CA1307822C - Floating negative automatic grounding switch - Google Patents
Floating negative automatic grounding switchInfo
- Publication number
- CA1307822C CA1307822C CA000519582A CA519582A CA1307822C CA 1307822 C CA1307822 C CA 1307822C CA 000519582 A CA000519582 A CA 000519582A CA 519582 A CA519582 A CA 519582A CA 1307822 C CA1307822 C CA 1307822C
- Authority
- CA
- Canada
- Prior art keywords
- ground
- negative bus
- voltage
- negative
- bus
- 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 - Lifetime
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
- H02H3/14—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 occurrence of voltage on parts normally at earth potential
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0038—Details of emergency protective circuit arrangements concerning the connection of the detecting means, e.g. for reducing their number
- H02H1/0053—Means for storing the measured quantities during a predetermined time
-
- 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
- H02H3/02—Details
- H02H3/05—Details with means for increasing reliability, e.g. redundancy arrangements
-
- 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
- H02H3/44—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 the rate of change of electrical quantities
- H02H3/445—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 the rate of change of electrical quantities of DC quantities
Landscapes
- Electric Propulsion And Braking For Vehicles (AREA)
Abstract
FLOATING NEGATIVE AUTOMATIC GROUNDING SWITCH
ABSTRACT OF THE DISCLOSURE:
In certain DC powered systems, particularly traction power systems such as electrified railways, it is becoming the practise to isolate both the positive and negative busses -- from which the system load derives its power -- from ground. However, the rolling stock (streetcars or train cars) maintains the chassis of the car at the potential of the negative bus; and under certain conditions its potential may float away from ground potential, to the extent that any person standing at ground potential and touching the chassis might receive an electric shock. The present invention provides a circuit having sensing means which determines whether the potential of the negative bus relative to ground has exceeded a predetermined limit, either positive or negative with respect to ground, or whether the rate of change of potential of the negative bus with respect to ground exceeds a predetermined rate; and in either event, the sensing means acts to cause a switch means which is usually a normally open silicon controlled rectifier arranged to face in the appropriate direction, to close and thereby to cause a short circuit between the negative bus and ground. The switch is retained in closed condition until the current through it reduces to substantially zero. In certain circumstances, it may be that it is the positive bus which is monitored. In any event, the circuit acts to ground the monitored bus under abnormal conditions, rather than to lift the monitored bus away from ground potential as would be the usual case.
ABSTRACT OF THE DISCLOSURE:
In certain DC powered systems, particularly traction power systems such as electrified railways, it is becoming the practise to isolate both the positive and negative busses -- from which the system load derives its power -- from ground. However, the rolling stock (streetcars or train cars) maintains the chassis of the car at the potential of the negative bus; and under certain conditions its potential may float away from ground potential, to the extent that any person standing at ground potential and touching the chassis might receive an electric shock. The present invention provides a circuit having sensing means which determines whether the potential of the negative bus relative to ground has exceeded a predetermined limit, either positive or negative with respect to ground, or whether the rate of change of potential of the negative bus with respect to ground exceeds a predetermined rate; and in either event, the sensing means acts to cause a switch means which is usually a normally open silicon controlled rectifier arranged to face in the appropriate direction, to close and thereby to cause a short circuit between the negative bus and ground. The switch is retained in closed condition until the current through it reduces to substantially zero. In certain circumstances, it may be that it is the positive bus which is monitored. In any event, the circuit acts to ground the monitored bus under abnormal conditions, rather than to lift the monitored bus away from ground potential as would be the usual case.
Description
~7~3~2 FLOATING NEGATIVE AUTOMATIC GROU~ING SWITC~l FIELD OE' THE_INVE~TION:
This invention relates to a circuit which monitors a floating bus in a direct current powered system such as a traction power system, for example a subway or other electrified railway. The invention is particularly adapted to provide a circuit for temporarily grounding the negative bus in a traction power system in the event that the voltage of the otherwise isolated negative bus floats to a prospectively hazardous potential with respect to ground potential.
BACKGROU~D OF THE INVENTION:
There have been, far many years, electrified railway lines which use direct current (DC) so as to provide traction power to rolling stock. For example, stree-t cars and certain 15 electrified railway lines may operate from steel rails at the negative side of the system and a suspended overhead wire at the positive side of the system. Other electrified railway systems, such as subway or underground train systems and the like, may comprise a third rail which is electrified at the positive 20 voltage of the trac~ion power system.
In all such systems as described above, it has for many -~ years been the usual practise that one or both of the rails over which the rolling stock moves is maintained at ground potential.
; ~ However, this has created serious problems in that the steel 25 rails which are otherwise in contact with -the earth and are therefore substantially at ground potential, may introduce stray electrical currents to local buried utilities such as hydro ,: .
, ; (electric power), water or sewer lines, telephone cables and the -- ~ .
. ' .
``` ~3~713~
like, or gas pipes, that may be in the general vicinity of the transit service. The steel rails may possibly be directly in contact with the earth, either directly by placement, or at the power source, or both.
In any event, any stray current which may occur can have a serious and detrimental effect on any buried utilities in the area, to the extent that there may be power losses, discontinuities in service from those utilities to local ; consumers, or hazardous conditions resulting.
To overcome and mitigate those problems, decisions have been made by operators oE transit systems or transit authorities, notably the Toronto Transit Comlr,ission, to isolate or insulate ~ both the negative and positive busses of the railway traction ; electrical power system, especially in third rail subway systems 15 and the like. Such decisions have therefore resulted in the electrical insulation of the rolling stock which moves over the steel rails, from ground. In most such systems it is the negative or return side of the DC power which is isolated from ground --the positive side being in any event removed from ground -- thus 20 resulting in a floating negative bus.
By providing insulation by which the running rails and third rail of the transit system are insulated from ground, and :
removing ground connections at the power source, stray currents to local buried utilities have been reduced to an acceptable 25 level. This in turn, however, has created another serious problem, which may directly affect the safety of the riders or operators of the rolling stock. The problem is that, under certain circumstances such as when a heavily loaded train is accelerating away from a station, the floating negative rail may ~, -. , ~ '7~;~Z
move away from substantially ground potential by as much as 300 volts.
Obviously, any one standing at ground potential who then touches the chassis of the rolling stock, may be liable to receive a potentially hazardous electrical shock~
In the event that the negative bus is permitted to float away from ground potential to a substantially or potentially hazardous extent, it is necessary to provide means to overcome the potential shock hazards. The present invention does that by 10 providing a circuit which monitors the condition of the negative bus with respect to ground, and which causes a switch to temporarily close in the event that the potential of the negative bus with respect to ground potential exceeds a ~predetermined limit.
The present invention also provides a circuit which may .
monitor the rate of change of the negative bus with respect to groundl and in the event that the rate of change exceeds a predetermined rate the sensing means will again cause the switch to close.
The net effect of closing the switch is, of course, to thereby ground the negative bus; and once the switch has been closed the current through the switch is monitored and the switch is retained in its closed position until the current through the switch reduces to substantially zero.
Of course, in the manner described hereaEter, the entire monitoring and grounding system can be connected to appropriate recorders so as to provide strip charts or the like by which the operation of the circuit may be continuously monitored and/or ~reviewed. This may provide the traction power system operators ~ 3 ;: ~
:~ - - . . . . .
, '' ~ . ~ . ' ~ ' , `` ~3~
the opportunity to predicte a major system failure before it occurs, and thereby to effect the necessary maintenance or design to preclude such Eailure. For the most part, state of the art microprocessors and solid state circuitry can be employed, so as to enable a maintenance operator to review and adjust the limits of the operating parameters beyond which the circuits of the present invention will cause an alarm status and will ground the monitored bus.
The conventional safety and potential hazard monitoring 10 systems in other than DC traction power systems are usually devised to determine if one or the other side of an electric power system becomes accidentally yrounded, and thereafter to isolate the system from ground or to shut down the system. This, of course, is found in ground fault monitoring, as it may be 15 carried out in mines, swimming pools, and the like; because it is the usual ease that if a fault oceurs in an eleetrical system, it is ~eeause one of the sides of that system faulted with respeet to ground potential, such as to create a shock hazard.
Of eourse, in the cireumstanees eontemplated by the 20 present invention, and as diseussed above, one of the sides of the traction power system is usually considered to be substantially at ground potential; and it is the purpose of the present invention to ensure that the side of the traction power system which is normally at ground potential does not float away ~; 25 from ground potential to the extent that a hazard may oceur in the~event that any person may eome into eontact with the floated potential.
Thus, the present invention provides a circuit whieh is intended for installation in an eleetrieal system where the load , 4 .
. . .
`~ ~L307~
may vary by increasing or decreasing between zero load current and a maximum current which is determined by the nature of the load, and where the loa~ is connected across a DC voltage source having a positive bus and a negative bus, both of which are isolated from ground potential. Sensing means are provided to determine the condition of voltage of one of the busses --usually the negative or return bus -- with respect to ground, and particularly to determine if the potential of the monitored bus with respect to ~round exceeds a predetermined voltage limit.
lO Switch means are provi~ed which, under the control oi the sensing means, will close and thereby temporarily ground the monitored bus under certain conditions of the voltage of that monitored bus having exceeded a predetermined limit or the rate of change of voltage of the ~lonitored bus away from ground potential having 15 exceeded a predetermined rate. In any event, when the switch means is closed, it is retained closed so as to temporarily ground the monitored bus -- that is, to cause its potential to reduce either in a positive or negative sense to become ground potential -- at which it is maintained for so long as there is 20 any substantial current ~low through the switch means from the monitored bus to ground.
The usual switch means that are provided in keeping with the present invention are one or more silicon controlled rectifiers, usually ones which have a high current rating, but 25 other switch means such as mechanical devices may be employed under certain circumstances and if the device can be triggered so as to open and close very rapidly and without failure.
United States patent 4,446,500, issued May l, 1984 teaches a remotely controlled circuit which has mechanical switch . -:: :
.
'' ' ~
. .
.
~ ~3~r41azV2 means. The circuit is otherwise a substantially solid statecircuit; but in any eventt it operates to disengage a load from the circuit upon the occurence of a fault condition.
A typical ground fault detection system is that taught in United Stated patent 3,754,221 issued August 21, 1973. There, a floating DC system is monitored, and has switches which are operable to resistances that are connected to the positive and negative busses. A fault condition is detected by sensing the voltage level between the switching device and ground, and ground faults are detected by alternately checking the positive and negative sides of the system against predetermined known ; reference resistance. Here again, however, the monitoring system seeks to determine if the floating DC system has accidentally gone to ground potential at other side.
15One other prior patent showing ground fault detection in a DC power system is taught in United States patent 4,188,574 issued February 12, 19~0. In that patent, a DC high voltage power supply is arranged with a detector which seeks to determine the existance of a lower impedance which suggests a ground fault between part of a floating secondary circuit in the high voltage DC supply and the chassis ground, which fault is manifested by an AC voltage. Once again, however, it is the purpose of that circuit to determine the presence of a ground fault current, rather than to determine the floating of an isolated bus away fro~n ground potential.
By employing silicon controlled rectifiers which have a high current rating, the present invention provides a circuit which not only will operate very rapidly, but which is substantially fail safe. That is because the triggering rate of ::
: ~:
~3~'7~2~
silicon controlled rectifiers is extremely ~ast; and moreover, a silicon controlled recitifer is such that i~ it ~ails, it will fail closed and therefore will short the monitored -- usually negative -- bus to ground. Since the operation of the circuit itself will be monitored, it is a simple matter to replace a silicon controlled rectifier. While that may result in a short term stray current conditions as discussed above, it ensures that the persons operating or as passengers in the traction power system are not placed in ]eopardy.
10While it is possible to set up a sensing circuit according to the present invention which is unidirectional, such arrangement is not usual unless it is known in which direction the monitored bus will always float -- for e~ample, in an M/A
(motor/alternator) circuit. Since, however, it is possible that ; 15 the floating bus may gain in potential with respect to ground either in a positive or negative sense, the more usual arrangement is for the system to be arranged to sense changes in potential of the monitored bus in either direction.
: :
In an extensive traction power system such as a subway ~ 20 system in a major city, as many as one circuit according to the ;~ present invention per station may be installed in the system. In any event, any such traction power system is divided into a number of sections, each receiving power from its own sub-station, and each such section would normally have a circuit ~25 according to the present invention installed within it.
BRIEF DESCRIPTION OF 'rHE DkAWING:
The present invention will be described hereafter in assoclation with the drawing, which illustrates a typical :
; ~ ~ 7 . , . :
~'~ ' ` '' ' ' : -~30'~ Z
traction power circuit having a monitoring and grounding switch circuit as taught hereby.
DESCRIPTION OF THE PREFERRED EMBODI~ENTS
As noted, it is the principal purpose of the present invention to provide a monitoring system which monitors one of the floating or isolated busses of a DC system, and to determine the voltage of the monitored bus with respect to ground potential, or the change of voltage of that monitored bus, beyond 10 certain predetermined limits. In the usual case, it is the negative bus that is monitored, and the monitoring is set up to determine whether the negative bus floats to a potential which may be either positive or negative with respect to ground.
The circuit 10 shows a load 12 being connected between a 15 positive bus 14 and a negative bus 16. The positive and negative busses 14 and 16 are in a DC traction power system, and the load 12 is the driving motors or the car or train. The negative bus is shown as being connected to one side of a sensing and switching circuit or system generally indicated at 18; the other 20 side of the sensing and switching system 18 being connected to a ground bus 20 which is grounded at 22. Within the sensing and switching circuit 18 there are voltage sensors 24 and 26, which are arranged to sense the voltage of the negative bus 16 with respect to the ground bus 20, in a reverse or forward direction, : :
25 respectively. Associated with the voltage sensors 24 and 26 are gate` triggers 28 and 30, which drive respective silicon controlled recti~iers 32 and 34. There may also be an additional pair of SCR's 36 and 38 in parallel with the silicon controlled rectifiers 32 and 34, respectively; and the SCR's 36 and 38 are .
~307~32~
usually of the same rating as the SCR's 32 and 34 so as to be in parallel redundancy with respect to the primary SCR's 32 and 34.
Each of the respective pairs of parallel redundant silicon controlled rectifiers is usually rated so as to carry the full current for the particular installation of the-e~e=~ according hereto.
In any event, the silicon controlled rectifiers 32 and 34 are connected in such a sense and in such a manner that they are normally open, but that they may close so as to permit 10 current flow through them from the negative bus to the ground bus, depending on the sense or direction in which the negative - bus has floated away from substantially ground potential. Thus, in the event that the negative bus floats to a predetermined voltage that is positive with respect to ground potential, a 15 voltage sensor 24 will trigger the gate trigger 28, causing the silicon control rectifier 32 (and usually also SCR 36) to close and thereby short the negative bus 16 to the ground bus 20; and likewise, SCR 3~ (and with it, SC~ 38) will be closed if the negative bus 16 floats to a negative potential with respect to 20 the ground bus 20 beyond a predetermined limit.
If either o the SCR's 32 or 34 is closed, (or SCR's 36 or 38), so as to short the negative bus 16 to ground, there will be current flow in the tie line 40. That current flow is sensed ~; by a current sensor 42, for a purpose discussed later. For as 25 long as there is a current sensor 42 detects a current in the tie line 40, the respecti~e SCR 32 or 34 (and/or 36 or 38) remains in a closed and therefore shorting condition. When the current in the tie line 40 reduces substantially to zero, or the voltage i:
across the respective SCR reverses the respective SCR 32 or 34 :
:
~'7~322 (or 36 or 3~) will autolnatically open, arld thereby once again - isolate the negative bus 16 from ground.
As stated above, it is possible that only the reverse or forward side of the sensing and switch circuit 18 may be installed, if it is known in which direction the negative bus 16 will always float away from ground potential. It is more usual, however, to install both of the reverse and forward sides of the sensing and switching circuit 18, so as to be able to determine an alarm condition if the negative bus floats in either direction 10 with respect to ground. It is also common to at least connect the negative bus 16 an~ the ground bus 20 to voltage recorder terminals 43 and 44, respectively; and the current sensor 42 may be connected to an alarln 46/ which may operate in the event that the current through the tie line 40 exceeds a predetermined 15 limit, or under other time-related circumstances. The voltage sensing means 24 and Z6 may also be set, by providing appropriate microprocessor circuits therein, to cause the associated SCR 32 or 34 (or 36 or 38) to close in the event that the rate of change ; of voltage of the negative bus 16 with respect to ground exceeds 20 a predetermined rate An excessive rate of change of potential of the negative bus 16 may be indicative of an incipient failure, or of a prospective hazardous condition, and in either event the voltage sensors 24 and 26 will act to preclude those possible hazardous condltions by grounding the negative bus 16.
In a typical subway system, without the installation of a circuit according to the present invention, it is possible that the negative rail 16 may float off ground potential by as much as 300 volts~ That condition may occur, for example, when a very heavy train is accelerating away from a subway station; and in ., ~ . - .
~ 3~ 22 any event, it represents a potentially very hazardous condition.
Thus, it is usual for the voltage sensors 24 and 26 to be adjusted to determine if the voltage of the negative bus may float away from ground potential by as little as 5 volts DC or up to as much as 150 volts DC. secause it is possible that such an increase of voltage of the negative bus with respect to ground may occur almost in any event as a car or train is accelerating away from a station, it is usual for the sensors 24 or 26 to have a timing circuit by which a set period of time will begin to run 10 at the instant that the voltage being sensed exceeds a predetermined limit. After a very short period of time, usually not more than several msec., the voltage sensor 24 or 26 will check once again to determine if the potential oE the negative bus 16 still remains above the predetermined voltage limit. If 15 so, the respective gate trigger 28 or 30 will operate, and the respective SCR 32 or 34 (and/or 36 or 38) will close. On the other hand, the voltage sensors 24 and 26 may also be set so that if the voltage of the negative bus 16 exceeds a predetermined level, even momentarily, the respective SCR 32 or 34 will be 20 caused to close. For example, that operation may occur at about volts DC, but that limit could be set higher or lower, at wi ll .
In the event that either of the SCR's 32 or 34 ~and/or 36 or 38) is closed, the current sensor 42 will determine the 25 current flow within the tie line 40. The current sensor 42 and alarm 46 are interactive to operate, or not, under certain different conditions. For example, if within up to about 20 or 25 msec. the current through the tie line 40 increases to not more than about 1500 amperes, or stays at a level not exceeding about ... ~ 1 1 ::
, ~ i,.. .... .. .
15~ amperes, nothing more will occur than that an internal counter will be incremented for purposes of overall system monitoring. However, if within a predetermined time period of, say 20 or 25 msec., the current flow has increased to and has remained above about 2500 amperes, the alarm 46 will be activated. Likewise, under other circumstances if the current through the tie line 40 exceeds a predetermined level in any event, the alarm 46 may be activated.
Needless to say, it is possible to apply conventional 10 microprocessor controls and monitoring to the system of the present invention, and the system may be used in varying conditions. For example, password protection can be installed to protect the settings of predetermined voltage, current and timing limits. Because of component reliability, or known failure modes, 15 the system is substantially fail safe. It may be used in a unidirectional basis, such as with stand-by motor/alternator system. A bi-directional system will detect faults between the positive and negative busses, where the negative bus~ floats posltive with respect to earth, positive to earth faults where 20 earth becomes positive with respect to the negative bus, or negative to earth faults where the negative bus becomes positive with respect to earth. These differing faults may manifest a floating negative bus beyond a present value, failure of the power system to ground, or a physical short circuit such as a 25 piece of metal physically connecting the negative or positive bus to ground.
It shoula, of course, be noted that the present ~' :
invention is applicable in any DC powered electrical system where the load is connected between the positive and negative side of ~:
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the system, and both sides of the system are permitted to float with respect to ground potential. Thus, the electrical system does not necessarily physically have to contain positive, negative and/or ground busses, it is sufficient that the electrical sides of the system can be determined.
The ambit of the present invention is defined by the accompanying claims.
.
:
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This invention relates to a circuit which monitors a floating bus in a direct current powered system such as a traction power system, for example a subway or other electrified railway. The invention is particularly adapted to provide a circuit for temporarily grounding the negative bus in a traction power system in the event that the voltage of the otherwise isolated negative bus floats to a prospectively hazardous potential with respect to ground potential.
BACKGROU~D OF THE INVENTION:
There have been, far many years, electrified railway lines which use direct current (DC) so as to provide traction power to rolling stock. For example, stree-t cars and certain 15 electrified railway lines may operate from steel rails at the negative side of the system and a suspended overhead wire at the positive side of the system. Other electrified railway systems, such as subway or underground train systems and the like, may comprise a third rail which is electrified at the positive 20 voltage of the trac~ion power system.
In all such systems as described above, it has for many -~ years been the usual practise that one or both of the rails over which the rolling stock moves is maintained at ground potential.
; ~ However, this has created serious problems in that the steel 25 rails which are otherwise in contact with -the earth and are therefore substantially at ground potential, may introduce stray electrical currents to local buried utilities such as hydro ,: .
, ; (electric power), water or sewer lines, telephone cables and the -- ~ .
. ' .
``` ~3~713~
like, or gas pipes, that may be in the general vicinity of the transit service. The steel rails may possibly be directly in contact with the earth, either directly by placement, or at the power source, or both.
In any event, any stray current which may occur can have a serious and detrimental effect on any buried utilities in the area, to the extent that there may be power losses, discontinuities in service from those utilities to local ; consumers, or hazardous conditions resulting.
To overcome and mitigate those problems, decisions have been made by operators oE transit systems or transit authorities, notably the Toronto Transit Comlr,ission, to isolate or insulate ~ both the negative and positive busses of the railway traction ; electrical power system, especially in third rail subway systems 15 and the like. Such decisions have therefore resulted in the electrical insulation of the rolling stock which moves over the steel rails, from ground. In most such systems it is the negative or return side of the DC power which is isolated from ground --the positive side being in any event removed from ground -- thus 20 resulting in a floating negative bus.
By providing insulation by which the running rails and third rail of the transit system are insulated from ground, and :
removing ground connections at the power source, stray currents to local buried utilities have been reduced to an acceptable 25 level. This in turn, however, has created another serious problem, which may directly affect the safety of the riders or operators of the rolling stock. The problem is that, under certain circumstances such as when a heavily loaded train is accelerating away from a station, the floating negative rail may ~, -. , ~ '7~;~Z
move away from substantially ground potential by as much as 300 volts.
Obviously, any one standing at ground potential who then touches the chassis of the rolling stock, may be liable to receive a potentially hazardous electrical shock~
In the event that the negative bus is permitted to float away from ground potential to a substantially or potentially hazardous extent, it is necessary to provide means to overcome the potential shock hazards. The present invention does that by 10 providing a circuit which monitors the condition of the negative bus with respect to ground, and which causes a switch to temporarily close in the event that the potential of the negative bus with respect to ground potential exceeds a ~predetermined limit.
The present invention also provides a circuit which may .
monitor the rate of change of the negative bus with respect to groundl and in the event that the rate of change exceeds a predetermined rate the sensing means will again cause the switch to close.
The net effect of closing the switch is, of course, to thereby ground the negative bus; and once the switch has been closed the current through the switch is monitored and the switch is retained in its closed position until the current through the switch reduces to substantially zero.
Of course, in the manner described hereaEter, the entire monitoring and grounding system can be connected to appropriate recorders so as to provide strip charts or the like by which the operation of the circuit may be continuously monitored and/or ~reviewed. This may provide the traction power system operators ~ 3 ;: ~
:~ - - . . . . .
, '' ~ . ~ . ' ~ ' , `` ~3~
the opportunity to predicte a major system failure before it occurs, and thereby to effect the necessary maintenance or design to preclude such Eailure. For the most part, state of the art microprocessors and solid state circuitry can be employed, so as to enable a maintenance operator to review and adjust the limits of the operating parameters beyond which the circuits of the present invention will cause an alarm status and will ground the monitored bus.
The conventional safety and potential hazard monitoring 10 systems in other than DC traction power systems are usually devised to determine if one or the other side of an electric power system becomes accidentally yrounded, and thereafter to isolate the system from ground or to shut down the system. This, of course, is found in ground fault monitoring, as it may be 15 carried out in mines, swimming pools, and the like; because it is the usual ease that if a fault oceurs in an eleetrical system, it is ~eeause one of the sides of that system faulted with respeet to ground potential, such as to create a shock hazard.
Of eourse, in the cireumstanees eontemplated by the 20 present invention, and as diseussed above, one of the sides of the traction power system is usually considered to be substantially at ground potential; and it is the purpose of the present invention to ensure that the side of the traction power system which is normally at ground potential does not float away ~; 25 from ground potential to the extent that a hazard may oceur in the~event that any person may eome into eontact with the floated potential.
Thus, the present invention provides a circuit whieh is intended for installation in an eleetrieal system where the load , 4 .
. . .
`~ ~L307~
may vary by increasing or decreasing between zero load current and a maximum current which is determined by the nature of the load, and where the loa~ is connected across a DC voltage source having a positive bus and a negative bus, both of which are isolated from ground potential. Sensing means are provided to determine the condition of voltage of one of the busses --usually the negative or return bus -- with respect to ground, and particularly to determine if the potential of the monitored bus with respect to ~round exceeds a predetermined voltage limit.
lO Switch means are provi~ed which, under the control oi the sensing means, will close and thereby temporarily ground the monitored bus under certain conditions of the voltage of that monitored bus having exceeded a predetermined limit or the rate of change of voltage of the ~lonitored bus away from ground potential having 15 exceeded a predetermined rate. In any event, when the switch means is closed, it is retained closed so as to temporarily ground the monitored bus -- that is, to cause its potential to reduce either in a positive or negative sense to become ground potential -- at which it is maintained for so long as there is 20 any substantial current ~low through the switch means from the monitored bus to ground.
The usual switch means that are provided in keeping with the present invention are one or more silicon controlled rectifiers, usually ones which have a high current rating, but 25 other switch means such as mechanical devices may be employed under certain circumstances and if the device can be triggered so as to open and close very rapidly and without failure.
United States patent 4,446,500, issued May l, 1984 teaches a remotely controlled circuit which has mechanical switch . -:: :
.
'' ' ~
. .
.
~ ~3~r41azV2 means. The circuit is otherwise a substantially solid statecircuit; but in any eventt it operates to disengage a load from the circuit upon the occurence of a fault condition.
A typical ground fault detection system is that taught in United Stated patent 3,754,221 issued August 21, 1973. There, a floating DC system is monitored, and has switches which are operable to resistances that are connected to the positive and negative busses. A fault condition is detected by sensing the voltage level between the switching device and ground, and ground faults are detected by alternately checking the positive and negative sides of the system against predetermined known ; reference resistance. Here again, however, the monitoring system seeks to determine if the floating DC system has accidentally gone to ground potential at other side.
15One other prior patent showing ground fault detection in a DC power system is taught in United States patent 4,188,574 issued February 12, 19~0. In that patent, a DC high voltage power supply is arranged with a detector which seeks to determine the existance of a lower impedance which suggests a ground fault between part of a floating secondary circuit in the high voltage DC supply and the chassis ground, which fault is manifested by an AC voltage. Once again, however, it is the purpose of that circuit to determine the presence of a ground fault current, rather than to determine the floating of an isolated bus away fro~n ground potential.
By employing silicon controlled rectifiers which have a high current rating, the present invention provides a circuit which not only will operate very rapidly, but which is substantially fail safe. That is because the triggering rate of ::
: ~:
~3~'7~2~
silicon controlled rectifiers is extremely ~ast; and moreover, a silicon controlled recitifer is such that i~ it ~ails, it will fail closed and therefore will short the monitored -- usually negative -- bus to ground. Since the operation of the circuit itself will be monitored, it is a simple matter to replace a silicon controlled rectifier. While that may result in a short term stray current conditions as discussed above, it ensures that the persons operating or as passengers in the traction power system are not placed in ]eopardy.
10While it is possible to set up a sensing circuit according to the present invention which is unidirectional, such arrangement is not usual unless it is known in which direction the monitored bus will always float -- for e~ample, in an M/A
(motor/alternator) circuit. Since, however, it is possible that ; 15 the floating bus may gain in potential with respect to ground either in a positive or negative sense, the more usual arrangement is for the system to be arranged to sense changes in potential of the monitored bus in either direction.
: :
In an extensive traction power system such as a subway ~ 20 system in a major city, as many as one circuit according to the ;~ present invention per station may be installed in the system. In any event, any such traction power system is divided into a number of sections, each receiving power from its own sub-station, and each such section would normally have a circuit ~25 according to the present invention installed within it.
BRIEF DESCRIPTION OF 'rHE DkAWING:
The present invention will be described hereafter in assoclation with the drawing, which illustrates a typical :
; ~ ~ 7 . , . :
~'~ ' ` '' ' ' : -~30'~ Z
traction power circuit having a monitoring and grounding switch circuit as taught hereby.
DESCRIPTION OF THE PREFERRED EMBODI~ENTS
As noted, it is the principal purpose of the present invention to provide a monitoring system which monitors one of the floating or isolated busses of a DC system, and to determine the voltage of the monitored bus with respect to ground potential, or the change of voltage of that monitored bus, beyond 10 certain predetermined limits. In the usual case, it is the negative bus that is monitored, and the monitoring is set up to determine whether the negative bus floats to a potential which may be either positive or negative with respect to ground.
The circuit 10 shows a load 12 being connected between a 15 positive bus 14 and a negative bus 16. The positive and negative busses 14 and 16 are in a DC traction power system, and the load 12 is the driving motors or the car or train. The negative bus is shown as being connected to one side of a sensing and switching circuit or system generally indicated at 18; the other 20 side of the sensing and switching system 18 being connected to a ground bus 20 which is grounded at 22. Within the sensing and switching circuit 18 there are voltage sensors 24 and 26, which are arranged to sense the voltage of the negative bus 16 with respect to the ground bus 20, in a reverse or forward direction, : :
25 respectively. Associated with the voltage sensors 24 and 26 are gate` triggers 28 and 30, which drive respective silicon controlled recti~iers 32 and 34. There may also be an additional pair of SCR's 36 and 38 in parallel with the silicon controlled rectifiers 32 and 34, respectively; and the SCR's 36 and 38 are .
~307~32~
usually of the same rating as the SCR's 32 and 34 so as to be in parallel redundancy with respect to the primary SCR's 32 and 34.
Each of the respective pairs of parallel redundant silicon controlled rectifiers is usually rated so as to carry the full current for the particular installation of the-e~e=~ according hereto.
In any event, the silicon controlled rectifiers 32 and 34 are connected in such a sense and in such a manner that they are normally open, but that they may close so as to permit 10 current flow through them from the negative bus to the ground bus, depending on the sense or direction in which the negative - bus has floated away from substantially ground potential. Thus, in the event that the negative bus floats to a predetermined voltage that is positive with respect to ground potential, a 15 voltage sensor 24 will trigger the gate trigger 28, causing the silicon control rectifier 32 (and usually also SCR 36) to close and thereby short the negative bus 16 to the ground bus 20; and likewise, SCR 3~ (and with it, SC~ 38) will be closed if the negative bus 16 floats to a negative potential with respect to 20 the ground bus 20 beyond a predetermined limit.
If either o the SCR's 32 or 34 is closed, (or SCR's 36 or 38), so as to short the negative bus 16 to ground, there will be current flow in the tie line 40. That current flow is sensed ~; by a current sensor 42, for a purpose discussed later. For as 25 long as there is a current sensor 42 detects a current in the tie line 40, the respecti~e SCR 32 or 34 (and/or 36 or 38) remains in a closed and therefore shorting condition. When the current in the tie line 40 reduces substantially to zero, or the voltage i:
across the respective SCR reverses the respective SCR 32 or 34 :
:
~'7~322 (or 36 or 3~) will autolnatically open, arld thereby once again - isolate the negative bus 16 from ground.
As stated above, it is possible that only the reverse or forward side of the sensing and switch circuit 18 may be installed, if it is known in which direction the negative bus 16 will always float away from ground potential. It is more usual, however, to install both of the reverse and forward sides of the sensing and switching circuit 18, so as to be able to determine an alarm condition if the negative bus floats in either direction 10 with respect to ground. It is also common to at least connect the negative bus 16 an~ the ground bus 20 to voltage recorder terminals 43 and 44, respectively; and the current sensor 42 may be connected to an alarln 46/ which may operate in the event that the current through the tie line 40 exceeds a predetermined 15 limit, or under other time-related circumstances. The voltage sensing means 24 and Z6 may also be set, by providing appropriate microprocessor circuits therein, to cause the associated SCR 32 or 34 (or 36 or 38) to close in the event that the rate of change ; of voltage of the negative bus 16 with respect to ground exceeds 20 a predetermined rate An excessive rate of change of potential of the negative bus 16 may be indicative of an incipient failure, or of a prospective hazardous condition, and in either event the voltage sensors 24 and 26 will act to preclude those possible hazardous condltions by grounding the negative bus 16.
In a typical subway system, without the installation of a circuit according to the present invention, it is possible that the negative rail 16 may float off ground potential by as much as 300 volts~ That condition may occur, for example, when a very heavy train is accelerating away from a subway station; and in ., ~ . - .
~ 3~ 22 any event, it represents a potentially very hazardous condition.
Thus, it is usual for the voltage sensors 24 and 26 to be adjusted to determine if the voltage of the negative bus may float away from ground potential by as little as 5 volts DC or up to as much as 150 volts DC. secause it is possible that such an increase of voltage of the negative bus with respect to ground may occur almost in any event as a car or train is accelerating away from a station, it is usual for the sensors 24 or 26 to have a timing circuit by which a set period of time will begin to run 10 at the instant that the voltage being sensed exceeds a predetermined limit. After a very short period of time, usually not more than several msec., the voltage sensor 24 or 26 will check once again to determine if the potential oE the negative bus 16 still remains above the predetermined voltage limit. If 15 so, the respective gate trigger 28 or 30 will operate, and the respective SCR 32 or 34 (and/or 36 or 38) will close. On the other hand, the voltage sensors 24 and 26 may also be set so that if the voltage of the negative bus 16 exceeds a predetermined level, even momentarily, the respective SCR 32 or 34 will be 20 caused to close. For example, that operation may occur at about volts DC, but that limit could be set higher or lower, at wi ll .
In the event that either of the SCR's 32 or 34 ~and/or 36 or 38) is closed, the current sensor 42 will determine the 25 current flow within the tie line 40. The current sensor 42 and alarm 46 are interactive to operate, or not, under certain different conditions. For example, if within up to about 20 or 25 msec. the current through the tie line 40 increases to not more than about 1500 amperes, or stays at a level not exceeding about ... ~ 1 1 ::
, ~ i,.. .... .. .
15~ amperes, nothing more will occur than that an internal counter will be incremented for purposes of overall system monitoring. However, if within a predetermined time period of, say 20 or 25 msec., the current flow has increased to and has remained above about 2500 amperes, the alarm 46 will be activated. Likewise, under other circumstances if the current through the tie line 40 exceeds a predetermined level in any event, the alarm 46 may be activated.
Needless to say, it is possible to apply conventional 10 microprocessor controls and monitoring to the system of the present invention, and the system may be used in varying conditions. For example, password protection can be installed to protect the settings of predetermined voltage, current and timing limits. Because of component reliability, or known failure modes, 15 the system is substantially fail safe. It may be used in a unidirectional basis, such as with stand-by motor/alternator system. A bi-directional system will detect faults between the positive and negative busses, where the negative bus~ floats posltive with respect to earth, positive to earth faults where 20 earth becomes positive with respect to the negative bus, or negative to earth faults where the negative bus becomes positive with respect to earth. These differing faults may manifest a floating negative bus beyond a present value, failure of the power system to ground, or a physical short circuit such as a 25 piece of metal physically connecting the negative or positive bus to ground.
It shoula, of course, be noted that the present ~' :
invention is applicable in any DC powered electrical system where the load is connected between the positive and negative side of ~:
~3~7~
the system, and both sides of the system are permitted to float with respect to ground potential. Thus, the electrical system does not necessarily physically have to contain positive, negative and/or ground busses, it is sufficient that the electrical sides of the system can be determined.
The ambit of the present invention is defined by the accompanying claims.
.
:
:~ ~, 13 ~ :
Claims (20)
1. . In a normally ungrounded electrical system having a load which may vary and increase or decrease between zero load current draw and a maximum load-related current draw, wherein said load is across a direct current (DC) voltage source having a positive bus and a negative bus, and wherein each of said positive and negative busses is isolated from ground potential, the improvement comprising:
sensing means for determining a condition of voltage of one of said busses with respect to ground beyond a predetermined voltage limit;
switch means for temporarily grounding said one of said busses to ground potential under the control of said sensing means; and monitoring means to retain said temporarily grounded bus at ground potential for so long as an anomalous condition permitting current flow through said switch means between said one of said busses and ground, continues to remain.
sensing means for determining a condition of voltage of one of said busses with respect to ground beyond a predetermined voltage limit;
switch means for temporarily grounding said one of said busses to ground potential under the control of said sensing means; and monitoring means to retain said temporarily grounded bus at ground potential for so long as an anomalous condition permitting current flow through said switch means between said one of said busses and ground, continues to remain.
2. The electrical system of claim 1, wherein said sensing means is arranged to sense the condition of said negative bus with respect to ground.
3. The electrical system of claim 2, wherein said sensing means senses the condition of the voltage of said negative bus, and is arranged to close said switch means when the voltage of said negative bus is positive with respect to said ground potential.
4. The electrical system of claim 2, wherein said sensing means senses the condition of the voltage of said negative bus, and is arranged to close said switch means when the voltage of said negative bus is negative with respect to said ground potential.
5. The electrical system of claim 2, wherein said sensing means senses the condition of the voltage of said negative bus, and is arranged to close said switch means when the voltage of said negative bus is either positive or negative with respect to said ground potential.
6. The electrical system of claim 5, wherein said sensing means is arranged to close said switch means if the voltage of said negative bus with respect to ground changes at a rate beyond a predetermined rate.
7. The electrical system of claim 5, wherein said monitoring means acts to retain said switch means in the closed condition until the negative bus voltage with respect to ground potential reverses from the polarity at the time that said switch means was closed.
8. The electrical system of claim 5, wherein said switch means remains in the closed condition until the current flow through said switch means reduces to a predetermined low current level.
9. The electrical system of claim 1, wherein said switch means is at least one silicon controlled rectifier which is arranged to be open under normal conditions, and which when closed permits current flow between said one of said busses and ground.
10. The electrical system of claim 5, wherein said switch means is at least two silicon controlled rectifiers arranged to face one each in opposite directions between said negative bus and ground, so that one or the other of said rectifiers will close and permit current flow between said negative bus and ground, depending on the positive or negative sense of the voltage of said negative bus with respect to ground, at the instant that its voltage with respect to ground exceeds said predetermined limit.
11. The electrical system of claim 10, wherein each of said silicon controlled rectifiers has a parallel silicon controlled rectifier facing in the same direction, in association therewith, for purposes of parallel redundancy thereof.
12. The electrical system of claim 6, wherein said switch means is at least two silicon controlled rectifiers arranged to face one each in opposite directions between said negative bus and ground, so that one or the other of said rectifiers will close and permit current flow from between negative bus and ground, depending on the positive or negative sense of the voltage of said negative bus with respect to ground, at the instant that the rate of change of its voltage exceeds said predetermined rate.
13. The electrical system of claim 2, wherein said sensing means is arranged to close said switch means if the voltage of said negative bus with respect to ground changes at a rate beyond a predetermined rate.
14. The electrical system of claim 2, wherein said monitoring means acts to retain said switch means in the closed condition until the negative bus voltage with respect to ground potential reverses from the polarity at the time that said switch means was closed.
15. The electrical system of claim 2, wherein said switch means remains in the closed condition until the current flow through said switch means reduces to a predetermined low current level.
16. The electrical system of claim 2, wherein said switch means is at least one silicon controlled rectifier which is arranged to be open under normal conditions, and which when closed permits current flow between said negative bus and ground.
17. The electrical system of claim 13, wherein said switch means is at least two silicon controlled rectifiers arranged to face one each in opposite directions between said negative bus and ground, so that one or the other of said rectifiers will close and permit current flow between said negative bus and ground, depending on the positive or negative sense of the voltage of said negative bus with respect to ground, at the instant that the rate of change of its voltage exceeds said predetermined rate.
18. The electrical system of claim 2, wherein said sensing means senses the condition of said negative bus and causes a timing operation to occur at the instant that the voltage of said negative bus exceeds a first predetermined limit; and wherein after a predetermined passage of time said sensing means senses said negative bus voltage, and causes said switch means to close if said negative bus voltage remains above said first predetermined limit.
19. The electrical system of claim 5, wherein said sensing means senses the condition of said negative bus and causes a timing operation to occur at the instant that the voltage of said negative bus exceeds a first predetermined limit; and wherein after a predetermined passage of time said sensing means senses said negative bus voltage, and causes said switch means to close if said negative bus voltage remains above said first predetermined limit.
20, The electrical system of claim 5, wherein said monitoring means is arranged to cause an alarm condition in the event that current through said switch either remains above a first predetermined level after a predetermined passage of time, or exceeds a second predetermined level.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000519582A CA1307822C (en) | 1986-10-01 | 1986-10-01 | Floating negative automatic grounding switch |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000519582A CA1307822C (en) | 1986-10-01 | 1986-10-01 | Floating negative automatic grounding switch |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1307822C true CA1307822C (en) | 1992-09-22 |
Family
ID=4134065
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000519582A Expired - Lifetime CA1307822C (en) | 1986-10-01 | 1986-10-01 | Floating negative automatic grounding switch |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1307822C (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2009000809A2 (en) | 2007-06-28 | 2008-12-31 | Siemens Aktiengesellschaft | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| DE102008049705A1 (en) * | 2008-09-30 | 2010-04-01 | Siemens Aktiengesellschaft | Device and method for limiting the rail potential in a DC track |
| DE102008049706A1 (en) * | 2008-09-30 | 2010-04-01 | Siemens Aktiengesellschaft | Device and method for limiting the rail potential in a DC track |
-
1986
- 1986-10-01 CA CA000519582A patent/CA1307822C/en not_active Expired - Lifetime
Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2009000809A2 (en) | 2007-06-28 | 2008-12-31 | Siemens Aktiengesellschaft | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| WO2009000809A3 (en) * | 2007-06-28 | 2009-04-30 | Siemens Ag | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| US7952849B2 (en) | 2007-06-28 | 2011-05-31 | Siemens Aktiengesellschaft | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| AU2008267285B2 (en) * | 2007-06-28 | 2013-05-02 | Siemens Aktiengesellschaft | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| CN101687466B (en) * | 2007-06-28 | 2013-07-10 | 西门子公司 | Rail vehicle with a wagon body and method for protective grounding of such a wagon body |
| DE102008049705A1 (en) * | 2008-09-30 | 2010-04-01 | Siemens Aktiengesellschaft | Device and method for limiting the rail potential in a DC track |
| DE102008049706A1 (en) * | 2008-09-30 | 2010-04-01 | Siemens Aktiengesellschaft | Device and method for limiting the rail potential in a DC track |
| WO2010037579A1 (en) * | 2008-09-30 | 2010-04-08 | Siemens Aktiengesellschaft | Device and method for limiting the rail potential in a dc railway |
| CN102210077A (en) * | 2008-09-30 | 2011-10-05 | 西门子公司 | Device and method for limiting the rail potential in a DC railway |
| CN102210077B (en) * | 2008-09-30 | 2014-07-16 | 西门子公司 | Device and method for limiting the rail potential in a DC railway |
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