GB2053595A

GB2053595A - Electronic overcurrent relay

Info

Publication number: GB2053595A
Application number: GB8019442A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1979-06-16
Filing date: 1980-06-13
Publication date: 1981-02-04
Also published as: FR2459567B1; JPS563524A; DE2924419A1; GB2053595B; FR2459567A1

Abstract

The invention relates to a electronic overcurrent relay. Current transformers 2a, 2b and 2c are connected in series with the three phases of a mains supply to a load 1. A further transformer 2d provides an operating voltage for the remaining electronic circuitry. After rectification by rectifiers 3a, 3b and 3c the outputs of the transformers are applied by way of a delay circuit R8, C11, R10 to an input of a pair of threshold circuits in unit 4. The upper and lower thresholds are defined by the voltages at inputs 4c and 4b which are obtained by dividing a constant voltage available at the output 4d. The outputs 4e, 4f and 4g indicate the value of the voltage at the input 4a relative to the inputs 4b and 4c and provide a visual display with the aid of the light emitting diodes LD12, LD13 and LD14. A relay amplifier 15 is driven by one of the outputs to disconnect the load in response to an excessive current. <IMAGE>

Description

SPECIFICATION Electronic overcurrent relay The invention relates to an electronic overcurrent relay and is associated with the field of control technology.

The purpose of known thermal overcurrent relay is to detect increased currents within a network system and to switch off such currents in good time. This can be achieved, for example, by means of bimetal regulators whose temperature rises with an increased current and which switch off the endangered phase of a mains supply by means of a contactor. The operation of such systems is limited because of physical conditions.

More extensive systems, which satisfy building regulation demands, have a very complex mechanical construction.

Overcurrent switching devices of the electronic kind comprise, for example, temperaturedependent resistors which are in thermal contact with a resistor connected in the main current path and perform a switching function when an increased current occurs. Switching systems of this kind are used to protect electric motors against thermal overloading. Means which provide electronic motor protection are disclosed in the German Offenlegungsschrift 1 5 63 572, German Offenlegungsschrift 20 60 483 and Technische Mitteilungen AEG-Telefunken 1969, No. 6, pages 383 to 392. Optimum relationships between accuracy and moderate price can be realised only with difficulty in such known devices.

The invention seeks to provide an electronic overcurrent relay which, although having a greater tripping accuracy, is not more expensive than known solutions to the probJem and has a variable tripping characteristic.

According to the invention, there is provided an electronic overcurrent relay comprising current transformers for sensing the load currents of several phases of a load, means for rectifying the output signals of the current transformer, a window discriminator having an input connected by way of a delay circuit to the output of the means for rectifying, a voltage divider connected to a constant voltage for applying voltages to the window discriminator to define the upper and lower limits of the window, a relay amplifier connected to an output of the window discriminator to disconnect the load in response to an excessive current and display elements connected to the outputs of the discriminator to indicate the prevailing operating state of the discriminator.

It is especially the universal use obtained by the adjustable tripping characteristics which is advantageous in the overcurrent relay according to the invention. Furthermore, the apparatus is of simple construction and can be manufactured at low cost by the use of integrated components.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a circuit diagram of an overcurrent relay of the invention.

The current flow of the three phases R, S, T of a three-phase network, connected to a motor 1, is to be monitored. Current transformers 2a, 2b, 2c, are connected to the three-phase mains and generate voltages in dependence of the current which flows through the phases. These currents are rectified by three rectifiers 3a, 3b, 3c, which are interconnected to produce one voltage.

The primary of a voltage transformer 2d is connected in series with the primary winding of the current transformer 2a, i.e. into the phase T. A bridge rectifier 3d, connected downstream and having filtering and stabilising means, supplies the operating voltage UB for the in-built electronic components.

The combined, filtered and stabilized d.c.

voltage obtained from the three rectifiers 3a, 3b, 3c is connected via a RC current to the input 4a (middle of the window) of a window discriminator.

Window discriminators are commercially available integrated circuits which are used, for example in control technology, for selecting specific values within a specific tolerance width of the required set point (comparator with window edges). A window discriminator analyses the magnitude of the input voltage by reference to two voltages which are externally defined. The window within which the circuit responds can be defined by a top or bottom limit.

A Schmitt trigger characteristic functions at the switching points (window edges). Components of this kind are sold, for example by Messrs Siemens, under the designation TCA 965.

The window discriminator 4 together with a relay stage 1 5 connected downstream thereof forms the electronic overcurrent relay. The window discriminator 4 is connected to the operating voltage UB which is formed by the rectifier bridge 3d. The input voltage at point 4a of the window discriminator represents the load current (nominal current IN) derived from the bridge circuits 3c, 3b, 3d.The window edges are to be formed, for example from 1.05 and 1.1 5 times the value of the nominal current 1N To this end, the bottom window edge (bottom current limit input) 4b is provided with a reference voltage which corresponds to 1.05 times the value of the nominal current and the top window edge (top current limit input) 4c contains a voltage which corresponds to 1.15 times the value of the nominal current 1N The voltages are obtained from the reference voltage source 4d of the window discriminator and are connected via a variable resistor R5 and divider resistors R6, R7 to the inputs 4b and 4c. The ratio at the inputs 4b and 4c is fixed by virtue of the divider resistors R6, R7 and the magnitude of the input voltage is defined by the variable resistor R5.Advantageously, the variable resistor R5 can be provided with an adjusting scale giving a finely graded display.

A signal change takes place at the outputs 4e and 4f of the window discriminator if the bottom current limit input 4b (1.05 times the value of IN) is exceeded, if a variable voltage is connected to the input 4a of the window discriminator. A signal change occurs at outputs 4f and 49 if the top current limit input 4c is exceeded.

The RC network at the input 4a of the window discriminator comprises a variable resistor R8, a resistor R9, a protective resistor R10 and a capacitor C1 1. These components are provided for the time-dependent change of load current (nominal current) I,, formed by the bridge circuits 3c, 3b, 3d. This current, representing an image of the true motor current, can increase by a factor of several times when the motor is in the starting condition (short circuit current of the three-phase motor) and can substantially exceed the preset maximum value of 1.15 x N N. The delay element R8, R9, C1 1 is connected upstream of the input 4a of the window discriminator to prevent the motor being immediately switched off in such cases.The charging time constant can be adjusted by means of the variable resistor R8.

The switched outputs 4e, 4f, 4g of the window discriminator are connected by means of light emitting diodes LD12, RD13, LD14, with series resistors to the operating voltage UD. A visual indication of the prevailing state of the system can thus be obtained in an advantageous manner. If the system operates at its bottom range (1.05 x IN) the light emitting diode LD12 will be connected and the middle region is indicated by the light emitting diode Lid 13 while the top range (which leads to tripping) is indicated by the light emitting diode LD14.

A relay amplifier 15, comprising a series resistor, transistor, relay with parallel diode and operating in accordance with the closed circuit principle, is connected to the output 49. When the top limiting value (1.5 x IN) is reached, such a known closed circuit system drives the relay coil via the transistor so that a switching function is performed (switching off the motor 1).

The switching amplifier 1 5 can of course also be connected to the outputs 4e and 4f in order to obtain different switching criteria. The relay amplifier can also comprise an electrically driven mechanical manual/automatic changeover system.

The window discriminator provides a tripping command if the adjustable limit current is exceeded and the RC network defines the time characteristic of the input voltage at the input 4a of the window discriminator.

In one embodiment of the overcurrent relay according to the invention, a voltage controlled oscillator (VCO) followed by a counter, connected between the input 4a and the output 4g of the window discriminator 4, is used to obtain a longer time constant at the input 4a of the window discriminator 4 or to externally simulate a very wide range. Voltage division at the input can then be obtained by means of a semi-conductor resistor with a time constant and an adjustable resistor.

These means of extending the circuit are not shown in the drawing.

In another embodiment the voltage from the inputs 4a and 4b can be connected to a microcomputer which can be biased with additional data (for example simulation of a bimetal within the network system, simulation of the temperature rise condition of the motor etc.) so that the tripping time for the window discriminator can be calculated by means of time constants which can be fed into the system. This embodiment is also not shown in the drawing.

Claims

1. An electronic overcurrent relay, comprising current transformers for sensing the load currents of several phases of a load, means for rectifying the output signals of the current transformers, a window discriminator having an input connected by way of a delay circuit to the output of the means for rectifying, a voltage divider connected to a constant voltage for applying voltages to the window discriminator to define the upper and lower limits of the window, a relay amplifier connected to an output of the window discriminator to disconnect the load in response to an excessive current and display elements connected to the outputs of the discriminator to indicate the prevailing operating start of the discriminator.

2. An overcurrent relay as claimed in claim 1, in which the current transformers comprise voltage coils which are loaded with a high resistance.

3. An overcurrent relay as claimed in claim 1 or 2 in which an additional transformer is connected to one phase of the load to provide an operating voltage for the overcurrent relay.

4. An overcurrent relay as claimed in any preceding claim, in which the circuit for the input of the window discriminator comprises an RC network incorporating a variable resistor.

5. An overcurrent relay as claimed in any preceding claim in which the voltage divider at the inputs of the window discriminator is variable to permit a desired voltage ratio to be adjusted for the bottom and top current limit input.

6. An overcurrent relay as claimed in any of claims 1 to 4, in which the voltage divider includes a variable resistor and forms a different voltage level with respect to the input connected to the delay circuit.

7. An overcurrent relay as claimed in claim 6, in which the variable voltage divider resistor is constructed as a potentiometer with a setting scale having a finely graded display.

8. An overcurrent relay as claimed in any preceding claim in which the relay amplifier is connected to be driven by all outputs of the window discriminator.

9. An overcurrent relay as claimed in claim 1, in which the delay circuit is a voltage controlled oscillator followed by a counter and switched dividers.

10. An overcurrent relay as claimed in claim 1, in which the delay circuit forms part of a microcomputer.

11. An electronic overcurrent relay constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.