CA1116236A - Electronic ground detector for armature supplies - Google Patents
Electronic ground detector for armature suppliesInfo
- Publication number
- CA1116236A CA1116236A CA324,203A CA324203A CA1116236A CA 1116236 A CA1116236 A CA 1116236A CA 324203 A CA324203 A CA 324203A CA 1116236 A CA1116236 A CA 1116236A
- Authority
- CA
- Canada
- Prior art keywords
- signal
- fault
- ground
- ground fault
- detector
- 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.)
- Expired
Links
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
- H02H3/16—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 fault current to earth, frame or mass
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/16—Measuring impedance of element or network through which a current is passing from another source, e.g. cable, power line
- G01R27/18—Measuring resistance to earth, i.e. line to ground
-
- 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
- G01R31/34—Testing dynamo-electric machines
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/08—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors
- H02H7/0833—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors for electric motors with control arrangements
Abstract
ELECTRONIC GROUND DETECTOR FOR ARMATURE SUPPLIES
ABSTRACT OF THE DISCLOSURE
This invention relates to a ground fault detector for a d.c. machine having a convertor power source. The detector includes a resistance circuit device that detects unbalanced currents corresponding to an asymmetrical ground fault in the machine. An undesired common mode voltage from the convertor is also present. A ground sensor connected to a mid-point of the resistance circuit filters out the harmonic voltage so that the ground sensor produces at its output a fault signal whose magnitude represents the ground fault current. The detector also includes a terminal sensor for sensing the convertor output voltage and for providing a terminal output signal proportional thereto. An attenuator attenuates this output signal by a predetermined amount which results in a threshold signal. Both the fault signal and threshold signal vary as the convertor output voltage varies. However, a trip signal is generated when the magnitude of the fault signal exceeds the magnitude of the threshold signal. The trip signal is independent of variations in convertor output voltage. Previous ground fault detectors have been desensitized by harmonic voltages from the convertor which resulted in false ground fault readings. The present detector filters out the harmonic voltages and is independent of variations in convertor output voltage.
ABSTRACT OF THE DISCLOSURE
This invention relates to a ground fault detector for a d.c. machine having a convertor power source. The detector includes a resistance circuit device that detects unbalanced currents corresponding to an asymmetrical ground fault in the machine. An undesired common mode voltage from the convertor is also present. A ground sensor connected to a mid-point of the resistance circuit filters out the harmonic voltage so that the ground sensor produces at its output a fault signal whose magnitude represents the ground fault current. The detector also includes a terminal sensor for sensing the convertor output voltage and for providing a terminal output signal proportional thereto. An attenuator attenuates this output signal by a predetermined amount which results in a threshold signal. Both the fault signal and threshold signal vary as the convertor output voltage varies. However, a trip signal is generated when the magnitude of the fault signal exceeds the magnitude of the threshold signal. The trip signal is independent of variations in convertor output voltage. Previous ground fault detectors have been desensitized by harmonic voltages from the convertor which resulted in false ground fault readings. The present detector filters out the harmonic voltages and is independent of variations in convertor output voltage.
Description
23~i :'`
ELECTRONIC GROUND DETECTOR FOR ARMATURE S~PPLIES
This invention relates to a ground fault detector for a d.c. machine having a convertor power source.
A ground fault may at some time occur in a .:~
h 5 d.c. machine and it is desirable to detect such a n fault because an undetected fault may seriously ; damage or destroy the machine. One known manner ~`~ in which to detect the presence of a ground fault in a d.c. machine having a convertor power source ` 10 is to connect a balanced resistor bridge network `~
across the terminals of the d.c. machine. This ground fault detection network has a mid-point where a null value of voltage is present when there is no asymmetrical ground fault in the d~c. machine.
15 The null value of voItage is ascertained by . balancing the resistance on either side of the mid-point. In the event of an asymmetrical ground fault in the machine, unbalanced currents develop a d.c. value of fault voltage at the 20 mid-point. This developed fault voltage is the - result of the asymmetrical graund in the motor introducing a ground reistance to the bridge ` network.
One disadvantage of the ground fault detection ;~
network utilizing a convertor power source is that it is desensitized by a thyristor conduction sensar ~: :
~ ~ .
.. . . . .
. ~
z36 : `
:;.
.
normally connected across the machine terminals The thyristor conduction sensor provides an important `: function for dual convertor operation in that it provides a logic signal when all converters are not ; 5 conducting. The presence of this signal allows for the .
- transfer of control o~ thyristor gating from one convertor to the other. The thyristor conduction sensor `~ comprises an unbalanced resistance network that has an unbalanced point connected to ground potential.
The unbalanced resistance introduced by the thyristor conducting sensor must be compensated for by the ground fault detection network.
~nother disadvantage with the ground fault detection network is that the value of voltage present ~; 15 at the mid-point in the event of a ground fault is dependent upon the value o~ the convertor output voltage. As the value of convertor output voltage rises so does the d.c. value of fault voltage developed at the , .~
: mid-point. If a critical threshold voltage is ~- 20 arbitrarly chosen above which the fault voltage is toexceed in order to trip the motor~ then in some situations the machine will not be tripped and will continue to operate while experiencing a tolerable `~ ground fault. However,~ a ground fault having a first ``; 25 fault voltage above the threshold voltage when the converter is operating at 80% of its rated output voltage may also have a second fault voltage below the threshold voltage when the convertor is operating at 20~ rated output. In this situation, either the first ~ 30 fault voltage is indicative of a tolerable fault and -` the machine is unnecessarily tripped or the second fault voltage is indicative of a fault that may damage the - machine and the machine is not tripped.
` Another disadvantage with the ground fault detection network is that, when the value of fault voltage at the : ' , ' ' " . ' " ' " ' ` , ~:
.: '!~,.,,..`.
;' `' , . ~ ' .:,, ~ .
mid-point is measured with a ground potential in the system, an undesired voltage corresponding to the alternating common mode harmonic voltage from the , convertor is also sensed. This harmonic voltage which is dependent upon phase angle, can be half as large as ~, the magnitude of the convertor output voltage.
' Therefore, the common mode harmonic voltage may be one or two orders of magnitude larger than a d.c. ground `~ fault voltage in the machine. This large fluctuating harmonic voltage may swamp out any fault voltage signals from the machine.
Accordingly, it is a feature of this invention to provide a ground fault detector for a d.c. machine - that reduces the value of common mode harmonic voltage present when the fault signal is sensed.
It is an additional feature of this invention to provide a ground fault detector for a d.c. machine that generates a trip signal which is independent from variations in the convertor output voltage.
The present invention relates to a ground fault detector for a d.c. machine having a convertor power source. The convertor has at its output an undesirable common mode harmonic voltage. The ground fault detector detects unbalanced currents resulting from an asymmetrical ground fault in the machine. The detector includes a filter that filters out at least a portion of the harmonic voltage and provides at its output a fault signal whose magnitude is proportional to the ground fault current. The portion of the harmonic voltage at the output is less than the value of the fault signal. The detector also senses the convertor output voltage applied to the machine and provides a terminal output signal proportional thereto.
An attenuator attenuates the terminal output signal ~y a predetermined amount to produce a threshold signal whose magnitude is proportional to the attenuated -' : . ~ " ' i ' :
' '. ' , ' , ": ~` `. :
; ~
~LgiZ3Çi .
; CASE-2576 terminal outpu-t signal. The detector compares the ~--` magnitudes of the ault signal and threshold signal and generates a trip signal when the ~; fault signal exceeds the threshold signal. By generating ~`~ 5 the threshold signal from the converter output and not ~s choosing it arbitarily the detector provides a threshold signal that varies with virations in the ~; convertor output voltage. Because the fault signal also ; varies with variations i~ the convertor output voltage, , 10 the trip signal, which is a comparision of the fault and threshold signals, is independent of variations in the convertor output voltage applied to the machine.
Additionally, the ground detector may be easily r adapted to detect a symmetrical ground fault in the machine. The ground detector may be adapted to simulate an asymmetrical ground fault by unbalancing the currents detected. A symmetrical ground fault is present when the trip signal is not generated during this simulation.
In accordance with an aspect of this invention there is provided a d.c. machine having a pair of terminals, power for the machine being provided by a convertor having in its output an undesired harmonic ~` voltage, the detector comprising: a ground sensing means connected to the terminals and having circuit ` means for detecting unbalanced currents resulting from an asymmetrical ground fault in the machine and having ` filtering means for filtering out at least a portion of the harmonic voltage, the ground sensing means producing at an output thereof a fault signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of the harmonic voltage at the output being less than the value of the fault signal.
In accordance with the foregoing aspect of this invention the detector may further comprise: terminal :.
.
~ :, ` ` ` . ` . `' ` "`, ~. .: ` ' ~
` . : ` , , : `
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'; sensing means connected to the terminals for detecting ; the output of the convertor and for providing a :` terminal output signal proportional thereto, attenuating means connected to the terminal sensing means ~or , 5 receiving the terminal output signal by a predetermined amount to provide a threshold signal whose magnitude represents the attenuated terminal output signal, and comparing means for comparing the magnitudes of the fault signal and the threshold signal and generating a trip signal when said fault signal exceeds said threshold signal.
~ For a better understanding of the nature and i objects of the invention reference may be had by way of example to the accompanying diagrammatic drawings in which:
Figure 1 is a schematic diagram showing the ground fault detector, convertor and d.c. machine, and Figure 2 is a circuit diagram for the ground sensor of Figure 1.
-~ 20 The preferred embodiment of the invention is shown in Figure 1. A d.c. machine 10 has terminals 12 and 14.
Terminal 12 has a positive voltage polarity while terminal 14 has a negative voltage polarity. Convertor 16 has its output connected across terminals 12 and 14 so as to supply d.c. power to machine 10. Convertor 16 converts a.c. power from its input (not shown) into d.c. power at its output. A ground fault detector ` shown within broken line 18 is provided to detect ground fault currents in machine 10.
The ground fault detector CQmpriSes a ground - sensing means shown within broken line 20. The ground sensing means 20 includes a resistance device 22, a ground sensor 24, and a magnitude device 26. The resistance device comprises a pair of resistors 28 and ~
a resistor 30. Resistors 28 are chosen to be equal to `
one another so as to provide a null value of voltage . .
- . .
., ,.
z~
:`
~ . .
~; - 6 -- at mid-point 32 during the absence of a ground fault in machine 10. It should be understood that resistors 28 may be adjustable if desired so that they may compensate ~ for any resistive unbalance that is not a direct result : 5 of a ground ~ault. For this purpose resistors 28 may be replaced by one or more potentiometers. Resistor 30 connected between point 32 and ground potential permits ground sensor 24 to sense any fault voltage " signal developed at mid-point 32 with respect to ground potential. It should also be understood that when sensing the fault signal across resistor 30 the sensitivity of the detector is enhanced by reducing the - affects of unbalances at mid-point 32 due to a thyristor conduction sensor bridge network (not shown).
In the event of an asymmetrical ground fault in machine 10 the resistances of the machine on opposite sides of ground fault are not e~ual. This results in unbalanced currents developing a fault signal at mid-point 32.
Also present at mid-point 32 is a harmonic alternating voltage that corresponds to the undesired common mode voltage generated by convertor 16. Ground sensor 24 filters out the harmonic voltage while allowing the fault signal to pass therethrough to magnitude device 26. Magnitude device 26 conforms any polarity associated with the magnitude of the fault signal to a first polarity.
Referring now to figure 2 a circuit diagram for the ground sensor 24 of figure 1 is shown. The input of the ground sensor is connected via resistor 100 to the inverting terminal of amplifier 102. The non-inverting terminal of amplifier 102 is connected to ground potential. A feedback path is provided from the output of amplifier 102 through potentiometer 104 connected in series with the parallel combination of resistor 106 and capacitor 108 to the inverting terminalO Potentiometer 104 controls the gain of amplifier 102. The frequency ~" ~
:`' ~ , , ` ' , ~! ` ;
~ .
~;
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~ 7 -- response of amplifier 102 is controlled by capacitor 108 and resistor 106. Capacitor 108 is chosen to provide a minimum value of time constant required to attenuate the convertor's harmonic common mode voltages and thus discriminates against changes in the convertor " output voltage. In this manner the output of the ground sensor comprises a d.c. value of the fault signal ` greater than the value of the attenuated harmonic voltages.
Referring back to figure 1, the ground fault detector further includes a terminal sensin~ means shown within broken line 29. The terminal sensing means 29 includes a terminal voltage sensor 31 which serves ; as an isolator and which senses the d.c. terminal voltage across terminals 12 and 14. The terminal voltage sensor provides a terminal signal representative of the d.c. converter output voltage~ Magnitude device 33 is included within terminal sensing means 29. Device 33 conforms any polarity associated with the magnitude of the terminal signal to a second polarity opposite to ` the first polarity associated with device 26.
The terminal signal from terminal sensing means 29 is fed simultaneously into attenuators 34 and 36.
The attenuators each attenuate the magnitude of the terminal signal by a predetermined amount associated with that attenuator. The predetermined amount of attenuation for attenuator 36 is chosen so as to give a threshold signal at the output of attenuator 36 having a magnitude above which the fault signal from ground sensing means 20 is indicative of a destructive fault current in machine lO. The predetermined amount of attenuation for attenuator 34 is chosen to be slightly larger than the attenuation of attenuator 36 so that the warning threshold signal is produced by attenuator 34.
The threshold signal from attenuator 36 is summed ,.
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..` -in summer 38 with the fault signal from ground sensing means 20. Because the polarities of the signals are opposite, the output of summer 38 is the difference of the two signals. When the fault signal exceeds the threshold siynal, amplifier 40 amplifies the summer's output signal by an amount so as to actuate logic 42 -` and generate a trip output signal. The trip output signal stops the operation of machine 10. In this manner summer 38, amplifier 40 and logic 42 act as a comparing means that compares the magnitudes of the fault signal and the threshold signal and generates a trip signal in the event that the magnitude of the fault signal exceeds that of the threshold signal.
Similarily the warning threshold signal from attenuator 34 is summed in summer 44 with the fault signal. When the fault signal exceeds the warning threshold signal, amplifier 46 amplifies the summation thereby actuating, logic 48 and generating a warning signal.
LGgic 42 and 48 act to generate and hold a trip and warning signal respectively. This ensures the detection of a momentary ground fault.
Additionally the ground detector may be adapted to detect large symmetrical ground faults in the machine.
; 25 It should be noted that the ground detector without the symmetrical ground fault detection is still a viable ground detector because symmetrical ground faults in machine 10 are unlikely. ~evertheless switch arrangement Sl-S2 provide means to control symmetrical `~ 30 ground detection. Sl is provided in the ground sensing means 20 and switch S2 following summer 38. Switch Sl - is open and S2 is in position 1 as shown when the detector - is operating to detect as~mmetrical ground faults. When " testing for a symmetrical ground fault switch Sl will be closed and switch S2 will be in position 2. By closiny switch Sl, resistance 50 is introduced into the resistance ~.
, `. . ! ' . , ' ,, ~ ;
~ ' ',, ' ', ' "~' ' ' ' ,. ~ '' 3~;
' ^ CASE-2576 ` _ g _ - device 22. Introduc-tion of resistance 50 unbalances the resistance device 22 and simulates an asymmetrical fault that should develope a fault signal at mid-point 32 if no actual symmetrical fault is present in machine 10. When switch Sl is closed switch S2 is simultaneously moved into position 2. The interconnection of the switches is indicated diagrammatically by dashed line 54. When switch S2 is in position 2 it inhibits any signal from the summer that would normally generate a trip signal. Diode 52 is a light emitting diode and will give a visual indication if a signal from summer 38 is present. ~n this manner diode 52 gives a visual indication of the simulated asymmetrical ground fault.
However, in the even~ of a symmetrical ground fault in machine 10 the fault signal developed at mid-point 32 during the simulation is less in magnitude than the threshold signal. Hence, summer 38 outputs a signal that does not generate a trip signal and is not indicated visually by diode 52. During the simulation, -the - 20 absence of a visual response by diode 52 represents the presence of a symmetrical ground fault in machine 10 .
The operation of the ground detector is now described for detection of an asymmetrical ground fault.
During this operation switch Sl is open and S2 is in position 1 as shown in figure 1. When an asymmetrical ground fault is present in machine 10 it should be understood that a parallel path of unbalanced resistance is developed between mid-point 32 and the ground fault in machine 10. This imbalance of parallel resistive . paths develops a ground fault signal at mid-point 32 in addition to the undesired harmonic voltage from convertor 16. Ground sensor 24 filters at least a portion of the harmonic voltage so as to produce at its output a fault signal whose magnitude is greater than the portion of the harmonic voltage at the output.
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' CASE-2576 The polarity of the magnitude of the fault signal is conformed to a first polarity by magnitude device 26.
Terminal voltage means 29 senses the terminal voltage and conforms its polarity to a second polarity opposite to the first polarity. The magnitude of the value of ` terminal output voltage is attenuated by attenuators 34 and 36 so as to respectively obtain a warning threshold signal and a threshold signal each having a polarity that may be opposite to the polarity of the fault signal. The operation of the detector and more particularly summer 38, amplifier 40, logic 42, summer 44, amplifier 46 and logic 48 is now described for different magnitudes of fault signal.
In the event of a small asymmetrical ground fault the magnitude of the fault signal is less than both that of the warning threshold signal and the threshold signal. In this event no trip or warning signal is generated by the detector. In the event that an asymmetrical ground fault in machine 10 has a fault current associated with it that is not so large as to damage ma~hine lO but worthy of recognition then the ` magnitude of the fault signal exceeds that of the warning threshold signal which results in summer 44, - amplifier 46 and logic 48 generating a warning signal.
In this event the magnitudes of the ground output signal is less than that of the threshold signal so no trip signal is generated by summer 3~, amplifier 40 and logic 42. Lastly, in the event that an asymmetrical ground fault having a ground fault current capable of damaging machine lO, then the magnitude of the ground output signal exceeds that of both the warning threshold signal and threshold signal. In this event a warning signal is genexated as previously mentioned and summer 38, amplifier 40 and logic 42 generate a trip signal that acts to stop machine 10 by cutting power to its terminals.
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It should be understood that the previous discussion of figures 1 and 2 have been directed to the preferred embodiment and that other alternate embodiments may be realized therefrom by a man skilled in the art. One alternative would be to replace each of summers 38 and 44 with a comparator.
Another alternative would be to combine the attenuators and the terminal voltage sensor so that attenuation was provided prior to the magnitude device. Also the simulation of an asymmetrical ground fault may be accomplished by unbalancing one of resistors 28 by switching in an additional resistance either in parallel with or directly in series with this one resistor. Lastly the polarities of the fault signal and threshold signals may be conformed to be the same polarity and a differential amplifier may be used instead of a summing amplifier.
.
,; :
ELECTRONIC GROUND DETECTOR FOR ARMATURE S~PPLIES
This invention relates to a ground fault detector for a d.c. machine having a convertor power source.
A ground fault may at some time occur in a .:~
h 5 d.c. machine and it is desirable to detect such a n fault because an undetected fault may seriously ; damage or destroy the machine. One known manner ~`~ in which to detect the presence of a ground fault in a d.c. machine having a convertor power source ` 10 is to connect a balanced resistor bridge network `~
across the terminals of the d.c. machine. This ground fault detection network has a mid-point where a null value of voltage is present when there is no asymmetrical ground fault in the d~c. machine.
15 The null value of voItage is ascertained by . balancing the resistance on either side of the mid-point. In the event of an asymmetrical ground fault in the machine, unbalanced currents develop a d.c. value of fault voltage at the 20 mid-point. This developed fault voltage is the - result of the asymmetrical graund in the motor introducing a ground reistance to the bridge ` network.
One disadvantage of the ground fault detection ;~
network utilizing a convertor power source is that it is desensitized by a thyristor conduction sensar ~: :
~ ~ .
.. . . . .
. ~
z36 : `
:;.
.
normally connected across the machine terminals The thyristor conduction sensor provides an important `: function for dual convertor operation in that it provides a logic signal when all converters are not ; 5 conducting. The presence of this signal allows for the .
- transfer of control o~ thyristor gating from one convertor to the other. The thyristor conduction sensor `~ comprises an unbalanced resistance network that has an unbalanced point connected to ground potential.
The unbalanced resistance introduced by the thyristor conducting sensor must be compensated for by the ground fault detection network.
~nother disadvantage with the ground fault detection network is that the value of voltage present ~; 15 at the mid-point in the event of a ground fault is dependent upon the value o~ the convertor output voltage. As the value of convertor output voltage rises so does the d.c. value of fault voltage developed at the , .~
: mid-point. If a critical threshold voltage is ~- 20 arbitrarly chosen above which the fault voltage is toexceed in order to trip the motor~ then in some situations the machine will not be tripped and will continue to operate while experiencing a tolerable `~ ground fault. However,~ a ground fault having a first ``; 25 fault voltage above the threshold voltage when the converter is operating at 80% of its rated output voltage may also have a second fault voltage below the threshold voltage when the convertor is operating at 20~ rated output. In this situation, either the first ~ 30 fault voltage is indicative of a tolerable fault and -` the machine is unnecessarily tripped or the second fault voltage is indicative of a fault that may damage the - machine and the machine is not tripped.
` Another disadvantage with the ground fault detection network is that, when the value of fault voltage at the : ' , ' ' " . ' " ' " ' ` , ~:
.: '!~,.,,..`.
;' `' , . ~ ' .:,, ~ .
mid-point is measured with a ground potential in the system, an undesired voltage corresponding to the alternating common mode harmonic voltage from the , convertor is also sensed. This harmonic voltage which is dependent upon phase angle, can be half as large as ~, the magnitude of the convertor output voltage.
' Therefore, the common mode harmonic voltage may be one or two orders of magnitude larger than a d.c. ground `~ fault voltage in the machine. This large fluctuating harmonic voltage may swamp out any fault voltage signals from the machine.
Accordingly, it is a feature of this invention to provide a ground fault detector for a d.c. machine - that reduces the value of common mode harmonic voltage present when the fault signal is sensed.
It is an additional feature of this invention to provide a ground fault detector for a d.c. machine that generates a trip signal which is independent from variations in the convertor output voltage.
The present invention relates to a ground fault detector for a d.c. machine having a convertor power source. The convertor has at its output an undesirable common mode harmonic voltage. The ground fault detector detects unbalanced currents resulting from an asymmetrical ground fault in the machine. The detector includes a filter that filters out at least a portion of the harmonic voltage and provides at its output a fault signal whose magnitude is proportional to the ground fault current. The portion of the harmonic voltage at the output is less than the value of the fault signal. The detector also senses the convertor output voltage applied to the machine and provides a terminal output signal proportional thereto.
An attenuator attenuates the terminal output signal ~y a predetermined amount to produce a threshold signal whose magnitude is proportional to the attenuated -' : . ~ " ' i ' :
' '. ' , ' , ": ~` `. :
; ~
~LgiZ3Çi .
; CASE-2576 terminal outpu-t signal. The detector compares the ~--` magnitudes of the ault signal and threshold signal and generates a trip signal when the ~; fault signal exceeds the threshold signal. By generating ~`~ 5 the threshold signal from the converter output and not ~s choosing it arbitarily the detector provides a threshold signal that varies with virations in the ~; convertor output voltage. Because the fault signal also ; varies with variations i~ the convertor output voltage, , 10 the trip signal, which is a comparision of the fault and threshold signals, is independent of variations in the convertor output voltage applied to the machine.
Additionally, the ground detector may be easily r adapted to detect a symmetrical ground fault in the machine. The ground detector may be adapted to simulate an asymmetrical ground fault by unbalancing the currents detected. A symmetrical ground fault is present when the trip signal is not generated during this simulation.
In accordance with an aspect of this invention there is provided a d.c. machine having a pair of terminals, power for the machine being provided by a convertor having in its output an undesired harmonic ~` voltage, the detector comprising: a ground sensing means connected to the terminals and having circuit ` means for detecting unbalanced currents resulting from an asymmetrical ground fault in the machine and having ` filtering means for filtering out at least a portion of the harmonic voltage, the ground sensing means producing at an output thereof a fault signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of the harmonic voltage at the output being less than the value of the fault signal.
In accordance with the foregoing aspect of this invention the detector may further comprise: terminal :.
.
~ :, ` ` ` . ` . `' ` "`, ~. .: ` ' ~
` . : ` , , : `
`` . ` .
- `:: ' ' ~ . ' ~. `.:,, ` ' . . .
'; sensing means connected to the terminals for detecting ; the output of the convertor and for providing a :` terminal output signal proportional thereto, attenuating means connected to the terminal sensing means ~or , 5 receiving the terminal output signal by a predetermined amount to provide a threshold signal whose magnitude represents the attenuated terminal output signal, and comparing means for comparing the magnitudes of the fault signal and the threshold signal and generating a trip signal when said fault signal exceeds said threshold signal.
~ For a better understanding of the nature and i objects of the invention reference may be had by way of example to the accompanying diagrammatic drawings in which:
Figure 1 is a schematic diagram showing the ground fault detector, convertor and d.c. machine, and Figure 2 is a circuit diagram for the ground sensor of Figure 1.
-~ 20 The preferred embodiment of the invention is shown in Figure 1. A d.c. machine 10 has terminals 12 and 14.
Terminal 12 has a positive voltage polarity while terminal 14 has a negative voltage polarity. Convertor 16 has its output connected across terminals 12 and 14 so as to supply d.c. power to machine 10. Convertor 16 converts a.c. power from its input (not shown) into d.c. power at its output. A ground fault detector ` shown within broken line 18 is provided to detect ground fault currents in machine 10.
The ground fault detector CQmpriSes a ground - sensing means shown within broken line 20. The ground sensing means 20 includes a resistance device 22, a ground sensor 24, and a magnitude device 26. The resistance device comprises a pair of resistors 28 and ~
a resistor 30. Resistors 28 are chosen to be equal to `
one another so as to provide a null value of voltage . .
- . .
., ,.
z~
:`
~ . .
~; - 6 -- at mid-point 32 during the absence of a ground fault in machine 10. It should be understood that resistors 28 may be adjustable if desired so that they may compensate ~ for any resistive unbalance that is not a direct result : 5 of a ground ~ault. For this purpose resistors 28 may be replaced by one or more potentiometers. Resistor 30 connected between point 32 and ground potential permits ground sensor 24 to sense any fault voltage " signal developed at mid-point 32 with respect to ground potential. It should also be understood that when sensing the fault signal across resistor 30 the sensitivity of the detector is enhanced by reducing the - affects of unbalances at mid-point 32 due to a thyristor conduction sensor bridge network (not shown).
In the event of an asymmetrical ground fault in machine 10 the resistances of the machine on opposite sides of ground fault are not e~ual. This results in unbalanced currents developing a fault signal at mid-point 32.
Also present at mid-point 32 is a harmonic alternating voltage that corresponds to the undesired common mode voltage generated by convertor 16. Ground sensor 24 filters out the harmonic voltage while allowing the fault signal to pass therethrough to magnitude device 26. Magnitude device 26 conforms any polarity associated with the magnitude of the fault signal to a first polarity.
Referring now to figure 2 a circuit diagram for the ground sensor 24 of figure 1 is shown. The input of the ground sensor is connected via resistor 100 to the inverting terminal of amplifier 102. The non-inverting terminal of amplifier 102 is connected to ground potential. A feedback path is provided from the output of amplifier 102 through potentiometer 104 connected in series with the parallel combination of resistor 106 and capacitor 108 to the inverting terminalO Potentiometer 104 controls the gain of amplifier 102. The frequency ~" ~
:`' ~ , , ` ' , ~! ` ;
~ .
~;
. .
~ 7 -- response of amplifier 102 is controlled by capacitor 108 and resistor 106. Capacitor 108 is chosen to provide a minimum value of time constant required to attenuate the convertor's harmonic common mode voltages and thus discriminates against changes in the convertor " output voltage. In this manner the output of the ground sensor comprises a d.c. value of the fault signal ` greater than the value of the attenuated harmonic voltages.
Referring back to figure 1, the ground fault detector further includes a terminal sensin~ means shown within broken line 29. The terminal sensing means 29 includes a terminal voltage sensor 31 which serves ; as an isolator and which senses the d.c. terminal voltage across terminals 12 and 14. The terminal voltage sensor provides a terminal signal representative of the d.c. converter output voltage~ Magnitude device 33 is included within terminal sensing means 29. Device 33 conforms any polarity associated with the magnitude of the terminal signal to a second polarity opposite to ` the first polarity associated with device 26.
The terminal signal from terminal sensing means 29 is fed simultaneously into attenuators 34 and 36.
The attenuators each attenuate the magnitude of the terminal signal by a predetermined amount associated with that attenuator. The predetermined amount of attenuation for attenuator 36 is chosen so as to give a threshold signal at the output of attenuator 36 having a magnitude above which the fault signal from ground sensing means 20 is indicative of a destructive fault current in machine lO. The predetermined amount of attenuation for attenuator 34 is chosen to be slightly larger than the attenuation of attenuator 36 so that the warning threshold signal is produced by attenuator 34.
The threshold signal from attenuator 36 is summed ,.
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..` -in summer 38 with the fault signal from ground sensing means 20. Because the polarities of the signals are opposite, the output of summer 38 is the difference of the two signals. When the fault signal exceeds the threshold siynal, amplifier 40 amplifies the summer's output signal by an amount so as to actuate logic 42 -` and generate a trip output signal. The trip output signal stops the operation of machine 10. In this manner summer 38, amplifier 40 and logic 42 act as a comparing means that compares the magnitudes of the fault signal and the threshold signal and generates a trip signal in the event that the magnitude of the fault signal exceeds that of the threshold signal.
Similarily the warning threshold signal from attenuator 34 is summed in summer 44 with the fault signal. When the fault signal exceeds the warning threshold signal, amplifier 46 amplifies the summation thereby actuating, logic 48 and generating a warning signal.
LGgic 42 and 48 act to generate and hold a trip and warning signal respectively. This ensures the detection of a momentary ground fault.
Additionally the ground detector may be adapted to detect large symmetrical ground faults in the machine.
; 25 It should be noted that the ground detector without the symmetrical ground fault detection is still a viable ground detector because symmetrical ground faults in machine 10 are unlikely. ~evertheless switch arrangement Sl-S2 provide means to control symmetrical `~ 30 ground detection. Sl is provided in the ground sensing means 20 and switch S2 following summer 38. Switch Sl - is open and S2 is in position 1 as shown when the detector - is operating to detect as~mmetrical ground faults. When " testing for a symmetrical ground fault switch Sl will be closed and switch S2 will be in position 2. By closiny switch Sl, resistance 50 is introduced into the resistance ~.
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' ^ CASE-2576 ` _ g _ - device 22. Introduc-tion of resistance 50 unbalances the resistance device 22 and simulates an asymmetrical fault that should develope a fault signal at mid-point 32 if no actual symmetrical fault is present in machine 10. When switch Sl is closed switch S2 is simultaneously moved into position 2. The interconnection of the switches is indicated diagrammatically by dashed line 54. When switch S2 is in position 2 it inhibits any signal from the summer that would normally generate a trip signal. Diode 52 is a light emitting diode and will give a visual indication if a signal from summer 38 is present. ~n this manner diode 52 gives a visual indication of the simulated asymmetrical ground fault.
However, in the even~ of a symmetrical ground fault in machine 10 the fault signal developed at mid-point 32 during the simulation is less in magnitude than the threshold signal. Hence, summer 38 outputs a signal that does not generate a trip signal and is not indicated visually by diode 52. During the simulation, -the - 20 absence of a visual response by diode 52 represents the presence of a symmetrical ground fault in machine 10 .
The operation of the ground detector is now described for detection of an asymmetrical ground fault.
During this operation switch Sl is open and S2 is in position 1 as shown in figure 1. When an asymmetrical ground fault is present in machine 10 it should be understood that a parallel path of unbalanced resistance is developed between mid-point 32 and the ground fault in machine 10. This imbalance of parallel resistive . paths develops a ground fault signal at mid-point 32 in addition to the undesired harmonic voltage from convertor 16. Ground sensor 24 filters at least a portion of the harmonic voltage so as to produce at its output a fault signal whose magnitude is greater than the portion of the harmonic voltage at the output.
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' CASE-2576 The polarity of the magnitude of the fault signal is conformed to a first polarity by magnitude device 26.
Terminal voltage means 29 senses the terminal voltage and conforms its polarity to a second polarity opposite to the first polarity. The magnitude of the value of ` terminal output voltage is attenuated by attenuators 34 and 36 so as to respectively obtain a warning threshold signal and a threshold signal each having a polarity that may be opposite to the polarity of the fault signal. The operation of the detector and more particularly summer 38, amplifier 40, logic 42, summer 44, amplifier 46 and logic 48 is now described for different magnitudes of fault signal.
In the event of a small asymmetrical ground fault the magnitude of the fault signal is less than both that of the warning threshold signal and the threshold signal. In this event no trip or warning signal is generated by the detector. In the event that an asymmetrical ground fault in machine 10 has a fault current associated with it that is not so large as to damage ma~hine lO but worthy of recognition then the ` magnitude of the fault signal exceeds that of the warning threshold signal which results in summer 44, - amplifier 46 and logic 48 generating a warning signal.
In this event the magnitudes of the ground output signal is less than that of the threshold signal so no trip signal is generated by summer 3~, amplifier 40 and logic 42. Lastly, in the event that an asymmetrical ground fault having a ground fault current capable of damaging machine lO, then the magnitude of the ground output signal exceeds that of both the warning threshold signal and threshold signal. In this event a warning signal is genexated as previously mentioned and summer 38, amplifier 40 and logic 42 generate a trip signal that acts to stop machine 10 by cutting power to its terminals.
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It should be understood that the previous discussion of figures 1 and 2 have been directed to the preferred embodiment and that other alternate embodiments may be realized therefrom by a man skilled in the art. One alternative would be to replace each of summers 38 and 44 with a comparator.
Another alternative would be to combine the attenuators and the terminal voltage sensor so that attenuation was provided prior to the magnitude device. Also the simulation of an asymmetrical ground fault may be accomplished by unbalancing one of resistors 28 by switching in an additional resistance either in parallel with or directly in series with this one resistor. Lastly the polarities of the fault signal and threshold signals may be conformed to be the same polarity and a differential amplifier may be used instead of a summing amplifier.
.
,; :
Claims (12)
1. A ground fault detector for a d.c. machine having a pair of terminals, power for said machine being provided by a convertor connected to said terminals, said convertor having in its output an undesired harmonic voltage, said detector comprising:
a ground sensing means connected to said terminals and having circuit means for detecting unbalanced currents resulting from an asymmetrical ground fault in said machine and having filtering means for filtering out at least a portion of said harmonic voltage, said ground sensing means producing at an output thereof a fault signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of said harmonic voltage at said output being less than the value of said fault signal.
a ground sensing means connected to said terminals and having circuit means for detecting unbalanced currents resulting from an asymmetrical ground fault in said machine and having filtering means for filtering out at least a portion of said harmonic voltage, said ground sensing means producing at an output thereof a fault signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of said harmonic voltage at said output being less than the value of said fault signal.
2. The ground fault detector of claim 2 wherein said circuit means comprises a balanced resistance network with a point at which said fault signal is detected, and said filtering means connected to said point for filtering out said portion of said harmonic voltage present at said point across a resistor to ground potential.
3. A ground fault detector for a d.c. machine having a pair of terminals, power for said machine being provided by a convertor connected to said terminals, said convertor having in its output an undesired common mode harmonic voltage, said detector comprising:
a ground sensing means connected to said terminals and having circuit means for detecting unbalanced currents resulting from an asymmetrical ground fault in said machine and having filtering means for filtering out at least a portion of said harmonic voltage, said ground sensing means providing at an output thereof a fault voltage signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of said harmonic signal at said output being less than the value of said fault signal, terminal sensing means connected to said terminals for detecting the output of said convertor and for providing a terminal output signal proportional thereto, and attenuating means connected to said terminal sensing means for receiving said terminal output signal, attenuating said terminal output signal by a predetermined amount to provide a threshold signal whose magnitude represents said attenuated terminal output signal, and comparing means for comparing the magnitudes of said fault signal and said threshold signal and generating a trip signal when said fault signal exceeds said threshold signal.
a ground sensing means connected to said terminals and having circuit means for detecting unbalanced currents resulting from an asymmetrical ground fault in said machine and having filtering means for filtering out at least a portion of said harmonic voltage, said ground sensing means providing at an output thereof a fault voltage signal whose magnitude is proportional to the ground fault current caused by an asymmetrical ground fault, the portion of said harmonic signal at said output being less than the value of said fault signal, terminal sensing means connected to said terminals for detecting the output of said convertor and for providing a terminal output signal proportional thereto, and attenuating means connected to said terminal sensing means for receiving said terminal output signal, attenuating said terminal output signal by a predetermined amount to provide a threshold signal whose magnitude represents said attenuated terminal output signal, and comparing means for comparing the magnitudes of said fault signal and said threshold signal and generating a trip signal when said fault signal exceeds said threshold signal.
4. The ground fault detector of claim 3 further comprising a switching means for simulating an asymmetrical ground fault by unbalancing the currents detected by said circuit means and indicating means indicating a symmetrical ground fault in said machine when said trip signal is not generated during the simulation.
5. The ground fault detector of claim 3 wherein said circuit means comprises a balanced resistor network having a mid-point at which said unbalanced currents are detected.
6. The ground fault detector of claim 5 wherein said filtering means is connected to said mid-point and any unbalanced currents detected at said midpoint develop a voltage drop across a resistor to ground potential.
7. The ground fault detector of claim 3 further including magnitude means associated with said comparing means for conforming the polarities of said fault signal and said threshold signal to opposing polarities.
8. The ground fault detector of claim 7 wherein said comparing means comprises a summer whose summation generates said trip signal when its polarity is the same as the polarity of said fault signal.
9. The ground fault detector of claim 8 wherein amplification means is provided to amplify the summation sufficiently to acuate logic into generating and holding said trip signal.
10. The ground fault detector of claim 3 wherein at least a portion of said harmonic voltage is filtered out by integration thereof.
11. The ground fault detector of claim 3 wherein said filtering means is adaptable for varying its integration time constant to correspond to a phase angle associated with said harmonic voltage.
12. The ground fault detector of claim 3 further including an additional attenuating means connected to said terminal sensing means for receiving said terminal output signal, additionally attenuating said terminal output signal by an amount greater than said predetermined amount to provide a warning threshold signal whose magnitude represents said additional attenuated terminal output signal, and warning comparing means for comparing the magnitudes of said fault signal and said warning threshold signal and
12. The ground fault detector of claim 3 further including an additional attenuating means connected to said terminal sensing means for receiving said terminal output signal, additionally attenuating said terminal output signal by an amount greater than said predetermined amount to provide a warning threshold signal whose magnitude represents said additional attenuated terminal output signal, and warning comparing means for comparing the magnitudes of said fault signal and said warning threshold signal and
Claim 12 continued:
generating a warning signal when said fault signal exceeds said warning threshold signal.
generating a warning signal when said fault signal exceeds said warning threshold signal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA324,203A CA1116236A (en) | 1979-03-23 | 1979-03-23 | Electronic ground detector for armature supplies |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA324,203A CA1116236A (en) | 1979-03-23 | 1979-03-23 | Electronic ground detector for armature supplies |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1116236A true CA1116236A (en) | 1982-01-12 |
Family
ID=4113830
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA324,203A Expired CA1116236A (en) | 1979-03-23 | 1979-03-23 | Electronic ground detector for armature supplies |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1116236A (en) |
-
1979
- 1979-03-23 CA CA324,203A patent/CA1116236A/en not_active Expired
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