GB2181561A - Short circuit current protection - Google Patents

Short circuit current protection Download PDF

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Publication number
GB2181561A
GB2181561A GB08624352A GB8624352A GB2181561A GB 2181561 A GB2181561 A GB 2181561A GB 08624352 A GB08624352 A GB 08624352A GB 8624352 A GB8624352 A GB 8624352A GB 2181561 A GB2181561 A GB 2181561A
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GB
United Kingdom
Prior art keywords
voltage
point
predetermined
half cycle
value
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB08624352A
Other versions
GB8624352D0 (en
Inventor
Christopher Robert Melbourne
Christos Charalambou Marouchos
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Balfour Beatty PLC
Original Assignee
BICC PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by BICC PLC filed Critical BICC PLC
Publication of GB8624352D0 publication Critical patent/GB8624352D0/en
Publication of GB2181561A publication Critical patent/GB2181561A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/08—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to excess current
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/52—Testing for short-circuits, leakage current or ground faults
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H1/00—Details of emergency protective circuit arrangements
    • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Current Or Voltage (AREA)

Abstract

Apparatus for determining the prospective short circuit current between any two points in an electrical installation (not shown) comprises a micro-processor (11) for comparing the voltage at the peak value (14) of a half cycle of the operating voltage before and after the switching of a thyristor (3), the switching causing a test current to flow between the two points by way of two terminals (1,2) and an impedance (20). The voltage is pre-sampled at a predetermined pre-sampling point (16) during the half-cycle and a variable gain amplifier (5) amplifies the voltage signal with again depending on the value of the voltage at the pre-sampling point (16). The micro-processor (11) estimates, from the value of the voltage at the pre-sampling point (16), the likely value of the voltage at the peak value (14), and adjusts the gain of the amplifier (9) to make the change in voltage at the peak value (14) greater than a pre-set threshold value. <IMAGE>

Description

SPECIFICATION Short circuit current protection This invention relates to the protection against short circuit current between conductors of an electrical installation.
National and International standards, in particular the 15th Edition of the I.E.E. Wiring Regulations, require protective devices be provided in an electrical installation to break any short circuit current in conductors of each circuit ofthe installation before such current can cause danger due to thermal and mechanical effects produced in conductors and connections.
To meet this requirement it is increasingly being specified that the prospective short circuit current at every relevant position of the complete electrical installation shall be ascertained.
In the applicant's UK Patent No. 2097942B and published application No. 21 19935A there is described apparatus for determining the prospective short circuit current at any position in an electrical installation. The object of the present invention isto provide an improved method and apparatus which allows accurate determination ofthe prospective shortcircuitcurrentwhilst maintaining a relatively simple and efficient circuit design.
Accordingly a method of determining the prospective short circuit current at any point in an electrical installation comprises the steps of measuring and storing the voltage at a first predetermined point of a half cycle of the operating voltage between conductors at any point in the electrical installation; causing a test current to flow between said conductors for a short period, said test current being of sufficient magnitude to effect a reduction in the voltage at said first predetermined point in a subsequent half cycle; pre-sampling the voltage at a predetermined pre-sampling point, the said pre-sampling point being before the first predetermined point in the subsequent half cycle; passing the voltage signal through a variable gain amplifier, the gain ofthe amplifier being automatically adjusted in response to the value ofthe voltage at the pre-sampling point; and measuring andstoringthevalueofthevoltageatthefirst predetermined point in the subsequent half cycle.
As described in the applicant's aforementioned UK Patent, an indication of the prospective short circuit current can be obtained by comparing the voltage before and after the flow of a test current. However, the voltage drop due to the flow of the current is relatively small, for example a drop of'0.001'volts in a 2 volt signal. This corresponds to a voltage drop of 0.15volts in a 300 volt supply voltage. By pre-sampling the voltage signal and adjusting the gain ofthe amplifier accordingly, a more accurate determination of this minute voltage drop can be performed.
The method preferably includes the steps of estimating, from the value ofthe voltage at the pre-sampling point, the likelyvalue of the voltage at the first predetermined point in the subsequent half cycle and adjusting the gain of the amplifier such that the difference between the amplified voltage at the said first predetermined point and its value atthe said point in the previous half cycle is greaterthan a predetermined threshold value. Conveniently the predetermined threshold value is at least 10% of the value ofthe voltage of the first predetermined point, and preferably at least 40% thereof.
The voltage between the conductors is conveniently measured by generating a reference voltage and comparing the voltage signal to be measured with said reference voltage. The reference voltage may then be subsequently varied such that it is approximately equal to the voltage signal atthe first predetermined point.
Alternatively the voltage between the conductors is measured by generating first and second reference voltages, the voltage between the conductors being subtracted from the first reference voltage to form an inverted signal, which inverted signal is then compared with the second reference voltage. In this instance the first reference voltage may be varied such that the voltage atthefirst predetermined point is approximately equal to the second reference voltage.
The first predetermined point, i.e. the point in the half cycle at which voltage measurements are made, is conveniently the peakvalue of the half cycle.
However, other points in the half cycle may conceivably be taken as the first predetermined point for voltage measurement.
The method conveniently includes the additional step of comparing the stored values of the voltage at the first predetermined point fortwo or more half cycles and initiating flow ofthe test current between the conductors only if the compared values ofthe two or more half cycles are within a predetermined error differential one from another. This therefore constitutes a check that the operating voltage, i.e.
mains supply, is relatively stable before the prospective short circuit currenttest is performed. In the eventthatthe supply is temporarily unstable,the compared values will be spaced sufficiently to lie outside the error differential such that the initiation of the test current will be inhibited. Thus the chances that any voltage drop will be due to random fluctuations in the operating voltage ratherthan due to the flow ofthetest current are significantly reduced.
Similarly, the method conveniently includes the steps of measuring and storing values for the period of each half cycle, comparing the stored values ofthe periods oftwo or more half cycles, and initiating flow of the test current between the conductors only if the compared values ofthe two or more half cycles are within a predetermined error differential onefrom another. Thus where the difference in the periods of half cycles is used to calculate the power factor as in the applicants application 211 9935A, the chances of erroneous readings being made due to fluctuations in the operating voltage, are reduced.
The invention further resides in apparatus for carrying outthe method of determiningthe prospective short circuit current as previously described. In particular apparatus for determining the prospective short circuit current at any point in an electrical installation comprises means for measuring and storing the voltage at a first predetermined point of a half cycle of the operating voltage between conductors at any point in the electrical installation; switching means for connection and disconnection of an impedance between said conductors to cause a currenttoflow therebetweenfora short period, the impedance being such that a currentflows of sufficient magnitude to effect a reduction in the voltage atthe said first predetermined point in a subsequent half cycle; pre-sampling means for measuring the voltage at a predetermined pre-sampling point, the said pre-sampling point being before the first predetermined point in the subsequent half cycle; a variable gain amplifier, adapted to amplify the voltage signal with a gain depending on the value of the voltage at the pre-sampling point; and means for measuring the amplified voltage at the first predetermined point in the subsequent half cycle.
The apparatus preferably includes a micro-processor adapted to estimate, from the voltage measurement ofthe pre-sampling means, the likely value ofthe voltage atthefirst predetermined point in the subsequent half cycle and supply control signalsto the amplifierto adjustthe gain thereof such that the difference between the amplified voltage at the said first predetermined point and its value atthe said point in the previous half cycle is greater than a predetermined threshold value.
The micro-processor conveniently includes a read only memory and a random access memory, the read only memory having stored therein predetermined factors relating to the voltage at the pre-sampling point, the random access memory storing voltage measurements bythe pre-sampling means and comparing them with the predetermined voltage factors stored in the read only memoryto derive the likely value of the voltage atthe first predetermined point in the subsequent half cycle.
The switching means must be capable of switching a currenttypically of the order of 100 ampsthrough the impedance, at a predetermined point in the voltage cycle. The switching means is therefore preferably a thyristor.
One embodiment ofthe invention will now be furtherdescribed, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of apparatus according to the invention; Figure2 is a sketch of a typical voltage signal at pointA in the circuit of Figure 1; Figures3 and 4are sketches of typical voltage signals at point B in the circuit of Figure 1; and Figure 5 is a sketch ofa voltage signal obtained at point C-in the circuit of Figure 1.
Referring to Figure 1 there is provided two terminals 1,2 for connection between the points in the electrical installation across which the prospective short circuit current is to be established.
Athyristor 3 and an impedance 20 are connected across the terminals 1 and 2. A rectifier 21 and a potential divider comprising variable resistors 4 and 5 are also included. The potential divider is adjusted by a microprocessor 11 to produce a voltage signal on line 6 having a maximum amplitude of between 1.63 and 2.45 volts. This voltage signal Vjn is shown in Figure 2.
Line 6 is connected to the negative input of a comparator 7, the positive input of which is connected to a line 8 carrying a reference voltage Vref.
The output from the comparator is an inverted signal VreVin, as shown in Figure3.
The output ofthe comparator7 is fed via line7toa variable gain amplifier9, the analogue output of which is connected to an analogue to digital converter 10 and input to the micro-processor 11.
Signals from the micro-processor are converted back by a digital to analogue converter 12 and used to supply the reference voltage on line 8 for the comparator 7. The micro-processor also supplies control signals via line 13to vary the gain ofthe amplifier 9.
The micro-processor 11 compares the value of a predetermined pointofthe inverted signal, e.g.the peakvoltage 14, with a second reference voltage, conveniently zerb volts. With the gain ofthe amplifier 9setatunity,themicro-processorthen adjuststhe value of the first reference voltage Vret on line 8so thatthe peak value approaches the zero volts reference voltage 15.
This is the situation prior to firing of the test current between the terminals 1 and 2. The micro-processor 11 actuates a thyristor 3 to switch in the impedance 20 for one half cycle and cause a current in excess of 100 amps to flow. The passage ofthis current causes the voltageto drop fractionally and the signal obtained from the comparator 7 is as shown in Figure 4. During this half cycle the microprocessor 11 pre-samples the voltage at a pre-sampling point as shown at 16 in Figure 4. The value ofthe voltage at the pre-sampling point 16 is compared with pre-programmed values stored in a read only memory to determine the appropriate gain for the amplifier 9. Accordingly the micro-processor sends signals via line 13 to the amplifier 9 to increase its gain, producing an output as shown in Figure 5.Thus the difference 25 between the amplified peak 17 and the zero volt reference 15, is due to the action ofthe amplifier9, significant compared to the voltage at the peak 14. In this way a highly accurate reading ofthe voltagedifferencecan be made asthe micro-processor is no longer attempting to measure the very small difference between the peak 14 and the reference voltage 15, but a much largerdifference between the reference voltage and the amplified peak 17.
The micro-processor 11 makes a decision on whether or not to fire the thyristorto switch in the test current based on its calculation of the stability ofthe operating voltage. To do this it compares the value of the peak voltage 14 of two preceding half cycles. If the values are within a preset acceptable margin of error, the micro-processorfires the thyristor switch so thatthe test current will flow during the next half cycle. Should the two values differ by more than the acceptable margin, then the micro-processor 11 inhibits the firming of the test current and repeats the above described comparison on the next half cycle.
in this way readings of the prospective short circuit are only made when the operating voltage is relatively stable, hence reducing the risk of obtaining erroneous results. As an alternative to the peak voltage 14, the voltage may conceivably be pre-sampled every half cycle and the voltage at the pre-sampling point 16 be used for comparison.

Claims (18)

1. A method of determining the prospective short circuitcurrentatany point in an electrical installation comprising the steps of measuring and storing the voltage at a first predetermined point of a half cycle ofthe operating voltage between conductors at any point in the electrical installation; causing a test currentto flow between said conductors for a short period, said test current being of sufficient magnitudeto effect a reduction in the voltage at said first predetermined point in a subsequent half cycle; pre-sampling the voltage at a predetermined pre-sampling point, the said pre-sampling point being before the first predetermined point in the subsequent half cycle; passing the voltage signal through a variable gain amplifier, the gain ofthe amplifier being automatically adjusted in response tothevalue of thevoltage atthe pre-sampling point; and measuring and storing the value ofthe voltage at the first predetermined point in the subsequent half cycle.
2. Amethodaccordingto Claim 1 includingthe steps of estìmating, from the value of the voitage at the pre-sampling point,the likely value of the voltage at the first predetermined point in the subsequent half cycle and adjusting the gain of the amplifier such that the difference between the amplified voltage at the said first predetermined point and its value at the said point in the previous half cycle is greaterthan a predetermined threshold value.
3. A method according to Claim 2 wherein the predetermined threshold value is at least 10% of the value ofthe voltage at the first predetermined point.
4. A method according to Claim 3 wherein the predetermined threshold value is at least 40% of the value ofthe voltage at the first predetermined point.
5. A method according to any of Claims 1 to 4 wherein the voltage between the conductors is measured by generating a reference voltage and comparing the voltage signal to be measured with said reference voltage.
6. A method according to Claim wherein the reference voltage is varied such that it is approximately equal to the voltage signal at the first predetermined point.
7. A method according to any of Claims 1 to 4 wherein the voltage between the conductors is measured by generating first and second reference voltages, the voltage between the conductors being substrated from the first reference voltage to form an inverted signal, which inverted signal is then compared with the second reference voltage.
8. A method according to Claim 7 wherein the first reference voltage is varied such that the voltage at the first predetermined point is approximately equal to the second reference voltage.
9. A method according to any of Claims 1 to 8 wherein the first predetermined point is the peak value ofthe half cycle.
10. A method according to any of Claims 1 to 9 including the additional step of comparing the stored values of the voltage atthe first predetermined point for two or more half cycles and initiating flow ofthe test current between the conductors only if the compared values of the two or more half cycles are within a predetermined error differential one from another.
11. A method according to any of Claims 1 to 10 including the additional steps of measuring and storing valuesforthe period of each halfcycle, comparing the stored values of the periods of two or more half cycles, and initiating flow ofthe test current between the conductors only if the compared values of the two or more half cycles are within a predetermined error differential one from another.
12. A method as claimed in Claim 1 and substantially as hereinbefore described.
13. Apparatus for carrying out a method of determining the prospective short circuit current at any point in an electrical installation according to any of Claims 1 to 12.
14. Apparatus for determining the prospective short current at any point in an electrical installation comprising means for measuring and storing the voltage at a first predetermined point of a half cycle of the operating voltage between conductors at any point in the electrical installation; switching means for connection and disconnection of an impedance between said conductorsto cause a currenttoflow therebetween for a short period, the impedance being such that a currentflowsofsufficient magnitude to effect a reduction in the voltage atthe said first predetermined point in a subsequent half cycle; pre-sampling means for measuring the voltage at a predetermined pre-sampling point, the said pre-samplaing point being before the first predetermined point in the subsequent half cycle; a variable gain amplifier, adapted to amplify the voltage signal with a gain depending on the value of the voltage at the pre-sampling point; and means for measuring the amplified voltage atthe first predetermined point in the subsequent half cycle.
15. Apparatus according to Claim 14includinga micro-processor adapted to estimate, from the voltage measurement ofthe pre-sampling means, the likely value of the voltage atthefirst predetermined point in the subsequent half cycle and supply control signals to the amplifier to adjustthe gain thereof such that the difference between the amplified voltage at the said first predetermined point and its value at the said point in the previous half cycle is greater than a predetermined threshold value.
16. Apparatus according to Claim 15 wherein the micro-processor includes a read only memory and a random access memory, the read only memory having stored therein predetermined values of voltage at the pre-sampling point, the random access memory storing voltage measurements by the pre-sampling means and comparing them with the predetermined values stored in the read only memory to derive the likely value of the voltage at the first predetermined point in the subsequent half cycle.
17. Apparatus according to any of Claims 14 to 16 wherein the switching means comprises a thyristor.
18. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.
GB08624352A 1985-10-14 1986-10-10 Short circuit current protection Withdrawn GB2181561A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB858525288A GB8525288D0 (en) 1985-10-14 1985-10-14 Short circuit current protection

Publications (2)

Publication Number Publication Date
GB8624352D0 GB8624352D0 (en) 1986-11-12
GB2181561A true GB2181561A (en) 1987-04-23

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GB858525288A Pending GB8525288D0 (en) 1985-10-14 1985-10-14 Short circuit current protection
GB08624352A Withdrawn GB2181561A (en) 1985-10-14 1986-10-10 Short circuit current protection

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Application Number Title Priority Date Filing Date
GB858525288A Pending GB8525288D0 (en) 1985-10-14 1985-10-14 Short circuit current protection

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0342597A2 (en) * 1988-05-16 1989-11-23 Hitachi, Ltd. Abnormality system for a high voltage power supply apparatus
EP0479099A1 (en) * 1990-09-25 1992-04-08 Pacesetter, Inc. Blood oxygen sensor having leakage compensation
FR2669739A1 (en) * 1990-11-28 1992-05-29 Merlin Gerin INSULATION CONTROLLER WITH IMPROVED PRECISION.

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0342597A2 (en) * 1988-05-16 1989-11-23 Hitachi, Ltd. Abnormality system for a high voltage power supply apparatus
EP0342597A3 (en) * 1988-05-16 1990-12-19 Hitachi, Ltd. Abnormality system and method for a high voltage power supply apparatus
EP0479099A1 (en) * 1990-09-25 1992-04-08 Pacesetter, Inc. Blood oxygen sensor having leakage compensation
FR2669739A1 (en) * 1990-11-28 1992-05-29 Merlin Gerin INSULATION CONTROLLER WITH IMPROVED PRECISION.
EP0489665A1 (en) * 1990-11-28 1992-06-10 Schneider Electric Sa Insulation monitor with better precision
US5303164A (en) * 1990-11-28 1994-04-12 Merlin Gerin Insulation monitor with improved precision

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Publication number Publication date
GB8525288D0 (en) 1985-11-20
GB8624352D0 (en) 1986-11-12

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WAP Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1)