WO1994007149A1

WO1994007149A1 - Capacitive voltage dividing circuit for power measurement

Info

Publication number: WO1994007149A1
Application number: PCT/AU1993/000471
Authority: WO
Inventors: Gregory James Nunn; David Russell Murray
Original assignee: Nu-Lec Pty. Ltd.
Priority date: 1992-09-14
Filing date: 1993-09-14
Publication date: 1994-03-31
Also published as: CN1087998A

Abstract

A circuit is disclosed for determining power supplied to a load via circuit breaker. The circuit includes a bushing with a screen (31) associated with a conductor (30) forming a coupling capacitor, which along with a capacitor (32) forms part of a capacitive voltage divider network, a current transformer (35) and a processor circuit (33) receiving a divided down voltage signal (V) and a current signal (I) from the transformer (35) for providing a real time indication of the current, voltage and power supplied to the load.

Description

TITLE

CAPACITIVE VOLTAGE DIVIDING CIRCUIT FOR POWER MEASUREMENT

THIS INVENTION relates to a circuit for determining characteristics of electrical supply to a load by a line. In particular, the invention concerns a circuit for detecting and measuring the potential of the line so that once the current flowing in the line is determined it is then possible to secure an indication of real time power supplied to the load and real time voltage and current signals can be processed for purposes other than securing a measure of power such as protection, measurement or diagnostics in real time.

BACKGROUND OF THE INVENTION The invention will be described by way of example with reference to three phase circuit breakers and in particular with reference to circuit breakers. It should be appreciated that this is by way of example only and that the circuit of the invention may also be used to advantage in other applications and with other circuit breakers or switches.

Some circuit breakers may typically be arranged in a housing which may be pole mounted. The housing is filled with sulphur hexaflouride (SF₆) gas which has excellent electrical insulating properties. A pole mounted communications cubicle, normally mounted low on the pole for ease of access, houses an operator control panel and micro-electronics. This cubicle provides the protection functions and controls and monitors the circuit breaker. The communications cubicle is connected to the circuit breaker by an unpluggable umbilical cable. The circuit breaker and the communications cubicle together comprise a remotely controlled and monitored pole mounted circuit breaker (or recloser). The circuit breaker is closed by a controlled pulse of current from a storage capacitor in the communications cubicle through a close solenoid. This attracts an operating plate which in turn closes contacts in vacuum interrupters via push rods extending between the plate and the vacuum interrupters. The contacts are held in the closed position by a latch resting on a trip bar. Opening of the contacts is achieved by releasing a controlled pulse of current from a capacitor through a trip coil. The trip coil attracts a trip armature bar which turns the trip bar and releases the latch. An opening spring and contact pressure springs accelerate the opening of the contacts in the vacuum interrupter.

Bushings which extend through and into the housing insulate the line conductors and the circuit conductors from the housing and enable the line conductors to enter the housing and allow the circuit conductors to be coupled to one end of the vacuum interrupters. A current transformer is arranged to extend around each of the circuit conductors in the vicinity of the bushing. This transformer enables the circuit current of the circuit conductor to be detected. A respective current transformer is associated with each circuit conductor.

It has been the practice in the past to include a voltage transformer for each phase to enable the phase voltage to be determined and to allow power supplied to the load by the circuit conductors to be measured. Even for low voltages the necessary voltage transformers are expensive. This is particularly so in circuit breakers for medium to high voltage applications.

OBJECT OF THE INVENTION It is an object of the present invention to at least minimise the disadvantages of known circuit breakers or switches by providing an alternative way of detecting and measuring phase voltage.

SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a circuit for determining electrical power supplied to a load via a circuit breaker or switch controlling at least one phase, the circuit breaker or switch having at least one line conductor coupled to it and the line conductor extends through an insulator bushing, at least one load conductor may be coupled to the circuit breaker or switch and the load conductor extending through an insulator bushing, at least one said bushing in a circuit established for the load conductor having a conductive screen through which the conductor extends and being separated from the conductor by the material from which the bushing is made, the screen and conductor forming a coupling capacitor, a respective second capacitor connected in series with each said coupling capacitor and said capacitors forming a capacitive voltage divider network providing a signal representative of line voltage, a current transformer associate with the phase for providing a current signal and a processor circuit for receiving an output from the current transformer and the voltage signal for providing a real time indication of the current, voltage and power supplied to the load.

DESCRIPTION OF PREFERRED EMBODIMENTS It should be appreciated that the circuit breaker or switch may be a single phase or multi-phase (two or more) breaker or switch. Where more than one phase is involved the arrangement described is duplicated for each phase with the voltage and current signals secured for each phase being coupled to a common processor circuit.

The description which follows is particularly applicable for multi-phase breakers or switches but the invention should not be taken as being so limited and may be employed for single phase breakers or switches also.

It should be appreciated that the current transformers may be associated either with the line conductors or with the load conductors extending from the circuit breakers. Preferably the current transformers are associated with the load conductors. The bushings with the screens which form the coupling capacitor of the two series connected capacitors of the divider network may be either the bushings associated with the line conductors or those bushings associated with the load conductors. Preferably the screens within the bushings for the load conductors are the ones employed for providing the coupling capacitors.

The coupling capacitors may be of any suitable capacitance value. Preferably the coupling capacitors are about 30pF.

The processor circuit includes an analog to digital convertor (A/D convertor) for digitising both the divided voltage signals received from the series connected coupling capacitor and the current signals. The A/D convertor may sample the voltage and current signals at any convenient sample rate. Preferably sampling occurs at about twenty times the supply frequency.

The processor circuit also multiplies the current and voltage signals to obtain a signal indicative of power consumed by a load coupled to each of the circuit conductors or for enabling the voltage and current signals to be processed in real time for other purposes. The processor circuit may include a memory for storing information representative of power consumed by each of the loads and of the voltage and current signals and have a display for displaying information.

DESCRIPTION OF DRAWINGS A particular preferred embodiment of the invention will now be described by way of example with reference to the drawings in which:

Figure 1 is a cross sectional view of a circuit breaker; and

Figure 2 is a schematic diagram of a circuit according to an embodiment of the invention.

Figure 1 shows a pole mounted circuit breaker having a housing 1. Plate 2 has mounted to it a closing solenoid 3 and contact push rods 4. Rods 4 extend into vacuum interrupters 5 having contacts 21. Latch 18 extends from plate 2 and engages a trip bar 17. Trip armature 16 is moved by trip coil 15. Opening spring 19 tends to bias the contacts 21 to the open position. Line conductors and load conductors 11 extend into the housing 1 and pass through insulator bushings. A current transformer 7 is associated with each load conductor and is located adjacent the bushings 9. In this Figure a voltage transformer 8 is shown. This transformer is eliminated by the circuit of the invention. In the Figure flexible connections 6 extend between the line conductors and the vacuum interrupters. The rods 4 are mounted on a contact pressure spring 20. Lightning arrestors 14 are mounted to the housing 1.

A switch cable entry module 12 is mounted within the housing 1. A cover 13 extends over the entry point for cable 22.

Bushing 9 associated with the load conductor has an insulator screen imbedded within it and an electrical connection exists between that screen and the remainder of the circuit of the invention as best shown in Figure 2.

Figure 2 shows a conductor 30 passing through a conducting screen 31 embedded within one of the bushings

9 shown in Figure 1. That screen together with the conductor 30 form a capacitor with the material of the bushing being the dielectric for the capacitor. That capacitor is in series with capacitor 32 forming a divider network. A line voltage representative signal V is then made available to processor circuit 33. The contacts of the breaker are shown at 34. Current transformer 35 provides a current signal I for the circuit 33. Conductor 36 extends from the breaker. Conductors 30 and 36 can be either the line and load conductors. Obviously only the connections for one phase have been shown. Like connections for the other phases are employed.

The processor circuit 33 includes an A/D converter, a microprocessor, memory and a display or some other means for accessing information secured by the microprocessor. The current and voltage signals are sampled, digitised and multiplied to secure an indication of real time power consumed by a lead coupled to the circuit breaker.

Instead of processing the current and voltage signals for securing a real time measure of power these signals can be processed for other purposes such as for protection, measurements or diagnostics in real time.

By employing the screen 31 with the insulator bushes as one capacitor in a capacitive voltage dividing network it is possible with the circuit of the invention to determine real time power consumption without employing an expensive voltage transformer to secure a voltage signal for the calculation of power.

Claims

CLAIMS:

1. A circuit for determining electrical power supplied to a load via a circuit breaker or switch controlling at least one phase, the circuit breaker or switch having at least one line conductor coupled to it and the line conductor extends through an insulator bushing, at least one load conductor may be coupled to the circuit breaker or switch and the load conductor extending through an insulator bushing, at least one said bushing in a circuit established for the load conductor having a conductive screen through which the conductor extends and being separated from the conductor by the material from which the bushing is made, the screen and conductor forming a coupling capacitor, a respective second capacitor connected in series with each said coupling capacitor and said capacitors forming a capacitive voltage divider network providing a signal representative of line voltage, a current transformer associate with the phase for providing a current signal and a processor circuit for receiving an output from the current transformer and the voltage signal for providing a real time indication of the current, voltage and power supplied to the load.

2. The circuit of Claim 1 where the circuit breaker or switch controls two phases and has three line conductors coupled to it and extending through respective said insulator bushings, two load conductors coupled to the circuit breaker or switch and the load conductors extending respective said insulator bushings, at least one of the bushings in a circuit established for each said load conductor having a conductive screen through which a respective said conductor extends and being separated from the conductor by the material from which the bushing is made, each said screen and the respective said conductor forming a coupling capacitor, a respective second capacitor connected in series with each said coupling capacitor and forming capacitive voltage divider networks each providing a signal representative of the voltage, a respective said current transformer associated with each said phase for providing respective said current signals and the processor circuit for receiving an output from each said current transformer and the voltage signal from each said divider network for providing a real time indication of the current, voltage and power supplied to the load.

3. The circuit of Claim 1 wherein the bushing with the screen is associated with the load conductor.

4. The circuit of Claim 2 wherein the bushings with the screens are associated with the load conductors.

5. The circuit of Claim 1 wherein the coupling capacitor has a capacitance of about 30pF.

6. The circuit of Claim 2 wherein each of the coupling capacitors has a capacitance of about 30pF.

7. The circuit of Claim 1 or 2 wherein the processor circuit includes an analog to digital convertor, a multiplier circuit and a memory.