WO2004111665A1

WO2004111665A1 - An electrical fault monitoring and control unit

Info

Publication number: WO2004111665A1
Application number: PCT/IB2004/001034
Authority: WO
Inventors: Gianfranco Campetti; Erich Jakob Haussermann
Original assignee: Powertech Industries (Proprietary) Limited
Priority date: 2003-06-12
Filing date: 2004-04-05
Publication date: 2004-12-23
Also published as: ZA200509158B

Abstract

A method of and an electrical fault monitoring apparatus for mon itoring an electrical load in an alternating current electrical installation are disclosed. An input terminal is connected to the electrical load to receive current/voltage input signals associated with the electrical load. A monitor in communication with the input terminal, monitors the input signals. An operator pre-defines threshold settings suitable for the particular load. A comparator is responsive to the monitor and compares the monitored input signals with the threshold settings and triggers an event if the monitored input signal violates a threshold setting. An output signal is then produced which activates ancillary equipment controlling the supply of power to the load. Tests are conducted at predetermined intervals and a record log is automatically kept of the tests done.

Description

AN ELECTRICAL FAULT MONITORING AND CONTROL UNIT

THIS INVENTION relates to the monitoring and protection of an electrical load. More particularly, the invention relates to a method of performing scheduled tests on an electrical load, to an electrical fault monitoring apparatus and to an electrical installation.

In an electrical installation which includes an electrical load network made up of component electrical loads (e.g. electric motors, furnaces, or the like) it is important that a fault in one component load should not interrupt or compromise the operation of another component load, or of the network as a whole.

In a network installation, in which electric motors of varying power outputs are used, the motors will have different current requirements during start-up. In the interests of optimal control and continuity of operation, each component load in such a network, is preferably protected by its own fault monitoring apparatus, which has been specially matched to the load requirements (e.g. during start-up) of the component load, or which has been specifically set or adjusted to match such load requirements.

In order to optimize the continuity of operation, and to minimize unscheduled downtime of the installation or network as a whole, or its component loads, it is necessary to provide proper apparatus, and to follow certain procedures. According to the invention, in an alternating current electrical installation in which electrical power is supplied to at least one electrical load, a method of performing scheduled tests on the electrical load includes fitting the electrical load with a fault monitoring apparatus matched to electrical current and voltage requirements of the load; at predetermined intervals performing tests on the fault monitoring apparatus to check if the apparatus is in proper functioning order; generating an automated record of data relating to such tests including the time of when the tests were carried out; and creating a log of the last occasion that a test was carried out.

The method may include the further step of generating a reminder to an operator that the load is due for manual testing an adjustable, predefined period after a previous test.

The invention also provides an electrical fault monitoring apparatus for monitoring an electrical load in an alternating current electrical installation, the apparatus including at least one input terminal having input connections connectable to the electrical load to be monitored, for receiving current/voltage input signals associated with the electrical load, during use; a monitor in communication with the input terminal, for monitoring the current/voltage input signals; operator adjustable threshold definition means in which current/voltage threshold settings for the input signals associated with the load during use, can be defined; access means connected to the operator adjustable threshold definition means for providing access to the threshold settings, by an operator; a comparator responsive to the monitor and operable to compare the monitored input signals with the threshold settings defined in the threshold definition means, and in response, to trigger an event if a monitored input signal violates a defined threshold setting; and output means operatively connected to the comparator, for producing an output signal in response to the triggering of the event by the comparator, the output means having switchable output connections connectable to ancillary equipment.

The apparatus may include an analog input port for receiving analog input signals from the electrical load.

The apparatus may include signal conditioning circuitry arranged as a low pass filter, connected between the input connections and the analog input port, for attenuating high frequency current/voltage input signals received via the input connections.

The monitor may include an amplitude meter.

The monitor may include a processor, operatively connected to the input connections, the processor being operable to execute a set of instructions, and when executing the set of instructions, forming the threshold definition means and the comparator.

The apparatus may include a program memory, in which the set of instructions is stored, the instructions being arranged in tasks, each of which is selectively executable by the processor. The memory may be a non-volatile solid state memory. For example, the memory may be a flash electrically programable read-only memory (Flash EPROM), an electrically erasable programmable read-only memory (EEPROM), a battery backed-up random-access memory (RAM), or the like.

One of the tasks may be arranged as a task scheduler, operable to schedule the execution of selected tasks at predetermined time intervals.

The processor may be operable, under control of the task scheduler, to monitor the input signals at predefined time intervals to determine if the monitored signals are within the defined threshold settings.

The task scheduler may be operable to synchronise the execution of tasks with a phase of alternating current fed to the load in use. For example the tasks may be executed once every 20 milliseconds, i.e. at a frequency of 50Hz.

The output means may include an output port operable to generate a test pulse, the output connections being connectable to a test input winding of a transformer for injecting the test pulse originating from the output port into the transformer, the monitor being arranged via the input port to monitor a current/voltage input signal caused by the test pulse at a secondary winding of the transformer, and in response, to trigger an event if the monitored value does not conform to a predefined signal value.

The processor may have synchronisation means operable to synchronise the generation of the test pulse via the output port, and in response, the monitoring of the test pulse via the input port, with a phase of alternating current fed to the load in use.

The apparatus may include a built-in tester, operable to test the integrity of the processor in operation, the built-in tester having a watchdog reset circuit, operable to reset the processor if the execution of instructions is inadvertently halted.

The processor may include a restart register for detecting the cause of a processor reset.

The processor may include restoring means for restoring the switching state of the output connections after an inadvertent processor reset, to its state prior to the reset.

The apparatus may include authorization verification means to render the access means accessible to an authorized operator only.

The access means may include a keypad connected to the processor for receiving inputs from an operator and in response to direct the operation of the processor. For example, the access means may receive threshold settings via the keypad. The threshold settings may for example, include a maximum peak-to-peak voltage supplied to the load, a maximum peak-to-peak current supplied to the load, an average voltage supplied to the load, an average current supplied to the load, the maximum electrical power supplied to the load and a maximum phase angle between supply voltage and supply current.

The authorization verification means may include password recognition means, in use to read a password keyed in via the keypad and to provide access to the threshold settings only when a valid password is received.

The access means may include a display connected to the processor, for displaying the recorded data to an operator. For example, the display may include a set of light emitting diodes (LED's), a liquid crystal display (LCD), or the like.

The access means may include a data interface connected to the processor, the data interface permitting access to an operator at a remote location. For example, the data interface may be an RS232 or an RS485 serial interface.

The data interface may include a configuration terminal for receiving inputs from an operator and in response to direct the operation of the processor. The configuration terminal may include an infra-red transceiver for receiving inputs from an operator via an optical communication device and for transmitting selected operational data via an optical communication device to an operator.

The apparatus may include adjustable time delay means operable to trigger the event a period of time after a threshold setting has been violated.

The processor may be operable to execute an algorithm to calculate the period of time as a function of the magnitude with which the monitored input signal violates the defined input threshold setting.

The algorithm may be arranged, when executed, to calculate the period of time to be inversely proportional to the magnitude with which the monitored input signal violates the defined input threshold setting. For example, the period of time may be associated with a particular monitored current signal, where a low current signal is associated with a long period of time and a higher current signal is associated with a shorter period of time.

The invention extends also to an electrical fault monitoring apparatus for monitoring an electrical load in an alternating current electrical installation, the apparatus including at least one input terminal having input connections connectable to the electrical load to be monitored, for receiving current/voltage input signals associated with the electrical load, during use; a monitor in communication with the input terminal, for monitoring the current/voltage input signals; threshold definition means in which current/voltage threshold settings for the input signals associated with the load during use, can be defined; a comparator responsive to the monitor and operable to compare the monitored input signals with the threshold settings defined in the threshold definition means, and in response, to trigger an event if a monitored input signal violates a defined threshold setting; output means operatively connected to the comparator, for producing an output signal in response to the triggering of the event by the comparator, the output means having switchable output connections connectable to ancillary equipment; a recorder for recording operational data, including the monitored input signal values, the recorder being operatively connected to the monitor and the comparator; and access means connected to the recorder for providing access to the recorded values, by an operator.

The apparatus may include a manual tester operatively connected to the comparator, the tester permitting an operator manually to trigger an event simulating a threshold violation.

The apparatus may include a real-time clock, operable to record the time at which the apparatus is manually tested as part of the operational data. The apparatus may include reminder generating means, operable to generate a reminder to an operator that the apparatus is due for manual testing, a predefined period after a previous test. The reminder generating means may include adjustment means to permit variation of the predefined period.

The recorder may include a non-volatile solid state data memory for storing the operational data recorded by the recorder. The data memory may be arranged as a first-in-first-out cyclic buffer, for storing the most recent operational data. For example, in the event that a monitored input value violates a threshold setting, a user may be able to retrieve the stored input values via the access facility from the memory, thereby to establish which events preceded the violation of the threshold setting.

The invention also provides an electrical fault monitoring apparatus for monitoring an electrical load in an alternating current electrical installation, the apparatus including at least one input terminal having input connections, connectable to the electrical load to be monitored, for receiving current/voltage input signals associated with the electrical load, during use; a monitor in communication with the input terminal, for monitoring at least one of a peak-to-peak voltage supplied to the load, a peak-to-peak current supplied to the load, an average voltage supplied to load, an average current supplied to the load, electrical power supplied to the load, a phase angle between supply voltage and supply current, a current transformer health signal, a switchgear control voltage signal, a sensitive earth fault signal, a pilot wire signal, a frozen contact signal, and a manual test signal; threshold definition means in which threshold settings for the monitored input signals can be defined; a comparator responsive to the monitor and operable to compare the monitored input signals with the threshold settings defined in the threshold definition means, and in response, to trigger an event if a monitored input signal violates a defined threshold setting; and output means operatively connected to the comparator, for producing an output signal in response to the triggering of the event by the comparator, the output means having switchable output connections connectable to ancillary equipment, such as contactors, relays, or the like.

The monitor may include a digital input port which is electrically isolated from the input connections, the digital input port being capable of measuring discrete inputs from the electrical load.

The output means may include a digital output port, drivingly connected to a switch mechanism such as a relay which may be controllably connected to the electrical load. The relay may for example be a double-pole double-throw (DPDT) relay having at least one set of normally open (N/O) and normally closed (HIC) contacts. The electrical load and an electrical power source may be connected to the contacts of the relay such that power will be supplied to the electrical load when the relay is activated, or such that power will be supplied to the electrical load when the relay is deactivated. The relay therefore permits the load to be connected to the electrical power source via the contacts such that the load will be isolated when the relay is not activated i.e. in a so called "fail- safe" arrangement. Depending on the required configuration, the relay may be selected to have a different number of contact sets i.e. the relay may have two sets of contacts for a single phase configuration.

The output port and the input port may be connectable to an external circuit via the output- and input connectors, the output port being operable to generate an electrical excitation signal, and the input port being operable to measure a return voltage/current, in use, to measure an external resistance in the circuit for detecting an earth-fault lockout, a frozen contact, a damaged pilot wire, or the like.

The invention extends to an alternating current electrical installation comprising a network of electrical loads, which includes at least one fault monitoring apparatus, as described herein, operatively connected to at least one electrical load in the network, the apparatus having its output connections controllably connected to an electrical load via ancillary equipment, for interrupting the electrical power supply to the electrical load when a defined current/voltage threshold setting for that load is violated.

The invention will now be described, by way of example only, with reference to the following diagrammatic drawings.

In the drawings, Figure 1 shows a schematic block diagram of an electrical fault monitoring apparatus, in accordance with the invention; Figure 2 shows a schematic block diagram of another embodiment of an electrical fault monitoring apparatus, in accordance with the invention;

Figure 3 shows a schematic block diagram of a further embodiment of an electrical fault monitoring apparatus in accordance with the invention;

Figure 4 shows a schematic block diagram of yet another embodiment of an electrical fault monitoring apparatus in accordance with the invention; Figure 5 shows sets of response delay times in time delay means, plotted against sets of current signal values; and

Figure 6 shows a schematic block diagram of an alternating current electrical installation in accordance with the invention.

Referring to Figure 1 , reference numeral 10, refers to an electrical fault monitoring apparatus, in accordance with the invention. The apparatus 10 includes an input terminal, having input connections 12 and 14, which are connectable to an electrical load to be monitored (not shown). The input connections 12 and 14 are connected to a processor 18 via signal conditioning circuitry 16.

The processor 18 in this example is an 8-bit CMOS microcontroller e.g. a PIC18F252 from Microchip.

The signal conditioning circuitry 16 includes a dual stage active low-pass filter for each of the input connections 12 and 14. The low-pass filters are tuned not to attenuate voltage signals at an alternating current frequency of 50Hz to 60Hz, and to attenuate voltage signals at higher frequencies, thereby to suppress unwanted spurious high frequency voltage signals. The filtered voltage signals are received by the processor via analog input ports, indicated by reference numeral 18.2.

The apparatus 10 further includes output means in the form of an output driver circuit 20, drivingly connected to an output port of the processor 18. The output driver circuit 20, is connected to a switchable output connection 22, for producing an output signal at the output connection 22 in response to receiving a signal from the processor 18. The output driver circuit 20 is arranged as an operational amplifier relay driver, for activating ancillary equipment, in the form of switchgear, connectable to the apparatus 10 via the output connection 22.

Access means including a liquid crystal display (LCD) 24 is drivingly connected to an output of the processor 18 for displaying data generated by the processor 18, to an operator.

In addition to the output driver circuit 20, the apparatus 10 includes output means in the form of a data interface 26. The data interface 26 transmits data, generated by the processor, to a remote location. The data interface 26 includes an RS485 driver which is optically isolated with high speed transistor-transistor logic (TTL) compatible opto- coupler devices.

The data interface is arranged as a configuration terminal which includes a SCADA interface 26.2, by means of which inputs from a remote location can be received and by means of which the processor can be directed to configure the functional operation of the apparatus 10. The configuration terminal 26.2 receives inputs from a remote location and in response directs the operation of the processor 18.

In addition to the LCD 24, the access means also includes a keypad 28 on which inputs from an operator can be keyed in to direct the operation of the processor 18 and to define threshold settings for various loads. The apparatus 10 includes a tester having a test button 28.2 located on the keypad 28, which can be pressed by an operator manually to test the operation of the apparatus 10, as described below.

A real time clock 30 is connected to the processor 18. The real time clock 30 keeps and updates the time of day and the date. In operation, the processor retrieves the time/date from the real time clock

30. In this example, the processor 18 transmits and stores the time/date as part of the data which is generated by the processor 18.

The access means also includes a set of light emitting diodes (LED's) 32 provided on a front panel (not shown) of the apparatus 10. The LED's 32 indicate the status of the apparatus by being selectively switched on or off by the processor 18.

The LCD 24 provides reminder generating means which can be activated by the processor 18, for reminding an operator that a manual test of the apparatus 10 needs to be performed. The reminder generating means on the LCD 24 is provided in the form of a blinking reminder message which is displayed when the manual test is due. The apparatus 10 further includes a non-volatile solid state memory 34 which is connected to the processor via a serial inter-IC- communication (I2C) data bus. The memory 34 is arranged as a first-in- first-out cyclic buffer which stores most recent data generated by the processor 18, by overwriting the oldest stored data. Typically, the memory stores monitored input voltage signal values received via input connectors 12 and 14.

The apparatus 10 includes an infra-red optical transceiver terminal 36, which is connected to the processor 18 for transmitting and receiving data, generated by the processor 18 to a matched optical device (not shown). This feature is used by an operator to download or upload data from and to the apparatus. The terminal 36 includes a photo- transistor for receiving optical signals and an LED for transmitting optical signals. The photo-transistor and the LED are selected to operate in the infra-red band.

The processor 18 includes a digital input port 18.4 which is electrically isolated from a discrete input terminal 19 by an opto-isolator 38. The discrete input terminal 19 receives discrete input signals, thereby to monitor discrete signals generated by a particular load. In this example, the discrete input signal is generated by an optically isolated control voltage measured across open contacts of the switchgear (not shown in Figure 1).

The processor 18 has an output port 18.6, drivably connected to a driver circuit 40. The driver circuit 40 is arranged as a self- test pulse circuit, which generates an output pulse at terminal 41 in response to a test pulse received from the processor 18 via the output port 18.6. The driver circuit 40 includes an operational amplifier which is drivably connected to a NPN power transistor. The terminal 41 is connectable to a primary test winding of a transformer (not shown), for injecting a test pulse into the test winding.

The apparatus 10 includes an internal test circuit 42 in the form of resistor voltage dividers, which trigger an event in the processor 18 when a measured voltage is not within predefined thresholds. In operation, the internal test circuit 42 is used to trigger the processor 18 to perform certain tasks when an event occurs. Similarly a power-off detection circuit 44 in the form of resistor voltage dividers and an operational amplifier trigger the processor 18 to perform certain tasks when the voltage supply to the apparatus 10 drops below a certain threshold value.

The processor 18 has an internal restart register (not shown) in which the cause of a processor shut down, and subsequent restart, is stored before the voltage drops below a certain threshold level. In the event of a spurious restart, the processor 18 resumes operation as before the restart by switching the output driver circuit 20 to its condition before the restart.

An alarm driver circuit 46 is connected to the processor 18. The processor 18 can be configured to drive an alarm relay 50 via the alarm driver circuit 46 when a predefined event occurs. When an event is triggered in the processor 18, a drive circuit 48 activates a hard wired magnetic indicator thereby to display that a trip has occurred.

In operation, the processor 18 executes instructions and thereby defines a monitor which samples the analog input port 18.2 at a frequency of 2000Hz and the processor 18 determines an amplitude for each input value. The processor thus defines an amplitude meter. The threshold settings, previously entered by an operator, are retrieved from the memory 34 by the processor, the processor thereby forming the threshold definition means.

The amplitude of the signal is digitally compared to the predefined threshold settings by the processor 18, and if any one of the threshold settings is violated, the processor 18 triggers an event, the processor 18 thereby forming a comparator. The processor 18 then switches the output driver circuit 20, thereby switching the switchable output connection 22, which is connected to ancillary equipment, which in turn is connected to the electrical load, for controlling the supply of electrical power to the load.

The processor 18 is programmed with an algorithm that switches the switchable output connections 22 only a predefined time delay after the threshold setting has been violated. The algorithm employs a mathematical formula which defines an inverse time relationship between the magnitude with which the threshold setting is violated and the delay time. In operation, the monitored input signal values are stored in the memory 34, by the processor, the processor 18 in combination with the memory 34, thereby providing a recorder for recording operational data which includes the monitored input signal values.

The processor 18, in combination with the keypad 28, the

LCD 24, the set of LED's 32, the data interface 26 and the optical transceiver terminal 36, provides access means and adjustment means through which the monitored values, the threshold settings, the response delay times and the recorded data are accessible to an operator.

The keypad 28 and the processor 18 further provide authorization verification means permitting an operator to enter a password to verify the operator's authorization to access the access means.

Referring to Figure 2 of the drawings, reference numeral 100 generally refers to another embodiment of the electrical fault monitoring apparatus.

The electrical fault monitoring apparatus 100 includes an input terminal 102, access means 104, output means in the form of an output terminal 106, and a processor 108. The processor 108 includes storage means 108.2, time and date measurement means in the form of a real-time clock 108.4, a microprocessor 108.6, a built-in tester (BIT) 108.8 and a manual tester 108.10. The processor 108 has an input port 102 having an analog input port 102.1 and an analog input/output (I/O) port 102.2. The analog input port 102.1 is connectable to a secondary winding of a voltage transformer (not shown) to sense voltage on a mains supply to an electrical load. The analog I/O port 102.2 includes analog input circuitry 102.21 connectable to a secondary winding of a core-balance current transformer (not shown) to sense when a leakage current in the electrical load exceeds a predetermined threshold. The predetermined threshold is typically operator selectable in the processor 108 to any signal value in the range of 2OmA to 50OmA. By using different current transformers, the range is adjustable up to a signal value of 10A. In addition, the analog I/O port 102.2 has an analog drive circuit 102.22 which is connectable to a test winding of the current transformer. The analog I/O port 102.2 is configured to produce an electrical excitation signal via the analog drive circuit 102.22, and to measure the return voltage and/or -current, for example on the test winding of the current transformer via the analog input circuitry 102.21. In different embodiments of the invention additional analog I/O ports which are connectable to other analog circuits, may be provided as part of the input terminal 102. For example, the analog circuits may include a pilot wire monitor which determines the continuity of an external circuit, an earth fault lockout circuit which monitors an earth leakage current, or the like.

The access facility 104 includes an operator interface 110 and a data interface 112 in the form of an RS232 port with an optical transceiver (not shown). The optical transceiver is optically connectable to a remote computer 120 for remote monitoring of the electrical fault monitoring apparatus 100. The operator interface 110 includes a keypad 110.1 , a display in the form of light emitting diode (LED) indicators 110.3, a liquid crystal display (LCD) 110.2 and optionally a key reader 110.4 (shown in broken lines) such as a so-called Dallas tag interface, and a hidden switch 110.5 (also shown in broken lines).

The output terminal 106 is connected to an output relay 116 which has double pole double throw (DPDT) contacts (not shown) in which one set of contacts is normally open (N/O) and another set of contacts is normally closed (N/C). The contacts of the output relay 116 are connectable intermediate an electrical load (not shown) and an electrical power source (not shown) so that the electrical load and the power source may either be connected via the N/O contacts or the N/C contacts. If the electrical load is connected to the N/O contacts, the electrical load is connected to the electrical power source in a so-called "fail-safe" arrangement such that if the relay 116 fails, the N/O contacts will open, thereby disconnecting the electrical load from the electrical power source. The contacts may also be connected to an alarm unit (not shown).

The storage means 108.2 is an Electrically Erasable/Programmable Read-Only Memory (EEPROM), which is configured to store configuration parameters. In addition, the microprocessor 108.6 includes internal Flash Electrically Programmable

Read- Only Memory (Flash EPROM) (not shown) configured to store and execute instructions in the form of executable code therefrom. The executable code is arranged in functional tasks which are scheduled to execute at least once every 20 milliseconds ie. at a frequency of 50Hz, to be synchronised with the alternating current (AC) phase of the AC electrical power source. It is to be appreciated that this frequency can vary from country to country.

The microprocessor 108.6 is programmed to read input signal values at the input port 102, to monitor output signal values at the output terminal 106, and to store the input signal values and the output signal values in the memory 108.2. The memory is arranged as a cyclic first-in-first-out (FIFO) buffer which stores the most recent input signal values and output signal values and which erases the oldest signal values.

The buiit-in tester 108.8 is arranged to test the internal functionality and operation of the processor 108. The built-in tester 108.8 includes a watchdog timer (not shown) that has to receive a refresh pulse once every 20 milliseconds ie. at a frequency of 50Hz. The refresh pulse is generated by an instruction which is executed by the microprocessor 108.6. If the watchdog timer does not receive the pulse within 20 milliseconds, the watchdog timer will reset the micro processor 108.6.

The processor module 108 also includes a manual tester in the form of a test button on the keypad 110.1. If the test button has not been pressed for a predetermined period of time, the operator interface

110 displays a reminder on the LCD 110.2 or illuminates an LED indicator

110.3 to remind the user to press the test button, the LCD 110.2 and the

LED 110.3 providing reminder generator means. If the test button is pressed, the processor 108 causes the output terminal 106 to switch the relay 116 to open the N/O contacts, thereby interrupting the supply of electrical power to the electrical load. A switched mode power supply 118 is connected to the processor 108, and is connectable to an electrical power source (not shown). The switched mode power supply module 118 regulates the supply of electrical power to the processor 108.

In operation, the processor 108 is programmed with current/voltage input threshold settings suitable for the various loads, via the operator interface 110, which include any combination of a maximum current threshold setting, an average current threshold setting, and a maximum power threshold setting for the electrical load. The threshold settings are thus selected to match the electrical load to be monitored. The maximum power threshold setting is obtained by calculating the coefficient of the voltage- and current values measured, for example at the input ports 102.1 and 102.2 respectively. The threshold settings also define the maximum allowable phase angle between the current and voltage values and a response delay time. The response delay time is a predefined time delay between the time when the parameter threshold settings are violated and the time when the output signal is produced at the output terminal 106. The response delay time is calculated, by the microprocessor 108.6, by means of an algorithm, as a function of the current that is detected at the input port 102.2. In this example, three calculation algorithms, shown in Figure 5, can be pre-selected by an operator. The three algorithms provide for different time/current response delay curves 5.1, 5.2 and 5.3 respectively. The time/current response delay curves follow an inverse exponential relationship between the monitored current values and the response delay time. The response delay time of the electrical fault monitoring apparatus 100 can be programmed to be calculated according to any one of the three response delay time curves shown in Figure 5. Curve 5.1 defines a response delay time for each current value monitored and curve 5.3 defines a longer response delay time for each current value. Curve 5.2 is an example of a user-defined curve, which can be programmed to follow any desired algorithm.

Referring to Figure 3 of the drawings, reference numeral 150 generally indicates another embodiment of an electrical fault monitoring apparatus. The electrical fault monitoring apparatus 150 is similar to the electrical fault monitoring apparatus 100, shown in Figure 2, and the same reference numerals have been used to refer to same or similar parts.

The electrical fault monitoring apparatus 150 includes an input port 102 and access means 104. The access means 104 includes an operator interface 110 and a data interface 112 which are connected to an optical interface 112.1 and to an electrical data interface 112.2. The electrical fault monitoring apparatus 150 further includes an output terminal 106 (shown in broken lines) which is connected to an output relay 116 and a switched mode power supply 118 which is connectable to an electrical power source (not shown).

A keypad 110.1 , which is part of the operator interface 110 has buttons 110.11 to 110.16. The operator interface 110 also includes a liquid crystal display (LCD) 110.2, and status display means 110.3, in the form of three LED's 110.31 , 110.32 and 110.33.

In the embodiment shown in Figure 3, the processor 108 is connected to a core-balance current transformer 152 via an analog I/O port 102.2 to sense a leakage current in an electrical load, and to a voltage transformer 154 via an analog input port 102.1 to sense a high tension voltage on a mains supply to the electrical load. The processor 108 further includes a SCADA interface 156 in the form of a 93.75KB RS485 serial data interface on which a communication protocol can be implemented.

Referring to Figure 4 of the drawings, reference numeral 200 generally indicates another embodiment of an electrical fault monitoring apparatus. The electrical fault monitoring apparatus 200 is similar to the electrical fault monitoring apparatus 150, shown in Figure 3, and the same reference numerals have been used to refer to same or similar parts.

The operator interface 110 is connected to an (optional) remote display unit 128, which may be located remote from the operator interface 110, and which displays similar information as displayed on the LCD 110.2.

The electrical fault monitoring apparatus 200 includes a multifunction module 202 for monitoring various input signals from an electrical load. A processor 108 is connected directly to current transformers 204, 206 and a core-balance current transformer 212 via an analog I/O port 102.4 and the analog input port 102.1 respectively.

The multifunction module 202 is connected to a current transformer 208 to sense a high current supply to an electrical load, to an optional voltage transformer 210 (shown in broken lines) to sense a voltage on a mains supply to an electrical load, to a pilot wire remote module 214 and to three resistor voltage dividers 216, 218, 220, respectively.

The multifunction module 202 includes an earth fault monitoring unit 222 (shown in broken lines), a pilot wire monitoring unit

224 (shown in broken lines) and an earth fault lockout unit 226 (also shown in broken lines), which are connected to the processor module 108 via an inter-IC-communication (I2C) serial bus.

The electrical fault monitoring apparatus 200 of Figure 4 is shown connected in a three phase mains power supply configuration. The current transformer 204 is connected to phase one, the current transformer 206 is connected to phase two and the current transformer 208 is connected to phase three. The optional voltage transformer 210 is not used in the configuration shown in Figure 4.

The pilot wire remote module 214 is connected to a pilot wire which is routed together with the electrical wiring from the output relay 116 to the electrical load (not shown). In operation, the pilot wire is used to sense if the wiring to the electrical load is damaged.

Resistor voltage dividers 216, 218 and 220 are connected to three phases of the electrical load 256, in Figure 6, to sense if any one of the main protection/isolation device's contacts are damaged, or if a low resistance current path exists between one of the contacts and an earth conductor. The switch mode power supply 118 is connected to a single phase of a mains power supply 230 to supply power to the electrical fault monitoring apparatus 200.

One side of the normally open (N/O) set of contacts (not shown) of the output relay 116 is connected to an electrical power supply. The other side of the N/O set of contacts (the equipment-side) is connected to the control circuit (not shown), for switching the mains electrical supply to the electrical load.

The SCADA interface 156 is connected via an RS485 serial bus to a remote computer (not shown).

Before or after installation of the electrical fault monitoring apparatus 200, it can be configured with the desired threshold settings.

The electrical fault monitoring apparatus 200 is configured via an electrical data interface 112.2 or via the optical interface 112.1. The electrical fault monitoring apparatus 200 is programmed with a maximum current threshold setting for each of the three phases that are to be measured by the current transformers 204, 206 and 208 respectively, and a response delay time (see Figure 5) to match the load, is selected. The response delay time is not only selected to match each load that is to be monitored, but also according to the other loads in the rest of the electrical installation (see Figure 6). The pilot wire monitoring unit 224 is configured to generate an alarm signal by flashing the alarm LED 110.32 if the pilot wire remote module 214 detects that the pilot wire has been damaged. The earth fault lockout unit 226 is configured to prevent the output relay 116 to return to its normal state if any one of the resistor voltage dividers 216, 218 or 220 detects an earth leakage or a frozen contact.

Once configured, the electrical fault monitoring apparatus 200 can be operated to activate the output relay 116 thereby to close the

N/O contacts to supply power to the load. The electrical fault monitoring apparatus 200 will monitor voltage/current inputs at discrete time intervals, and when one of the preset thresholds is violated, the processor 108 will deactivate the relay 116 thereby opening the N/O contacts and interrupting the power supply to the load. Once interrupted, the resistor voltage dividers 216, 218 and 220 will monitor the load side of the relay contacts and will illuminate the alarm LED 110.32 if an earth fault lockout is present or if a frozen contact is detected.

The input currents detected by the current transformers 204,

206 and 208 as well as the status of the output relay 116, the pilot wire and the values detected by the resistor voltage dividers 216, 218 and 220 are stored in the memory 108.2 together with a time- and date stamp on a first-in-first-out basis. When a threshold setting is violated the last set of measured values is stored in the memory 108.2. The values can be retrieved via the data interface 112.2, the optical interface 112.1 , the SCADA interface 156 or the LCD display 110.2.

When the power to the load is interrupted, the trip LED 110.31 will be activated to indicate that the electrical fault monitoring apparatus 200 is in "tripped" condition. The electrical fault monitoring apparatus 200 can be configured to generate a reminder by flashing the alarm LED 110.32 if the test button 110.16 has not been pressed for a predefined period of time, for example, a week. If the test button 110.16 is pressed the supply to the electrical load is interrupted by the processor 108 opening the N/O contacts of the output relay 116, and the relay protection/isolation device (not shown) on the equipment side is then tested for an earth leakage or a frozen contact by the earth fault lockout unit 226. The time and date on which the test was performed are stored, and can be accessed, for control purposes. The unit is internally reset after a preset period of time after which the processor 108 activates the output relay 116 to close the N/O contacts, provided that none of the above fault conditions is present. Power may then be restored to the load by activating the main protection/isolation device.

Referring to Figure 6, an alternating current electrical installation 250 is shown. A three phase alternating current electrical power source 252 is connected via ancillary equipment in the form of a three phase contactor 254, to an electrical load 256, only one of which is shown. A fault monitoring apparatus 10 is controllably connected to the contactor via an output connection 258 and is set up so that its current/voltage threshold settings match the load. Similarly, other loads in the installation have fault monitoring apparatuses, matched to their respective loads, connected thereto, to monitor the currents/voltages fed to the loads. In the case of the load 256 shown, electrical currents supplied to the electrical load 256 are monitored by means of current transformers 260, 262 and 264 connected to an input port 266 of the fault monitoring apparatus 10. In the event that an electrical current detected by any one of the current transformers 260, 262 and 264 exceeds a threshold setting, predefined in the apparatus 10, the contactor 254 is switched via the output connection 258 thereby to interrupt the supply of electrical power to the load 256. The current/voltage values present immediately preceding the interruption are logged together with a time stamp to facilitate detection of the fault.

The executable code, stored in FLASH EPROM on the micro processor 108.6 stores computer executable instructions in separate tasks that can be executed sequentially when the apparatus 10, 100, 150 or 200 is in operation.

The tasks include the following: Output terminal switching This task switches the relay 116 in response to inputs from the analog I/O ports.

Operator interface updates

This task reads the keypad 110.1 and displays information on the LCD 110.2. and on the LED's 110.3.

Optical interface updates

The status of the processor 108 including the threshold settings, the recorded input values, the status at the output terminal 106 and the values stored in memory are sent to the optical interface 112.1.

The interface can also receive threshold settings.

Inter-IC-Communication (I2C) An I2C communication protocol is used by the processor 108 to access peripheral components, such as the real-time clock, the watchdog timer, or the like.

- Storage of threshold settings and response delay times

The threshold settings keyed in via the keypad 110.1 , or received via the optical interface 112.1 , are stored in the memory 108.2. Upon startup of the processor 108, the stored settings are read from the memory 108.2.

Logging of input values and output signals

This task stores the input values received from the analog I/O port 102 and stores the status of the output signals sequentially in the memory which is arranged as a cyclic first in first out (FIFO) buffer.

Input means monitoring

The input means is monitored once every 0.5 milli-seconds ie. at a frequency of 2000 Hz. Basic processing is done by the microprocessor 108.6, such as the peak-to-peak measurement of the sampled values, the comparison of the voltage values with the threshold settings, the accumulation of input values for determining average input values, or the synchronisation to a zero voltage crossing, i.e. the AC phase of the AC electrical supply.

- Monitoring of the internal supply voltage

The regulated voltage from the switched mode power supply 118 is monitored, and when it drops below 4.5V, the microprocessor 108.6 is reset.

Ancillary equipment testing

The voltages on each of two activation coils (not shown) of the output relay 116 are monitored to check if it is close to +12V when the output relay 116 is de-energized and if it is close to -12V when the relay is energized.

Current transformer checks

To test the external wiring of the current transformer, 15OmA pulses are applied to a test winding of the current transformer. The output voltage at the secondary winding of the current transformer is measured and if the 15OmA pulse can not be detected at the secondary winding, one of the LED's 110.3 will indicate a fault condition. The pulses are synchronised to the AC phase, and the checks are performed so that the monitoring of input values are not delayed beyond acceptable limits.

Control voltage check

The voltage across each contact pair of the relay 116 is checked and compared with the status at the output terminal 106. If a discrepancy is detected, the alarm LED 110.33 will blink to indicate a fault condition.

Task scheduling Each of the tasks is executed at least once every 20ms. The execution of the tasks is synchronised with the phase of the alternating current electrical supply i.e. 50 Hz. The transformer checks are thus also synchronised with the AC phase. Reading of discrete inputs

Eight optically isolated discrete inputs (not shown) are monitored.

Overcurrent monitoring

A current signal is monitored to detect if it is within a predefined threshold setting.

Pilot wire monitoring The resistivity of a pilot wire, is monitored and if the resistance value exceeds 150 Ohms, a fault is detected.

Earth fault lockout (ELO)

In a three phase electrical installation, when the output relay 116 switches the control circuit (not shown) to an open circuit condition, a leakage current to earth is monitored by each of the three resistor voltage dividers on the control circuit 216, 218 and 220, respectively. If the leakage current exceeds a predetermined threshold, an earth fault lockout condition is detected. The same operation can be performed in a single phase installation.

Frozen contact detection

The voltage is monitored on the contacts of the three phase contactor 254, see Figure 6, when the contacts are in the open circuit condition. If a voltage exceeding 20V is detected, the contacts have been damaged, and a fault is detected.

Manual Test If the test button 110.16 is pressed, an output is produced at the output terminai 106 which simulates a fault condition, and in response, the relay 116 is switched to an open circuit condition.

- Reminding facility

If the test button has not been pressed for a predefined period of time, a reminder segment on the LCD 110.2, or the LED 32.2 is activated.

- Authorization facility

The threshold settings and response delay times can only be updated once a valid password has been provided via the keypad buttons 110.11 - 110.14, thereby preventing an unauthorised access to the threshold settings, the response delay time settings and the recorded data.

In use, a method of performing scheduled tests on electrical loads is provided by fitting each of the loads with an apparatus 10, 100, 150, or 200, by using the manual test facility of the apparatus to trigger the testing of each electrical load, by keeping an automated record of data relating to each such test with the processor/memory combination, and, when needed, by creating a log of the last time a test was carried out by making the test data available to an operator through the access facility.

The inventor believes that the invention as illustrated provides a method and apparatus which ensures that scheduled tests are performed regularly and not left to the discretion of an operator to perform. By reason of the automatic storage of the test data, which is again not left to an operator, the cause of a fault and the evaluation thereof can be readily reconstructed. The electrical fault monitoring apparatus shown is also easy to install and to configure to meet customised requirements of a variety of different loads having varying power requirements.

Claims

1. In an alternating current electrical installation in which electrical power is supplied to at least one electrical load, a method of performing scheduled tests on the electrical load includes fitting the electrical load with a fault monitoring apparatus matched to electrical current and voltage requirements of the load; at predetermined intervals performing tests on the fault monitoring apparatus to check if the apparatus is in proper functioning order; generating an automated record of data relating to such tests including the time of when the tests were carried out; and creating a log of the last occasion that a test was carried out.

2. A method as claimed in claim 1 , which includes the further step of generating a reminder to an operator that the load is due for manual testing an adjustable, predefined period after a previous test.

3. An electrical fault monitoring apparatus for monitoring an electrical load in an alternating current electrical installation, the apparatus including at least one input terminal having input connections connectable to the electrical load to be monitored, for receiving current/voltage input signals associated with the electrical load, during use; a monitor in communication with the input terminal, for monitoring the current/voltage input signals; operator adjustable threshold definition means in which current/voltage threshold settings for the input signals associated with the load during use, can be defined; access means connected to the operator adjustable threshold definition means for providing access to the threshold settings, by an operator; a comparator responsive to the monitor and operable to compare the monitored input signals with the threshold settings defined in the threshold definition means, and in response, to trigger an event if a monitored input signal violates a defined threshold setting; and output means operatively connected to the comparator, for producing an output signal in response to the triggering of the event by the comparator, the output means having switchable output connections connectable to ancillary equipment.

4. An apparatus as claimed in claim 3, in which the input terminal is in the form of an analog input port for receiving analog input signals from the electrical load.

5. An apparatus as claimed in claim 4, which includes signal conditioning circuitry arranged as a low pass filter, connected between the input connections and the analog input port, for attenuating high frequency current/voltage input signals received via the input connections.

6. An apparatus as claimed in claim 4 or claim 5, in which the monitor includes an amplitude meter.

7. An apparatus as claimed in any one of the preceding claims 3 to 6, in which the monitor includes a processor, operatively connected to the input connections, the processor being operable to execute a set of instructions, and when executing the set of instructions, forming the threshold definition means and the comparator.

8. An apparatus as claimed in claim 7, which includes a program memory, in which the set of instructions is stored, the instructions being arranged in tasks, each of which is selectively executable by the processor.

9. An apparatus as claimed in claim 8, in which one of the tasks is arranged as a task scheduler, operable to schedule the execution of selected tasks at predetermined time intervals.

10. An apparatus as claimed in claim 9, in which the processor is operable, under control of the task scheduler, to monitor the input signals at predefined time intervals to determine if the monitored signals are within the defined threshold settings.

11. An apparatus as claimed in claim 10, in which the task scheduler is operable to synchronise the execution of tasks with a phase of alternating current fed to the load in use.

12. An apparatus as claimed in claim 11 , in which the output means includes an output port operable to generate a test pulse, the output connections being connectable to a test input winding of a transformer for injecting the test pulse originating from the output port into the transformer, the monitor being arranged via the input port to monitor a current/voltage input signal caused by the test pulse at a secondary winding of the transformer, and in response, to trigger an event if the monitored value does not conform to a predefined signal value.

13. An apparatus as claimed in claim 12, in which the processor has synchronisation means operable to synchronise the generation of the test pulse via the output port, and in response, the monitoring of the test pulse via the input port, with a phase of alternating current fed to the load

14. An electrical fault monitoring apparatus as claimed in any one of the preceding claims 7 to 13, which includes a built-in tester, operable to test the integrity of the processor in operation, the built-in tester having a watchdog reset circuit, operable to reset the processor if the execution of instructions is inadvertently halted.

15. An electrical fault monitoring apparatus as claimed in claim

14, in which the processor includes a restart register for detecting the cause of a processor reset.

16. An electrical fault monitoring apparatus as claimed in claim

15, in which the processor has restoring means for restoring the switching state of the output connections after an inadvertent processor reset, to its state prior to the reset.

17. An apparatus as claimed in any one of the preceding claims

7 to 16, which includes authorization verification means to render the access means accessible to an authorized operator only.

18. An apparatus as claimed in any one of the preceding claims

7 to 17, which includes adjustable time delay means operable to trigger the event a period of time after a threshold setting has been violated.

19. An apparatus as claimed in claim 18, in which the processor is operable to execute an algorithm to calculate the period of time as a function of the magnitude with which the monitored input signal violates the defined input threshold setting.

20. An apparatus as claimed in claim 19, in which the algorithm is arranged, when executed, to calculate the period of time to be inversely proportional to the magnitude with which the monitored input signal violates the defined input threshold setting.

21. An apparatus as claimed in any one of the preceding claims 3 to 20, which includes a recorder for recording operational data, including values of the monitored input signals, the recorder being operatively connected to the monitor and the comparator.

22. An apparatus as claimed in claim 21 , which includes a manual tester operatively connected to the comparator, the tester permitting an operator manually to trigger an event simulating a threshold violation.

23. An apparatus as claimed in claim 22, which includes a realtime clock, operable to record the time at which the apparatus is manually tested as part of the operational data.

24. An apparatus as claimed in claim 22 or claim 23, which includes reminder generating means, operable to generate a reminder to an operator that the apparatus is due for manual testing, a predefined period after a previous test.

25. An apparatus as claimed in claim 24, in which the reminder generating means includes adjustment means to permit variation of the predefined period.

26. An apparatus as claimed in any one of the preceding claims 21 to 25, in which the recorder includes a non-volatile solid state data memory for storing the operational data recorded by the recorder.

27. An apparatus as claimed in claim 26, in which the data memory is arranged as a first-in-first-out cyclic buffer, for storing the most recent operational data.

28. An apparatus as claimed in any one of the preceding claims 3 to 27, in which the monitor is arranged to monitor at least one of a peak- to-peak voltage supplied to the load, a peak-to-peak current supplied to the load, an average voltage supplied to the load, an average current supplied to the load, electrical power supplied to the load, a phase angle between supply voltage and supply current, a current transformer health signal, a switchgear control voltage signal, a sensitive earth fault signal, a pilot wire signal, a frozen contact signal, and a manual test signal.

29. An apparatus as claimed in claim any one of the preceding claims 3 to 28, in which the monitor includes a digital input port which is electrically isolated from the input connections, the digital input port being capable of measuring discrete inputs from the electrical load.

30. An alternating current electrical installation comprising a network of electrical loads, which includes at least one fault monitoring apparatus as claimed in any one of the preceding claims 3 to 29 operatively connected to at least one electrical load in the network, the apparatus having its output connections controllably connected to the electrical load via ancillary switching equipment, for interrupting the electrical power supply to the electrical load when a defined current/voltage threshold setting for that load is violated.