AP467A

AP467A - Apparatus for metering an electrical supply.

Info

Publication number: AP467A
Application number: APAP/P/1993/000551A
Authority: AP
Inventors: Der Walt Sarel Jacob Van
Original assignee: Nienaber And Van Der Walt Tms Pty Limited
Priority date: 1992-02-21
Filing date: 1993-05-24
Publication date: 1996-03-06
Also published as: ZA933498B; AP9300551A0

Abstract

Apparatus for metering an electrical supply comprises switch means such as a circuit breaker or contanctor which connects the electrical supply selectively to a load. A sensor measures the current or power drawn by the load, and generates a meter drive signal which is applied to a coil which drives a dynamometer-type watt hour meter. A reference circuit is provided which sets a reference current or power value and which generates a reference meter drive signal which is applied to the same coil or to a second coil to drive the meter mechanism in the opposite direction. A counter connected to the mechanism is incremented or decremented according to whether the load signal or the reference signal is greater. When there is no load or only a small load, the reference signal is greater than the load signal and the counter is incremented. When the load signal is greater than the reference signal, the counter is decremented. When the counter reaches zero, the switch means is operated to disconnect the load. The apparatus effectively prevents a consumer from using electricity at too great a rate.

Description

BACKGROUND OF THE INVENTION

This invention relates to apparatus for metering an electrical supply.

The supply of electrical energy to a consumer is normally monitored by means of a watt-hour meter, which records the energy consumption and which is read by a representative of the municipality at intervals. The difference in the watt-hour meter readings is used as the basis for debiting the consumer.

Various electricity supply arrangements have been proposed which attempt to control electricity consumption by forcing consumers to pay in advance for a certain number of units. A credit value corresponding to the number of units purchased is stored, and the electrical supply is interrupted when the credit value is exhausted. A disadvantage of such systems is that they generally cannot control the rate at which the credit value is consumed, with resultant inconvenience to the consumer when essential loads are interrupted.

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SUMMARY OF THE INVENTION

According to the invention an apparatus for metering an electrical supply comprises:

switch means arranged to connect the electrical supply selectively to a load;

sensor means arranged to measure the current or power drawn by the load and to generate a first meter drive signal proportional thereto, the first meter drive signal being applied to a meter mechanism to drive the meter mechanism in a first direction;

reference means for setting a reference current or power value and arranged to generate a second, reference meter drive signal proportional thereto, the reference drive signal being applied to the meter mechanism to drive it in a second direction opposed to the first direction;

counter means arranged to be incremented or decremented by the meter mechanism according to the direction in which the meter mechanism is driven; and control means responsive to the counter means to operate the switch means when the counter means reaches a predetermined value, so that the supply of current to the load is interrupted if the current or power drawn by the load over a predetermined period of time exceeds a reference rate of energy consumption related to the reference current or power value.

The counter means may be a mechanical counter or an electronic

BAD ORIGINAL A counter, for example.

The control means may include detector means arranged to detect a condition of the counter corresponding to the predetermined value thereof.

The control means is preferably adapted to calculate the integral of the difference between the first meter drive signal and the second, reference meter drive signal, and to interrupt the supply of current to the load when the calculated integral reaches a predetermined value.

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The predetermined value may be zero or another value.

Preferably, the meter mechanism is a dynamometer-type mechanism and the first and second meter drive signals are current signals which are applied in opposition to at least one coil to drive the mechanism with a resultant difference signal.

Alternatively, the second meter drive signals are applied to respective first and second coils arranged to drive the mechanism in opposite directions.

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The reference means may be arranged to derive the reference drive signal from an AC mains electrical supply which supplies the load.

In a preferred embodiment the reference means includes a network of switch elements responsive to variations in the voltage of the AC mains electrical supply, and a plurality of correction components connected to respective switch elements, so that the correction components are switched in or out of circuit selectively to maintain the reference drive signal substantially constant against variations in the voltage of the AC mains electrical supply.

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Figure 1 (

Figure 2

The xorrection components may comprise a plurality of resistors in a ladder network arranged to control the magnitude of the reference drive signal.

Alternatively, the correction components may comprise a plurality of tappings on a second coil.

BRIEF DESCRIPTION OF THE DRAWINGS is a block schematic diagram of apparatus for metering an electrical supply according to the invention; and is a schematic circuit diagram of electrical circuitry of the apparatus.

DESCRIPTION OF AN EMBODIMENT

The invention is aimed at controlling the rate of consumption of electrical energy to correspond to a given periodic payment by a consumer. The principle is of particular use in electricity supply arrangements in which a consumer pre-purchases a number of units of electricity and is allocated a credit value, so that the electrical supply to the consumer is maintained until the credit value is exhausted.

One version of the invention is designed to work with a conventional dynamometer type watt-hour meter. In such meters, a metallic disc is placed in an alternating current flux path, which induces an alternating current into the disc. By arranging the geometry of the coils producing the alternating current field in such a way that the flux is induced by current proportional to the voltage in the circuit and a second flux proportional to the current in the circuit is established, a net torque will be created, proportional to the voltage/current product, which will cause the disc to rotate. The disc drives a mechanical or electronic counter

BAD ORIGINAL ft through a reduction mechanism so that the count represents a value in kilowatt hours.

According to the invention, a credit current related to the value of a credit purchased from an electricity supplier is applied to the current sensing coil of the watt-hour meter, in opposition to the load current. The counter is arranged so that in the absence of a load current, it generates a count proportional to the magnitude of the credit current. When a load exists, the meter will count at a rate proportional to the difference between the credit and load currents. If the load current is larger than the credit current, the meter will reduce its count. The credit current is typically obtained from the same source as the load current, and can be introduced into the same current coil as the load current by deriving it from an isolation transformer. Alternatively, the credit current can be introduced into a second current coil adjacent to the load current coil. The second current coil can even be mounted on the same magnetic yoke as the load current coil.

The system is arranged so that when the output of the counter reaches zero, a switch is operated to interrupt the load current. For example, an electrical contactor can be arranged so that its coil is de-energised and its contacts are opened when the counter output reaches zero, or a shunt-trip circuit breaker can be arranged to trip when the counter output reaches zero. If the counter is a mechanical counter, an optical detector, a proximity detector or a magnetic detector can be used to achieve this function. It is also possible to use a direct mechanical coupling between the mechanical counter output and a switch device.

Once the load current is interrupted, the counter will begin counting again, at a rate determined by the magnitude of the credit current alone. Once the counter output reaches a predetermined minimum value, the current to the load is restored automatically if the switch means is a contactor, or can be restored by moving the operating handle of a shuntBAD ORIGINAL trip circuit breaker to its ON position. The effect of this arrangement is that a consumer is prevented from consuming energy at an average rate greater than a predetermined rate corresponding to the value of the credit current. Effectively, the circuit measures the integral of the difference between the power actually drawn by the consumer, and a reference energy consumption rate. This allows the consumer to accumulate credit during off-peak periods (eg. at night), which can be used during peak periods (eg. for cooking). There is no limit on the time period during which such accumulation can occur.

The effect of supply voltage variations is countered by the introduction of a circuit sensing the voltage variation and making small adjustments to the credit current in order to maintain a constant power signal. This can be dispensed with in applications where a small error of say, ± 10% is acceptable.

The value of the credit current is preferably adjustable, either continuously or in a number of steps. The system can also be implemented electromagnetically or electronically, and can be applied to single phase or polyphase systems.

Figure 1 shows the basic components of metering apparatus according to the invention. A dynamometer disc 10 of a kilowatt-hour meter is arranged to rotate under the influence of a voltage coil 12 and a load current coil 14. A credit current coil 16 is arranged adjacent to the current coil 14 and opposes the effect of the load current coil. A credit current control circuit 18 applies a credit current to the credit current coil 16, the credit current having a magnitude corresponding to the value of an energy credit purchased from the electricity supplier. The credit current corresponds to a credit power, the credit power being the rate of energy use corresponding to the total energy credit in a predetermined time period (typically one month). In other words, if the consumer uses energy continuously at the credit power, or energy

BAD ORIGINAL ft consumption rate, for one month, the energy credit will last for exactly one month.

A switch 20 controls the supply of current to the coil 22 of a contactor 24, which in turns controls the electrical supply to the consumer. A sensor (not shown) monitors the output of the counter of the kilowatthour meter, and opens the switch 20 when the count reaches zero. This opens the contacts of the contactor 24, cutting off the electrical supply to the consumer. The credit current control circuit can be switched in and out of circuit by a radio control circuit 26 with a latching switch 28. This feature enhances control of electricity use by the supplier if consumer does not pay.

The credit current control circuit is shown in greater detail in Figure 2. In the credit current control circuit, a transformer 30 and a pair of rectifiers 32 and 34 with associated smoothing capacitors 36 and 38 are arranged to provide a 50,8 volt DC supply proportional to the AC mains supply voltage. A voltage reference circuit comprising a zener diode 40 and a transistor 42 provides an output voltage which amplifies a +. 10 % variation in the AC mains supply voltage to +. 25 V DC. This output voltage is fed into two ladder networks 44 and 46 of zener diodes with associated current limiting resistors and reed relays R1 to R4 and R5 to R8. The ladder networks are arranged so that an increasing number of the reed relays are switched in or out as the output voltage from the voltage reference circuit deviates further from the nominal value. Diodes 48 and 50 feed positive and negative variations to the respective ladder networks.

The relays R1 to R8 are arranged to connect respective resistors in parallel with respective 60 Ω fixed credit rate resistors 52 and 54. In the event of a negative voltage deviation, additional resistors are switched in parallel with the fixed credit rate resistor 54, while further resistors are connected in series with the fixed resistor 52 in the event of positive

BAD ORIGINAL ft variations in the supply voltage. This allows the credit current rate to be maintained so that it represents the credit power within a 2,5% error margin.

It should be understood that the resistor values shown in Figure 2 are chosen for use with a particular kilowatt-hour meter, and a particular credit current. Of course, the resistor values will vary when other types of meters are used, although the principle of operation remains the same.

{ Instead of using the circuit of Figure 2 to connect different combinations of resistors in series and parallel, the circuit can be configured to switch tappings on the credit current coil, to achieve the same result.

Although the apparatus shown in Figure 1 is a single phase device, the invention can readily be applied to a three phase or a polyphase kilowatt-hour meter in an analogous manner. In such a case, a credit current can be introduced into one of the current coils of such a meter, or a credit current coil can be mounted on the same magnetic yoke as one of the current coils of the polyphase meter.

Claims

1. Apparatus for metering an electrical supply comprising:

switch means arranged to connect the electrical supply selectively to a load;

2. Apparatus according to claim 1 wherein the counter means is a mechanical counter.

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3. Apparatus according to claim 1 wherein the counter means is an electronic counter.

4. Apparatus according to any one of claims 1 to 3 wherein the control means includes detector means arranged to detect a condition of the counter corresponding to the predetermined value thereof.

5. Apparatus according to any one of claims 1 to 4 wherein the control means is adapted to calculate the integral of the difference between the first meter drive signal and the second, reference meter drive signal, and to interrupt the supply of current to the load when the calculated integral reaches a predetermined value.

6. Apparatus according to claim 5 wherein the predetermined value is zero.

7. Apparatus according to any one of claims 1 to 6 wherein the meter mechanism is a dynamometer-type mechanism and the first and second meter drive signals are current signals which are applied in opposition to at least one coil to drive the mechanism with a resultant difference signal.

8. Apparatus according to claim 7 wherein the first and second meter drive signals are applied to respective first and second coils arranged to drive the mechanism in opposite directions.

9. Apparatus according to claim 7 or claim 8 wherein the reference means is arranged to derive the reference drive signal from an AC mains electrical supply which supplies the load.

10. Apparatus according to claim 9 wherein the reference means

BAD ORIGINAL A includes a network of switch elements responsive to variations in the voltage of the AC mains electrical supply, and a plurality of correction components connected to respective switch elements, so that the correction components are switched in or out of circuit selectively to maintain the reference drive signal substantially constant against variations in the voltage of the AC mains electrical supply.

11. Apparatus according to claim 10 wherein the correction components comprise a plurality of resistors in a ladder network arranged to control the magnitude of the reference drive signal.

12. Apparatus according to claim 10 wherein the correction components comprise a plurality of tappings on a second coil.

13. Apparatus for metering an electrical supply substantially as herein described with reference to the accompanying drawings.