GB2118726A

GB2118726A - Mains power measurement

Info

Publication number: GB2118726A
Application number: GB08210722A
Authority: GB
Inventors: Peter Henry Saul
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1982-04-13
Filing date: 1982-04-13
Publication date: 1983-11-02
Also published as: GB2118726B

Abstract

A sigma-delta modulator is used to measure mains power by controlling the control counter of the sigma-delta modulator in accordance with the voltage of the supply and using a signal proportional to line current as input to the modulator.

Description

SPECIFICATION Electric power measurement The present invention relates to electric power measurement and more particularly to apparatus for the measurement of power supplied to a consumer.

In measuring the power supplied to a consumer by the mains electricity there are a number of problems which must be overcome:- 1 ) The dynamic range of the meter must be greater than 1000:1.

2) The accuracy must be better than 1% even at the extremes of dynamic range.

3) The current and voltage terms must both be measured and accurately multiplied.

4) Power consumption by the metering apparatus must be small.

5) "Standby" power consumption i.e. during power failure conditions must be lower or ideally zero.

It is an object of the present invention to provide an electronic metering apparatus which overcomes the above problems and in particular achieves multiplication of the current and voltage terms to the required accuracy.

Accordingly the present invention provides an electronic metering apparatus for electrical power measurement of a consumer including current sensing means producing a d.c. signal proportional to the consumer current, a first sigma-delta modulator connected to the output of the current sensing means and including voltage sensing means producing a d.c. signal proportional to the consumer voltage, a second sigma-delta modulator connected to the output of the voltage sensing means, a control counter for said first sigma-delta modulator said control counter being controlled from a Read only Memory (ROM) the output of which is controlled by the output of said first sigma-delta modulator.

Embodiments of the present invention will now be described with reference to the accompanying drawing which shows in block diagrammatic form electronic apparatus for measuring electricity consumption according to the present invention.

In the present invention the assumption is made that the current and voltage supplied can be sampled accurately by conventional technique, and converted to D.C. voltage suitable for measurement. The current supplied term can then be measured by a conventional 'voltmeter' type of technique, requiring a resolution better than the product of the dynamic range (1000:1 ) and the accuracy (1%, or 100:1) overall, the resolution required is therefore 100,000:1, or about 1 7 binary bits, but the accuracy need only be 1%.

This level of precision and resolution is relatively easily achieved. The voltage supplied can be similarly measured, and although the accuracy again needs to be better than 1% the dynamic range is smaller, and therefore the resolution need only be about 1 part in 1000.

Multiplication of the voltage and current terms in analogue form is clearly not practicable, as 1% accuracy analogue multiplies are currently close to the state of the art even with very restricted dynamic range. Conventional multiplication in digital form requires a 17 xl 0 bit multiplier, again a very difficult and costly item, or a lengthy and expensive microprocessor controlled software routine.

It is proposed in the present invention that a simple voltmeter circuit of the charge-balancing (or sigma-delta) type can be modified to include multiplication at little extra cost in hardware.

Referring now to the drawing the current in a consumer cable 10 is measured by conventional current sensing means 12 and rectified by a rectifier 14 to produce a d.c. output proportional to the current.

The line voltage is rectified by a rectifier 1 6 to produce a d.c. output proportional to the line voltage. The d.c. voltage Vdc is fed to the input of a sigma-delta modulator and counter 18, the output of which is fed to a Read Only Memory 20.

The output of the Read Only Memory 20 is used to control a control counter 22 which in turn together with the d.c. signal proportional to current controls a second sigma-delta modulator 24, the output 26 of which provides the measurement of power. A clock circuit 28 is provided to synchronise the control counter 22 and sigma-delta modulator and counter 18, and sigma-delta modulator 24.

The operation of the measuring apparatus is as follows:-- Current is measured in the conventional manner by current sensor 1 2 and rectifier 14 but the control counter for the sigmadelta modulator 24 is of variable length set by the output of the ROM. The output of the ROM 20 is obtained by a "look up" table stored in the ROM being addressed by the output of the first sigmadelta counter circuit 1 8 which is in turn determined by the line voltage.

The output data generated by a sigma-delta modulator is a linear function of the input voltage applied to the sigma-delta modulator and also a linear function of the integration time allowed.

The data is therefore easily shown to be a product of the voltage and the integration time. If, as in this case the integration time is proportional to a second voltage then the data output is a product of the two voltages applied to the respective sigma-delta modulators.

The multiply function is therefore performed by the modification of the length of control counter 22 and therefore requires little modification to standard, known circuit elements.

In some cases, it may be desired to insert into the product term the phase angle of the current.

This can be achieved by measuring the time delay between zero crossings of the voltage and current wave forms respectively. At zero phase angle, the delay is zero, while at all angles up to 1 80C the phase can be measured as a digital quantity from the number of clock periods occurring between the zero crossings. This number can be converted to the appropriate control signal in a ROM and used to control the length of the counter 1 8, thus producing a product term in a similar way to that described above. The data output is then the product of the input voltage, current and average phase angle, Realization of the circuit would be on a CMOS or a bipolar integrated circuit, with an advantage to the former in power consumption.Retention of data during power down conditions can be via a battery backup supply provided with the system or by retention of data in a non-volatile memory, either on the name chip as the power measurement scheme, or on a separate chip.

Claims (filed on 8/4/83) 1. An electronic metering apparatus for electrical power measurement of a consumer including current sensing means producing a d.c, signal proportional to the consumer current, a first sigma-delta modulator connected to the output of the current sensing means and including voltage sensing means producing a d.c. signal proportional to the consumer voltage, a second sigma-delta modulator connected to the output of the voltage sensing means, a control counter for said first sigma-delta modulator said control counter being controlled from a Read Only Memory (ROM) the output of which is controlled by the output of said first sigma-delta modulator, 2. An electronic metering apparatus for electrical power measurement substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. producing a product term in a similar way to that described above. The data output is then the product of the input voltage, current and average phase angle, Realization of the circuit would be on a CMOS or a bipolar integrated circuit, with an advantage to the former in power consumption. Retention of data during power down conditions can be via a battery backup supply provided with the system or by retention of data in a non-volatile memory, either on the name chip as the power measurement scheme, or on a separate chip. Claims (filed on 8/4/83)

1. An electronic metering apparatus for electrical power measurement of a consumer including current sensing means producing a d.c, signal proportional to the consumer current, a first sigma-delta modulator connected to the output of the current sensing means and including voltage sensing means producing a d.c. signal proportional to the consumer voltage, a second sigma-delta modulator connected to the output of the voltage sensing means, a control counter for said first sigma-delta modulator said control counter being controlled from a Read Only Memory (ROM) the output of which is controlled by the output of said first sigma-delta modulator,

2. An electronic metering apparatus for electrical power measurement substantially as described with reference to the accompanying drawing.