EP0496732A1

EP0496732A1 - Electronic watt-hour meter

Info

Publication number: EP0496732A1
Application number: EP90909589A
Authority: EP
Inventors: Georgy Katrib
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-09-07
Filing date: 1990-07-03
Publication date: 1992-08-05
Also published as: EP0496732A4; WO1991003740A1; JPH05500708A

Abstract

Un signal proportionnel au courant de charge est intégré pendant les demi-cycles positifs (ou négatifs) de la tension d'alimentation. Lorsque la sortie de l'intégrateur (7) franchit la ligne de deux limites, une tension de référence (avec la polarité appropriée) est également intégrée afin d'amener la sortie de l'intégrateur à l'intérieur de deux limites. Pendant le temps d'intégration de la tension de référence, on utilise les impulsions émises par une tension à un convertisseur de fréquence (3) afin d'augmenter ou de diminuer un compteur (2) selon la polarité de la tension de référence. On rend la fréquence de la tension transmise au convertisseur de fréquence proportionnelle à la tension d'alimentation. Après l'écoulement d'une période de temps, le contenu du compteur change d'une quantité proportionnelle à l'énergie active consommée pendant cette période.A signal proportional to the load current is integrated during the positive (or negative) half-cycles of the supply voltage. When the integrator output (7) crosses the line of two limits, a reference voltage (with the appropriate polarity) is also integrated in order to bring the integrator output within two limits. During the integration time of the reference voltage, the pulses emitted by a voltage are used at a frequency converter (3) in order to increase or decrease a counter (2) according to the polarity of the reference voltage. The frequency of the voltage transmitted to the frequency converter is made proportional to the supply voltage. After the expiration of a period of time, the content of the counter changes by an amount proportional to the active energy consumed during this period.

Description

DESCRIPTION

Title:

Electronic watt-hour meter.

Technical field:

Measurements of active and reactive electricity consumption, suitable for use in single and three-phase supply systems.

Background art:

Most of the electronic watt-hour meters, available at present, use different methods to multiply two signals proportional to the instantaneous values of the supply voltage and the load current to obtain the instantaneous power. The time integration of this power is proportional to the consumed energy. The main differences between these methods are in the multiplication and integration stages. The different methods of multiplication are:

1- Variable conductance multiplication:

The logarithms of the voltage and current signals are obtained and added ; the antilogarithm of the result is proportional to the instantaneous power.

2- Hall effect multiplication:

A semiconductor, carrying the current signal, is placed in a transverse magnetic field proportional to the supply voltage. A voltage, perpendicular to the current and the magnetic field is induced. This voltage is proportional to the instantaneous power.

3- Time division multiplication:

This is a sampling technique that produces a pulse train. Each pulse has its width proportional to one signal (voltage or current) and its height proportional to the other signal. The pulse area is therefore proportional to the power at the sampling point.

4- Digital multiplication:

The voltage and current signals are sample and converted into digital form. A microprocessor is used to perform the multiplication and calculate the power at the sampling point. The integration method used depends on the output of the integration stage. Analogue integration is used when the multiplier has an analogue signal as its output, and digital integration is used when the multiplication is performed by a microprocessor. In this case , the microprocessor calculates the energy from knowing the power at each sample and the time between samples.

Disclosure of invention:

The invention follows a different approach and does not use any of the multiplication methods described above. The basic principle of the technique used will now be explained:

The instantaneous value of the supply voltage is given by the formula:

(1)

where:

'V' is the rms value of the supply voltage, and 'w' is the angular frequency.

The instantaneous load current can also be represented by the formula:

(2)

where:

'I' is the rms value of the load current, and

'θ' is the phase angle between the load current and the supply voltage.

If a signal proportional to the load current is integrated for a half-cycle, the integrator output at the end of the integration half-cycle is given by the formula:

(3)

where:

'K_o' is the ratio between the current signal and the load current,

'K₁' is the integrator time constant,

' α ' is the integration starting angle which is the angle between the integration starting point and the first zero-crossing point of the supply voltage to the positive values. Formula (3) shows that when the integration is performed during the positive half-cycle of the supply voltage (α = 0) or during the negative half-cycle of the supply voltage ( α = π), then 'A' will be proportional to 'I cos θ / K₁ w'.

Formula (3) further shows that if the integration is performed when the supply voltage slope is positive

( α = - π / 2 ) or when the supply voltage slope is negative ( α = + π / 2 ), then 'A' will be proportional to 'I sin θ / K₁ w'.

Since 'A' has been obtained by integrating the load current for a half-cycle, any even harmonics present in the load current will have no effect on the value of 'A'. However, the effect of any odd harmonics component of an order 'm' depends on its phase relation with the main component. This will vary between zero and a maximum value equal to the effect of an equal main component current divided by 'm'. Therefore, these odd harmonics and any D.C. component in the load current should be attenuated to the required level.

The following methods can be used for watt-hour and var-hour measurements:

1- A signal proportional to the load current is integrated as described above. Every integration halfcycle, the integrator output changes by the value 'A' as given by formula (3). After the end of '6' integration half-cycles, the current signal is switched off and a reference D.C. voltage 'V_o' is integrated. The polarity of this D.C. voltage is chosen so that the integrator output moves linearly towards zero voltage. The counting time 'T_c' which is the time needed for the integrator output to reach zero is:

The value of 'V_o' is chosen so that the integrator output should reach zero within a fixed number of halfcycles 'F' during which the load current is not integrated. Therefore a measurement will be obtained every 'G+F' half-cycles. The time equivalent of these 'G+F' half-cycles will be called the measuring time 'T_m' and is given by:

T_m = (G+F) π / w (5)

The pulses produced by a pulse generating means (oscillating at frequency 'f_c' ) are counted during the counting time. If 'f_c' is made proportional to the supply voltage ( f_c = K₂ V), then, the number of these pulses is: n = f_c T_c

From (5) and (6):

n = K T_m V I cos (θ - α ) (7) where:

Formula (7) shows that 'n' is proportional to the active or reactive energy consumed by the load during the measuring time. If the counts 'n' are accumulated for a period of time, the total number 'N' will be proportional to the watt-hour or var-hour (depending on the integration half-cycle starting point as described earlier) consumed by the load during that period. As can be seen from (7), 'N' is independent of the supply frequency and the integrator capacitor.

2- The current signal is integrated as described before, and a positive or negative reference voltage is integrated, whenever needed, in order to keep the integrator output at zero. The values of these reference voltages are chosen to produce equal slopes 'V_o / K₁' with different direction at the integrator output. The pulses produced by a voltage to frequency converter (oscillating at frequency proportional to the supply voltage 'f_c = k₂ V') are used to increment a counting means when the positive reference voltage is integrated and to decrement the counting means when the negative reference voltage is integrated (or vice versa). After 'G' integration half-cycles, the contents of the counting means will be changed by the value 'n' represented by formula (6) above. The time equivalent to 'G' integration half-cycles is:

T = G π / w (8) From (6) and (8): n = N = B T V I cos ( θ - α ) ( 9 ) where :

'B' is a constant equal to

Formula (9) shows that 'N' is proportional to the active or reactive energy consumed by the load during any length of time 'T'.

In practice, the integrator output is kept within two limits. Only when the integrator output moves outside the allowed range, the suitable reference voltage is integrated to bring it back inside the limits. Also, the switching of these reference voltages is synchronized with the pulses 'f_c'. The total error introduced by this arrangement is less than the energy

needed to move the integrator output from one limit to the other. This error becomes negligible within a couple of seconds after the measurement starts. On the other hand, the synchronization of the reference voltages integration with 'f_c ' , allows their integration time to be equivalent to an integer number of pulses. This eliminates any error due to a missing fraction of a pulse during counting and enables the use of lower values of 'f_c' without impairing the measurement accuracy.

Another improvement is to invert the current signal and integrate the non-inverted current signal during the integration half-cycles and the inverted current signal during the other half-cycles. This compensates any D.C. component, offset voltage and temperature drifts in the current input and filter circuits.

Brief description of drawings

Fig. (1) is the expected wave-form shown for the case of the first method with an inductive load and 'α = 0', 'G = 1', and 'F = 1'. In this figure:

(a) is the supply voltage wave-form,

(b) is the load current wave-form,

(c) is the input of the integrator, and

(d) is the output of the integrator. The counting time 'T_c' and 'A' are shown. Fig. (2) is the expected wave-form in the case of the second method with an inductive load and without current inversion. In this figure:

(a) is the supply voltage wave-form,

(b) is the load current wave-form,

(c) is the integrator input,

(d) is the integrator output, and

(e) is the pulses 'f_c'.

Fig. (3) is a block diagram of the watt-hour and var-hour meters. In this figure:

(1) is the Voltage Sensing circuit. It provides the supply voltage signal which is a suitable low voltage A.C. signal proportional to the supply voltage,

(2) is the Half-cycle Finder circuit which determines when the load current signal is to be integrated. In the case of the watt-hour meter, it tests whether the supply voltage is positive or negative; and in the case of the var-hour meter, it tests whether the supply voltage slope is negative or positive. This circuit receives its input from the Voltage Sensing circuit and the information from its output are sent to the Control circuit,

(3) is the Voltage-Controlled Oscillator (V.C.O.) circuit. It generates the measuring pulses which are counted during the counting time as determined by the Control circuit. The frequency of these pulses 'f_c' is proportional to the supply voltage. In the case of the second method, the Control circuit synchronizes the switching of the reference voltages with these pulses, (4) is the Current Sensing circuit. It provides the load current signal which is a low voltage A.C. signal proportional to the load current. In the case of the second method the inverse of this signal may also be required. The phase angle between this signal (or its inverse) and the load voltage signal should be equal to the phase shift between the main component of the load current and the main component of the supply voltage,

(5) is the Reference voltages circuit. It provides the reference voltages that are needed to force the integration circuit output to move in the direction selected by the control circuit,

(6) is the switching circuit. It receives the signals from the Current Sensing and the Reference Voltages circuits, and connect the suitable signals (as required by the control circuit) to the Integration circuit input.

(7) is the Integration circuit which integrates the outputs of the switching circuit,

(8) is the Level Sensing circuit. It informs the

Control circuit with the condition of the integrator output. It senses when the integrator output reaches zero in the case of the first method, and when the integrator output is outside a predetermined range in the case of the second method,

(9) is the Counter and Display which counts the measuring pulses received from the Control Means, divides the count by a suitable scale, and displays the meter readings,

(10) is the Control circuit. It receives the outputs of the Level Sensing, the V.C.O., and the Halfcycles Finder circuits; and controls the following:

a) the function of the switching circuit,

b) the switching of the counting pulses to the Counter and Display,

c) the selection of the counting direction (up/down)

Best mode for carrying out the invention

The best mode for carrying out the invention is to use the second method described above. The supply voltage signal is obtained using a suitable voltage divider and a band-pass filter tuned to the main component frequency. This filter increases the accuracy of detecting the zero-crossing points of the supply voltage.

The supply voltage signal is rectified and filtered, the resulting D.C. voltage is used to drive a voltage to frequency converter whose output will oscillate at a frequency proportional to the supply voltage. The converter pulses from this circuit are used by the control circuit to initiate and stop the integration of the reference voltages, this permit the use of low oscillation frequencies, and one of the readily available voltage to frequency converter can be used. The Control circuit passes these pulses together with the counting direction signal to the counter during the integration of the reference voltages.

The supply voltage signal is also used to determine the integration half-cycles. In the case of the watthour meter, a zero-crossing detector is used to define the positive and negative half-cycles. In the case of var-hour meter, a 90 degree phase shift is introduced by differentiating the supply voltage signal then the zerocrossing points are found to determine when the supply voltage slope is negative or positive. The differentiator output will be clean enough since its input signal is filtered by the band pass filter.

A shunt is used as a current sensor, the load current signal is obtained by amplifying and filtering the voltage drop across this shunt. The filter used is a band pass filter similar to the one used with the supply voltage signal, this arrangement will compensate any phase shift introduced by the first filter. The inverse of the load current signal is also obtained. The load current signal is integrated during the integration half-cycles, and its inverse is integrated during the other half-cycles. The integrator output is monitored and when it drifts outside a first range, a constant reference voltage is applied to the integrator input at the first rising (or falling) edge of a pulse from the voltage to frequency converter. The polarity of this reference voltage is selected to bring the integrator output back to within a second range smaller than the first. After the integrator output enter the second range, the reference voltage is switched off at the first rising (or falling) edge of a pulse at the output of the voltage to frequency converter. During the reference voltage integration, the counter is incremented or decremented (according to the sign of the reference voltage) at every falling (or rising) edge of the voltage to frequency converter output.

The counter content is displayed after a suitable scaling

Industrial applicability

Traditionally, induction type watt-hour meters are used for electricity consumption measurements. However, economics and more flexible tariff requirements increases the demand for electronic meters.

the simplicity of the method allow the production of low cost electronic single phase electricity meters suitable for use for residential electricity consumption measurements.

Three-phase measurement is possible by using tow or three units that share the same reference voltages, counter and display. In this case, proper arrangements are made to prevent integrating a reference voltage in more than one circuit at the same time.

Electronic meters are suitable for use as a basis of flexible load management system and for remote meters reading.

When a high number of meters are needed in one area, space and cost can be saved by sharing the voltage sensing, the half-cycle finder, and the voltage to frequency converter circuits between all the meters connected to the same phase, in addition to sharing the reference voltages circuit between all meters.

Claims

The claims defining the invention are as follows: 1- An electronic watt-hour and var-hour metering apparatus comprising:

First input sensing means for providing a first

A.C. signal. The main component of the said first A.C. signal is proportional to the main component of the first input;

Second input sensing means for providing a second A.C. signal. The main component of the said second A.C. signal is proportional to the main component of the second input. The frequency of the said second input is equal to the frequency of the said first input. The phase shift between the said first A.C. signal and the said second A.C. signal must be equal to the phase shift between the said first input and the said second input.

Integration half-cycles finding means to determine the integration half-cycles in which the said first A.C. signal is to be integrated. In the case of the watt-hour meter, the said integration half-cycles are defined by the positive (or negative) half-cycles of the said second A.C. signal; and in the case of the var-hour meter, the said integration half-cycles are defined by the said second A.C signal half-cycles that have negative (or positive) slope.

Oscillator means to produce the counting pulses. The frequency of the said counting pulses is proportional to the said second A.C. signal.

Reference voltages producing means to produce two reference D.C. voltages with different polarities. Only one reference voltage is needed if the measured energy flows only in one direction.

Switching means to connect and disconnect the said first A.C. signal and the appropriate reference D.C. voltage to the integration means. The operation of this circuit is controlled by the said control means.

Integration means for integrating the signals received from the said switching means.

Level sensing means to monitor the voltage level at the output of the integration means. It informs the control means wether the said voltage level is positive, negative or zero.

Counter and display means for counting the said counting pulses during the integration of one of the said reference voltages, dividing the count by a suitable scale, and storing and displaying the results of measurements. The counting direction is controlled by the control means.

Control means to control the operation of the switching means and the counter and display means. This control means being connected to the output of the said integration half-cycles finding means, the level sensing means, and the oscillator means. It uses the received information to instruct the said switching means to switch on the said first A.C. signals during the said integration half-cycles., and to switch on the appropriate one of the said D.C. reference voltage after the end of a fixed number of integration half-cycles until the output of the said integration means reaches zero. At the same time the said control means passes the said counting pulses to the said counting and display means and controls the counting direction according to the signals received from the said level sensing means.

2-An electronic watt-hour and var-hour metering apparatus comprising:

First input sensing means for providing a first A.C. signal. The main component of the said first A.C. signal is proportional to the main component of the first input. The said first input sensing means can also provide the inverse of the said first A.C. signal,

Integration half-cycles finding means to determine the integration half-cycles. In the case of the watt hour meter, the said integration half-cycles are defined by the positive (or negative) half-cycles of the said second A.C. signal; and in the case of the var-hour meter, the said integration half-cycles are defined by the said second A.C signal half-cycles that have negative (or positive) slope.

Reference voltages producing means to produce two reference D.C. voltages with different polarities.

Switching means to connect and disconnect the said first A.C. signal or its inverse and the appropriate reference D.C. voltage to the integration means. The operation of this circuit is controlled by the control means.

Level sensing means to monitor the voltage level at the output of the said integration means. It informs the control means wether the said voltage level is higher or lower than the limits of a first and a second ranges. The said second range is contained in the said first range.

Control means to control the operation of the switching means and the counter and display means. This control means being connected to the output of the said integration half-cycles finding means, the level sensing means, and the oscillator means. It uses the received information to instruct the said switching means to switch on the said first A.C. signals during the said integration half-cycles, and the inverse of the said first A.C signal (if provided) during the other halfcycles, and to switch on the appropriate one of the said D.C. reference voltage if the said level sensing means indicates that the output of the said integration means is outside the said first range and to switch the said D.C. voltage off if the said level sensing means indicates that the output of the said integration means has returned to the inside the said second range, the on and off switching of the said reference voltage is synchronized with the said counting pulses. At the same time the said control means passes the said counting pulses to the said counting and display means and controls the counting direction according to the signals received from the said level sensing means.