AU597005B2 - Automatic ground fault protection for an electric power system - Google Patents

Description

CO,?RM0NEAhJTE OF AtJSTRAIA1, PATENr I ACT, 1952 COHPLETE SPECIFICATION Form e q ulation 13 (2)

(ORIGINAL)

FOR OFFICE USE 59705 Short Title: Int. C: 2.plication Nuber: C 42,2-7 Lodged, Complete Specification-Lodged; Accepted3 Lapsed: Published.

u fLf~ o t t Priority: ei t q r A~lated Art: 4 4 TO BE COMAPLETED BY APPLICR IT Name of Applicant GENERAL ELECTRIC COMPANY 4 Adress of Applicant: 1 River Road, Schenectady, New York, U.S.A.

P Actual Inventor: Address for Servicei Ajith Kuttanair KUMAR, Myron Lee SMITH William 1ohn WHITE ARTHUR S. CAVr CO., Patent and Trade Mark Attorneys, 1 Alfred Street, Sydney, New South Wales, Ajastralia, 2000.

Complete Specification for the invention entitled: JQTOP(VC C-(ZOU0 FR)Aui 1PR0'C6C'V I oON T:Q aec-r 'c poea, s-/sTGmr) The following statement is a V!ull description of this invention, including the,,best method of performinhg it knoin to me-:- -1- ASC-49 0~ i.rr~r rm* !77 Background of the Invention This invention relates generally to an electric power system in which a variable amount of electric power is supplied to an electric load circuit from a controllable source of power, and it relates more particularly to improved means for protecting such a system in the event of abnormally high magnitudes of ground leakage current in the system.

The invention is described in thsp context of a propulsion system for a large self-propelled traction vehicle, such as a locomotive, wherein a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) is used to drive an, electrical transmission comprising generating means for supplying electric current to a plurality of direct current traction motors whose rotors are drivingly coupled t.hroijgh speed-reducing *gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator.

09CC These voltages are rectified and applied to the armature and/or field windings of the traction motors.

During the "motoring" or propulsion mode of operation, a U' locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in di~icrete steps between zero and maximum, and so tha t the engine develops whztever 'level of power the traction and auxiliary loads demand.

-2- Engine horsepower is' proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulatinn governor which adjusts the quantity of pressurized diesel fue'i fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches"s between a low power position (Ni) and a maximum power position The throttle handle is part of the control console located in the operator's cab of the locomotive.

I. The position of the throttle handle determines the engine speed setting of the governor.

For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of Ott, horsepower (assuming maximum output torque). When the throttle Ott,"notch 8 is selected, maximum speed 1,050 rpm) and maximum rated gross horsepower 4,000) are realized. Under normal ***conditions the engine power at each notch equals the power *.demanded by the. electric propulsion system which is supplied by 0 10 .0the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxi ay qipmns The output power (KVA) of the main alternator is proportional to the product of the rims magnitudes of generated :.~.voltage and load current. The volta5,e magnitude varies with the rotational speed of the engine, and It is also a function of the amount of current in the alternator armature and field windings, respectively. For the purpose of accurately controlling and regulating its power output, it is common pr~actice to adjust the field strength of the traction alternator to compensate for load changes and to minimize the error between actual and desired KVA.

The desired power depends on the specific speed setting of the -3engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant: optimum electrical power output for each position of the throttle handle. The alternator output regulating function is performed by an asscciated controller which is responsive to the throttle position and to a plurality of feedback signals representative, respectively, of certain parameters or quantities (such as the magnitudes of the alternator output voltage and current) of the electric propulsion system.

In an electric propulsion system, all of the power components (alternator, rectifier, traction motors, and their interconnecting contactors and cables) need to be well insulated to avoid harmful short circuits between the electrically energized parts of these components and ground. The insulation has to withstand very harsh conditions on a locomotive, including constant vibration, frequent mechanical shocks, infrequent maintenance, occasional electrical overloads, a wide range of ambient temperatures, and an atmosphere that can be very wet and/or dirty. If the insulation of a component were damaged, or if its dielectric strength deteriorates, or if moisture or an accumu.lation of dirt were to provide a relatively low resistance path through or on the surface of the insulation, then undesirably high leakage current can flow between the component and the locomotive frame which is at ground potential. Such an insulation breakdown can be accompanied by ionization discharges or flashovers. The discharge will start before the voltage level reaches its ultimate breakdown value. The dirtier and wetter the insulation, the lower the discharge starting voltage relative to the actual breakdown value. Without proper detection and timely protection, there is a real danger that an initiaily harmless electrical discharge will soon grow or propagate tu\ an extent that causes serious or irreparable daimage to the insulation system and possibly to the equipment itself.

i i- r I i, 9 44 94 9 99 44 *r 9 9**9 d 4 4 p 4i 44 9 *i S 9* 9 It is conventional practice to provide ground fault protection for locomotive propulsion systems. In a typical prior art practice, the operating coil of a ground relay is connected between the locomotive frame (ground) and a point between the armature and series field windings of one of the d-c traction motors of the propulsion system. This is the only point of the system that is intentionally grounded, and normally the ground leakage current in the relay coil has a negligible magnitude.

However, in the event of a ground fault, the leakage current magnitude increases above the "pickup" point 0.25 ampere) of the relay, whereupon the ground relay initiates the opening of a contactor in series with the alternator field and thereby shuts down the electrical propulsion system. At the same time, an alarm bell is sounded and an appropriate light on an annunciator is turned on. The locomotive operator can then manually reset the propulsion system and restore traction power. The reset mechanism is arranged to lockout after three tries. This prior art ground relay is sensitive enough to respond to any potentially harmful degradation of the insulation system. But a propulsion outage due to ground relay action may sometimes be unnecessary, as when the increase in leakage current is due primarily to moisture in the insulation system, and any such outage will undesirably reduce the productivity of the locomotive.

Summary of the Invention (non-limiting) A general objective of the present invention is to provide improved means for protecting an electric power system in automatic response to the detection of actual or incipient ground faults.

A more specific ob..ective is the provision, for an electric power system including means for varying the amount of power supplied from a controllable source to an electric load circuit, of ground leakage current responsive means that automatically initiates a series of power limiting and restoring measures to *k S r* ,i $4 i; We..

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4l V. 4 *4 4 protect the power components of the system from serious damage due to ground faults without unnecessarily disrupting normal operation thereof.

In carrying out the invention in one form, electric power is supplied to an electric load circuit from a suitable source of power, and the amount of power is varied as a function of the value of a variable control signal that is provided by associated control means. Normally the control signal value is determined by a given command signal in combination with other selected input signals to the control means. A representative feedback signal is derived from ground leakage current in either the power source or the load circuit. The control means includes ground fault responsive means activated when the feedback signal indicates that the magnitude of leakage current is abnormally high to modify the value of the control signal in the following manner: If the ,kage current rises to a magnitude higher than a predetermined deration threshold level but not higher than a predetermined maximum permissible limit, the control signal is limited so that the power output of the source is reduced to a, fraction of its normally desired amount (which fraction is inversely proportional to the leakage current magnitude in excess of the threshold level), and If the leakage current magnitude rises above the maximum linit, the control signal is limited so that the power output is restricted to zero for at least a predetermined time interval 15 seconds) and at the end of that interval the zero-powi- restriction is automatically removed if the leakage current magnitude is then below a certain reset point (which is appreciably lower than the maximum limit).

The ground fault responsive means is so arranged that it will not automatically remove the zero-power restriction after the leakage current magniiuda has remained continuously above the aforesaid r -6reset point for a predetermined span of time after increasing above the maximum limit, or after the ground fault responsive means has a history of being repeatedly activated different times within a predetermined period 30 minutes) immediately preceding the time at which leakage current again increases above the maximum limit. In either case, a "permanent" rather than a temporary fault in the ground insulation is assumed, and the electric power system remains shut down until an authorized maintainer finds and corrects the problem and then manually resets the ground fault protection means. But a temporary ground fault (which typically is caused by excessive moisture) is allowed to cure itself (as by drying oi!t so that the normal dielectric strength of the insulating medium is restored) o during the time power is fractionally reduced, or during the ,r short interval 15 seconds) of zero power before this restriction is automatically removed, thereby avoiding an o* unnecessary or prolonged loss of power.

o The invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying o drawings.

Brief Description of the Drawings Fig. 1 is a schematic diaqram of an electrical propulsion 'c" system for a traction vehicle, including a thermal prime mover (such as a diesel engine), a traction alternator, a plurality of traction motors, and a controller; 1Fig. 2 is an expanded block diagram of the controller (shown as a single block in Fig. 1) which produces output signals for controlling the field excitation of the alternator and the rotational speed of the engine; Fig. 3 is a diagram of an "equivalent circuit" that is used to illustrate the manner in which the controller normally produces the alternator field excitation control signal and also -7to illustrate its significant interfaces with the system ground fault protection means of the present invention; Fig. 4 is a flow chart that explains the preferred manner of providing automatic ground fault protection in accordance with the present invention; and Figs. 5, 6, 7 and 8 are flow charts that explain the operations of the preferred embodiments of the four subroutines that are shown as single steps in Fig. 4.

Description of the Preferred Embodiment The propulsion system shown in Fig. 1 includes a variable-speed prime mover 11 mechanically coupled to the rotor of a dynamoelectric machine 12 comprising a 3-phase alternating current synchronous generator, also referred to as the main traction alternator. The main alternator has a set of three *4 star-connected armature windings on its stator. In operation it Sgenerates 3-phase voltages in these windings, which voltages are applied to a-c input terminals of at least one 3-phase, *double-way uncontrolled power rectifier bridge 13. The rectified electric power output of the bridge 13 is supplied, via a d-c bus 14 and individual contactors (15C, 16C), to an electric load circuit comprising parallel-connected armature windings of a ,plurality of variable-speed d-c traction motors, only two of which (15,16) are identified in Fig. 1. The described power components 11-16 are all located on board a self-propelled .traction vehicle such as a locomotive. In practice each traction S motor is hung on a different axle of the locomotive, and its shaft is coupled to the associated axle by speed-reduction S 'gearing (not shown). There are usually two or three axles per truck, and there are two trucks per locomotive.

The traction motors have non-rotating field windings (not shown) that are respectively connected in series with the windings on their rotatable armatures during the motoring or propulsion mode of operation. However, for braking or retarding the locomotive the armature windings of the traction motors are 4.

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44 disconnected from the power rectifier 13 and connected to a conventional dynamic braking resistor grid (not shown), and the motor field windings are reconnected in series with each other for energization by the rectified output of the main alternator 12. (Alternatively, a-c traction motors could be used, in which case suitably controlled electric power inverters would be connected between the respective motors and the d-c bus 14.) Field windings 12F on the rotor of the main alternator 12 are connected for energization through a contactor 12C to the output of a suitable source 17 of regulated excitation current.

Preferably the source 17 comprises a 3-phase controlled rectifier bridge the input terminals 18 of which receive alternating voltages from a pri,,,e mover-driven auxiliary alternator that can actually comprise an auxiliary set of 3-phase armature windings on the same frame as the main alternator 12. This source includes conventional means for varying the magnitude of the direct current that it supplies to the alternator field as necessary to minimize any magnitude difference between a variable control signal on an input line 19 and a feedback signal which during motoring is representative of the average magnitude V of the output voltage of the power rectifier 13. The latter nl&tage magnitude is a known function of the magnitude of excitation current in the field windings 12F and of the magnitude of output current in the armature windings of the main alternator 12, respectiveily, -nr1 it also varies with the speed of the prime mover 11. It is sensed by a conventional voltage sensing module connected across the d-c output terminals of the power rectifier.

A current detecting module 22 of relatively low resistance approximately 125 ohms) is connected between the neutral S of the alternator stator windings and the grounded chassis or frame of the locomotive, and it provides on an output line 23 a feedback signal representative of the magnitude (IGND) of ground leakage current in the electric preopulsion system. It will be apparent that IGND is a measure of curren't flowing, 'via the

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-9module 22, between the neutral S and any ground fault in the stator windings of the main alternator 12, in the power rectifier 13, or in the electric load circuit that is connected to the power rectifier. The latter circuit includes the field windings of the traction motors 15, 16, etc. and, in the motoring mode of operaticn, the motor armature windings as well.

The prime mover 11 that drives the alternator field 12F is a thermal or internal-combustion engine or equivalent. On a diesel-electric locomotive, the motive power is typically provided by a high-horsepower, turbocharged, 4-stroke, 16-cylinder diesel engine. Such an engine has a number of ancillary systems, some of which are represented by labeled blocks in Fig. 1. A diesel engine fuel system 24 conventionally includes a fuel tank, fuel pumps and nozzles for injecting fuel o. oil into the respective power cylinders which are arranged in two I. 4 rows or banks on opposite sides of the engine, tappet rods U 'cooperating with fuel cams on a pair of camshafts for actuating the respective injectors at the proper times during each full s turn of the crankshaft, and a pair of fuel pump racks for controlling how much fuel oil flows into a cylinder each time the associated injector is actuated. The position of each fuel pump o- 0* rack, and hence the quantity of fuel that is being supplied to the engine, is controlled by an output piston of an engine speed governor system 25 to which both racks are linked. The governor regulates engine speed by automatically displacing the racks, within predetermined limits, in a direction and by an amount that minimizes any difference between actual and desired speeds of the engine crankshaft. The desired speed is set by a variable speed control signal received from an associated controller 26, which signal is herein called the speed command signal or the speed call signal. An engine speed signal RPM indicates the actual rotational speed of the engine crankshaft and hence of the alternator field.

N; A The speed command signal for the engine governor system and the excitation control signal for the alternator field regulator 17 are provided by the controller 26. In a normal motoring or propulion mode of operation, the values of these signals are determined by the value of a command signal that is given to the controller by a manually operated throttle 27 to which the controller is coupled. A locomotive throttle conventionally has eight power positions or notches plus idle and shutdown. N1 corresponds to a minimum desired engine speed (power), while N8 corresponds to maximum speed and full power. When dynamic braking of a moving locomotive is desired, the operator moves the throttle handle to its idle position and manipulates a manually operated lever of a convwtional brake controller 28 so that the miain controller 26 is now supplied with a variable "brake call" signal that will determine the value of the alternator excitation control signal. (In the braking mode, S* a feedback signal which is representative of the magnitude of the r0,: current being supplied to the motor field windings from the g rectified output of the main alternator 12 will be supplied to the alternator field regulator 17 and there subtracted from the control signal on line 19 to determine the differenr, or error signal to which the regulator responds.) In a consist of two or •4 more locomotives, only the lead unit is usually attended, and the main controller on board each trail unit will receive, over trainlines, encoded signals that indicate the throttle position S* or brake call selected by the operator in the lead unit.

For each power level of the engine there is a corresponding desired load. The controller 26 is suitably arranged to p t "translate the notch information from the throttle 27 ioto a control signal of appropriate magnitude on the input line 19 of the alternator field regulator 17, whereby in motoring the traction power is regulated to match the called-for power so long as the alternator output voltage and load current are both within predetermined limits. For this purpose, and for the purpose of 11 7' i i I UI Illllll~ CII IP~ -11- *rt 9 9 t $9 90 9 9q4 9.

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deration unloading the engine) in the event of certain abnormal conditions, it is necessary to supply the controller 26 with information about various operating conditions and parameters of the propulsion system.

More particularly, the controller 26 typically receives the above-mentioned engine speed signal RPM, the voltage feedback signal V, and current feedback signals 11, 12, etc. which are representative, respectively, of the magnitude of current in the armature windings of the individual traction motors. It also receives a load control signal issued by the governor system if the engine cannot develop the power demanded and still maintain the called-for speed. (The load control signal is effective, when issued, to reduce the magnitude of the control signal on the line 19 so as to weaken the alternator field until a new balance point is reached.) As is illustrated in Fig. 1, the controller is supplied with additional data including: "VOLT MAX" and "CUR MAX" data that establish absolute maximum limits for the alternator output voltage and current, respectively; "CRANK" data indicating whether or not an engine starting cranking) routine is being executed; and relevant inputs from other selected sources, as represented by the block labeled "OTHER." The alternator field regulator 17 communicates with the controller via a multiiine serial data link or bus 21. The controller 26 also communicates with "CONTACTOR DRIVERS" (block 29) which are suitably constructed and arranged to actuate the alternator field contactor 12C and the individual traction motor con%,ctors 15C, 16C, etc. in accordance with commands from the controller.

For the purpose of responding to ground faults in the propulsion system, the controller 26 is supplied, via the output line 23 of the current detecting module 22, with the aforesaid feedback signal whose value varies with the magnitude IGND of ground leakage current. If this signal inddiates tfht 4IND if abnormally high, the controller executes cptai (tve 74 in the cab of the locomotive.

In the presently preferred embodiment of the invention, the controller 26 comprises a microcomputer. Persons skilled in the art will understand that a microcomputer is actually a coordinated system of commercially available components a:d associated electrical circuits and elements that can be programmed to perform a variety of desired functions. In a typical microcomputer, which is illustrated in Fig. 2, a central processing unit (CPU) executes an operating program stored in an erasable and electrically reprogrammable read only memory (EPROM) which also stores tables and data utilized in the program.

o0 Contained within the CPU are conventional counters, registers, I .accumulators, flip flops (flags), etc., along with a precision oscillator which provides a high-frequency clock signal. The microcomputer also includes a random access memory (RAM) into which data may be temporarily stored and from which data may be read at various address locations determined by the program stcred in the EPROM. These components are interconnected by appropriate address, data, and control buses. In one practical embodiment of the invention, an Intel 8086 microprocessor is used.

The other blocks shown in Fig. 2 represent conventional peripheral and interface components that interconnect the microcomputer and the external circuits. More particularly, the block labeled is an input/output circuit for supplying the i microcomputer with data representative of the selected throttle 2 position or the brake ommnand and with digital signals representative of the readin' of various voltage, current and other feedback sensing modules associated with the locomotive propulsion system. The latter signals are derived from an analog-to-digital converter 31 connected via a conventional multiplexer 32 to a plurality of signal conditioners to which the -26minutes, and a fifth one occurs within 30 minutes after the -13sensor outputs are respectively applied. The signal conditioners serve the conventional dual purposes of buffering and biasing the analog sensor output signals. As is indicated in Fig. 2, the input/output circuit also interconnects the microcomputer with the alter.nator field regulator via the multiline bus 21, with the engine speed governor, with the display module, with the contactor drivers, and with a digital-to-analog signal converter 33 whose output is connected to the line 19.

The controller 26 is programmed to produce, on the line 19, a control signal having a magnitude that depends on either the throttle position selected by the locomotive -perator (in the normal motoring mode of operation) or the brake command selected by the operator (in the dynamic braking mode). The presently preferred manner in which this is accomplished during motoring is described in a copending U.S. patent application S.N. 747,628 filed on June 24, 1985, in the joint names of E. T. Balch, R. E.

Bilski and A. K. Kumar and assigned to General Electric Company, °which patent application is incorporated herein by reference. A simplified block diagram of some of its presently significant functions is shown in Fig. 3 which will now be described.

As is explained in the referenced application, the S- alternator excitation control programs (reference No. 41 in Fig.

3) include routines for, providing, on two channels labeled "PWR" and "V respectively, numbers representing reference values of traction power and of a common voltage and current limit.

Both of these values are on a per axle basis. Normally they vary 4 ,with the throttle position, being highest at notch 8. But the S* normal values we appropriately modified by a rate limit function in the event of a step change in the call data and by a deration function in response to a wheelslip or certain other temporary abnormal conditions.

As is illustrated in Fig. 3, the modified power reference value from the excitation control programs is fed to an "unbalance correction" function 42 which is also supplied, on a 11 r,# ~~~1XIC-WII- li.~ -14line labeled "KVA(AV)," with datum representative of the actual kilowatts of power output (per axle) of the traction alternator 12. (Suitable signal processing programs are included in the block 41 for the purpose of deriving the latter datum from the feedback signals V, II, 12, etc.) The output of the unbalance correction function represents the desired value of the alternator output. It will differ from the modified power reference input when necessary to correct for any appreciable power unbalance among the various traction motors of the locomotive.

The modified V I reference value is fed to a "reference limits" function 43 which is also supplied with data representative of the maximum current and voltage limits established by CUR-MAX and VOLT MAX, respectively. The latter limit is hereinafter called VMAX. In the function 43 the common V I reference input is deployed to provide a limited current 0* reference value that varies with the input up to the maximum limit of current and to provide a separate limited voltage reference value that also varies with the input up to the limit established by VMAX.

The limited voltage reference value is compared with the actual value of the alternator voltage feedback signal V to derive a voltage error value equal to their difference. This error is then processed in accordance with a programmed compensation routine to derive a voltage control value that is representative of the voltage error value. The compensation S routine introduces a proportional plus integral transfer funcion (see reference No. 44 i. Fig. the gain of which is determined 0, by datum that depends on the throttle position and other parameters of the locomotive and its controls. Thus the voltage control value varies as a function of the time integral of its associated error value. Similar routines (not shown in Fig. 3) are provided for comparing the limited current reference value with the actual va'ie of the motor current feedback signal I(MAX) -4 l ]i i

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from the most loaded traction motor to derive a current error value equal to their difference, and for comparing the desired power value with the actual power demand of the most loaded motor, as found by Multiplying V by I(MAX), to derive a power error value equal to their difference, if any, The latter two error values are then processed in accordance with programmed compensation routines similar to the transfer function 44 to derive current and power control values that are respectively representative of the current and power error values. All three of the control values are supplied to a gate 45 that selects the least value for passing to a limit function 46 from which an output signal VC is derived, and accordingly the value of VC corresponds to the smallest control value. Means 47 for clamping the value of the output signal VC to zero is pro"' etween the limit function 46 and the digital-to-analog converter 33.

9 r The value of VC determines the magnitude of the analoy control signal that the controller 26 supplies, via the line 19, to the alternator field regulator 17 (Fig. Jn the motoring I mode of operation the field regulator will respond to the latter signal by varying the field strength of the traction alternator as necessary to minimize any difference between the value of the voltage feedback signal V and the value of the output signal VC.

So long as both V and I(MAX) are within a limit that varies with r,o the throttle position and are not above their respective maximum limits as imposed by the function 43, the value of VC is determined by the power control value which will now be smaller than either the voltage or current control value. Consequently *the alternator output voltage is maintained at whatever level results in essentially zero error between actual and desired traction power. But if V (or IMAX) tends to exceed its limited reference value, the voltage (or current) control value is driven lower than the power control value and the value of VC acccOrdingly decreases, whereby the alternator output is adjusted to whatever level results in zero voltage (or current) error.

ii .llxVLn-- -16- In accordance with the present invention, the value of the control signal on line 19 is reduced to zero, thereby restricting the power output of the main alternator 12 to zero, by activating the clamping means 47 for at least a predetermined minimum interval of time approximately 15 seconds) if the magnitude of ground leakage current in the propulsion system rises above a predetermined maximum permissible lmit.

Concurrently with the start of this clamping action, contactor opening commands are given to the contactor drivers 29, the normal excitation control programs 41 are disabled or turned off, and the occurrence of the ground fault is logged in the display module 30. For the purpose of detecting and responding to such a ground fault, the controller 26 includes ground fault responsive o. means which in Fig, 3 is symbolized by a block 48 labeled "GNOD .i FAULT PROTECTION PROGRAM" and by a block 49 labeled "DISABLE EXC PROGRAMS, MAKE VC=O, OPEN PWR CONTACTORS." While a zero-power restriction is in effect, any excessive ,moisture that may have been the cause of the ground fault can dry out, in which case the fault will be self-curing. At the end of the aforesaid minimum interval of time, the ground fault protection program 48 will automatically remove the zero-power restriction by deactivating the clamping means 47, turning on the n*o excitation control programs, and issuing contactor closing commands, unless IGND did not decrease below a predetermined reset limit less than approximately 70% of the maximum limit) within a predetermined span of time approximately no 0, nine seconds) measured from the start of this interval, or (2) the ground fault responsive means has repeatedly activated the "e clamping means different times within a predetermined period approximately 30 minutes) immediately preceding the time at which IGND increases above its maximum permissible limit, where n is a predetermined whole number If either one of the latter conditions is true, a "permanent" or non-self-curing ground fault is assumed, and the ground fault I; i I c-U -r -17protection function must be manually reset to remove the zero-power restriction.

The ground fault protection program 48 is also effective, if and when the feedback signal on the output line 23 of the ground current detecting module 22 (Fiq. 1) indicates that IGND is higher than a predetermined deration threshold level but has not exceeded the aforesaid maximum limit, to modify the value of the control signal in a manner that reduces the power output of the alternator to a fraction of its normally desired amount. The latter function is preferably accomplished by reducing VMAX if the propulsion system is operating in its motoring mode or by reducing the value of the brake call if the system is operating ra ,in its dynamic braking mode, with the amount of reduction being proportional to the magnitude of leakage current in excess of the deration threshold level. As was explained hereinbefore, ground leakage current tends to increase, and the ionization discharge starting voltage tends to decrease, as moisture increases. By fractionally reducing the alternator output when the leakage current is in a "medium" range when IGND has increased to an abnormally high level but is not above its maximum permissible limit), potentially harmful discharges can be avoided or at least minimized without a total loss of traction power, and the alternator voltage amplitude is allowed to increase as the ground insulation medium dehydrates and its dielectric strength gradually returns to normal.

Although this ground fault protection function could be implemented in a variety of different ways to obtain the results summarized above, the presently preferred way is to program the microcomputer 26 to execute the routine that is illustrated in Fig. 4. This routine is repeated once every 60 milliseconds. It starts with an initializing subroutine 51, the basic steps of which are shown in Fig. 5. This subroutine begins at at irquiry point 52 which determines whether or not the locomotive is in its engine cranking mode. If it is, the next and final step 53 in :i- -ii- I l- .I -18the ground fault protection routine will start a timer #1 and will then reset second and third timers and a pair of counters.

Otherwise the subroutine 51 proceeds from point 52 to an inquiry point 54 where the status of the first timer is tested. So long as this timer is active not over), the ground fault protection routine ends here. In effect, the ground fault protection function is disabled while the engine 11 is being started and thereafter for the period of time approximately 10 seconds) that timer #1 is running.

Once the timer #1 has timed out, the initializing subroutine 51 will proceed from point 54 to another inquiry point 55 which determines whether or not the propulsion system is operating in its dynamic braking mode, and this is followed by a step which 41000 presets the deration set point or threshold level and the maximum i permissible limit of ground leakage current. If the system is not in a brake mode, the presetting step 56 will load a first predetermined number (Ml) corresponding to the desired threshold rrO ,,,level 0.5 ampere) into a "KI" register and will also load a second predetermined number (M2) corresponding to the desired maximum limit 1.0 ampere) into a "K2" register.

Alternatively, in the dynamic brake mode only, the presetting .4 step 57 will load a third predetermined number (BI) corresponding to the desired threshold level 0.25 ampere) into the KI *register and will also load another number (B2) corresponding to the desired maximum limit 0.5 ampere) into the K2 register. In the example given, the number in the KI register is approximately one-half of the number in the K2 register.

Preferably B1 is a lower number than M1 and B2 is a lower number than M2, whereby the sensitivity of the ground fault protection means is increased when the propulsion system is switched to its dynamic braking mode of operation. This is both desirable and permissible because the normal amount of ground leakage current that inherently exists in the electric power system and that is represented by the feedback signal on line 23 will be much lower

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-19- 0 #0 *04 0 41 S 0 0 04 4 V so 0 so 00 0 0 I1 0 o *004 4 00 00 00 4 0t *0 00 during the braking mode (when the armature windings of the traction motors are disconnected from the power rectifier) than during the motoring mode.

After the presetting step 56 or 57, the subroutine shown in Fig. 5 proceeds to an inquiry point 58 where the status of a ground counter ("GND CNTR") is tested. The operation of the ground counter will soon be explained in connection with the description of the grounded subroutine shown in Fig. 6, If the count in this counter is not greater than 2, the control is returned directly to the next step 61 of the main ground fault protection routine (Fig. But otherwise, a step 59 is executed to replace the number in the K2 register with a lower number before returning to the step 61 in the main routine. This lower number corresponds to the desired reset level of ground leakage current. Preferably, the reset point is appreciably lower than approximately two-thirds of) the maximum limit.

In effect, step 59 introduces desirable "hysteresis" in the operation of the ground fault protection means.

After completing the initializing subroutine 51, the ground fault protection routine executes the step 61 of reading and saving the present value of the feedback signal on line 23, which value corresponds to the magnitude (IGND) of ground leakage current in the propulsion system. As is indicated in Fig. 4, the routine then proceeds to an inquiry point 62 where the saved value is compared with the number stored in register K2 to determine which one is greater. So long as the ground counter has not counted more than two consecutive passes through the grounded subroutine (Fig. the answer to inquiry 62 is affirmative only if IGND is above its preset maximum permissible limit, and thereafter the answer will continue to be affirmative until IGND has decreased below the aforesaid reset level. If tha answer to the inqtir 62 is affirmative, the grounded subroutine 63 is called.

The presently preferred embodiment of the grounded subroutine 63 will now be described with reference to Fig. 6. It begins by resetting a timer #2 (step 64). Then, at step 65, the above-mentioned ground counter is automatically incremented whatever count is stored in a dedicated address of the microcomputer memory is increased by This is followed by testing, at 67, the status of the ground counter. If the count is 1, the grounded subroutine is aborted here; otherwise it proceeds to another inquiry point 69 where the status of the ground counter is retested to determine whether or not the count is 2. If the answer is affirmative, a ground fault is assumed and the grounded subroutine responds by executing a ser-'es of steps 71-79. As will soon be apparent, the count in the ground counter will never reach 2 unless IGND is above the preset maximum limit (K2) for two consecutive passes tn'ough the *--grounded subroutine, and thus the steps 71-79 are delayed until IGND has remained above this limit for 60 milliseconds after this condition is initially detected. In effect, the grounded j e .subroutine ignores the first time IGND rises above the preset maximum limit, and the rest of this subroutine will not be executed if the initial indication of a ground fault were actually caused by a harmless electrical transient ("noise") .ihat subsides in less than 60 milliseconds.

If the count in the ground counter is 2, a temporary ground fault counter ("TEMP FAULT CNTR") is automatically incremented by 1 (step 71), and then its status is tested at 72. So long as L the count in the temporary ground fault counter is not greater than a predetermined whole number (typically the answer i to the inquiry 72 is negative and the grounded subroutine proceeds to another inquiry point 73 where the status of a timer #3 is checked. If this timer is active not over), the subroutine proceeds directly from point 73 to the next step 74, but otherwise the timer #3 is started (at step 75) before step 74 is executed. Timer #3 is set to be active to continue

TVT

I-

.~ci i L I -iC ii -21- Pp 4 4 4r04* 0404 #4 00 P 4,04 44 *i 9 4* 40 4C 4 4 4 4 0C running) for a predetermined period of time after being started, and in a typical application of the invention T is approximately 30 minutes. Step 74 will restrict the puwer output of the main alternator 12 to zero by activating the above-described function shown in block 49 of Fig. 3, and in addition it will enter a "KILL PWR" message in the display module This is followed by a step 76 which starts a timer #4 and sets a "ground" flag in its true state. Timer #4 is the automatic reset timer; it is typically set to be active for an interval of approximately 15 seconds after being started. From step 76 of the grounded subroutine 63, the control is returned to the main routine which ends at this point.

As is indicated in Fig. 6, if the count stored in the temporary ground fault counter were equal to n 3) just before the grounded subroutine is executed, step 71 will incre,..

the count to n 1 and consequently the answer to the inquiry 72 will now be affirmative rather than negative. In this event, a "permanent" ground fault is assumed, and the subroutine will proceed from the inquiy point 72 to a step 77 which resets timers #2 and #3 and also resets the temporary ground fault counter. Step 77 is followed by steps 78 and 79. In step 78, a predetermined number is loaded into the ground counter, thereby freezing its count at this number. Step 79 will restrict the alternator power output to zero in the same manner as step 74, and in addition it will enter a "WONT LOAD" message in the display module 30. From step 79 of the grounded subroutine 63, the control is returned to the main routine which ends at this point.

If the answer to inquiry 62 in the main routine (Fig. 4) remains affirmative for 120 milliseconds or longer, the grounded subroutine (Fig. 6) will be executed more than two consecutive times. On the third mnsecutive pass through this subroutine, the aiswer to inquiry 69 will be negative, and the subroutine will then proceed from the inquiry point 69 to yet another l I-~~'YX -22inquiry point 81 where the status of the ground counter is retested to determine whether or not the count is greater than N.

If not, the control is returned to the main routine which ends at this point. Thereafter, the inquiry 81 is repeatedly executed at intervals until IGND decreases below its reset level and inquiry 62 yields a negative answer. However, once the grounded subroutine 63 has been executed N consecutive times, on the next pass through this subroutine the answer to inquiry 81 will be affirmative. In this event, a "permanent" ground fault is assumed, and steps 77-79 are executed before ending the ground fault protection routine. In a typical application of the invention, N is 150 which is reached in a time span of nine seconds. This length of time is appreciably shorter than the aforesaid 15-second reset interval.

r rning now to the description of the ground fault protection routine shown in Fig. 4, it will be apparent that the grounded subroutine 63 is not called before IGND increases above its predetermined nir ximum permissible limit or after <oo. IGND has exceeded this limit for at least 60 ms. and then ,decreases below its predetermined reset level. In either case, the answer to inquiry 62 is negative, and the main routine will proceed from point 62 to an inquiry point 82 which checks the state of the ground flag. If the ground flag is not in its true state, the main routine will prr ,ed directly from point 82 to the next step 83. Otherwise it proceeds to the step 83 via an iriquiry point 84 where the status of the automatic reset timer #4 is checked. So long as this timer is active, the routine will proceed directly from point 84 to the next step 83. But once the reset timer #4 times out at the end of the aforesaid interval, an additional step 85 is executed before step 83. In step 85 the power restrictions that were imposed by step 74 of the grounded subroutine 63 are automatically reversed or removed, the "KILL PWR" message 'in the display module is cancelled or reset, and the ground f'lag is set in a false state. This permits i~-illV*li~-YIIIIIl IIIC-~~Ua~- ~LLLPI~-^I~- -23the power output of the main alternator 12 to be restored to whatever level is determined by the normal operation of the excitation control programs 41 (Fig. 3).

In step 83 of the main routine, the count in the ground counter is reset to zero. This is followed by an inquiry point 86 where the status of the temporary ground fault counter is tested to determine whether or not its count is zero. If the answer is affirmative, the routine proceeds directly from point 86 to another inquiry point 87, but otherwise a ground forget subroutine 88 is called before executing the inquiry 87. The ground forget subroutine is shown in Fig. 8 which will be described later. As is indicated in Fig. 4, the inquiry point 87 compares the saved value of the leakage current feedback signal with the number stored in register KI to determine which one is greater. The answer to inquiry 87 is affirmative if IGND is higher than its preset deration threshold level, in which case a deration subroutine 89 is called.

3.

The presently preferred embodiment of the deration subroutine 89 will now be described with reference to Fig. 7. It begins at an inquiry point 90 which determines whether or not the 'propulsion system is operating in a dynamic braking mode. If not, the maximum voltage limit VMAX is fetched (step 91) and then reduced by a step 92 which calculates the product of VMAX and a fraction equal to one minus the ratio of actual to maximum deviations of IGND fiom the deration threshold level. The actual deviation corresponds to the saved value of the leakage current feedback signal less the number stored in register KI, and the maximum deviation corresponds to the known difference between these quantities if IGND were to increase to 4 predetermined :magnitude at which 100% deration VC 0) is desired. The latter magnitude cart be approximately the same as, but preferably is slightly more than, the previously mentioned maximum permissible limit of ground leakage current when the propulsion 7_ -L I i I -24system is motoring. The product of VMAX and the aforesaid fraction is saved for the excitation control program.

Alternatively, if the inquiry point 90 reveals that the propulsion system is operating in a braking mode, the value of the brake call signal is fetched by a step 93 of the deration subroutine. Then, at a step 94, it is reduced by calculating the product of the fetched value and a fraction equal to one minus the ratio of actual to maximum deviations of IGND from the predetermined deration threshold level of ground leakage current during dynamic braking. The product of the brake cai value and this fraction is saved for the excitation control program, After executing either step 92 or step 94, the deration subroutine 89 returns to a final step 95 of the main ground fault protection routine (Fig. 4) where a "LOAD LIMITED" message is entered in the display module 30 and a "derate" flag is set. Once the derate flag is set, the excitation control programs (41) will use the reduced value of VMAX that was saved at step 92 of the deration subroutine (or, in the dynamic brake mode, the reduced brake call value that was saved at step 94).

o, So long is IGND is not higher than its predetermined deration threshold level, the answer to inquiry 87 will be negative. In this event, as 's shown in Fig. 4, the final step 97 of the ground fault protection routine is to clear the derate flag. Now the excitation control programs (41) wi1i use the normal VMAX and brake call values.

*Anytime the count in the temporary ground fault counter is greater than zero when the aforesaid inquiry 86 is executed, the ground fault protection routine will proceed from inquiry point 86 to inquiry point 87 via the ground forget subroutine 88 which will now be described. Fig. 8 illustrates the presently preferred embodiment of this subroutine. It begins by checking, at an inquiry point 98, the status of timer If this timer is active (indicating that IGND has increased above its maximum permissible limit and step 71 of the grounded subroutine 63 has j- i I s tt tI Jar 6* *6~ 6i 6:*6* 6 9 66 9i *i 6 6i 6 incremented the temporary ground fault counter at least once during the immediately preceding 30-minute period), the control returns directly from point 98 of the ground forget subroutine to the inquiry point 87 of the main ground fault protection routine (Fig. But once 30 minutes has elapsed since the last time the timer #3 was started, the answer to inquiry 98 is affirmative and the ground forget subroutine will proceed to another inquiry point 99 where the status of timer #2 is checked.

Noting that timer #2 is reset by step 64 each time the grounded subroutine 63 is executed, it will be apparent that this timer is initially inactive over), and consequently the ground forget subroutine shown in Fig. 8 proceeds from point 99 to a step 100 where the temporary ground fault counte is automatically decremented by 1. Step 100 is followed by a step 101 which starts timer and the ground forget subroutine then returns to the main ground fault protection routine. Timer #2 is set to be active, after being started, for a predetermined interval of time equal to approximately approximately minutes). Once this timer is active, the ground foget subroutine 88 will return directly from inquiry point 99 to the rain ground fault protection routine, and its counterdecrementing step 100 will not again be executed until the end of the last-mentioned 10-minute interval (assuming no new ground faults occur during this interval). In this manner, the temporary ground fault counter automatically forgets or loses one count if there is no ground fault when timer #3 indicates the end of the 30-minute period, and so long as timer #3 is not restarted, this counter will thereafter forget one more count at the end of each successive 10-minute interval until the count therein is reduced to zero. It will now be apparent that the temporary ground fault counter will ai umulate a count of 4 if a ground fault recurs four times during any 30-minute period, or if ground faults are detected three different times during a period, a fourth one is detected during the next di I i.

-26minutes, and a fifth one occurs within 30 minutes after the fourth.

While a preferred embodiment of the invention has been shown and described by way of example, many modifications will undoubtedly oc.ur to persons skilled in the art. The concluding claims are therefore intended to cover all such modifications as fall within the true spirit and scope of the invention.

0 0% o 0 00 0 0 *9 *0 0 0 00?

Claims (14)

1. In an electric power system including a controllable source of power, an electric load circuit, means for interconnecting the source and load circuit, and means associated with the source for varying its power output as a function of the value of a variable control signal, improved means for automatically providing ground fault protection comprising: a) control means normally operative in response to a given command signal and other selected input signals for determining the value of said control signal and thereby controlling, as desired, the amount of electric power that said source supplies to said load circuit; and Sb) current detecting means for supplying said control means 't with a feedback signal representative of the magnitude of ground leakage current in the electric power system; c) said control means including ground fault responsive means activated when said feedback signal indicates that the magnitude of leakage current is abnormally high to modify the value of said control signal irn a manner that restricts the power output of said source to zero for at least a predetermined minimum interval of time if the leakage current magrntude is abova a predetermined maximum permissible limit; d) said ground fault responsive means being effective, at the end of said minimum interval, automatically to remove said zero-power restriction if the leakage current magnitude decreased below a predetermined reset level within a predetermined span of time after increasing above said maximum limit; and S" e) said control means including additional means for modifying the value of said control signal in a manner that reduces said power output to a fraction of its normally desired amount if said feedback signal indicates that the leakage current nl t; :i* 1~ L~r magnitude is higher than a predetermined deration threshold level but has not exceeded said maximum limit.

2. The ground fault protection means as in claim 1 in which said additional means is effective to reduce said power output proportionately to the magnitude of leakage current in excess of said threshold level.

3. The ground fault protection means as in claim 1 in which said reset level is appreciably lower than said maximum limit.

4. The ground fault protection means as in claim 1, in which said reset level is less than approximately 70% of said maximum limit. The ground fault protection means as in claim 1, in which said ground fault responsive means will not automatically remove said zero-power restriction after the leakage current magnitude has remained continuously above said reset level for said predetermined span of time which is shorter than said minimum interval. e

6. The ground fault protection means as in claim 1, in which said ground fault responsive means is ineffective to automatically remove said zero-power restriction after it has been repeatedly activated different times within a predetermined period immediately preceding the time at which S, o leakage current magnitude increases above said maximum limit.

7. The ground fault protection means as in claim 6, in *which said ground fault responsive means will not automatically o remove said zero-power restriction after the leakage current d magnitude has remained continuously above said reset level for said predetermined span of time uKk h4rer hcp FS(cM( a -29-

8. The ground fault protection means as in claim 1, in which said ground fault responsive means includes timing means that is started concurrently with the leakage current magnitude increasing above said maximum limit and that continues running, once started, for a predetermined period of time and counting means that adds one count each time leakage current magnitude increases above said maximum limit, said counting means being arranged automatically to forget one count whenever said timing means indicates the end of said period of time and (ii) thereafter, so long as said timing means is not restarted, at the end of successive predetermined intervals each of which is approximately-T(wiere n is a predetermined whole number), said ground fault responsive means being ineffective to automatically remove said zero-power restriction once said counting means accumulates a count of n 1.

9. The ground fault protective means as in claim 8, in :which said timing means and said counting means are automatically reset in response to either the leakage current magnitude remaining continuously above said reset level for said opredetermined span of time or said counting means accumulating a count of n 1. The ground fault protection means of claim 1 for an :electric power system wherein said controllable source of power C is an a-c generator having a set of 3-phase star-connected stator windings and a rotating field excited by the power varying means.

11. The ground fault protection means as in claim 10, in which said current detecting means is connected between ground and the neutral of said stator windings.

12. The ground fault protection means of claim 11, wherein said electric power system is the propulsion system on board a locomotive, and said electric load circuit comprises a plurality of traction motors. a

13. The ground fault protection means of claim 12, wherein said interconnecting means includes an uncontrolled power rectifier, and said traction motors are d-c motors, with each of the motors having both armature windings and field windings.

14. The ground fault protection means of claim 13, wherein said load circuit comprises the armature and field windings of said motors when the propulsion system is operating in a "motoring" mode but comprises only the field windings of said motors when the propulsion system is operating in a "dynamic braking" mode, and wherein said ground fault response means includes means for presetting said maximum limit to a first level if the system is not operating in its dynamic braking mode or to a predetermined lower level if the system is operating in its dynamic braking mode. The ground fault protection means as in claim 14, in whiih said threshold level is approximately one-half of said maximum limit.

16. The ground fault protection means as in claim 14, in ,o which said reset level is approximately two-thirds of said maximum limit.

17. Ground fault protection means substantially as herein described with reference to any one of the accoupanying drawings thereof. *i8. ny now! Weans ot forth heroin, or any nl rcan -i- conpcrent .SQt forth herein, the said noanc or conponont bein g a DAED tlis 22nd day of October 1986 G GENERALt ELECTRIC CCOPANY SBy Its Patent Attorneys ARFTUR S. CAVE 00. w^y r