Abstract:
A current sensing device and a residual current detection device are described having a temperature compensation capability so that residual current can be directly measured to a high degree of precision in the background of a high load current. The residual current device comprises a plurality of resistive shunts for connection in respective ones of a plurality of lines through which current can flow to and from a load, and a detector means sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing through the shunts.

Description:
FIELD OF THE INVENTION 
     This invention relates to a current sensor with a temperature compensation capability intended for use in an electrical apparatus such as a residual current correction device, a current meter or a power meter. 
     DISCUSSION OF THE BACKGROUND ART 
     It is an aim of the present invention to provide a current sensor in economical form which includes temperature sensing means for facilitating temperature compensation. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a current sensor comprising a metallic link member having two end portions of conductive material and an intermediate portion interconnecting the end portions, said intermediate portion being formed of a resistive material, and an integrated circuit analog to digital converter mounted on said link member, said converter having analog input terminals connected to respective ones of said two end portions and digital output terminals for connection to a processing apparatus, wherein a temperature sensor is provided on or within said intermediate portion. 
     The temperature sensor is preferably an electronic semiconductor temperature sensor and may be mounted directly onto the intermediate portion with a thermally suitable conducting glue. The temperature sensor may be built into (i.e. integrated) into the integrated circuit analog to digital converter in which case it will form part of a semiconductor die mounted directly onto the intermediate portion. 
     Embodiments of the invention have the advantage that the temperature sensor will follow the temperature of the intermediate portion very closely. It is therefore possible to compensate for changes in the resistance of the intermediate portion resulting from temperature variations as the current flowing therethrough changes. 
     Conveniently, the converter is attached to the intermediate portion by a layer of electrically insulative adhesive material and the analog input terminals of the converter are connected to the end portions by wire bonds. 
     The converter preferably includes a delta-sigma modulator which provides a high frequency one-bit digital data. One or more decimation filtering stages may be included in the converter. 
     The converter may also have a voltage reference terminal for connection to a reference voltage source, the converter operating to provide digital output signals respectively representing the current flowing through said intermediate portion and digital output signals representing the voltage on one of said end portions. 
     Embodiments of the invention may be advantageously employed in residual current devices. Conventionally, residual current is detected utilising a current transformer having primary windings through which, in the case of a single phase device, load current flows in opposite directions so that if the return current is different from the outwardly flowing current because of current leakage an output current signal is induced in a secondary winding of the transformer. In the case of a multi-phase device, primary windings of the transformer are connected in all of the phase lines and the neutral line. In normal situations, when there is no current leakage, the net current induced in the secondary winding is zero and therefore no output is detected. 
     Sophisticated materials have been developed for the core of the current transformer, which enable considerable accuracy to be obtained when the currents flowing in the primary windings are substantially sinusoidal. However, switch mode power supplies are often used for computers and other equipment and there is an increasing tendency for such equipment to cause dc offsets in the currents. Such developments have made detectors utilising current transformers less reliable and prone to false tripping or failure to detect a dc current leakage. 
     This is a particular problem in the case of directly actuated electro-mechanical devices, where the current transformer secondary winding actually drives an actuator. The situation is not much improved, when including an electronic detection and amplification means connected to the secondary winding, as there are still problems with high frequency transients and dc offsets. A very small dc current level can cause the core to saturate thereby seriously impairing the ability of the detector to detect current leakage. 
     It is also an aim of the present invention to provide a residual current detection device in which the above mentioned problems are substantially overcome in a simple and efficacious manner. 
     In accordance with the invention there is further provided a residual current detection device comprising a plurality of resistive shunts for connection in respective ones of a plurality of lines though which current can flow to and from a load, and detector means sensitive tothe voltage developed across each of the shunts to detect any imbalance between the currents flowing though the shunts, wherein a temperature compensation means is provided for facilitating compensation for fluctuations in shunt resistance with variations in temperature. 
     In preferred embodiments, the temperature compensation means is a temperature sensor provided on or within each of said plurality of resistive shunts. 
     Preferably, the detector means comprises an analog to digital converter for each shunt and a processor for receiving the digital signals from the converters and determining whether a current imbalance exists. In this case, the temperature sensor may be built into (i.e. integrated) into the analog to digital converter. The temperature sensor is preferably an electronic semiconductor temperature sensor mounted directly onto the shunt with a thermally conducting glue. 
     Each shunt preferably takes the form of a composite strip having conductive portions at its ends and a resistive portion interconnecting the conductive portions. Such composite strips can be mass produced inexpensively to very high tolerances which makes them extremely suitable for this purpose. 
     The analog to digital converter for each shunt may include a delta-sigma modulator, which generates a high frequency single digital data stream which is converted by decimation filtering to a multibit digital data stream at a lower frequency. 
     The analog to digital converter for each shunt is preferably connected to the processor through an isolation barrier so that the converter can float at the voltage level of the shunt which it serves. The decimation filtering may be effected entirely in the converter, entirely in the processor or split between the converter and the processor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic perspective view of an example of the invention as applied to a single phase device, 
     FIG. 2 is a block diagram of an another example of the invention as applied to a three phase device, 
     FIG. 3 is a perspective view showing one of the current sensing devices embodying the present invention, 
     FIG. 4 is a sectional view of the current sensing device of FIG. 3, 
     FIG. 5 is an elevation of the device of FIG. 3, 
     FIG. 6 is a block diagram of a simple form of the electronic circuit of a single current sensor device, 
     FIG. 7 is a block diagram of an alternative form of the electronic circuit, 
     FIG. 8 is a block diagram of yet another form of the electronic circuit, and 
     FIG. 9 is a block diagram of a form of the electronic circuit which incorporates a temperature sensor in accordance with and embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the device shown in FIG. 1, a substrate  10  supports two composite conductor strips  11 ,  12 . Each of these includes end portions  13  of copper and an intermediate portion  14  of a resistive material such as manganin. The strips are formed by slicing up a sandwich formed by electron beam welding the copper portions to opposite sides of the manganin portion. The shunts formed by the resistive portions manufactured by this method can have a nominal resistance of 0.2 mΩ to a tolerance of less than 5%. If the two shunts  14  used on one device are pressed from adjacent portions of the sandwich stock, they are matched to within 2%. Calibration of the shunts built into a unit at two different temperatures can virtually eliminate shunt errors. In this way, at least two temperature measurements are made. Two temperature measurements are taken because the difference in shunt A from shunt B is linear when the devices are adjacent to one another. 
     However, it is desirable to provide for direct compensation for temperature fluctuations arising from current fluctuations especially in a single current detector. The resistivity of precision resistance materials does not change with temperature. Compared to pure metals such as copper or aluminium, with Temperature Coefficient of Resistance (TCR) values close to 4000 ppm/DegC., the TCR values of Manganin or Zeranin are more than a factor of 400 better—but still not zero. 
     In reality, the plot of Resistance Vs Temperature (R(T)-curve) is not strictly linear and it is common practice to describe the curves by a third order polynomial. In general this is: 
     R(T)=R o *(1+a o *T+b o *T 2 +c o *T 3 ) where T=Temperature in DegC. &amp; R o =Resistance at 0 DegC.. 
     At a more practical Reference Temperature of 20 Deg C.. we can rewrite this as R(T)=R20*[1+a o *(T−20)+b o *(T−20) 2 +c o *T(−20) 3 ] 
     The typical curves for the resistance materials Manganin and Zeranin curves are determined by the main composition of the alloys and vary very little from batch to batch. The production spread is less than 5 to 10 ppm/DegC.. These slight differences in the TCR value can be expressed in a tiny change of the first order coefficient “a” in the above equation and the second and third coefficients are basically not changed. For example a dR(T)/R20-curve for different batches is just rotated around the 20 DegC. point and the curve itself is unchanged. This explains the calibration of the shunts at two different temperatures mentioned above. 
     However in accordance with the present invention, it may be desired to calibrate each individual shunt in an RCD OR in the case of the Current Sensor (Single Shunt) we could calibrate it separately. Varying as a third order polynomial at least 4 if not more points for a good calibration would be needed. 
     As described by the above equations if we know the temperature and our Ref resistance R20 for example we can with a suitable number of points find the coefficients and calibrate the shunt. 
     In the preferred embodiment, the temperature sensor itself is built into (integrated) and is a part of the ASIC which includes the analogue to digital converter ADC. In other words the temperature sensor will be an electronic semiconductor temperature sensor in the ADC. The ADC is mounted as a semiconductor die directly onto the shunts with a suitable thermally conducting glue and will therefore track the temperature of the Manganin (shunt) very accurately. 
     Preferably, the temperature sensor is input and sampled via the voltage modulator. It could have its own modulator (see RCD FIG. 9 showing the added temperature channel) or be multiplexed into the voltage channel (see FIG.  2 ). 
     Is possible to avoid having to make several stable temperatures to make the measurements. Instead we propose to measure at 20 Deg C. for example and then apply a known current which heats up the shunts and make several measurements during this process until the shunt arrives at its new steady state temperature as a result of the applied current. 
     In the example shown in FIG. 1, there is a separate signal preprocessing ASIC  15  mounted on each of the shunts  14  and connected to the copper end portions  13  of the associated conductor strips. The two ASICs  15  are connected to via an isolation transformer array  16  to a main processor  17 . The ASICs  15  operate to convert the two voltages across the shunts into a digital signal stream which is communicated to the processor  17  via the isolation transformer array. The main processor is programmed to provide a drive signal to a trip actuator  18 . 
     The actual preferred structural configuration of the current sensors is shown in FIGS. 3 to  5 . These show leads  40  connecting two analog input terminals of the ASIC to the two copper end portions  13 . Other leads connect other terminals of the ASIC  15  to a lead frame  40   a  by means of which all other external connections are made. FIG. 5 shows in dotted lines a block  42  of encapsulation material and FIG. 4 shows an electrically insulative adhesive layer  41  by means of which the ASIC is attached to the intermediate portion  14 , which may be of manganin or zeranin of the composite strip  14 ,  15 . The strips are formed by slicing up a sandwich formed by electron beam welding of the copper bars to opposite sides of a manganin bar. The temperature sensor is preferably integrated within the ADC of the ASIC  15 . 
     FIG. 6 shows that within the ASIC  15  there is provided a single delta-sigma modulator  15   a.  There is also an analog input circuit which has its input terminals connected to the copper end portions  13 . The output of the ASIC  15  in this case consists of a high frequency one-bit data signal train. In use, the ASIC output is connected via a transformer or other isolation barrier  16  to a processor  17 . The processor in this arrangement is configured to carry out one or more decimation filtering operations to convert the one-bit signal stream into a multi-bit value at a lower frequency. 
     The processor  17  may typically be configured to receive signals from a plurality of the detectors and to sum these signals to ascertain whether the current flows through the detectors are balanced. Such an arrangement can be used for residual current correction allowing an actuator to trip a switch if an unbalanced condition is found to exist. The processor  17  may alternatively or additionally compare the instantaneous current level with a trip level so that overcurrent tripping can be controlled. 
     FIG. 2 shows in rather more electrical detail a three phase device. In this case there are four shunts  14 , one in each phase line and a fourth in the neutral line. The ASICs  15  of FIG. 1 are shown as four separate blocks  20 ,  21 ,  22 , and  23 , and there is a power supply unit  24  which draws power from the phase lines on the mains side of the shunts  14  and provides controlled voltages to the processor  17 . Power is supplied to the four blocks  20  to  23  via isolation barriers  25  which make up the array  16 . Each block of the ASIC includes an analog to digital converter in the form of a delta-sigma modulator which provides a high frequency one bit digital data stream. A multiplexer may be included in each converter so that the converter can provide to the processor, through the respective isolation barrier, signals representing both current in the associated shunt and the voltage at one end of it. The processor uses these signals to monitor the current in each shunt and to operate the actuator  18  if an imbalance occurs. 
     It will be noted that the voltage sensing connections to the ASICs are made via resistor chains connected between each phase line and the neutral. Each such resistor chain comprises an outer pair of precision resistors of relatively low ohmic value and an intermediate resistor of relatively high ohmic value. These resistor chains allow the RCD to be provided with an independent reference. If the neutral ADC is taken as the selected system reference, then the operating software of the main processor can use the multiple signals derived from the several resistor chains to calibrate each phase against the neutral reference. 
     The CPU is programmed to carry out the necessary calculations to determine the existence of an imbalance and can determine the true RMS value of the residual current, which conventional devices fail to do correctly particularly in the case of non-sinusoidal current waveforms. The CPU may be programmed to enable it to determine from the data it receives whether a particular event is, in fact, an unacceptable leakage more reliably than conventional devices. For example, the CPU can take into account the historic performance of the unit when setting the leakage current threshold and may ignore events which have a recognisable “signature”. In this way improved tolerance to nuisance tripping can be obtained 
     Decimation filtering of the high frequency one bit data stream is required to reduce each data stream to a multi-bit digital signal at a predetermined sample frequency. By way of example, each current signal may be a 23-bit signal at a sample rate of 64 times the mains frequency, but lower resolution at lower sample rates can be employed when non-linear, rather than linear conversion is acceptable. The decimation filtering is typically a function of the processor, filtering of the four data streams being executed simultaneously so that sample values are derived for all four shunts simultaneously. A circuit employing such an arrangement is shown in FIG. 6 as described above. 
     In an alternative embodiment as shown in FIG. 7, one or more stages of the decimation filtration may be executed by hardware included within the ASIC. This includes a serial output driver  15   b  to transmit the bits of the multi-bit digital signal produced by the filtration stage  15   c  serially to the processor. Multi-bit digital words are transmitted serially across the isolation barriers instead of a one-bit signal stream. The filtration stages may be split between the ASIC and the processor. With this arrangement, the configuration of the processor can be simplified as part or all of the decimation filtration operation is carried out in the ASIC. 
     Where current and voltage are both to be monitored as in the system shown in FIG. 2, the circuit  15  may be as shown in FIG. 8 with separate modulations and filtering components for the two signal streams and a common serial interface. Alternatively separate serial interfaces may be employed. The ASIC of FIG. 8 has a further analog input which can be connected to a reference voltage source. Two analog input stages  21   a  and  21   b  are present and these feed signals to two independent delta-sigma modulators  15   d,    15   e.  As shown, there are two independent decimation filtration stages  15   f,    15   g  for the two one-bit digital signal streams. The outputs of the stages  15   f,    15   g  may, as shown, be connected to a common serial output stage or (not shown) separate serial output stages may be provided. 
     It will be appreciated that the arrangement of FIG. 8 may be modified by the omission of the two filtration stages  15   f,    15   g  where all filtration is to be carried out by the processor. 
     Where voltage as well as current is monitored by the processor, precise calibration of the shunts can be achieved. This allows more accurate determination of the current balance in RCD applications. Moreover, as voltage and current are both being monitored to a high level of precision, accurate power consumption metering can be obtained. 
     Where the devices of the invention are used in RCD and overcurrent trip systems, the processor can be programmed to recognise the transients which may occur when loads are switched in and out of circuit to avoid false tripping. Many other convenient functions can be programmed into the processor, made possible by the high precision of the current measurements capable of being carried out. 
     FIG. 9 shows an arrangement similar to the one of FIG. 8 except for the addition of a temperature sensor in accordance with embodiments of the present invention. The temperature sensor is input and sampled via the voltage modulator. The sensor could have its own modulator (FIG. 9) or be multiplexed into the voltage channel as mentioned above. 
     The arrangements described enable very accurate detection of current imbalance to be effected even in the presence of switching transients and DC offsets. The problems which arise from potential saturation of the current transformer core are avoided completely. 
     Since the CPU receives actual line current and voltage data from each of the blocks  20  to  23 , it can be programmed to perform other calculations, such as current limit and power consumption. Thus an RCD device constructed as described above can also provide the functions of a conventional circuit breaker and/or those of a power consumption meter without any additional sensing or analog-to-digital components being required.