|Title of Invention||
A CURRENT MEASUREMENT DEVICE
|Abstract||57) Abstract:- The present invention relates to a current measurement device comprising a mutual inductance transformer and an integration circuit which integrates the signal from the transformer characterised in that the measurement device comprises a dither circuit which adds a substantially rectangular signal to the signal from the transformer before the input of the integration circuit in order that the integration circuit provides a signal representing the measured current perturbed by a substantially triangular dither signal. PRICE; THRITY RUPEES I|
5 The present invention concerns a current measurement circuit comprising a mutually coupled transformer and an integration circuit which integrates the signal of the mutually coupled transformer.
10 The use of mutually coupled transformers (otherwise known as inductively coupled transformers) for measuring currents possesses certain advantages in comparison with other measuring devices. In particuler, in the field of electricity metering, the use of a mutually coupled transformer avoids
15 the problems of saturation by dc currents which arise with iron-core transformers and enables the measurement of the current separately from other measurements, for example measurements of the current on the other phases of a multiphase network. However, mutually coupled transformers
2 0 present a particular problem in that the signal supplied by the transformer is the derivative of the measured current. Thus, the use of a mutually coupled transformer requires an integration circuit to process the signal.
2 5 A measurement circuit including a mutually coupled
transformer and an integration circuit are described in the paieni application EP 0 403 330. The iniegravion circuit of this application is particularly adapted to suppress low frequency signals and parasitic dc signals which can be produced at the
3 0 output of conventional integration circuits.
It is often the case that signals representing a measurement of current are converted by an analogue digital converter for later processing. A known technique for improving the 3 5 resolution of an analogue digital converter uses a dither signal added to the measured signal before conversion by the
converter. This dither signal can be a random signal or a periodic signal, for example a triangular signal or a signal having the form of a ramp. Using this technique the value of the measured signal can be determined by an analysis of the 5 converted values of the signal with a resolution finer than that normally possible with the available quantisation steps of the converter. The principals of the dither technique have been known for a long time and examples of measurement and communication systems which use a dither signal for 10 improving the performance of a converter are described in US 4 187 466, EP 0 181 719 and EP 0 613 256. The patent EP 0 181 719 in particular describes the principles of a system which adds a triangular dither signal to the current signal before conversion.
It is thus desirable to have a system for measuring currents which combines the advantages associated with a mutually coupled transformer with those associated with the use of a dither signal. The generation of a dither signal can present
2 0 problems in relation to the cost of components and the
complexity of the system which generates it. The present invention aims to provide a system which provides the advantages of the two techniques but which avoids the problems of complexity and duplication of components.
To this end, the present invention is characterized in that the measurement circuit further comprises a dither circuit which adds a substantially rectangular signal to the signal from the transformer before the input of the integration circuit in order 30 that the integration circuit provides a signal representing the measured current perturbed by a substantially triangular dither signal.
In general, a square wave or rectangular signal is not
3 5 appropriate as a dither signal but it may be easily generated in
an electronic circuit, for example at the output of a
microprocessor. The present invention provides a circuit in which a current signal modulated by a triangular dither signal is produced and which can be sent to an analogue to digital converter, but which avoids duplication of components due to 5 the fact that the integration circuit performs both the functions of integration of the signal from the mutually coupled transformer and of creation of a dither signal.
As described above, the value of the measured signal by the 10 transformer may then be calculated after conversion by the steps of signal processing with a resolution finer than that possible with only an analogue to digital converter.
The integration circuit may be a digital or analogue integrator 15 of a conventional type. However, preferably, a circuit similar to that of EP 0 403 330 is used, which comprises an amplifier having an associated gain resistance and a feedback loop, the feedback loop comprising a feedback capacitor, the integration circuit further comprising a second integrator mounted at the
2 0 output of the amplifier, upstream of the feedback loop.
_ By these means, the integration circuit suppresses low frequencies and parasite dc signals which can arise in conventional analogue circuits. Preferably, the integration 15 ciricuit can further comprise means for correction of the phase delays in the integratd signal, in particular the delay introduced by the gain lesistancc of the integrator, comprising :. a passive filter mounted upstream of the amplifier. Where the current measurent circuit forms part of the input circuit of an
3 0 electricity meter, including a voltage measurement circuit, the
passive filter can be dimensioned in order that all the phase delays between the voltage and current paths are corrected by this filter, the voltage in the network being measured directly without changing its phase. Thus, the voltage channel is free 3 5 from constraints regarding the use of components used for correction of phase, which is advantageous if the voltage
channel is also used for communication of data.
As discussed above, a square wave or rectangular signal is easily generated by an electronic circuit. In one embodiment, 5 the rectangular signal can be generated using a pulse width modulated signal (PWM), provided at an output of a microprocessor, which signal is passed via a switching means comprising at least one latch in order to divide the average frequency of the PWM signal and to create the square wave.
The use of a dither signal having the form of a simple triangular waveform can cause problems associated with the relation between the amplitude of the signal and the quantisation steps of the converter. In particular, a problem
15 arises if the amplitude of the dither signal is not equal to an integral number of quantisation steps of the converter. In order to overcome these problems, the dither circuit preferably supplies a rectangular signal which is further modulated by a modulation signal. This signal is then
2 0 integrated in order to become a modulated triangular signal.
The use of a signal having this form overcomes the problems associated with the differences between the amplitude of the non-modulated triangular signal and the quantisation levels of
2 5 the converter. In effect, the modulation signal introduces a
component to the converted values which is dependant on the differences between the amplitude of the triangular signal and the quantisation steps of the converter and which can be suppressed after conversion in the same processing operation
3 0 that suppresses the dither signal, for example, by using a
Preferably, the modulation signal is also generated using the PWM signal of the microprocessor. In one embodiment, the 3 5 two limits of the modulation signal may be generated using two integrating circuits which integrate the PWM signal (and
its inverse) in order to create each limit. However, it is possible to further reduce the number of components used. Preferably, the dither circuit comprises an integrator which integrates the pulse width modulated signal in order to create a triangular signal representing one limit of the modulation signal, this signal being supplied to switching means in order to create a substantially rectangular signal having only one limit which varies as per the signal from the integrator, the dither circuit further comprising a filter which filters low frequency components from the signal so as to provide a rectangular signal which is symetrically modulated. Considering the rectangular modulated signal before the filtering step, this signal comprises a frequency component corresponding to the frequency of the rectangular signal and a low frequency component corresponding to the triangular signal which modulates one of the two limits of the rectangular signal. The filter thus eliminates low frequency components and the signal assumes a symetrical shape, i.e. such that the two limits of the signal are equally modulated.
Accordingly the present invention provides a current measurement device comprising a mutual inductance transformer and an integration circuit which integrates the signal from said transformer characterised in that said measurement device comprises a dither circuit said dither circuit comprising means for generating a substantially rectangular signal and means for adding substantially rectangular signal to the signal from said transformer before the input of said integration circuit in order that said integration circuit provides a signal representing the measured current perturbed by a substantially triangular dither signal
'2 1 JUN 2002 DUPLICATE
The invention will be best understood in the light of the following
description of one embodiment of the invention, which is given by way of an
illustrative and non-limiting example, with reference to the accompanying
drawings in which:
Figure 1 shows an example of an input circuit for an electricity meter
comprising a current measurement circuit as per the present invention;
Figure 2 shows the steps of creation of the dither signal added to the current
signal measured by the circuit of Figure 1.
As is shown in Figure 1, the input circuit for an electricity meter as per the
embodiment comprises a current measurement circuit including a dither
circuit 1 and an
integrating circuit 2, which integrates the signal from an inductance transformer 3 (mutually coupled transformer) combined with a square wave supplied by the dither circuit, a voltage measurement circuit 4 and an analogue to digital 5 converter 5.
The dither circuit 1 comprises a latch 6 which receives the PWM signal of a microprocessor associated with this circuit. The latch divides the average frequency of the PWM signal in
10 order to create a square wave. The modulation signal is produced by the switch 8 which cuts the reference voltage Vref in response to the PWM signal of the microprocessor, so as to produce, after filtering by the resistance and capacitor 9 and 11 and addition of the reference voltage Vref by the
15 resistances 9, 10 and 12, the modulated Vref signal. This signal
is modulated in a triangular form at the frequency of the PWM
cycle and having a depth of modulation which is dependant on
the relation R12
R9 + R10
This signal is then cut by the second switch 7 in response ,to the. square wave produced by the PWM signal passed via the latch 6 so as to provide an assymetrically modulated square wave signal. The capacitor 13 serves to equalise the rise and
2 5 fail times of the modulation signal.
As shown in Figure 1, this signal, modulated at its upper limiit, is treated by a symetrising circuit comprising a capacitor 15, which eliminates the low frequencies which render the
3 0 modulated square wave assymetric so as to generate a
trapezoidally modulated square wave, and resistances 16, 17 and a capacitor 18 which function io compensate for the distortion of the trapezoidal signal output from the capacitor 15. In effect, the symetrising circuit comprising the 3 5 components 15 to 18 constitutes a pass band filter, which passes the frequency of the square wave and its first
harmonics and which excludes the frequences which distort -the symetrical form of the signal. Referring to Figure 2 a square wave signal modulated and symetric is thus produced.
As shown in Figure 1 the current measurement circuit further comprises a mutual inductance transformer 3, the signal of which represents the derivative of the current which passes in the phase Fl of an a.c. network. This signal is supplied to the integrator circuit 2 and after to the analogue-digital converter 5. The integration circuit 2 comprises an amplifier 20 having a feedback loop formed of a capacitor 21 and a resistance 22 which control the integration of the signal of the transformer. The circuit further comprises a coupling capacitor 23 connected at the output of the amplifier upstream of the feedback loop, which changes the characteristics of the circuit in order to suppress low frequencies and dc signals, and a gain limiting resistance 24 which prevents the saturation of the amplifier. A voltage divider 24, 25 is connected at the output of the circuit upstream of the converter 5. In effect, these elements of the circuit correspond to the circuit described in EP 0 403 330 which supplies a signal corresponding to the integral of the signal of the transformer without parasitic effects and distortions of the signal.
The current measurement circuit forms part of an input circuit of an electricity meter which further comprises a voltage measurement circuit 4, comprising a voltage divider comprised .of two resistances 26, 27. The input circuit of the present invention is different from that of the circuit of EP 0 403 330 in that there is no phase correcting element in the voltage signal path, the capacitor 28 functioning only to attenuate undesired high frequencies. 1 hat is, the phase correction elements are formed by the capacitor 29 and the resistance 30 which function in combination with the resistances 22 and 31 to correct the phase in the current path so that the current and voltage paths are in phase at the input of the analogue to
digital converter. The current path further comprises a capacitor 32 which attenuates undesired high frequencies. The capacitors 28 and 32 are optional and may be omitted in other embodiments.
The signal from the dither circuit is added to the signal received from the transformer 3 and supplied to the integration circuit at the point 33. The combination of the signals is then integrated in order to create a signal
10 representing the image of the current measured by the
transformer dithered by a dither signal having the form of a triangular waveform modulated by a trapezoid. The amplitude of the waveform and of the modulation signal are chosen to create a dither signal comprising a triangular signal 30 having
15 an amplitude corresponding to several quantisation steps of the converter modulated at each limit by an amplitude corresponding to at least half a quantisation step. The signal is then supplied to the input of the analogue to digital converter. As described in the introduction above, the use of a signal
2 0 having this form improves the resolution of the signal by the converter. After the step of conversion the dither signal can be removed using a digital decimation filter or other known means, for example, by the conversion of the anlogue dither signal to a digital signal and the subtraction of this signal from the signal representing the combined values of the current and dither signal. The current and voltage signal thus calculated can be used in a calculation of the energy consumed etc.
1. A current measurement device comprising a mutual inductance transformer and an integration circuit which integrates the signal from said transformer characterised in that said measurement device comprises a dither circuit said dither circuit comprising means for generating a substantially rectangular signal and means for adding substantially rectangular signal to the signal from said transformer before the input of said integration circuit in order that said integration circuit provides a signal representing the measured current perturbed by a substantially triangular dither signal.
2. The current measurement device as claimed in claim 1 in which said dither circuit comprises modulation means for modulating said substantially rectangular signal.
3. The current measurement device as claimed in claim 1 or 2 in which said means for generating said substantially rectangular signal comprises switching means having at least one latch for receiving a pulse width modulated signal and for dividing the frequency of this signal to generate said substantially rectangular signal.
4. The current measurement device as claimed in claim 3 in which the dither circuit comprises an integrator which integrates the pulse width modulated signal in order to create a triangular signal representing one limit
of the modulation, this signal being supplied to switching means in order to create a signal substantially rectangular having only one limit which varies as per the signal from the integrator, the dither circuit further comprising a filter which filters low frequency components so as to provide a rectangular signal which is symetrically modulated.
5. The current measurement device as claimed in any one of the
preceding claims wherein the said integration circuit comprises an amplifier
having a gain limiting resistance and a feedback loop, with a feedback
capacitor, and a second capacitor connected at the output of the amplifier
upstream of the feedback loop.
6. The current measurement device as claimed in claim 5 wherein
between the transformer and the amplifier a passive filter is provided for
correcting the phase between the integrated signal and the signal received
from the transformer.
7. An electricity meter comprising a current measurement device
as claimed in claim 6.
8. A current measurement device, substantially as herein described
with reference to the accompanying drawings.
16 Ml Ml ' 3
|Indian Patent Application Number||625/MAS/1995|
|PG Journal Number||30/2009|
|Date of Filing||25-May-1995|
|Name of Patentee||SCHLUMBERGER INDUSTRIES S.A|
|Applicant Address||50 AVENUE JEAN-JAURES 92120 MONTROUGE|
|PCT International Classification Number||G01R27/26|
|PCT International Application Number||N/A|
|PCT International Filing date|