GB2259785A - Power meters - Google Patents

Abstract

In a power meter, voltage and current pulse signals V/Fv and V/Fi (Figure 2) obtained (4, 5 Figure 1) by frequency modulating two carrier frequency signals are used to latch (14, 15) the output of a clock counter (12, 40) to respective subtractors (20, 23); each subtractor subtracts a previous count value from a freshly counted value; the differences, representing the periods, are inverted (21, 24) and from each output is subtracted (22, 25) a reference value corresponding to zero voltage and zero current; and the new differences (38, 39) representing the supply voltage and load current are then multiplied in a four-quadrant multiplier (26). The reference values for the subtractors are obtained by averaging the incoming frequency modulated voltage and current signals over a relatively long period e.g. 40 seconds using a counter (18, 34). a latch (19, 35) and a timer (17, 36). A calibration arrangement is described with reference to Figure 5 (not shown), and the meter may include a card-reading facility as in Figures 6-10 (not shown). <IMAGE>

Description

Title: Improvements in and relating to Power Meters Field of invention This invention concerns metering apparatus for the measurement of electrical power and methods and apparatus for continuously generating and updating reference values within such apparatus corresponding to zero current or zero voltage.

Backqround to the invention It is known to measure voltage and current to ascertain the power consumption from a supply typically an alternating current supply, by generating signals corresponding to the voltage and current, producing a product of the two signals and integrating the result over a period of time.

n recent years developnent work has been concer.tratec on mergers incorporating solid state devices for perfo rining some of the computations. Potentially such devices allow more accurate measurement than older electromechanical designs of meter and will also enable remote reading to be incorporated more readily into such meters.

It is an object of the present invention to provide a method and apparatus whereby one or more zero current or zero voltage reference values, required by such a meter, can be generated.

Summary of the invention According to one aspect of the present invention in power measuring apparatus adapted to produce an output signal proportional to the product of an alternating supply voltage and alternating current flowing through a load connected thereto, in which a first signal is derived proportional to the said supply voltage and a second signal is derived proportional to the said load current so that a product signal can be produced by a microprocessor corresponding to the power, and wherein third and fourth signals are produced by frequency modulating two constant frequency carrier signals using respectively the said first and second signals, and the said third and fourth signals are supplied to the microprocessor for multiplication to produce the power product output signal, at least one of the said third and fourth signals is averaged by signal processing means, to provide at least one reference value for the apparatus, corresponding to zero voltage or zero current or both.

Since the voltage and current values are to be multIplIed, and the product will be zero if either is zero, it 5 only necessary to ensure that one of the computed values reliably reverts to zero when no power is being drawn.

Since the current waveform can become very unsymmetrical in the presence of certain loads (such as 1/2 wave rectifying citcuits) preferably the averaging is applied only to the said third signal (ie the voltage related signal).

The advantage of the invention is greater accuracy and stability with time and changes in temperature.

Conveniently the average value is obtained by accumulatIng the pulses of the voltage related or current related frequency varying signal over a relatively long period of time and using the accumulated value to control a reference value corresponding to the mean voltage (or current) frequency value. In the case of a sinusoidal waveform, the frequency variations from the mean frequency value caused by each positive going excursion of the sinusoid should be matched by the frequency variations from the mean frequency value caused by each negative going excursion, so that after N periods of the modulating sinusoid (during each of which the frequency of the pulses describing the sinusoid varies from F to (F+f), back to F, down to (F-f) and back to F), the total number of pulses accumulated will be NF.Therefore by repetitively accumulating this value over similar numbers of periods, the value of N can be considered to be a constant and the accumulated value will in each case be proportional to F, which is the frequency corresponding to zero voltage (or current).

Typically, where the frequency of the alternation vo~-aJe or current signal is 50Hz, the value of F is 2KHZ.

Preferably the value of N is of the order of 2000, although lower and higher values of N may be employed depending on the level of accuracy required of the "zero" current/"zero" voltage frequency value.

Should the modulating signal (voltage or current) waveform be non-sinusoidal due for example to relative load or supply characteristics, the value will not be NF bu. will be t4(F+f), where f is the effective + dc element introduced into the "zero" frequency value due to the nonlinearity of the sinusoidal waveform.

In accordance with another aspect of the invention metering apparatus as aforesaid may comprise: (a) additional pulse counter means adapted to receive and accumulate the voltage-related frequency signal pulses during measuring sequences, (b) means for resetting the additional pulse counter to a preset value, typically to zero at the beginning of each measuring sequence, (c) timer means for determining the duration of each measuring sequence (thereby to determine the number of periods N of the alternating voltage waveform during which the pulses are to be accumulated), and (d) latch means for latching the accumulated pulse count from the said additional pulse counter at the end of each measuring sequence.

In accordance with another aspect of the invention, metering apparatus as aforesaid may comprise: (a) further pulse counter means adapted to receive and accumulate current-related frequency signal pulses, during measuring sequences, (b) means for resetting the said further pulse counter to a preset value, typically zero at the beginning of a each measuring sequence, (c) further timer means for determining the duration of each measuring sequence (thereby to determine the number of periods N of the alternating current waveform during which the pulses are to be accumulated), and (d) further latch means for latching the accumulated pulse count from the said further pulse counter at the end of each measuring sequence.

According to a preferred feature of the invention the said additional pulse counter may be adapted to operate continually whilst the meter is operating, so as to provide a continually updated frequency value corresponding to the mean supply voltage.

According to another preferred feature of the invention the said further counter may be adapted to be operated only once, during or after manufacture, at a time when the meter is set to measure a supply voltage and current being delivered therefrom to a load, which is selected so that the current waveform is essentially sinusoidal, and the latched current value at the end of the one-cff measurin sequence is stored permanently in the meter or use as the reference frequency value corresponding to zero current.

Mitering apparatus according to the inventior. ray include both said additional and said further counters and associated timers and latches.

Where the same measuring period is to be applied to both voltage and current related frequency signals, a common tier may be employed. A typical timing period is t0 seconds.

The latched value from the said additional counter and/or the said further counter, may be converted to a value equal to the number of pulses occurring with a standard period (eg one second) or the period of the alternating supply voltage waveform (which in the case of a centrally generated electricity supply will be constant and will typically be of the order of 1/50th of a second).

Since the signals are in binary form they can be transmitted via electrical isolating devices to enable a user accessible port to be associated with the apparatus.

Typically opto-isolators are employed.

In an electrical power measuring apparatus as aforesaid the said third and fourth signals may be transmitted to the microprocessor via electrical isolating devices such as opto-isolators. However it is to be understood that such isolation of the microprocessor from the third and fourth signals is optional and not essential in all cases.

The first and second signals are preferably in the for of analogue voltages and may be derived using potential dividers and shunts in known manner. The signals may be obtained without electrical isolation from the supply line but if desired, isolation may be achieved using one or more transformers.

Where one or both of the first and second signals is or are an electrical current this is preferably converted into a voltage in any convenient manner so that both first and second signals are in the form of electrical voltages.

The signal to frequency conversion is most simply effected using voltage controlled pulse generators in which the instantaneous frequency of the pulses from each pulse generator is controlled by the instantaneous value of one of the first and second signal voltages respectively.

Processing the third and fourth frequency varying signals to obtain a power signal may comprise the following steps: 1. generating a constant frequency clock signal having a frequency many times that of the mean frequency of the said third and fourth signals, 2. continually entering said clock pulses into two counters, 3. capturing the value of one counter at the beginning of each of the pulses making up the said third signal and likewise capturing the value of the other counter at the beginning of each of the pulses making up the saId fourth signal, 4. subtracting the previously captured value from the currently captured value derived from each counter to form third and fourth difference signals, 5. forming the arithmetical reciprocal of each cf tne said third and fourth difference signals, 6. subtracting from the reciprocals of each of said third d fourth difference signals frequency values equal to the mean frequency of each of the said third and fourth signals respectively to form fifth and sixth signals, the mean frequency of each of the said third and fourth signals having been generated by the said additional, and the said further pulse counting means and latching means and further latching means, and 7. supplying the fifth and sixth signals to a quadrature multiplying device and supplying the output thereof as the power signal.

If the signal to frequency conversion of the voltage and current signals operates without error then the frequency value to be subtracted from the third and fourth difference signals is a fixed frequency which equates to the value of the frequency corresponding to zero voltage and zero current where the supply is an alternating supply having a generally sinusoidal waveform.

The invention also lies in a method of generating reference frequency values corresponding to zero voltage and zero current for use in a power measuring method in which there is to be produced a product of signals proportional to the voltage of an electrical supply and the current flowing through a load connected thereto, wherein the power measuring method includes the steps of deriving a first signal proportional to the said supply voltage, deriving a second signal proportional to the said load current, frequency modulating two constant frequency carrier signals by the said first and second signals to produce third and fourth signals respectively, normalising the third and fourth signals with reference to reference frequency values corresponding to zero voltage and zero current, and multiplying the two normalised third and fourth signals to form a product signal which is proportional to the power being absorbed by the load connected to the electrical supply, in which the said third and fourth signals are each averaged and the average value of each is employed on the reference frequency value zero voltage and zero current reference signals, as aforesaid.

The invention also lies in apparatus for processing the aforementioned third and fourth signals comprising: 1. a clock pulse generator for producing a clock pulse signal having a frequency many times that of the mean frequency of the third and fourth signals, 2. two counters to which the clock pulses are supplied continually, 3. circuit means for capturing the value in each of the counters, 4. circuit means responsive to the said third and fourth signals for controlling the capture means to capture the counter values at the beginning of each pulse which appears in the said third and fourth signals respectively, 5. means for storing each captured value, 6. means for subtracting the stored captured value from the currently captured value before the currently captured value is inserted into the storage means to form third and fourth difference signals, 7. circuit means adapted to form the arithmetical reciprocal of each of the said third and fourth difference signals, 8. means for deriving the mean frequency of the said third and fourth signals, 9. circuit means for subtracting the said mean values from the reciprocals of the two difference signals to form fifth and sixth signals, and 10. quadrature multiplying means receptive of the said fifth and sixth signals to provide an output signal corresponding to the power being absorbed by the load connected to the said electrical supply.

Preferably a microprocessor is employed for effecting the multiplication of the fifth and sixth signals and the processor is programmed to compute therefrom not only the apparent power but also the apparent instantaneous power, the real energy and power and also the reactive energy and power supplied to the load.

Power measuring apparatus embodying the invention is equally applicable to single or multiple phase electrical supply metering. For single phase metering it is only necessary to measure the current in one line whereas for multiphase supplies, it is of course necessary to measure at least the current in each of the phases and if appropriate the voltage relating to each phase to produce three power signals which must be summed in order to produce a total power signal for the multiple phase supply.

Any of the apparatus as aforesaid may be combined with a coin freed mechanism or card reader to provide for prepayment of electricity.

Likewise any of the apparatus as aforesaid may be combined with means for remote reading and remote control of the meter by any telemetering method or by power line signal modulation techniques and the meter accordingly may include receiver and/or transmitter means for decoding data transmitted to the meter and appropriately modulating a carrier for transmission of data from the registers contained in the meter relating to example to the accumulated power measured by the meter.

Any of the apparatus as aforesaid may include switch means for interrupting the supply of electric current from the meter to the load. The switch means may be remotely controllable as by telemetry (for example power line signal transmission) . Where receiver means is included for said remote control the receiver may be adapted to generate a control signal for operating the switch means on or off in response to the receipt and decoding of an appropriate command signal. Provision may be made for local operation of the switch means.

Any of the apparatus as aforesaid may include display means such as an LCD display or the like for displaying in alpha numeric characters the accumulated power measured by the meter.

The voltage to frequency conversion may be achieved by controlling the frequency of a free running oscillator set to produce pulses of a frequency F (where F is significantly greater than the frequency of the oscillating current supply) so that as the instantaneous voltage of the supply increases from zero up to a positive peak then decreases through zero to a negative peak and back again to zero during a cycle, so the pulse production rate increases from the rate at zero voltage up to the positive maximum and then decreases through zero pulse rate and continues to decrease until the negative maximum is reached and thereafter increases once again to the zero voltage pulse rate at the end of each cycle.

By reversing the polarity of the supply voltage (or proportion there of which is fed to the pulse repetition rate controlling terminal of the oscillator), so the frequency can be varied in an opposite sense during each cycle, firstly decreasing (as the supply voltage increases) and then increasing (as the supply voltage collapses and reverses polarity) and lastly decreasing again as the supply voltage once again increases towards zero to complete the cycle.

An integrated circuit timing device whose relaxation period is controlled at least in part by a potential difference between two of its pins, may be employed.

Such a device comprises a type 555 timer.

The mean pulse repetition rate (ie when zero voltage appears between the said two pins), of such a device, is determined by an external RC circuit and typically a mean repetition rate of 2000 Hz is employed if the supply frequency is 50Hz.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified block circuit diagram of a power measuring meter, Figure 2 is a block circuit diagram of the elements which make up the processor employed in Figure 1, and show the additional counters and latches required by the invention, Figure 3 comprises a block circuit diagram of the voltage and current to frequency converters of Figure 1, Figure 4 illustrates a simplified form of meter based on Figure 1 in which no opto-isolation of the processor is required from the voltage to frequency and current to frequency circuits, Figure 5 illustrates how two or more meters can be connected to a so-called substandard meter for simultaneous calibration, Figure 6 illustrates a card controlled meter in its protective housing, Figure 7 is a perspective view of the interior of the meter with the front housing cover and card reader removed, Figure 8 is a perspective view of the card reader from one side, Figure 9 is a similar view of the card reader from the other side, and Figure 10 is a perspective view of the rear of the meter housing with the pcb and card reader removed, to reveal the contactor and current shunt.

In order to better understand the generation of the reference values corresponding to zero voltage and/or zero current, the construction and operation of a meter to which the invention may be applied will first be described with reference to the drawings.

General construction and operation Referring to Figure 1, the instantaneous potential of the mains supply is measured between the live line 3 and the neutral line 11 of a domestic supply. The voltage is converted to a pulse train by the voltage to frequency converter 5 which has a free running frequency of approximately 2,000 Hz which is modulated up or down in frequency depending on the polarity and amplitude of the potential excursion. Typically the base frequency is modulated by the supply voltage waveform signal to produce a frequency modulated signal with a deviation of plus or minus 200 Hz. The deviation is proportional to the instantaneous voltage of the waveform. The pulse train produced by the frequency to voltage converter 5 is coupled to input 1 of a processor 8 via an opto-isolator 7.

The instantaneous current flowing is measured by measuring the potential generated across a shunt resistor 2 connected in series with a live wire 3 of the supply to the consumer. This voltage is converted to a pulse train by a voltage to frequency converter 4 which also has a free running oscillator operating at a frequency of approximately 2,000 Hz. This base frequency is modulated by the supply current waveform signal to produce a frequency modulated signal with a deviation of plus or minus 200 Hz full scale. The deviation is proportional to the instantaneous current flowing to the consumer and the pulse train is coupled via an opto-isolator 6 to a second input of the processor 8.

The processor 8 provides signals for driving a display 106 (typically liquid crystal display) to indicate under normal operation, the accumulated numerical value of units of power measured by the meter, (as will be described in more detail with reference to Figure 2).

Likewise a contactor 108 may also be controlled by an output signal from the processor.

The contactor is preferably a pulse operated device held into its last switched state by a spring or permanent magnet or both.

The contactor is only required if ON/OFF control of the supply to the consumer/load is required as in the case of a coin or a card controlled meter or a meter which is te be remotely controlled by the supply authority as by power line modulation.

Where provided, a power line signalling receiver or transmitter/receiver 112 provides control signals for the processor or receives data from the processor for transmission to the supply authority - as for example to indicate a fault condition in the meter or relay to the authority the accumulated value of measured power.

The card reader lle likewise is controllable by signals from the processor and in turn produces electrical signals serving as signal inputs to the processor. Thus the processor may produce a signal to enable the card reader, cancel data on a card after it has been read, may receive data read from an inserted prepayment card for validating the card and can indicate to the processor the number of units to be permitted before requiring a further card to be inserted.

DC power for the voltage/current to frequency converter circuits 4 and 5 and the driving elements of the optoisolators is derived from a halfwave rectifying circuit made up of diode 116 and reservoir/smoothing capacitor 118. Typically the DC voltage requirement is of the order of a few volts for example 5-15 volts, and since the ac supply voltage will normally be 240 volts RMS or the like, a tapping 120 on the primary winding 122 of a transformer 124 can provide the lower ac voltage needed as input to the rectifying citcuit 116/118.

Where isolation of the processor is required (as is essential if user accessible ports are provided on the meter), the dc power for the processor (and display, car reader, powerline communication receiver/transmitter, contactor etc when fitted) is derived from a second rectifying circuit supplied from the secondary winding 126 of the transformer 124 and comprising diode 128 and smoothing/reservoir capacitor 130.

The processor may be an integrated circuit capable of performing all the functions in Figure 2, or an array of devices which collectively can perform the said functions.

In order to obtain the values of voltage and current, the processor has to measure the time between pulses. In the example shown this is achieved by the same method for both current and voltage, and will be described with reference to Figure 2.

For simplicity the processor will be assumed to be a single integrated circuit ie a customised microprocessor chip.

Also for simplicity the controlling (central processing) element of the customised device of Figure 2 is not shown, nor are the signal paths between it and the individual processing elements shown in Figure 2.

A 16 bit counter 40 is continuously clocked by a crystal controlled oscillator 12 running at 5MHz.

The output of the counter is connected in parallel to the inputs of two 16 bit latches 14 and 15. Latch 14 relates to the voltage and latch 15 to current.

The clock inputs of the respective latches are fed with the pulse trains of lines 9 and 10 respectively.

On the positive edge of a pulse on line 10, the value of the counter 40 is stored in the latch 14. The edge also provides an interrupt via the interrupt request register 13. The interrupt causes the value stored in the latch 14 to be read. To obtain a number indicative of the period between pulses, the value of the previous count is subtracted from the newly counted value in a subtractor 20. To produce a value proportional to frequency, the period is inverted by inverter stage 21 to produce the arithmetical reciprocal.

Reference value generation in accordance with the invention Simultaneously with the above counting and latching sequence, another counter 18 and latch 19 average the incoming voltage related frequency over a 40 second period determined by a timer 17. This average voltage related frequency is then subtracted by 22 from the instantaneous voltage related frequency to give numerical output value at 38 proportional to the instantaneous voltage. This feature allows the the voltage "base" frequency to be continuously autozeroed to cater for long term drift.

A second channel accepts the signal representative of current along line 9 and produces a value 39 proportional to the instantaneous current in exactly the same way as the signal 38 is produced corresponding to voltage.

The current value channel has a similar arrangement to the voltage channel for producing an average value of the current related frequency, but typically this is only performed once during calibration, during manufacture.

The items concerned are labelled 34, 35 and 36. The average or base current related frequency value is subtracted from the values of instantaneous current frequency which arise during use, giving a frequency value proportional to the instantaneous current, for supply as signal 39.

Power computation The two signals 38 and 39 are the inputs to a four quadrant multiplier 26. As these two signals are asynchronous, the multiplication is caused to occur at regularly occuring intervals of time displaced by, for example, 500 microseconds. A suitable timing or interrupt circuit 27 produces the necessary control signals for the multiplier. The latter uses the input signal present at 38 and 39 at each instant in time and each result is then passed to an accumulator 28 which keeps a total of the power consumed. The running total in the accumulator is compared in the comparator 29 with a number, from register 30 which is equivalent to a 1/1000th of a kilowatt hour.

When this value is reached or exceeded a current pulse is generated causing the kilowatt hour register 33 to be incremented by one and if desired a front panel light emitting diode (LED) 37 can be triggered to flash. The value in 30 is also subtracted from the register (accumulator) 28 in response to the generation of the count pulse.

It is to be noted that if the value in the register 28 is greater than the value from 30 the excess will remain in the register 28 and will count towards the new accumulating value in the register. This significantly improves the accurancy of the measuring technique since in this way no part of any power signal computed by multiplier 26 will he lost and over a long period of time the shortfall in the kilowatt hour register 33 would be quite considerable if the overflow amounts left in the accumulator register 28 were (in fact) to be disregarded.

The number used to indicate a 1/1000th of a kilowatt hour is at least initially adjustable to enable calibration of the meter against a standard. This provides a method for calibration of the meter during manufacture and if required following any subsequent refurbishment.

The processor 8 may include memory means (not shown) in which one or more programmes or instructions can be stored for recall in response to appropriate interrupts and/or input signals, to cause the processor to perform the functions described in relation to Figure 2.

Figure 3 illustrates a preferred circuit for providing two frequency modulated signals corresponding to the instantaneous values of supply voltage and load current of Figure 1.

Certain of the components and connections are common to elements in Figure 1 and to this end the same reference numerals have been employed.

The shunt should develop the smallest possible potential difference vI. To this end a differential amplifier 86 is employed to generate a larger signal V1 for supply to pin 5 of a type 555 timer 88.

Mean frequency controlling circuit elements 90 and 92 provide potentials for pins 2, 6 and 7 and a charging/discharging capacitor 94 is connected between pin 2 (which is also connected to pin 6) and the live line.

Pin 3 provides the FI output signal which may be supplied directly to the processor 8 (as shown in Figure 4) or via an opto-isolator 6 as shown in Figure 1.

A second 555 timer 96 forms the basis of the supply voltage to frequency converter 5. A small fraction of the supply voltage (between the LIVE line 3 and the NEUTRAL line 11) is produced by a potential divider made up of resistors 84 and 98. The desired small fraction appears across the resistor 98. This potential difference appears between pins 1 and 3 of 96. As before the mean frequency of operation of 96 is controlled by RC elements 100, 102 and 104 and typically the resistors 100 or 102 or both are made adjustable so that the frequency of 5 can be made the same as that of 4. (Alternatively or additionally the resistor 90 or 92 or both may be made adjustable).

As before the IF signal is derived from pin 3 of the 555 device and is supplied either directly (as shown in Figure 4) or via an opto-isolator 7 as shown in Figure 1, to the second input of the processor 8.

Figure 4 merely shows diagrammatically how in a simple meter (in which there is no user accessible port such as coin freed mechanism or card reader) but merely a display with or without a contactor for local or remote ON/OFF control of the supply, the need for opto-isolators is removed. To this end the processor 8 is at LIVE rail polarity as is consequently the display 106 and actuator coil of the contactor 108 if provided. Where the latter is provided the LOAD terminal is connected to the terminal A of Figure 4 via the contacts 110. The latter is to advantage a spring and/or permanent magnet assisted contactor requiring positive and negative pulses only for operation to open and close the contacts 110.

Where the contactor is not required, the load is connected direct to terminal A.

Calibration is usually performed by comparing the power measured by a meter under test with the power measured by a "standard meter" set to measure the same voltage and current parameters over the same period of time. Although a so-called standard quality meter is ideally used, in practice meters which are not quite up to standards quality may be used as the reference and such meters are commonly referred to as sub-standard meters.

Such a meter is that produced by Landis and Gyr under the code TVE 102/1. These meters deliver an electrical pulse for each 1/500,000th of a kilowatt hour measured by the meter. Each such pulse is called a unit power pulse.

As described with reference to Figure 2 the numerical value with which the accumulated value being registered at 28 is to be compared (to determine when a 1/1000th of a kilowatt hour has been registered by the meter) can be adjusted for calibration purposes. This numerical value is held in the register 30.

Since the pulses to be accumulated by the meter under test should correspond to 1/1000th kilowatt hour, interface 74 is provided which includes divider devices (not shown), typically CMOS type CD 4510B connected to provide a 500:1 ratio, so that one pulse is delivered by the interface for every 500 pulses received from the "standard" meter 50.

The permanent value for register 30 is arrived at by feeding via optical port 32 pulses from the interface 74, to a counter 41. The value in counter 41 is initially set to zero by a reset pulse on line 44. This reset pulse may for example be the first to arrive of a sequence of pulses from the sub-standard meter or a specially generated reset pulse. The incrementing value in register accumulator 28 is also reset to zero by the same reset pulse on line 44.

If (as is arranged) both meters are set to measure the same voltage and current, pulses arriving from the sub standard meter via inter face 74 and port 32 increment the counter 41, and in a similar manner the numerical value in the register accumulator 28 is incremented by the action of the power measuring circuits of the meter under test as described with reference to Figure 2.

Counter 41 is set to generate an output pulse when N pulses have been received from the interface 74 this trigger is supplied to the divider 42 to divide the numerical value which has been accumulated in the accumulator register 28 by the value N to produce a numerical value for latching into the register 30.

The value of N may be 256 to simplify the division step and ensure a relatively long period in which the substandard meter output is compared with the meter or meters under test. To this end the arrival of the 257th pulse can be used to serve as the trigger to generate the divider instruction pulse along line 45.

It is to be understood however that the numerical value N is quite arbitrary and any value can be chosen which is convenient and sufficiently large enough to ensure that enough unit power pulses have been received to ensure an accurate value after division by N for insertion in the register 30.

After the comparator register 30 is latched it is preferably WzRITE-inhibited in any known manner to prevent unauthorised recalibration of the meter.

As shown in Figure 5 a sub-standard meter 50 may be connected between the L and N terminals of a supply 52 and to receive current from the terminals 68, 70 of the secondary of a current transfer 54.

One terminal 70 of the current transformer secondary is connected to the L terminal of the sub-standard meter 50 and in order to ensure that the same current passes through the current measuring circuits meters of all the LOAD terminal 56 if the sub-standard meter is connected to the LIVE terminal 60 of the first meter under test 58 and the LOAD terminal 62 of that meter is connected to the LIVE terminal 64 of the next meter under test 66 and so on, until the last meter in the chain where the LOAD terminal is connected to the terminal 68 of the load.

In Figure 5 only two meters are shown under test and it is therefore the LOAD terminal 72 of the second meter 66 which is connected to the load terminal 68.

In order to convey unit power pulses from the sub-standard meter 50 to a number of meters under test 58 and 66 etc, the interface unit 74 pulses serve to drive a chain of LEDs 76, 78 etc and cause the latter to flash in synchronism therewith. By positioning each LED 76, 78 etc opposite the opto communication ports 80, 82 respectively of the meters under test, so the pulses derived from the unit power pulses from the sub-standard meter 50 can be used to calibrate all of the meters in the chain.

The assembled meter of Figures 1 and 2 is shown in Figure 6 within a two part housing comprising a base unit 132 and front cover 134. The latter is adapted to be panel or wall mounted and the front cover includes a panel containing a viewing window 136 through which a display 106 can be seen. The slot of a card reader 106 is shown at 138 and finger operable control buttons 140 and 142 allow the meter to be programmed after appropriate instructions have been entered via an opto communications port 144 containing an LED 146 and light sensitive transistor 148.

The LED 37 which flashes when unit power pulses are generated is also visible through the window 150.

Removing the front cover and card reader allows the inside of the meter to be seen as shown in Figure 7. Here the display 106 is shown mounted on a small pcb 156 carrying also the receiver and transmitter units 146, 148 of the opto communications port 144, switches 152 and 154 operable by the press pads 140 and 142 in Figure 6, and the LED 37 (of Figure 2). The small pcb 156 is mounted by standoffs 158 and 160 from the main pcb 162 on which is mounted a central processor chip 164 and related power supplies and buffer circuit elements, the opto isolators, the 555 timer devices 88, 96, the differential amplifier 86 and related decoupling and signal coupling paths and devices. A slot 166 is provided into which the inboard end of a card reader can be fitted and located.

Cable connections are provided at 168, 170, 172 and 174 for connecting the Live, Neutral In, Neutral Out and Load (ie for example the live busbar of a domestic supply).

Figures 8 and 9 show the card reader as comprising a shallow box-like member 176 defining a slot 138 at one end and having on one face a DC motor 178 adapted to draw in and eject cards via a claw drive 180 and on the other face an erasing device comprising a pivotted arm 182 bearing a permanent magnet 184 which during reverse movement of the card (not shown) is moved under the action of the motor drive into contact with the magnetic stripe on the card to erase magnetic data stored thereon.

Reading and writing control circuits for receiving signals from and supplying signals to a read/write head (not shown) also mounted on one face of the box 176, are also carried by the latter.

Beneath the main pcb 162 as shown in Figure 10, is located a contactor 186 which for convenience supports the shunt 2 (of Figure 1) itself connected between one terminal 167 of the contactor and one of the cable connectors 168 etc of Figure 7.

Claims (18)

Claims:

1. Power measuring apparatus adapted to produce an output signal proportional to the product of an alternating supply voltage and alternating current flowing through a load connected thereto, in which a first signal is derived proportional to the said supply voltage and a second signal is derived proportional to the said load current so that a product signal can be produced by a microprocessor corresponding to the power, and wherein third and fourth signals are produced by frequency modulating two constant frequency carrier signals using respectively the said first and second signals, and the said third and fourth signals are supplied to the microprocessor for multiplication to produce the power product output signal, characterised by signal processing means by which at least one of the said third and fourth signals is averaged to provide at least one reference value for the apparatus.

2. Power measuring apparatus as claimed in claim 1 wherein the signal processing means applies the averaging to the voltage related varying frequency signal (the third signal).

3. Power measuring apparatus as claimed in claim 2 wherein the signal processing means is controlled to accumlate the pulses of the voltage related frequency varying signal over a relatively long period of time.

4. Power measuring apparatus as claimed in claim 3 wherein the reference value corresponds to the mean frequency of the voltage related frequency varying signal.

5. Power measuring apparatus as claimed in claim 4 wherein the averaging is performed repetitively over similar periods.

6. Power measuring apparatus as claimed in claim 5 wherein the mean frequency of the voltage related frequency varying signal is 2KHz.

7. Power measuring apparatus as claimed in claim 6 wherein the period over which the averaging is performed is of the order of 2000 periods of the alternating supply voltage.

8. Power measuring apparatus as claimed in claim 1 comprising (a) additional pulse counter means adapted to receive and accumulate the voltage-related frequency signal pulses during measuring sequences, (b) means for resetting the additional pulse counter to a preset value, typically zero at the beginning of each measuring sequence, (c) timer means for determining the duration of each measuring sequence (thereby to determine the number of periods N of the alternating voltage waveform during which the pulses are to be accumulated), and (d) latch means for latching the accumulated pulse count from the said additional pulse counter at the end of each measuring sequence.

9. Power measuring apparatus as claimed in claim 8 comprising (a) further pulse counter means adapted to receive and accumulate current-related frequency signal pulses, during measuring sequences, (b) means for resetting the said further pulse counter to a preset value, typically zero, at the beginning of a each measuring sequence, (c) further timer means for determining the duration of each measuring sequence (thereby to determine the number of periods N of the alternating current waveform during which the pulses are to be accumulated), and (d) further latch means for latching the accumulated pulse count from the said further pulse counter at the end of each measuring sequence.

10. Power measuring apparatus as claimed in claim 8 or 9 wherein the said additional pulse counter is adapted to operate continually while the meter is operating, so as to provide a continually updated frequency value corresponding to the mean supply voltage.

11. Power measuring apparatus as claimed in claim 9 or 10 which the said further counter is employed to generate a reference frequency value corresponding to zero current at a time when the meter is measuring power in conditions in which both the voltage and current waveforms are essentially sinusoidal and memory means is provided in the apparatus within which the latched current value so obtained is permanently stored for use as the reference frequency value corresponding to zero currnt.

12. Power measuring apparatus as claimed in any of claims 8 to 11 wherein the latched value from the said additional counter and/or the said further counter is converted to a value equal to the number of pulses occurring within a standard period (eg one second) or the period of the alternating supply voltage waveform if constant.

13. Power measuring apparatus as claimed in any of claims 9 to 12 wherein the first and second signals are analogue waveform signals and are derived using potential dividers and shunts.

14. Power measuring apparatus as claimed in claim 13 wherein signal to frequency conversion is effected using voltage controlled pulse generators in which the instantaneous frequency of the pulses from each pulse generator is controlled by the instantaneous value of one of the first and second signal voltages respectively.

15. Power measuring apparatus as claimed in claim 13 or 14 wherein processing the third and fourth frequency varying signals to obtain a power signal comprises the steps of:a) generating a constant frequency clock signal having a frequency many times that of the mean frequency of the said third and fourth signals, b) continually entering said clock pulses into two counters, c) capturing the value of one counter at the beginning of each of the pulses making up the said third signal and likewise capturing the value of the other counter at the beginning of each of the pulses making up the said fourth signal, d) subtracting the previous captured value from the currently captured value derived from each counter to form third and fourth difference signals, e) forming the arithmetical reciprocal of each of the said third and fourth difference signals, f) subtracting from the reciprocals of each of said third and fourth difference signals frequency values equal to the mean frequency of each of the said third and fourth signals respectively to form fifth and sixth signals, the mean frequency of each of the said third and fourth signals having been generated by the said additional and said further pulse counting means and latching means and further latching means, and g) supplying the fifth and sixth signals to a quadrature multiplying device and supplying the output thereof as the power signal.

16. A method of generating reference frequency values corresponding to zero voltage and zero current for use in a power measuring method in which there is to be produced a product of signals proportional to the voltage of an electrical supply and the current flowing through a load connected thereto, wherein the power measuring method includes the steps of deriving a first signal proportional to the said supply voltage, deriving a second signal proportional to the said load current, frequency modulating two constant frequency carrier signals by the said first and second signals to produce third and fourth signals respectively, normalising the third and fourth signals with reference to reference frequency values corresponding to zero voltage and zero current, and multiplying the two normalised third and fourth signals to form a product signal which is proportional to the power being absorbed by the load connected to the electrical supply, characterised in that the said third and fourth signals are each averaged and the average value of each is employed as the zero voltage and zero current reference signals as aforesaid.

17. Power measuring apparatus as claimed-in claim 1 constructed substantially as herein described and with reference to the accompanying drawings.

18. A method as claimed in claim 16 substantially as herein described with reference to and as illustrated in the accompanying drawings.