its working life. This increases the long term stability of the copper bar.

Claims

1. A current monitoring device comprising an electrically conductive element through which a current to be monitored passes in use; a voltage sensor for determining the potential difference between two points on the electrically conductive element; a temperature sensor for monitoring the temperature of the electrically conductive element; and a current processor for determining a value, related to the current flowing through the electrically conductive element, in accordance with a predetermined algorithm based on the determined potential difference and the monitored temperature so as to compensate for variations in the determined potential difference with temperature.

2. A device according to claim 1, wherein the temperature sensor monitors the temperature of the electrically conductive element at a single position along its length.

3. A device according to claim 2, wherein the position is located centrally between the said two points.

4. A device according to claim 2 or claim 3, wherein the electrically conductive element is substantially symmetrical about the temperature sensing position and between said two points.

5. A device according to any of the preceding claims, wherein the temperature sensor is provided on a semiconductor chip secured in heat conducting contact with the electrically conductive element.

6. A device according to claim 5, wherein the semiconductor chip also includes the voltage sensor.

7. A current monitoring device comprising an electrically conductive element through which a current to be monitored flows in use; a voltage sensor for determining the potential difference between two points on the electrically conductive element; and a current processor for determining a value, relating to the current flowing through the electrically conductive element, in accordance with a predetermined algorithm based on the determined potential difference, wherein the voltage sensor is coupled to the two points on the electrically conductive element via leads which extend at least partly within sections of the electrically conductive element so as to reduce inductive effects on the leads.

8. A device according to claim 7, wherein the electrically conductive element is tubular, the leads extending through the tubular element .

9. A device according to claim 7, wherein the electrically conductive element comprises a pair of spaced bars between which the leads extend.

10. A current monitoring device according to any of claims 7 to 9 and according to any of claims 1 to 6.

11. A device according to any of the preceding claims, wherein the electrically conductive element is made of copper .

12. A device according to any of the preceding claims, wherein the electrically conductive element has been pre- worked so as to have an anneal state substantially matching its end of life anneal state.

13. An electricity power meter for monitoring the use of electrical power supplied along dual lines, the meter comprising a current monitoring device according to any of the preceding claims; a voltage monitor for monitoring the potential difference between the two lines; and a power processor for computing power from the monitored current and potential difference.

14. A meter according to claim 13, when dependent on claim 5 or claim 6, wherein the voltage monitor and/or the power processor are also mounted on the same chip.

15. A meter according to claim 13 or claim 14, wherein the current and power processors are implemented by the same processor.