CA2082261A1 - Regenerative braking protection for an electrically-propelled traction vehicle - Google Patents

Abstract

An electric propulsion system for a traction vehicle includes a controllable power converter operable in either a propulsion or an electric braking mode in which power is withdrawn from a wayside source or returned to the source, respectively. The wayside source utilizes certain preselected frequencies for power generation and for signaling. It is desirable to detect these frequencies quickly and to be able to disconnect the vehicle from the source if some frequencies are being generated by the vehicle and others are not present on the system. Bandpass filters are coupled to the power converter to detect the preselected frequencies. Wayside transients sometimes encroach on these frequencies with large power spikes causing ringing of the filter circuits and delay in detecting the preselected frequencies. The system includes an active power limiter for limiting the peak amplitude of the signals coupled to the filters without interfering with the signal frequency content or affecting filter operation.

Description

WO 92/15469 P'CI/U592/00621

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REGENERATIVE BRAKING PROTECTION FOR
AN ELECTRICALLY-PROPELLED TRACTION VEHICLE

~- This appllcation ls a continuation-in-part of commonlyasslgned, co-pending U.S. Patent Applicatlon Serlal No.
~ (20LC-1601), filed March 8, 1991 and U.S. Patent Appllcatlon ; Serial No. (20LC-1482), f~led December 20, 1990, the d~sclosures of wh~ch are hereby lncorporated by reference. `

BACKG.ROUND OF THE INVENTION
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Th~s lnventlon relates generally to electrlcal propulslon systems used on tract~on vehlcles (such as self-propelled rapld ~; trans~t ra~l cars) to propel and retard the vehlcle, the ; 10 electrlc power lnput to the system belng obta~ned from ways~fde :~l conductors ~e.g., a thlrd ra~l) normally energ~zed by d~rect current derlved from electr~c power sources located at varlous ~, statlons along the r~ght-of-way that the vehlcle w~ll travel,~,f and tt relates more part~cularly to means for detect~ng whether or not the wayslde conductor w~tth wh~ch the veh~cle ~s ln ~l contact ~s so energlzed and for preventing any regenerative braklng operatlon of the propulslon system lf the wayside :j conductor ls not otherw~se energized and for dlsconnect~ng the vehlcle from the wayslde conductlon lf the vehicle propuls~on . ..
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system generate~ signal frequency components which may interfere with a wayside co~munication system.
~ A typical traction vehicle propulsion system - comprises two pairs of electri~ traction motors, with the rotatable shafts of each pair being mechanically coupled through suitable gearing to the respective axle-wheels set~ of a separate one of the two trucks ~that support the vehicle, two controllable electric : power converters each having relatively positive and negative direct current (d-c) source terminals and a set of load terminal~ connected to a different pair of traction motor3, a bi-directional current path in~luding an electro-mechan~cal line switch or circuit breaker for connecting the d-c terminal~ of both converter-~ to a set o~ current collectors protruding from the vehicle in sliding contact with a normally ~energized way3ide sourc~ of unipolarity voltage having .~a relatively low, constant magnitude, and suitable control means for operating the converters either in l20 a propulsion (motoring) mode when acceleration or ;.constant speed o~ the vehlcle is desired, or in an ;~electrical retarding ~braking) mode when deceleration i8 desired. Pre~erably the traction motors are .~ three-phase alternating current (a-c) induction motors, the converter~ are three-phase voltage source .inverters, and a low pa3s electrical ~ilter is ;~ connected between the a~ore3aid lins switch and the ~ d-c ter~inala o~ the inverters.
'~ In it~ motoring mode o~ operation, each inverter 30 is so controlled that the unipolarity voltage applied ~:
. to its source terminals ia converted into three-phase ~ :~
. alternating voltage of variable fundamental ~requency `, and amplitude at its load terminals, and the a-c ,... ., "-,j , ' .

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traction motors respond by producing torque to aceelerate the veh~cle or maintain its speed as desired. In the alternative electrical braking mode of operation, the inverter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direztion through the inverter and appears as direct current and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motors respond by absorbing Xinetic energy and slowing t~e vehicle.
Electrical braking is achieved by a combination of dyna~ic and regenerative braking. Dynamic braking i~ effected by qelectively connecting a dynamic braking resistance in parallel relationship with the d-c source terminals of the inverter. This resistance receives current from the inverter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on t~e other hand, is effected by returning to the wayside source power flowing in a reverse direction through the inverter during electrical braking. The regenerated power can be advantageou ly utilized by the propulsion systems of other traction vehicles ~haring the same wayside source o~ voltage and operating in their motoring mode. The two electrical braking modQs can be combined in desired proportions, ~ this mixing process being commonly referred to as ; "blending. n An electrical propulsion system, including a voltage sourca inverter for supplying a-c ~ .
traction motors, is disclosed in U.S. patent 4,904,918 -- Bailey, Rumar and Plette, granted on February 27, l990, and assignQd to General Electric Company.

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The wayside source of unipolarity voltage usually comprises two or more low-voltage d-c power generating plantQ or stations located near the right-of-way traveled by the trac~ion vehicle. In a typical station the d~c power is d~rived from commerc~ally available three-phasa a-c electric power by means of a polyphase power transSor~er in combination with an uncontrolled power rectifying bridge having a set oS
a-c input terminal3 connected to the transSormer secondary winding-~ (i.e., the low voltage windings) and a pair oS d-~ output terminals across which the unipolarity voltage is produced. At each station one of the d-c terminals oS th~ rectifying bridge ~conventionally the one whose potential is negative with respect to the other d-c terminal) is grounded, and suitable ~eans is provided for connecting the other ter~inal to a plurality of bare electrical conductor~ extending along different sections of the right-o~-way. Proximate ends of the conductors in ad~acent sections are separated Srom each other by relatively short, insulating gaps. Such gaps are ~ commonly ~ound at track crossings and switches and at ;:~ other strategic locations along the route traveled by `; the ~ehicle. As the vehicle is driven by its electrical propulsion ~ystem alonq each different sec~ion oS the right-o~-way, its current collectors are in sliding contact with the corresponding ; conductor until a conductor gap is reached, at which point the vehicle will be unpo~ered ~or a relatively short distancQ (e.g., as short as four or Sive feet) unti~ it~ leading current collector ma~es contact with the wayside conductor associated with the next section , oS the right-of-way. This external conductor is :~` :

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usually a third rail parallel to the pair o~ ralls forming the t~acX on which the vehicle travels, in which case the gaps are simply air gaps and the cuxrent collectors are spring-biased "shoes"
respectively supported in cantilever fashion on the two trucks o~ the vehicle.
There are certain times when the wayside :.~ conductor in one section o~ the right-of-way will be temporarily disconnected from it8 normal voltage lo source ~or track maintenance work or for some other purpose. In this event, the wayside conductor is . intend~d to be de-energized or "dead." If a vehi le enters such a de-energized section while its electric powex converter is operating in a regenerative braking mod~, there i~ a possibility that the regenerative current ~rom the vehicle will raise the electrical potential on the wayside conductor to an undesirably h$qh level, thereby endangaring maintenance people who believe the conductor i-~ dead. In order to prevent thi~ hazard ~rom occurring, suitabls ~eans for ; detecting whether or not the conductor is energized by a power generating station and for prqventing regenerativ~ braking if the conductor is not so energized are desired.
~; 25 It i~ known in the prior art, as disclosed in U.S. patent No. 4,~57,753, to provide a permissive control signal which is inserted into the electrical ~ power supplied by each wayside genQrating station to " the vehiclQ ~or establishing when regenerated power .',30 may sa~ely be returned ~rom the vehicle to the wayside conductor.
;;It is also known in the prior art, as described ,lin U.S. patent No. 4,326,154, to open the line switch ,~

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in thQ current path between the electric power converter and the current collectors on board the vehicle as these current collectors traverse each wayside conductor gap every time the vehicle moves from section to section of the right-of-way, thereby preventing either motoring or regenerative braking operatio~ of the propulsion system as the vehicle enters the next sQction. After the leading current collector ~ake~ contact with the wayslde conductor of the next sectio~, the lin~ æwitch i~ not reclosQd until voltage i detected on the current collectors and a current sensor indicates that appreciable current is flowing to auxiliary electrical load circuit3 on the vehicle.
15The above-referenced prior art regenerative braking protect~ve apparatus has shortcomings. If the propulgion 8y8tem werQ operating in a regenerative braking mode and the traction vehicle were traveling at a relatively high speed (e.g., 40 MPH) as the current collectors pa33 through a gap between a first energized wayside conductor and a second de-energized ~;l conductor, the gap would bQ traversed in an interval of time (e.g., 70 mi}liseconds) that i~ shorter than 1 a typical opening time ~e.g., lO0 milliseconds) o~ a i 25 conventional line switch. Consequently, an unde~irable ~pike o~ high voltage could be applied to the de-energized conductor before the line switch has time to open the current path between the converter and the current collectors. Furthermore, if the wayside conductor were disconnected from its power generating station while i~ contact with a vehicle whose propulsion syste~ is operating in a regenerative braking mode, the regeneFative current would fool ~he '~ ~
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protective apparatus so that neithsr the .~ current-collector voltage detector nor the auxiliary load current sensor would cause the line switch to open.
It is known in the prior art relating to electrical propulsion system~ for trolley buses to - interrupt regenerative braking current in the bi-directional current path bet~een eac~ of two trolley poles and an electric power converter whenever the tandem trolley poles of the bus traverse insulator . gaps in a pair of overhead power supply lines and :. thereafter to permit such current to be conducted to the power lines of the next section of right-o~-way only if the unipolarity voltage across such lines has - 15 proper polarity and magnitude and is not decreasing.
See U.S. patent No. 4,453,113 wherein the bi-directional path includes a diode bridge to ensure that the polarity of the voltage applied to the d-c source terminal~ of the converter during motoring ~20 operation will not change if line voltage polarity ,~ changes, a pair of the diodes are shunted by thyristors poled to conduct regenerative current, and a l~.ghtning arrestor capacitor is connected between '~l the trolley conductors. During re~enerativ~ braking ; 25 operation, both thyristors change from conducting to non-conductin~ ~tat2s whenever the trolleys come to the in~ulator gap becau6e current in the trolley `~:conductor~ then decrea~es abruptly to zero and the resulting increase of voltage across the lightning arreskor capacitor puts a reverse bias on the thyristor~. Later, th~ thyristors are leturned to their conducting ~tates in response to the concurrence :.of a numbar of conditions: the sensed line voltage `''`'` , . .. ..
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has proper polarity and magnitude and 18 not decreasing; traction motor current exceed~ a predetermined threshold magnitude; and the vehicle i3 moving faster than a predetermined speed.
A typical power conversion system which includes a voltage source inverter for supplying a-c traction motors is shown in U.S. Patent No. 3,890,551 to Plunkett, assigned to General Electric Company The Plunkett patent also includes a low pass electrical filter of the conventional series inductance (L), shunt capacitance (C) type between the voltage raising resistor and the inverter for attenuating harmonics generated by operation of the inverter and for pa~tially isolating the inverter from undesirable line transients. (A~ used herein, the term "harmonics"
refers to various component~ of the composite current and voltage wavsforms having frequencies that are ;~
multiples of the frequency of the Sundamental component of such waveforms.) In addition, the shunt capacitance of the filtQr at the DC terminal~ of the inverter provides the "~tiff" voltage required for proper operation of a voltage ~ource inverter.
The filter capacitors used to~ provide the filt~red DC link voltage in the. above described system~ are generally electrolytic capacitors and have a higher ~ailure rate than many other power i component~. Typically, the filter capacitors may range from 10000 to 100,000 microforads (MFD) and are formed from a plurality of parallel connected capacitors. For example, as many as 112 ind~vidual ; capacitors may be used to create a single 55,000 MFD
capacitance means. One of the primary functions of 1 these capacitors, in addition to "smoothing" the DC
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link yoltage i~ to reducP certain frequencies o~
current which can be introduced to the wayside conductors DC power source from the propulsion system.
As is well known, such wayside conductors are often positioned adjacent wayside signalling equipment in transit applications. The signalling equipment may operate at preselected frequencie~, such as, for example, 25 Hz, 60 Hz, 9S Hz, 200 Hz, or such other frequency as the transit authority may select. The signalling ~ystem may be used for communication to trans~t vehicles operating in the system or to indicate the presence of a transit vehicle within a particular block of the transit system. Other frequencieQ, such as 360 Hz, 720 Hz, and 990 Hz, are used for safety checks a3 iS explained in copending ; U.S. Patent Application ~erial No. 07/630,638, filed December 20, 1990, and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. Because of the importance of the signals on the signallinq system, it is desirable that transit vehicles not generate signals i in their respective propulsion systems which might :, Iint~r~ere wlth the ~ignalling system. To this end, th~ values o~ thQ capacitance means and ths inductance ;25 means in the power filter circuit are selected to ~;avoid oscillations or ringing at signalling ~requen¢ies or harmonics of these frequencies.
However, as noted above, the electrolytic capacitors used in the filter circuits are Xnown to have higher failure rate~ than other components. Accordingly, it is desirable to provide a method for periodically verifying the value of the capacitance means 80 that capacitor~ whose value has changed may be replaced.
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. 10 Such maintenance not only assures lntegrity o~ the filter circuit but can be used to direct maintenance . personnel to the capacitors in case of degradations - and ascures smoother operation of the propulsion : 5 system with adequate capacitance means.
Both the voltage dQtection circuits at 360 Hz, 720 Hz, and 990 Hz and the current detection circuits ` at 25 Hz, 60 Hz, 95 Hz, and 200 Hz are susceptible to : signal transient~ causing ringing of the filter circuits. Such transients are typically step-change signals caused by shoe bounce and line breaker opening and closing. Ringing forces the output of the filter circuits to appear higher than the actual frequency .. ~ component being sampled. In order to avoid false , lS indication~ due to such transients, it has been .~ proposed to establish a higher set-point for detection : of t~e actual signal compone~ts and to establish a time delay to assure that selected frequency i aomponents still exist after ringing due to such step change~ ha~ abated. However, it is de~irable to avoid the us~ o~ time delays in order to improve response time and the use of higher thresholds or set-points may allow 90mQ actual ~requency component to be undetected.
25SU~M~RY OF THE INV2NTION
~: A general ob~ective of the present invention i8 to provide improved protection for the electrical propulsion system of a traction vehicle normally l powered by an external sourc~ of unipolarity voltage delivsred to the vehicle via a sectionalized wayside conductor.
`' The above noted desirable features are :., implemented in a frequency detecting means hav~ng a . ' , : , .. 1 . . :,':.:
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pre-amplifier input circuit which limits the ampl~tude of step-change signals coupled to filter circuits within the fraquency detecting means to a predetermined magnitude~ In general, the invention comprises an electrical propulsion system on board a transit vehlcle including a controllable electric power converter having a set of load terminals adapted for connection to at least one traction motor.
Current collectors on the vehicle are disposed in sliding contact with a sectionalized external conductor (e.g., a third rail) that extends along a - dedicated right-of-way traveled by th~ vehicle, each section of the conductor being energized by an associated wayside source o~ unipolarity voltage to which it is nor~ally connected. The current collectors and d-c source terminals of the converter are interconnected by controllable electric switch means having alternative conducting and non-conducting states . .. .
In one asp~ct of the invention, tha aforesaid detecting mean~ comprises voltage ripple detecting means coupled to the vehicle's current collectors for enabling the switch means during electrical braking only lr any one of the current collectors is in contact with an external conductor section energized by voltage having an a-c ripple component of predetermined ~requency and at least a predetermined threshold magnitude, such ~requency being characteristic of the wayside voltage sources.
Enabling will terminate any time the conductor section is disconnected from its waysid~ voltage source or when the current collectors enter another ~, intersectional gap. If regenerative braking were in .`
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effect when the vehicle is traveling between ~' consecutive external conductor gaps, loss of enabling would cause the aforesaid circuit brPaker to open, and regenerative curre~t would then decrease to zero.
When the convertex is operating in the braking mode, there is no ripple of the aforesaid characteristic frequency in the voltage at its source terminals, and therefore the converter voltage will not cause the ripple detector to continue providing an enable cignal after the external conductor section is di~connscted from i~s wayside voltage source.
In another aspect of the invention, ths ripple ~-~
detecting means comprises an electrical filter network ; for deriving an output value representative of the amplitude of the characteristic-frequency ripple component of the external conductor voltage, and le~el detecting means for providing the enable signal if such output value exceeds a level correspondin~ to the aforesaid threshold amplitude. ~he filter network comprises a bandpas~ type active ~ilter characterized by maximally ~lat passband magnitude response, a center frequency substantially equal to the chara¢terlstic frequency, and a -3 decibels bandwidth that is a relatively small percentage of the center ~requency, rectifying means for rectifying any signal passing through the bandpass filter, and signal ~ sm~othing means ~or deriving th~ aforesaid output ;~ value which varies with the average magnitude of the rectified signal. The bandpass filter is connected to the vehicle's current collectors via a high-pass type filter characterized by equal-ripple passband ' magnitude response and a -3 decibels cutoff frequency ,.. .........
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PCT/~S92/00621 WO~2/15469 13 2`~3~?,~' that ~s lower than the center frequency of the bandpass filter.
In yet another aspect of the invention, the frequency detecting means comprises current detecting means coupled in circuit wlth the vehicle's current collectors for determining the presence o~ ripple currents having frequencies corresponding to frequencies used ~n the wayside signalling system. If such signals ar~ detected, the detecting mean~
disables the converter and disconnects the vehicle from the current collector~.
In each aspect o~ the invention, the voltage and current ~requency detecting me~ns incorporate a preamplifi~r circuit which limits the amplitude of any signal input to th~ filtar circuits to preselected maximu~ values. The limiting maximum values are selected to prevent ringing o~ the filter circuits in response to step-chang~ input signals caused by bouncing Or the current collectors or by cycling of the line brea~er~ connecting the vehicle to the current collectors. In a pre~erred mode, the preamplifier circuit includes an operational amplifier having a su~ming ~unction at an input terminal thereof connected to an output terminal through a pair o~
rsversely poled diodes. I~ the output signal amplitude exceeds the ~orward conduction voltage drop o~ either of the diodes, current to the summing ~unction through the forward biased diode drives the ampli~ier output in a direction to minimize the d~ode current, thus limiting s~gnal developed at the output terminal o~ the amplifier. The amplifier output signal is coupled to an input o~ a corresponding one of the fllter clrcults. `
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2 '~ ~(3 2 2 ~ 14 The invention will be better understood and its various objectives and advantages will be more fully appreciated from ~he following description taken in conjunction with the accompanying drawings. ~-~

5BRI~E~ 5~IPT~O~ OF THE DRAWI~GS ~.
,. FIG. 1 is a block diagram of an electrically :
; propelled rapid transit vehicle on a track the third rail o~ which is sect$onalized for energization by :
separate wayside voltage sources;
10FIG. 2 is a schematic circuit diagram o~ the propulsio~ system on board the vehicle shown in FIÇ.
1, which system includes the presently preferred :
embodiment o~ the subject invention, FI&. 3 i~ an expanded block diagram of the ripple `~
sensing means shown as a single block in FIG. 2;
. FIGS. 4A and 4B are graphs of magnitude attenuation vs. excitation frequency for active filters used in the ripple sensing means to obtain bandpass and high-pass characteristics, respect$vely;
~ 20 and -: FIGS. 5-8 are ~lowcharts that axplain how the controller o~ FIG. 2 is programmed in accordance with , the present invention to provide regenerative braking ~ protectlon. ..
.,~', ~ , 25DETATL~D ~ESCRIPTION QF THE INVE~TION
FIG. 1 illustrates symbolically the self-propelled rapid transit vehicle 10 having flanged ;~
wheels tnot shown) that are guided by a spaced-apart :
' pair o~ parallel steel rails 11 of a track in the .~30 right-of-way traveled by the vehicle. On board the vehicle there is an electrical propulsion system that :;

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- includes at least one traction motor drivingly coupled to the vehicle wheels. The propulsion system is shown in FIG. 2 which will soon be described. To enable its propulsion syst2m to receive electrical power from a stationary wayside source of unipolarity voltage, the vehicle 10 is equipped with a pair of conventional spring biased current collectors or pick-up shoes 12 and 13 that protrude laterally from the respective trucks of the vehicle. As the vehicle 10 moves on the guide rails 11 each of its shoes 12 and 13 make sliding contact with normally energized bare electrical conductors that extend along different sections of the right-of-way in parallel relationship to the track. The external conductors are commonly referred to as a th$rd rail, and FIG. 1 illustrate~
two ad~acent sections 15 and 16 o such a rail. The external conductor 15 is connected via a suitable ; circuit interrupter 17 to the relatively positive output terminal of an electric power station 19 that serveC as a wayside source of unipolarity voltage.
The negative output terminal of the station 19 is grounded, as are both of the rails 11. ~nother ~, circuit interrupter 20 is provided for conneating the external conductor 16 either to the same electric power station 19 or, as i9 shown in FIG. 1, to a ~' different station 22.
Each ~ the electri¢ power stations 19 and 22 typically comprises an uncontrolled power rectifying ' bridge having a pair of ralatively positive and negativ~ d-c output terminals, and a set of a-c input terminal~ connect~ad to the secondary windings of polyphase a-c power transforming means. The primary windings of the power transforming means are in turn ~''' ' ' , .
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energized by three-phase electricity supplied at commercial power frequencies (e.g., 60 Hz) over high tension power lines 24. It is common practice to arrange the transformer secondary windings in one of two alternative configurations: either three phases, in which case a three-phase double-way power rectifying bridge is used; or six phases, i~ which ca~a a six-ph~se double-way power rectifying bridge is u~ed. In either case, the power station will provide a relatively constant unipolarity output voltage the average magnitude of which is typically in a range from 600 volts normal to 800 volts maximum.
Normally both of the circuit interrupters 17 and 20 are closed, and ther~Sora both of the third rail conductors 15 and 16 are in fact energized by the wayside voltage source(s). Proximate ends of the adjacent conductors 15 and 16 are physically separated from each other by a distance D, commonly referr2d to as a third rail gap. The air gap D insulates the conductor 16 from the conductor 15 and enables either one to be electrically isolated from the other when maintenance work i8 belng performed on the assoc~atad ~, section of the track. In FIG. 1, the circuit ~ interrupter 20 i8 shown in its open state, whereby the ;l 25 conductor 16 is not now energized by the wayside voltag~ source 22. Note that the length of each third : rail gap D is greater than the distance d between the two pick-up shoes 12 and 13. Consequently, as the vehicle 10 moves from one section of the right-of-way ;! 30 to the next section and each of its two shoes 12, 13 traverse the intersectional gap in the third rail, the trailing shoe 13 (assuming the direction of movement indicated by the arrow in FIG. 1) will always separate ~:' '''' ' :` ~ ' :
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from the waysid~ conductor 15 before lts leading shoe 12 reaches the ad~acent conductor 16. The effective length of the third rail gap is D - d, a distance that can be as short as four or five feet in practice. So long as at least one of the two shoes 12, 13 of the YehiCla 10 i8 in sliding contact with an energized section of third rail (conductor 15 in FIG. 1), the shoe voltage (Vdc) will equal the unipolarity voltage of the way~ide source (station 19) to which this section ~ connected.
- The electrical propulsion system of the vehicle is illustrated ~n block form in FIG. 2. It comprises at least one pair of traction motors 26 and 28 electrically connected in parallel and mechanically coupled by ~uitable gearing to ~he respective axle-wheels set~ of a first one of the two trucks that support the vehicle, and at least one controllable electric power convsrter 30 having a set of load terminals electrically connected to both motor~. The converter 30 also has a pair o~ relatively po~itive and negative d-c terminal~ 31 and 32. Preferably, the ~, negativQ terminal 32 and the vehicle body ground are ,' interconnected by means o* the parallel combination of a rQsistor 32R and a diode 32D polQd to conduct current in a direction from terminal 32 to ground, and th~ positive terminal 31 o~ tha converter 30 i~
i connected to both of the previously described pick-up shoes 12 and 13 o~ the vehicle by means of a bi-directional current path including a line 33, a conventional electro~mechanical circuit breaker 34, an isolating switch 35, and controllable electrlc switch mean~ 36 having alternative conducting and non-conducting stat-s. Prorerably, each o~ tho w0~2/15469 PCT/US92/00621 2~2~i3.
. 18 traction motors 26 and 28 is a three-phase a-c induction motor having a full-load rating on the order of 300 horsepower more or less, and the converter 30 is a three-phase voltage source inverter of a . 5 conventional design. A controller 37 is electrically ; coupled to the inverter 30 which i~ thereby caused to ; operate either in a motoring mode under the control of an associated throttle handle 38 or alternatively in - an electrical braking mode under the control o~ an . 10 associated ~rake handle 39. In modern practice, the various functions performed by the controller 37 are .1 implemented by a suitably programmed microcomputer.
During motoring, i.e., when electrical power i5 being conveyed from the wayside voltage sourc~ to the traction ~otors, direct curront is supplied to the inverter 30 through its d-c terminals 31 and 32, and the inverter acts to convert th$s direct current into alternating current supplied through its load !`` terminals to the motor~ 26 and 28. In this operating mode, the inverter is so controlled as to vary the amplitude and frequency o~ the alternating voltaga at its a-c load terminals to provide the needed acceleration or constant speed o~ the vehicl~ 10. The well-known pulse width ~odulation control strategy can be used. In modern practice, GTO thyristors are prererred a~ the maln controllable electrical valves r the inverter, thereby avoiding the need for auxiliary thyristors and commutation circuit~.
As wa3 explained in the background section above, ;~ 30 electrical braking Or the vehicle lO is achieved by a combination o~ dynamic and regenerative braking.
Dynamic braking 1B effected by means o~ a dynamlc braking circuit connected to the d-c terminals 31 and ~'", `~, .. .
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32 of the inverter 30. This circuit co~prises a series combination of a resistor 40 and an electric power chopper 41 of conventional design. During electrical braking, each of the traction motors 26 and 28 operates as an electrical generator driven by the inertia of the vehicle lO, returning power to the propulsion system. This return power flows through the inverter 30 in a reverse direction from the direction o~ flow during ~otoring, and appears as unipolarity voltage and direct current at ths d-c ter~inals 31 and 32. During dynamic braking, at least some of the braking current i~ diverted through r~si8tor 40 whare el~ctrlc energy i~ dl~sipated in the form of heat. For controlling current in the dynamic braking re~$stor 40, the chopper 41 is repetitively turned on and off by the controller 37 so as to vary the average magnitude of current in the resistor 40 as desired. In accordance with common practice, whenever the voltagQ on tha line 33 rises above a predetermined level (e.g., 780 volts) with respect to ground potential, the chopper contro} automatically responds in a voltage regulating manner that limits further - voltage rise, thereby preventing this line voltage from exceedlng a safe maximu~ level.
Regenerative braking is effected by returning reversely-Ylowing power to the third ra~l. During this mod~ of braking, braking current from the po~itive d-c terminal 31 of the inverter 30 flows through the line 33, the switch means 34, the thyristor branch of the switch means 36, and the ~hoe 12 and l~ to the third rail and returns through 32R.
Regenerated power can be used for propelling other vehicles in contact with the same third rail ' .

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conductor. If the power demands of such other vehicles were insufficient to use all of the electrical braking energy, the voltage on line 33 would increase until the chopper controls respond by varying the on-off ratlo of the chopper 41 so that ; more energy is dissipated in the dynamic bra~ing :~ resistor 40.
A low-pass electrical fllter is connected between the circuit breaXer 34 and the d-c terminals of the inverter 30. As is shown in FI5. 2, this filter comprises a serie~ inductance L in the bi-directional current path between tho line 33 and the breaker 34, a first shunt cap~citance Cl directly connected between t~e poqitive and negative terminalc 31 and 32 o~ the inverter, and a second shunt capacitance C2 connected acro~s the dynamic braking circuit between the line 33 and ground. During motoring, th~ ~irst capacitance serves mainly as the required "stiffn voltage source for the inverter 30. During electrical braking, this particular filter cooperates with the resistor-diods combination 32R, 32D in an advantageous manner that i8 explained in the previou~ly referenced patent 4,904,918 -- Bailey et al. Although not shown in FIG. 2, it i8 normal practica to add to the propul~ion ~y~tem a ~econd palr o~ traction motors . drivingly coupled to the axlo-wheel sets of tha second truak o~ the vehicle 10 and electrically connected to the load terminal~ o~ a second voltage source inverter whosa positive d-c terminal is connected directly to the lina 33 and whoss negativQ d-c terminal is connected through anoth~r resistor-dioda combination ~` to ground, with a second dynamic bra~ing circuit being ~! connected in par~llol relatio~ship to the d-c , . W O 92/15469 P ~ /US92/00621 2~8~t'.' ~.

terminal~ of the second inverter, and preferably with . the "on" period o~ the two choppers being staggered with respect to each other during dynamic braking operation. The vertical arrows in FIG. 2 are intended to represent the interconnections between the - illustrated components of the propulsion system and such added components, such as those shown in the phantom outlined box A.
. The circuit breaker 34 is a controllable electric `. 10 switch having alternative conducting and ~,~ non-conducting state~. In its conducting state, which is normal, the ma~n contacts of the breaker 34 are ,; closed, whereas ln a non-conducting state such contacts are open and therefore current in ths : 15 bi-directional path will be interrupted. The breaker contacts are opened and closed by an associated operat~ng machanis~ (labelsd ~LBH in FIG 2) in response to appropriate command~ issued by th~
controller 37 whenever a state change is desired. As i8 indicated in FIG. 2, tho isolat$ng switch 35 ha3 a companion switch 35a that is used to connect auxiliary load circuits 42 to the switch means 36 and hence to .~ the shoe~ 12, 13 that are in sliding contact with the third rail. The auxiliary ¢ircuits comprise the vehicle'3 lighting and heating system~, air compressor~, battery charger, and the like.
In accordancQ with the present invention, the controllable ~witch means 36 of the propulsion system comprise~ a ~olid-state uncontrolled unidirectional electrical valve 43 in combination with a parallel solid-state controlled unidirectiona~ electric valve `l 44. The uncontrolled valve 43 ls a powerrated diode ;~ poled to conduct direct current in a direct$on from .

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WO92/15469 PCT~US92/00621 ~ ~ ~ 2 ~ g ~ .

the shoes 12, 13 to the positive d-c terminal 31 of the inverter 30. Thus, the switch means 36 will always be in a conducting state if the third rail is ~ ;
energized, the breaker 34 i5 closed, and the inverter 30 is operating in its motoring mode. The controlled valv~ 44 in the parallel branch of the switch means 36 is a power-rated thyristor poled inversely with respect to the diode 43. In one practical application of the invention, the thyristor 44 is rated to conduct load current having an average magnitude of approximately l,OOO amperes and to withstand a forward voltage having a peak magnitude over 3,000 volts when in its non-conducting state. As is shown in FIG. 2, a series RC snubbar circuit 45 is connected across the 15 diode 43 and the thyristor 44. `
Once a suitable flring signal is applied to the control electrod~ or gate of tXe thyristor 44 and while the inverter 30 is operating in an electrical braking mode, the switch means 36 i8 able to conduct regenerative braking current ~rom the po~itive j terminal 3l of the invarter via the line 33 to the shoes 12, 13 and hence to the section of the third rail with which either shoe iB in contact. This ; current conducting statQ will continue until the regenerative current decreases to zero, at which time the thyristor 44 will automatically change from conducting ~on) to non-conducting ~off) states. The thyristor aahieves this on-to-of~ transition by an inherent com~utation process whenever it is s~arved of load current. The thyristor gate i9 connected to suitable firing signal generating means 46 that i~
; operative to supply the aforesaid firing signal in ~ responsa to a discrete eommand issued by the . . ~

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controller 37 whenever it is desired to return the thyristor 44 to the on state. Preferably, the firing signal generating means 46 comprises a pulse stretcher that responds to the leading edge of the firing command by activating a burst generator for a ; predetermined short interval of time (e.g., approximately one millisecond). T~e burst generator in turn actiYates a suitable gate driver which then applie~ to tha gate of the thyristor 44 a firing signal comprising a train oS short pulses ; corresponding ln frequency to the output of the burst generator.
The operation of the controllable switch means 36 will now be summarized. Assume that the vehicle 10 i~
approaching the th~rd rail gap shown in FIG. 1 while the inverter 30 i~ operating in an electrical braking mode and regenerative current has been enabled by an earlier ~iring signal from the generator 46. Under such conditions, the circuit breaker 34 i~ closed, the thyristor 44 i9 on, and the switch means 36 is th~refore conducting regenerative current. ~t this time, the regenerated power delivered by the propulsion sy~tem on board the vehicle 10 to the third rail conductor 15 i8 being utilized by other electrical loads ~not shownl connected to the same conductor te.g., other vehicles operating in motoring modes and having shoQs in contact with the conductor 15). A~ a desirable result of such regeneration, the I power drain on the wayside voltage source 19 is ; 30 reduced a corre~pondlng amount.
As soon as the vehicle 10 reache~ the third rail gap D and it~ trailing shoe 13 separates from the end ; oS the conductor 15, the regenerative current . . ~ .

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necessarily decreases to zero. Consequently, the thyristor 44 immediately changes from conducting to non-conducting states. Because of the short time (typically 300 microseconds) required to complete this on-to-off transition, the thyristor 44 will have turned off before (and will not again be turned on when) the lead~ng shoe 12 of the vehicle subsequently makes contact with the proxi~ate end of the next third rail conductor 16 as the vehicle 10 moves past the gap D at maximum speed. In other words, when the shoe 12 first touches the next conductor 16, the switch means 36 will always be in a non-conducting state. At any time thereafter the thyristor 44 can be returned to its regenerative current conducting state by applying another ~iring signal to its gate, but until then the switch means 36 will remain in a non-conducting state and conseguently both of the shoes 12, 13 are electrically isolated from th~ voltage on the line 33 . .
of the bi-directional current path. Note that after the switch means 36 changes from conducting to non-conducting states, an appreciablQ voltage will remain on thQ line 33 a~ long a~ dynamic braking continue~ and the filter capacitances Cl and C2 have not discharged. Note al30 that it i5 com~on practice to open the slower operat$ng circuit breaker 34 each time a third rail gap is traversed.
After the vehiclQ ~hoes 12, 13 traverse the gap D, regenerative braXing operation 1~ deslred if the ~ next third rail conductor 16 is energized by a wayside `l 30 voltage source (i.e., if the circuit interrupter 20 i8 closed) but is not desired if the next conductor is i deenergized or "dead" (i.e., if the interrupter 20 is I open)~ If the conductor 16 were so energized, it ;

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would be safe to regenerate power and the controller 37 which is s~itably arranged to release the breaker 34 and command another thyristor firing signal. A
conventional way to detect this condition is to check the mag~itude of the wayside voltage as soon a5 the leading shoe makes contact with the next conductor.
For this purpose, the shoe voltage Vdc is supplied to the controller 37 which is suitably arranged to compare the actual magnitude of such voltage with a predetermined threshold magnitude ~e.g, 400 volts).
As long as Vdc is lower than this threshold, the next conductor 16 is assumed to be deenergized, the switch means 34 and 36 are not returned to their conducting states, and the third rail i-q protected from being inadvert~ntly energized by the voltage on line 33.
Such a voltage magnitude checking technigue is generally useful, but it does not ensure the desired regeneratlve braking protection under all possible conditions. One such condition would occur ir the interrupter 20 is open and another vehicle has a shoe in contact with the conductor 16 while operating in a regeneratlve braking mode. In this event, the regenerating propulsion system on board the other vehicle could raise the magnitude of Vdc above the 400-volt threshold even though the conductor 16 is di3connected rrOm thc wayside voltage source 22, Another condition that can cause the voltage mag~itude chQcking technique to respond ralsely would occur i~
the interrupter 20 were changed ~rom closed to open state~ a~ter the vehicle l0 move~ past the gap D and the swltch means 36 has b~n~properly returned to its ; regenerativa current conducting state. In this event, the vehicle's own regenerating propulsion system could ",1 .
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~ WO92/15469 PCT/US92/00621 . . .
2~ 26 ~aintaln the magnitude of its shoe voltage above the ; 400-volt threshold even though the interrupter 20 has opened to disconnect the conductor 16 from the source 22.
; 5 It has been observed that the instantaneous maqnitude of Vdc i8 not con~tant but undulates between maximum and minim~m levels due to the sinusoidal waveforms of the alternating voltages that are rectified in the previously described elPctrlc power stations l9 and 22. In other word~, the third rall voltage is actually a composite of a pure d-c voltage and a superi~posed a-c ripple component the amplitude of which is lower than tha ~agnitude of the d-c component. The characteristic ripple has a predeter~ined fundamental frequency. In the cas~ of third rail sections energized by stat$0n3 having ; three-phase rectifier bridges, this frequency will be six times greater than the commercial power frequency, and the characteristic ripple is hereinafter referred to as the 6X ripple component. On the other hand, for third rail sectio~s energized by station~ having six-phase rectifier bridges, the characteristic ripple component (12X) will have a fundamental frequency that is twelv~ ti~Q3 greater than the commercia~ power ~re~ency. ln ~ither cas~, the fundamental r~pple il Srequency i~ not the same as the Prequency oS any signi~icant harmonics generated by operation o~ the inverter 30 in a regenerative braking mode.

In accordance with the present invention, energization o~ third rail s~ctions by th2 wayside source o~ voltage i8 detected by prov-di~g suitable means for s2nsing the fundamental frequency o~ th~
characteristic a-c ripple component o~ the third rail . .

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voltage and for determining the amplitude of such component. Whenever such a ripple component is present and has at least a predetermined threshold amplitude, the ripple detecting means provides an enable or "ripple OK" signal. In FIG. 2, the ripple sensing mean3 i~ represented by a singla block 48 having an input line connected to the pick-up shoe~ 12 and 13 and two output lines 360 and 720 connected to the controller 37. The ripple sensor 48 providee on line 360 a value representative of the amplitude of any 6X ripple component of the shoe voltage Vdc, and it provides on llne 720 a value representative o~ the amplitude o~ any 12X ripple component. A~ will be explained when FIGS. 5-7 are described, the function o~ determining if either ou~put value of the ripple sensor exceed~ a predetermined minimum level is performed in the controller 37.
Preferably, the desired rippl~ sensing function is implemented with active electrical filters.
Suitable circuit components and inte~connections, desig~ approache~, and advantages o~ activo filters are well known to persons skilled in the art. See, or example, the textbook In~ uc~ion tQ~he_~h~o~y and Desig: 9 _A~ by L.~. Huelsman and P.E.
j 25 Allen ~McGraw-Hill Book Company, 1980), the disclo~ure o~ which is hereby incorporated herein by re~erencQ.
For the purpo~e o~ the presQnt di~closure, an "activa ~ilter~ i8 d~ined as a network of conventional resistors (R), capa~itor~ (C) and at least one operational amplifier ~s'OP AMpn) connected between input and output terminals and so arranged that whenever the input terminals are excited by a voltage o~ undulating magnitude the network will : ....................................................................... :: .
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develop an output signal representative of input voltaye excursions occurring at any frequency substantially within a predetermined range of frequencies and will greatly attenuate all other s components of input voltage. The resistors and capacitors in an active filter are passive elements of fixed ohmic and microfarad values, respectively, -whereas the OP A~P is the active device. oP AMPs perform several important ~unctions in active RC
filters. They enable a filter without inductors to exhibit the resonant effects of complex ~requency-magnitude relationships of second and higher order3; they permit significant reductions in the sizes and weights of resistors and capacitors used in l~ a fil~er that i8 tuned to a relatively low characteristic frequency; they provide electrical power to offset the power losses in the passive elements o~ the filtar. OP AMPs are particularly useful because of their relatively high input impedanc~, low output impQdance, high open-loop gain, low c03t, and hlgh reliability. To accomplish the desired ripple sensing o~ the present invention, a "bandpass" type o~ active filter need~ to be u~ed. A
bandpas3 Silter has a rrequency "pas~band" bounded by two ~requency "stopbands." In other words, it readily tran~mit~ or passes excitation signal frequencies within a ~elected ~requency range (the passband) while e~fectiv~ly blocking other-~requency components either ~' abov~ or below the selectQd passband. The frequency ~ 30 di~ference between high and low limit3 o~ the passband :~ ~s~nown as the "bandwidth~ of the ~ilter.
Active filters are conventionally designed so that their passband magnitude characteri~tics are .~ ', '~
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W O 92/15469 P ~ /US92/00621 29 2iQ(~
eit~er maximally flat ("Butterworth~) or equal-ripple ("Chebyshev"). In either case, as is axplained in the previously-cited text of Huelsman and Allen (see Chapter 2), an ldealized characteristic can be approached by increasing the order (also referred to as the nu~ber of poles) of the complex frequency-magnitude relationship that defines the voltage transfer function o~ the filter. A ~ilter having ~aximally flat ~agnituds response di~fsrs fro~
~ 15 a fllter having equal-ripple magnitude response in at : least two sisnificant respects: the latter filter has a sharper cutoff between passband and stopband (i.e., a steeper rolloff) and greater attenuation of stopband ~requencies, whereas the former filter has a more linear phas~ characteristic and ~etter trans~ent response. To obtain the 6harp filtering action and high degree o~ integrity required in the presently disclosed regenerative braking protective means, the ~; ripple cen~or 48 i8 formed by combining active filters characterized by both kinds of responses in a network a~ illustrated in FIG. 3. Thi3 combinat~on includes:
~1) an initial high-pass type ~ctive filter 51 that is ~excited by a voltage proportional to the shoe voltaga .~Vdc, the parameters o~ thi~ filter belng selected for :~ 25 equal-ripp~e magnitude re~ponsQ and for a "half-power"
cutoff frequency (fc) slightly under the lower ~hal~-power ~requency o~ the 6X ripple passband; (2) a !i~irst bandpas3 typs active ~ilter 52 that is excited by the a-c signal3 passing through the high-pass ,30 filter 51, the parameters Or this filter being ;selected ~or maximally rlat magnitude rQsponse, for a center frequency (fo) substantially equal to the fundamental frequency of t~e 6X ripple co~ponent of ! , ;'` ';' ' ''';` ''',,'.''.' ' ''''' ~' ~'.' ., ' ' :

w092/1~469 PCT/US9~/00621 2 ~ 2 r l ~ 1 Vdc, and for unity gain when the excitation frequency equals fo, with the bandwidth of the filter being relatively narrow (i.e., the difference between the two half-power frequencie~ respectively above and below fo is a small percentage, such as 5 or 6 percent, of fo); (3) an interstage high-pas~ type passive filter section 53 that i~ also excited by the a-c signals passing through the initial high-pass filter 51; and t4) a second bandpass type active filter 54 that i~ excited by the a-c signals passing through the high-pas~ filter 53 and is essentially a duplicate of the ~ilter 52 except for being tuned to a center frequency sub~tant~ally equal to the fundamental ~requency of the 12X ripple component of Vdc. ~Persons skilled in the art will understand that ; a half-power frequency i~ any excitation frequency at which the output voltage of a filter is 70.7 percent , of its peak magnitude which in turn $~ obtained, in !~ the case of a bandpass filter, at the center ~requency. In logarithmic unit~, the magnitude at this ~requency i8 down 3 decibels ~-3dB] fxom the peak Il magnitudQ. ) The ripple sensor 48 also includes a pair of precision rectifier circuits 5S and 56 connected to the output terminal~ Oe the respective bandpass l ~iltars 52 and 54. These circuits recti~y any ~ignals '.' passing through the bandpa~s filters. ~he rectified ;'! signal3 are rQspectively supplied to a pair of smoothing circuits 57 and 58 comprising conventional ` 30 low-pass ~ilters for deriving on the lines 360 and 720 ; corresponding output values _h t vary with the average .~ magnitudes of the signals supplied by the rectifier '~ circuits 5S and 56, re pectively. Consequently, the ~I .

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values on the output lines 360 and 720 are respectively representative of the amplitudes of the 6X and 12X ripple component3 of Vdc.
Preferably, the first bandpass filter 52 is a fourth order, unity gain active filter formed by connecting two separate second order filter modules in a conventional cascade arrange~e~t. A practical :design for each ~odule i~ described on pages 218-20 and 258-61 of the Hu~ls~an and Allen textbook~ The filter 52 comprises two duplicate modules of this kind interconnected with multiple feedback in the manner indicated in Figures 6.3-12 and 6.4-1 on pages 301 and 303 of the same text. These mod~les are synchronously tuned to a center frequency equal to th~ fundamental frequency of the 6X ripple ~oltage on th2 third rail.
For example, i~ th~ a-~ power frequency were 60 Hz, fo - 360 Hæ. If somQ drift were expected, a desirable bias could be provided by adding an extra 5 Hz to this center ~requency. The -3 dB bandwidth (BW~ of the j20 bàndpas~ filter 52 is approximately 5.5 percent of fo ; (e.g., 20 Hz). The re~ulting relationship be~ween the ;' voltage transfer ~unction of this filter and the excitation frequency is shown in FI~. 4~. The same rQsult~ could b~ obtained by alternatively designing each of tho cascaded second-order fllter madule~ in accordance wi~h the "Delyiannis-Friend" bandpass circuit described on pages 203-07 of the textbook by M.E. Van Valkenbu~g (Holt, ,, Rinehart and Winston, 1982).
; 30Preferably, the high-pass filter 51 is a third order, single-amplifier active filter si~ r to the ~Sallen and Rey" ~ilter shown in Figure 4.3-2 on page 164 of the Huel3man and Allen text. This filter is so ~''`,' ' '", .,, ~, .. . .

.
WO92/1~469 PCT/US92/00621 2~ 32 , designed that its passband ripple amplitude is 0.5 dB
and its -3 dB cutoff frequency fc is approximately 5.5 percent lower than the center frequenry of the bandpass filter 52. For example, fc = 340 Hz, which .5 is only 10 Hz less than the lower limit of the .jbandwidth of the 6X bandpass filter 52. The resulting ~relationship between the voltage transfer function of this ~ilter ~nd the excitation frequency is shown ~n FIG. 4B.
In operation, the ~ilter 51 and 52 selectively pass any 360 Hz a-c ripple in Vdc with virtually no . attenuation, and therefore the value on the output line 360 of the ripple sensor will be an accurate ;~ measure of ~he amplitude of such ripple. However, the ~iltering action o~ the~s two filter~ will greatly .~ attenuate any ripple voltage having frequencles above .. 370 Hz or below 350 Hz, wheraby the output value on . the line 360 is not appreclably increa3ed by any such ;' J stopband ripple (e.g., communication signals) as may be present in Vdc. The combined effects of the sharp . cuto~ contributed by the equal-ripple high-pass .` filter 51 and the narrow bandwidth contributed by the maximally ~lat bandpass ~ilter 52 will provide a !~ desirQd attenuation o~ more than -30 dB at an .~ 25 excitation ~requency o~ 300 Hz. Thus, unwantad low frequency noise below the passband o~ interest is e~featively eliminated ~rom the signal developed at the output terminaln o~ the bandpass ~ilter 52.
~: Pre~erably, thc second bandpass filter 54 is constructed and arranged in essentially the same manner a~ the ~irst bandpa~s ~ilter 52. However, the second ~ilter 54 i8 tuned to a center frequency (e.g., 720 Hz) that is twice the center ~requency o~ the 1"':, , . .
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~ WO92/154~9 PcT/uss2/oo62l 33 2~ 2Sl ~ first bandpass filter, and preferably the ohmic values ;~ of certain reslstors in the second filter are chosen so that this filter has a small positive gain (e.g., 1.1) when its excitation frequency and center frequency coincide. The -3 dB bandwidth of the second bandpass filter i5 approximately 40 Hz.
The high-pas~ filter section 53 between the input terminals of the second bandpass filter 54 and the initial high pas3 active ~ilter 51 is preferably ~
:. 10 conventional second order RC network having a -6 dB
cutoff frequency (e.g., 370 Hz~ that is higher than the center ~requency of the first bandpass filter 52.
In operation, the filters 51, 53, and 54 selectively !' pass any 720 Hz ripple in Vdc with virtually no ~: 15 attenuation, and therefore the value on the output ; line 72Q o~ the ripple sensor 43 Will be an accurate measure of the amplitude of such ripple. However, . their filtering action will greatly attenuate any ripple voltages having frequencie~ above 740 Hz or below 700 ~8~ whereby the output value on the lina 720 is not appreciably increased by any such stopband ripple as Day be pr~sent in Vdc.
The value~ on the two output lines 360 and 720 of the ripple sensing means 48 are used in the controller 2S 37 ~or the purposa o~ deciding whether or not the propulsion ~y3te~ will operate in a regenerative braking mode when eleatrical braking is de~ired. As previously oxplained, the thyristor branch o~ the . switch means 36 (sQe FIG. 2) will automatically change : 30 ~rom conducting to non-conducting states each time the vehiole 10 enters a third rail gap while operating in a regenerative braking mode. After the vehicle moves . past the gap and it~ leading shoe has made contact '~ ''" ' ~' ' : ' , .~........................................ . . .

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with the next third rail conductor, as soon a~ the controller 37 determines that there is a value on either of the two output lines 360 and 720 in excess of a predetermined minimum level it will issue a closing command to the breaker mechanism LB 60 as to reclose the circuit breaker 34 and a firing command that enable~ the firing signal generating mean~ 4S to apply a suitable firing si~nal to the gate of the thyristor 44 which then returns to its regeneratlve current conductinq state. In the remainder of thi~
specificat~on, a practical way to implement tha functions sum~arized ~n this paragraph w~ll be disclosed.
In the preferred mode of practicing thQ
invention, the controller 37 is a microcGntroller , comprising a coordinated system of commercially ,.. ..
availabla microcomputer component3 and a~sociated electrical circuits and elements that oan be programmad to per~orm a variety o~ desired functions.
A flowchart o~ pre~ently-relevant steps ~n one such progra~ Ls displayed in FIG. 5 and will now be descri~ed. The FIG. 5 program is automatically executed by the microcontroller 37 every lO
; millisecond~. It begins with a routine 61 of "checking" the ripple sensor output values on lines 360 and 720. Relevant details o~ the ripple checking , routine 61 w~ll soon be explained with re~erence to ;~ FIG. 7. As will be apparent when FIG. 7 i8 described, this routine includes means ~or setting a "ripple oKn flag in a true 3tate if either one o~ the ripple output values exceeds the minimum level.
The next step 62 in the FIG. 5 program decides whether or not an electrical brakinq mode of operation ... .
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is desired. If no braking mode is scheduled, other routines 64 that are not presently relevant will be performed. But if the vehicle operator has called for electrical braking, the progra~ proceeds from the step 62 to an inquiry 65 as to whether or not the brak~ng ~ mode was true previouRly. on the first pass through the FIG. S progra~ after the brake call is initiated, the answer to lnquiry 65 i8 negative and the progra~
: will proceed directly to a dyna~ic braking sequence of steps compri~ing a step 66 that issues an opening command to the pre~iously described breaker mechanism LB (which ~echani~ responds thereto by moving the circuit breaker 34 to its open or non-conducting : state, thereby disconnecting thR line 33 in th~
bi-directional current path from the switch means 36), a step 67 that loads a binary word unique to dynamic braking in a "present mode~ register, and a step 68 that initiates operation of the ~nverter 30 in an electrical braking mode. With the breaker 34 ln its ~ 20 non-conducting ~tatQ, the ~raking mode will be solely :~ dynami~ braking.
For as long as electrical brakinq continues to be called ~or, every timQ the FIG. S program is executed : a~ter the above-describQd ~irst pas~ through the dynamic ~raking sequence, the answer to the inquiry 65 will be a~irmative and th~ program will proceed from inquiry 65 to another inquiry step 70 that looks at th~ word ~aved in the prosen~ ~ode register to . determine whether or not the present mode is regenera~iva braking. Initially the present mode is ~ not regenerative braking, and therefore the program proceeds to yet another inquiry 6tep 71 that will determine whether to enable regenerative braking or :
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not. The step 71 actually comprises a series of inquiries the relevant parts of which are shown in FIG. 60 The ~irst in~ulry 72 in step 71 looks at the state o~ tha ripple OK flag and rasponds af~irmatively iS the state i~ tn~e. The ripple OX flag i~ true if the shoe voltage Vdc has either a 6X or a 12X ripple compon~nt o~ appreciable amplitude (see FI~. 7). A~
is indicated in FIG. 6, if the answer to inquiry 72 i~
yes there is another inquiry 73 to determine whether or not Vdc has a magnituda greater than 3 predeter~ined level G (e.g., 450 volts)~ (In practica, an af~irmativ~ answer i~ not obtained at inguiry 73 until Vdc has remained greater than G for 50 m~lllssconds.) It will ba apparent that i~ either one of the shoes 12 and 13 were in contact with a i deenergized section of the third rail, or if both shoes wer~ not in contact with the third rail (e.g., both shoes in a third rall gap), the answer to inquiry 73 would be no. However, if either shoe were in contact with an energized third rail conductor, Vdc would exceed G and the answer to the inquiry 73 is yes, in whi¢h event the next inquin 74 in the step 71 determine3 whether or not the magnitude o~ Vdc ls less than a predetermined high magnltude H (e.g., 825 volt~). An a~irmative answ~r to the inquiry step 71 ,' i9 obtained i~ the answer to its third inquiry 7~ is ye~, but a negative answer wlll be obtained i~ the answer to at least on~ o~ the three constituent inquiries 72, 73 and 74 is no.
As long as the answer to the inquiry step 71 i~
negative, the FIG. 5 program end~ here and the ~ propulsion system continues to operate in the dynamic .,'~ ' ,`', ,' .
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braking mode that was initiated by the above-described first pass through. However, when the answer to the ;inquiry 71 is affirmative, the program will proceed from this step ~o a regenerative braking sequence o~
steps comprising a step 75 that reset-~ (if necessary) the firing command, a step 76 that issues a clo~ing command to tha breaker mechanism LB (wh~ch mechanism responds thereto by returning the circuit breaker 34 to its conducting stat~ thereby reconnecting the line 33 to the switch mean 36), a step ?7 that issue3 a discrete firing command to the firing signal generating mean3 46 (which responds thereto by applying a æuitable ~iring signal to the gate of the thyristor 44, thereby changing thQ sw$tch means 36 fro~ a non-condu~ting state to a conducting state), a ~tep 78 that load~ a binary word unique to regenerative braking ln the present mode register, and a step 79 that initiates operatlon of the inverter 30 in a regenerative braking mode. With both the breaker 34 and the switch means 36 in conducting statQs, the electrical braking mode will actually be a blend of dynamic and regenerative braking.
The next time through tha FIG. 5 program after regeneratlv~ braXing commence3, the inquiry step 70 will have an a~firmativQ answer, and the program wlll there~ora proceed from inquiry 70 to an alternative inquiry step 80 that is the same as the multi-part inquiry step 71 shown in FIG. 6. As long as th~
answer to the inquiry 80 i8 a~irmative, the FIG. 5 ~;'30 program and3~here and the propulsion system continues ~to operat~ in the regenerative braking mode. However, ;~1if the third rail conductor with which the vehicle's shoes are in contact were disconnected fro~ its .,, ,,, , ~ . .
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.'.'.1 wayside source of voltage, or when the vehicle reaches the next third rail gap, the answer to the inquiry step 80 will be negative instead of positive, and the program will now proceed from this step to the dynamic braking sequence of steps 66-68 previously described.
As a result, regenerative current being conducted by the thyristor 44 decreases to zero (either because the circ~it breaXer 34 stopped conducting when ~tep 66 was executed, or because the vehicle shoe~ sntered the aforesaid gap, whichever occurs first). Now the previously conducting switch means 36 automatically changes to it~ non-conducting state where it will remain until the next tlme the regenerative braking sequencs of steps 75-79 are executed.
FIG. 7 shows relevant details of th~ previously mentioned ripple-checking routine 61. This routine begins with an inquiry step 81 that perform~ the function of a bistable level detector: if the actual value on output line 360 o~ the ripple sensing means 48 equal~ or exce2ds a certain l~vel K, the answer to inquiry 81 i3 af~$rmative: otherwisQ the answer i8 neg~tive. K is the value derived by the ripple sensor 48 whenev~r the 360 Hz ripple component of Vdc has a pre~etermined thr~hold amplitude that is relatively low but measurable ~e.g., approximately 0.25% o~ the average m~gnitude o~ normal wayside voltage). I~ this value i8 lower than ~, th~ routine 61 proceeds ~rom the inquiry 81 to a similar inquiry step 82 that compares the actual value on the output line 720 of the ripple sensor with K. If the answer to the inquiry 82 is al80 negative, there is neither a 360 Hz nor a 720 Hz ripple in the third rail voltage and the next step 83 is to set the ripple 0~ flag in a "false"

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:
WOg2/15469 PCT/US92/00621 2 '~ 3 ~
: 39 state. This signals that regenerative bra~ing i~
impermissible and prevent~ execution of the : regenerative braking seqUenCQ of steps 75-79 in the FIG. 5 program. On the other hand, if either the 360 Hz ripple component or the 720 Hz ripple component has an appreciable amplitude (i.e., an amplitude that equals or exceeds the aforesaid threshold), the ripple OK flag i3 set in its ~rue state by a step 85 (a~ter executing another inquiry step 84 that will soon be explained). In the manner descrlbed above with reSerence to FIGS. 5 and 6, the truG state of ~he ripple OX flag ~erve~ as a signal enabling regenerative brak~ng to be e~fected. ~y checking for either a characteristic 6X ripple component or a characteristic ~2X r~pple component of Vdc, tha ripplQ
detecting means will respond correctly regardless oS
whether the wayside voltage source energizing the .~ . third rail ~ectlon along which the vehicle 10 i8 ;~ traveling incorpcrates a three-phase or a six-phase power recti~ying bridge. Note that each time the ~, state of th~ ripple OX rlag changes from true to false while electrlcal braking is in eSSect, the enable signal will terminatQ and the inquiry step 80 in the . FIG. 5 pro~ram will respond by initiating execution of `' 25 the dyna~is braking seguence 66-68.
The inquiry ~tep 84 and the othQr steps occurring after step 83 or 85 o~ the ripple checking routine 61 ;.' are provlded because oS a potentlal problem that will now ba explained. During regenQrativQ ~raking, the d-c component o~ Vdc tQnds to increase to a relatively high magnitudQ iS the third rail i8 not suf~iciently receptive to the regenerated power, and therQ i8 a poGsibillty that the output value o~ the ripple , . '', .
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sensing ~eans 48 could fall below X even though the third rail is not disconnected from its wayside voltage source. In the event the ripple OK flag is changed from true to false states for this reason, the FIG. 5 program will change the electrical braking ~ode ~rom regenerative braking to dynamic br~king as ; previously described. Vdc will now decrease u~til the ripple sensor output value rises to X at which time the rippls OK ~lag rQturns to its true state, and consequently the FIG. 5 program will now restore regenerative braking.
Frequent cycling in and out of the regenerative braking mode is undesirable. In order to limit such cycling ~o a predetermi~ed ~aximum number (e.g.~ two) while the prQpulsion 3y8te~ continues to operate in an electrical braking mod~ and Vdc has an appreciable ~agnitude (e.g., greater than 50 volt~), the ripple checking routina 61 include~ a "regen disable" flag that initially i8 in a false state. The inquiry step 84 will not inhibit the above-described operation of the rippl~ detectlng means as long as the regen disable ~lag remaln~ in its false state. 3ut steps 87, 88 and 89 o~ th~ routine 61 will set this ~lag in it~ true ~tate each time thQ ripple OK ~lag changes 25 ~rom tru~ to ~al~e state~ while regenerative braking ~; ;
, i3 in ef~ect, and concurrently a step 90 will ~ increment by 1 the binary number in a "cycle counter"
;I register o~ the microcomputer. Whil8 the regen ~i disable Plag i8 true, the ripple OK flag i8 "lockedn ; 30 in it~ ~alse state by the inquiry step 84, whereby regenerative braking is prevented. So long as the cycle count is less than two, lOO milliseconds after step 89 wa~ executed the re~en disable ~lag will be .

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w092/15469 41 2 3 ~ ~2 ~ :
automatically re~et to it8 initial, false state by steps 91-96 of the routine 61, and as soon thereafter as either a 6X ox a 12X ripple component o~
appre~iable amplitude is detected the ripple OK flag is again set in its true state and regenerative - braking i8 able to resume. However, once the cycle count reaches two, the reset step 96 is bypassed, the regen disabl~ ~lag i8 not returned to ~ts fals~ ~tate, the ripple OK flag remains locked ln it3 ~al~e state, and consequently the regenerative braking sequence of steps 75-79 cannot ~e executed. Within 50 millisecond3 after Vdc falls below 50 volts (as will occur each time the vehicle'~ shoes 12, 13 ent~r a third rail gap) or the electrical braXing mod~ i8 diæcontinu~d, step 98 o~ the routine 61 will set th~
regen disable flag in its initial, false state, and a step 99 will reset the cycle counter to its initial count of 0.
In practice there i8 a possibility that the thyristor 44 will not stay turned on when step 77 of the FIG. 5 program i~ executèd (i. Q . when a firing command i8 issued) due to bouncing of the pick-up ~i shoes 12, 13 on th~ third rail or due to a very low initial magnitude o~ rege~erative current or due to some other transient condition (e.g., a coating o~ ice on the third rail conductorl that results in the ~; thyristor being back-biasQd temporarily. Under such abnormal conditions, regenerative braking could not actually commence (or would unintentionally be discontinued) even though the regeneratlve braking enabling sequence of steps 75-79 were properly executed. To addres~ this potential problem, the microcontroller 37 includes a special ~auto re-flre~
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` WO92/15469 PCT/US92/00621 2~2'~ 42 progra~ that i5 displayed in FIG. 8 and will now be described.
The auto re-fire program is automatically executed every 50 milliseconds. As is indicated in FIG. ~, it begins with an inquiry step lOl that looks at the word saved in th~ present mode register to determine whether or not regenerativa braking i8 the ; presently designated mode of operation. If not, the binary number stored ln a "timer~ register (TMR2) i8 set at 0 ~y a 5tep 102, and then the auto re-fire program ends. But if the propulsion cystem i8 intended to be operating in a ragenerativ~ braking mode, the auto re-flre program proceeds from inquiry 101 to a second inquiry 103 as to whether or not the number ~aved in ~MR2 i8 less than 25. If the answer i~ affirmative, the program will end after incrementing the timar register by one at a step 104 and resetting (if nece~sary) the firing command at a step lOS. Alternatively, i~ the answer to inquiry 103 were negative (as is trua eYery 25th pas~ through the ; auto re-~ire progra~), th~ next step 106 in this progra~ would be to determine whether or not the ~peed at which the vehicl~ lO i~ traveling exceeds a predetermined relatively low velocity (e.g., 2 MPH).
I~ not, the auto re-fire program end~ here regardless of whether or not the propulsion system is then operating in its regenerative braking mode.
Otherwise, two additional ~teps 107 and 108 arc executed be~ore cnding the program. Step 107 resets the number in the timer register to 0, and step 108 issues another discrete firing command to thQ firing signal generating mean~ 46. As a result, firing s1gnal~ are pcr1od1c~11y reappli-d to the g~tc o~ the `~' :'' .,,~ .

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~3 thyristGr 44 durlng regenerative braking operation.
If the thyrlstor fails to turn on when regenerative braking was supposed to commence, or if at any time it unintentionally turns off while regenerative braking is enabled, the auto re-fire program ensures that another ~iring signal is supplied wlthin 1.25 seconds, thereby returning (i~ necessary) the thyristor to it8 regenerative current conducting ~tate.
The two inverter~ 30 and 30A are controlled ~rom the common control means 37 which responds to alternative co~mand signals from interloc~ed throttle and brake controllers 38 and 39, respectively. The control mean~ 37 also receives feedback signals representative of sensed values o-~ voltage, current, and other selected varlable~ in each of the inverters 30 and 30A. To operate in a dynamic braking mode, the control means 37 derives a train of suitably timed periodic ~ignals that determine the repetitive on and off intervals of the choppers 41 and 41A, and it varies the ratio of these intervals as desired. This signal train i8 ~ed over a line 110 to the ~irst chopper 41 and also to suitablR means 112 ~or ! splitting it into a separatQ traln of periodic signals that are displaced from the signals of the oxiginal ;25 train on the line 110 by a length o~ time correspondlng to approximately one-haIf the period o~
~uch signal3. The sQparatQ signal train i8 ~ed over a linQ 114 to the second chopper 41A. In this manner, the two choppars are coordinated so as to operate alternately rather than in unison. That i8~ the "on"
periods of chopper 41A are staggered in time with respect to the "on" periods of chopper 41. This staggering reduces the amplitude and increases the .:

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wo92/1~469 PCT/US92/00621 2 ~ Q ~
44 :
freguency of the braking current traversing the line c~pacitor C2, thereby making it much easier for this capacitor, which i8 then acting as a filter for attenuating the harmonic~ generated by operation of both of the choppers, to perform its filtering functlon.
As previously discussed, it is critical to the operation of the propulsion system for transit .~ vehicle~ that frequencies corresponding to signalling - 10 frequencies not b~ induc~d into thR way3ide power system by the propulsion system. Continuous monitorlng o~ th~ lin~ current IL by thQ control 37 i~
utilized to assur~ that such signal ~requencie~ are :~, not present in lino current. Referring to FIG. 9, 15 therR i3 shown a simpli~ied block d~agram o~ a part of the propulsion control system including the monitoring ; of line voltage VL~ way~ide conductor voltage VT~ and line current ~L. Each ~ignal VL~ V`T~ and I~ i3 coupled ~ through re~pective bu~rer circuit~ 118, 116, and 120 'i 20 The signals from buf~ers 116, 120 are coupled to input terminal~ o~ electronic switche~ 122 and 124, ', respectively. The switches 122, 124 ar~ arran~ed to pas~ either the ~ignals ~rom the respective bu~fers 116, 118 or ~ignals rrom a freguency generator 126.
The signals developed at the output terminals o~
switches 124, 122 ~re coupled to input terminals o~
corresponding band-pass rilters 128, 130 and also to selected input ter~inals Or a multiplexer (MUX) 132.
Signals de~eloped at the output terminals of each of ; 30 the ~ilters 128, 130 are also coupled to selected input terminals o~ MUX 132. The wayside conductor voltage ~ignal Vr i8 coupled directly rrOm bufrer 118 to MUX 132. For purpose o~ illustration, the various ... . .
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componsnts such as resistors, diodes, capacitors, and other active ~evices have been omitted with the understanding that the use o~ such devices is well known in the art and they are subsumed within the blocks shown in FIG. 9. A selected signal from MUX
132 is coupled through a sample and hold circu~t (latch~ 134 and applied to an analog-to-digi~al ~A/D) converter 136. The digitized output of A/D converter 136 is coupled ~hrough a bu~er 138 onto addres~/data lines 140 for application to control 37. It will be appreciated that control 37 is a microcomputer based control which can be program~ad in a manner well known in the art to implement various selected control ; functions. While the above deRcribed check verifies th~ integr$ty o~ the most likely component to change its value, it i8 pos-~iblQ for other component~, including the inverter control or ohopper control, to also ~ary or ~ail. It i8 therefore desirable to confirm that ~ignal frequencies are not introduced ; 20 onto the wayslde conductors. Referring again to FIG.
9, the switches 124, 122 ar~ under the control of control 37 and are operative to pass either the VT and IL signals or the signals from generator 126 to the corresponding band-pass filters 128, 130. The ~ilter 128 i8 selected to pa58 only those signals having a predetermined signal rrequency, e.g., 25 Hz. A signal i out o~ ~ilter 128 is there~ore repre~entative of any component o~ the IL signal having a frequency near the signal rrequency during normal monitoring. During a test mode, a signal rrom filter 128 indicate~ that the filter and asaociated system are operating properly for passing signals Or the predetermined freguency.
While the I~ ~ignal i8 generally ~onitored to assure :; .' :''.:' ' ' ' ' ' . ' .
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that signals corresponding to local signal frequencies are not being induced into the wayside power source, it is also desirable to monitor the VT signal to assure that other frequencies, such as 360 Hz, are present during regeneratiVQ braking. The systems for monitoring VT and IL for freguencies associated with the wayiide power source are described in FIGS. 3A and ; 3B although th~ ~requency verification circuit i~
shown only in PIG. 9.
Referring briefly to FIG. 10, there i8 shown a pass-band characteristic of a typical band-pass filter 128. At thQ center frequency, i.e., the selected s~gnal frequency such as 25 Hz, the filter 128 has maximum transmission. ~ha upper and lower frequency test points, ~or example, the half-power points, ara deaigned to encompass upper and lower frequencie~
which ~ight inter~ere with the signal frequency. In the te~t mode, i.e., whsn the switch 124 passes signal~ ~rom th~ generator 126 to filter 128, the control 37 ~onitors the responsQ of the filter 128 to veri~y that its rQsponsa corresponds to the waveform o~ FIG. 10.
~hQ adv~ntagQ o~ the sy~tem of FIG. 9 is that the ~requency components Or the IL and VT ~ignals can be monitored with only a singl~ control loop, i.e., no redundant control loop~ and electronic "voting" are necQssary. Thls is possible since the sel~-test system o~ FIG. 9 assurei~ th~ accuracy o~ the system.
It will be recognized that the introduction ot signal ~requencie~ onto the wayside conductor is so critical ; that tha propulsion system in any transit vehicle producing such ~ignal ~requencies is i~mediately ;~ disabled. Thu~, while the system o~ FIG. 9 eliminatei .. .

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-WO92/1~469 PcT/uss2/oo62 2~ 4 redundancy, it still assures integrity of the systemby self-test.
FIG. ll is a schematic representation of a peak amplitude signal limiting circuit 132 coupled in circuit with a bandpas~ filter circuit of the type , shown in FIGS. 3A, 3B, and 9, such a5 ~ilter circuit , 128, in one ~orm of the present invention. The - current signal IL frO~ FIG . ~ coupled to ~nput terminal 134 from where it is processed through an a-c iO coupling circuit 136 comprising the series combination o~ capacitor 138 and resistor 140- The IL signal i8 then coupled through an electronic switch 142 to an inverting input terminal of an operational amplif$er 144 withln the ~iignal llmiting circuit 132. The electronic switch 142 i9i controlled by a signal from the control 37 as described previously. The switch 142 may comprise a field effect transistor (FET) or ~ other type of electronic switch well known in the art.
'~ Connected to the input line between the resistor 140 and switch 142 i9 a pair of reversely poled diodes 146. One terminal of thes~ diode~ i9i connected to a re~erence plane such as ground. ThQ diodes 146 limit the amplitude o~ voltage applied to thQ~ siwitch 142 when the switch i8 in itsi non-conducting state. As ; 25 previou~ly de~cribed, the switch is forced into a non-conducting ~tate during a test time period when a test ; siignal i~ inserted into the filters such as filte,r 128 ~ in ordQr to conrirm that a filter isiresponding at the ,~ proper rrequency. I~ FIG. 11, the test signal is inserted at terminal 148 through an a-c coupling network lS0 to the inverting input terminal o~
amplifier 144.
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WO92/ls46s PCT/US92/00621 2 ~

The operational amplifier circuit including the amplifier 144 is designed to limit the peak amplitude of the ripple current signal applied to the filter circuits such as filter circuit 128. The a~plifier circuit includes a feedback resistor 152 coupled between an output terminal and thQ inverting input term$nal. For purpo~es o~ de~cription, it will be noted that a su~ming ~unction 154 ~s formed at the - inverting input termindl of amplifier 144. In th~
particular arrangement, the non-inverting input terminal of amplifier 144 i8 connected to a reference plane selected to be at ground potential. Th~ output terminal o~ amplifier 144 i8 normally biased to zero voltage by a voltage divider cir~uit comprising f~rst and second r2sistor~ 156 ~nd 158 connected between tha output t~rminal and a sourcR of negative bias voltag2 and by a second pair of saries connected resistors 160 and 162 connected between the output terminal and a sourc~ o~ positive bias voltage. The resistors in the preferred embodiment arQ o~ the same resistive value and the bias voltages are o~ thQ same magnitude so that the output terminal i~ biased to zero volts.
Limiting o~ the slgnal dQveloped at the output terminal o~ ampli~ier 144 i8 achieved by a feedback ; 25 circuit including a voltage breakover devlce such as diode 164 and diode 166. The diodes 164 and 166 are !connected between the summing ~unction at the `~lnverting input terminal o~ amplifier 144 and a ~unction intermediate a respective pair o~ the resistors 156, 158, and 160, 162. I~ the signal at the output terminal oS operational amplifier 144 attempts to go positive to an extent that the voltage at the ~unction intermediate resistors 156 and 158 .` .
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.: , WO92/ls46s PCT/US92/00621 2 ~ ~ 2 ~

rises abova about 0.6 volt~, the diode 164 become~
conductive providing a feedback loop to the inverting input terminal of amplifier 144 thus limiting the output voltage to that value which will cause the diode 164 to conduct. Similarly, i~ the output terminal o~ ampll~ler 144 attempts to go negativa to an extent that th~ ~unction inter~ediate resistors 160 : and 162 become~ le88 than about 0.6 volts negative, the diod~ 166 will conduct and l~it the excursion o~
lo the signal at the output of amplifler 144 to the seleGted negative limit. Thue, the operational amplifier circuit 132 ~erves to l~mit the amplitude o~
the signal developed at it~ output terminal to a presele~ted value determined by the ratio bstween th~
resistor~ 156, 158, and 160, 162.
The curre~t s$gnal developed at the output terminal o~ amplifier 144 is coupled through a voltage divider network compri~ing resistors 168 and 170 and through current limiting resistor 172 to an input terminal Or the bandpass filter circuit 128. As previou~ly described, the filter circuit 128 and the other filter circuits utilized in thQ present inventlon are actlve ~ilter circuit~ and may be of the type de~cribed in the text Fun~tion Cirçuit~ De~iqn and Applic~tions by Wong and Ott published by McGraw-: Hill Hook Company, Inc., 1976. As described previously, these ~ilter circuits may be designed as bandpass ~ilters ~or any particular freguency. In the : illustrative embodiment, the circuits are designed for nominal frequencies of 25 Hz, 60 Hz, 95 Hz, 200 Hz, 360 Hz, 720 Hz, and 990 Hz. The lower frequency signals, i.e., those signals between 25 Hz and 200 Hz, ~ro typlc~lly used rOr w~ysldo cor~unication and exlst .~. ' '.

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. W O 92/15469 PC~r/US92/00621 2~2~

on communication lines running generally parallel to the wayside curren~ conductors. The higher frequencies, i.e., 360 Hz through sso Hz, are characteristic frequencies of the wayside power source. In the case of the lower frequency signals, it is desirable to detect whether or not the on board propulsion sys~e~ o~ the vehicle is generating signals which m~ght lnterfere with tha wayside signalling sy~te~. In the case of the high~r frequency signals, 0 it i8 de~iral~l2 to dQt~ Ct whether tho~e ~ignal~ are actually present on the current conductors since that indic:ates whether or not the wayside power source i5 act$ve. If signals having ~requency component3 corresponding to the lower ~raquencies is detected as being generated by the propulsion system~ it i~
nece~sary to disable tha propulsion 6y8tem SO ag to avoid any inter~erence with th6 wayside signalling system. Detection of the higher frequency signal~ i8 indicati~e o~ an accepta~le condition ~or operating ~
propul~ion system. However, i~ the higher frequency signals are not detected, it i8 indicative that the wayside conductor i8 not being powered by an external power sourcel and thererore it i9 desirable to disconnect the v~hl¢le rrom the waysida current collector~. Tha control 37 include3 a mlcrocomputer control which i8 e~ective to respond to detection or non-detection o~ these signals and to take appropriate l action to dlsconnect the propulsion system from the ;l wayside power sourcQ. In general, in the event it i8 desirable to disconnect the propulsion system, the control 37 removes the gating signals to the solid state switche~ in the power converter and causes the contactors which connect the converter to the wayside ,..~
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W~92/15469 PCT/US92/00621 2~2~

power source to drop out thu-~ separating the propulsion system from the wayside power source.
While the invention has been described in what is presently considered to be a preferred embodiment, other modifications and variations will become apparent to thosQ skilled in the art. Accordingly, it is desired that the invention not be limited to the specific disclo~ed embodi~ent but be interpreted within the ~ull spirit and ~cop~ o~ thc appended claims.

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Claims (16)

52What Is Claimed Is:

1. Improved regenerative braking protective means for an electrical propulsion system on board a traction vehicle, the system including a controllable electric power converter having a set of load terminals adapted to be connected to a normally energized wayside source of unipolarity voltage, the converter having alternative motoring and electrical braking modes of operation, the wayside source comprising a plurality of bare electrical conductors respectively extending along different sections of the right-of-way traveled by the vehicle, with proximate ends of the conductors in adjacent sections being separated from each other by relatively short, insulating gaps, the vehicle being equipped with at least one current collector in sliding contact with the wayside conductors as the vehicle moves along its right-of-way, the current collector and the converter's d-c terminals being interconnected by controllable electric switch means having alternative conducting and non-conducting states wherein the improvement comprises:
voltage ripple detecting means coupled to the vehicle's current collector for providing an enable signal if the current collector is in contact with the wayside conductor energized by voltage having an a-c ripple component of predetermined frequency and at least a predetermined threshold amplitude;
means operative when the converter is operating in its electrical braking mode for changing the controllable switch means from conducting to non-conducting states in response to the current collector traversing each wayside conductor gap as the vehicle moves from one section of the right-of-way to the next section; and means operatively associated with said voltage ripple detecting means for limiting the peak amplitude of the ripple component coupled thereto.

2. The regenerative braking protective means of claim 1 wherein said peak amplitude limiting means comprises an operational amplifier circuit, said circuit including an operational amplifier having an inverting input terminal and an output terminal, a summing junction connected to said inverting input terminal and being coupled for receiving a signal from the vehicle current collector representative of a-c voltage on the collector, and voltage breakover means coupled between said input terminal and said summing junction for coupling current therebetween when voltage at said output terminal exceeds voltage at said summing junction by a preselected magnitude.

3. The regenerative braking protective means of claim 2 wherein said voltage breakover means comprises first and second diodes connected in first and second current paths between said input terminal and said output terminal, said first diode being reversely poled with respect to said second diode.

4. The regenerative braking protective means of claim 2 and including a voltage divider circuit connected between said output terminal and a source of bias voltage, said divider circuit comprising at least a first and a second resistor coupled in series current path, said voltage breakover means being connected between said input terminal and a junction intermediate said first and second resistor, said voltage at said output terminal being limited to a voltage established at said voltage divider junction and the breakover voltage of said breakover means.

5. The regenerative braking protective means of claim 3 and including a first voltage divider circuit comprising at least first and second resistors connected between a relatively positive voltage source and said output terminal of said operational amplifier, a second voltage divider comprising at least another first and another second resistors connected between a relatively negative voltage source and said output terminal, each of said first and second diodes being coupled between said input terminal and a junction intermediate said first and second resistors and a junction intermediate said another first and second resistors, respectively.

6. The regenerative braking protective means of claim 2 wherein said output terminal os coupled to an input terminal of said voltage ripple detecting means.

7. The regenerative braking protective means of claim 6 and including a voltage divider circuit connected between said output terminal and a reference voltage terminal, said input terminal of said voltage ripple detecting means being coupled to a junction intermediate said voltage divider circuit for receiving a signal of amplitude less than the amplitude of a signal at said output terminal of said amplifier.

8. Improved protective means for an electrical propulsion system on board a traction vehicle, the system including a controllable electric power converter having a set of load terminals adapted to be connected to at least one traction motor and a pair of d-c terminals adapted to be connected to a normally energized wayside source of unipolarity voltage, the converter having alternative motoring and electrical braking modes of operation, the wayside source comprising a plurality of bare electrical conductors respectively extending along different sections of the right-of-way traveled by the vehicle, with proximate ends of the conductors in adjacent sections being separated from each other by relatively short, insulating gaps, the vehicle being equipped with at least one current collector in sliding contact with the wayside conductors as the vehicle moves along its right-of-way the current collector and the converter's d-c terminals being interconnected by controllable electric switch means having alternative conducting and non-conducting states wherein the improvement comprises:
frequency detecting means coupled to the vehicle's current collector for providing a disable signal if current at the current collector includes a predetermined frequency component of at least a predetermined threshold amplitude;
means responsive to the disable signal for disabling the converter and disconnecting the converter from the wayside conductors; and means coupled in circuit with said frequency detecting means for limiting the peak amplitude of at least the predetermined frequency component coupled to the frequency detecting means.

9. The protective means of claim 8 wherein said frequency detecting means includes a current sensor coupled in circuit with the vehicles current collector for providing a current signal representative of current in the current collector, said limiting means being connected in circuit with said current sensor for limiting the peak amplitude of the current signal coupled to the frequency detector.

10. The protective means of claim 9 wherein said limiting means comprises an operational amplifier having an inverting input terminal connected for receiving said current signal and including feedback means coupled between an output terminal of said amplifier and said inverting input terminal for limiting the peak amplitude of signals at said output terminal.

11. The protective means of claim 10 wherein said feedback means comprises voltage breakover means for conducting current to said input terminal only when voltage at said output terminals exceeds a predetermined value.

12. The protective means of claim 11 wherein said voltage breakover means comprises at least first an second reversely poled diodes connected in parallel circuit paths between said input terminal and said output terminal of said amplifier.

13. The protective means of claim 12 wherein said predetermined frequency is selected from the group comprising the frequencies 25 Hz, 60 Hz, 95 Hz, 200 Hz, 360 Hz, 720 Hz, and 990 Hz.

14. The protective means of claim 12 wherein said frequency detecting means comprises a filter network having an input terminal coupled to said output terminal of said operational amplifier, said filter network comprising an active type bandpass filter characterized in that step-change signals of amplitude greater than a preselected value causes the filter network to ring, the peak amplitude of the current signal being limited to a value less than said preselected value.

15. An electrical filter network for providing an output signal representative of the amplitude of a characteristic a-c ripple component of an input voltage of undulating magnitude, comprising:

a bandpass type active filter having input and output terminals, said bandpass filter being characterized by maximally flat passband magnitude response, a center frequency substantially equal to the fundamental frequency of the characteristic ripple component of input voltage, and a -3 dB bandwidth that is a relatively small percentage of said center frequency;
a high-pass type active filter connected to the input terminals of said bandpass filter and adapted to be excited by the input voltage, said high-pass filter being characterized by equal-ripple passband magnitude response and a -3 dB cutoff frequency that is lower than the center frequency of said bandpass filter;
an operational amplifier having a summing junction formed at an inverting input terminal thereof for receiving a signal representative of the a-c ripple component of the input voltage, an output terminal of said amplifier being coupled to supply the input voltage representative signal to said bandpass filter, and voltage breakover means coupled between said inverting input terminal and said output terminal of said amplifier for coupling current therebetween when the input voltage representative signal reaches a predetermined amplitude to thereby limit the input voltage representative signal to said predetermined amplitude;
rectifying means connected to said output terminals for rectifying any signal passing through said bandpass filter; and signal smoothing means connected to said rectifying means for deriving an output signal having a value that varies with the average magnitude of the rectified signal supplied by said rectifying means.

16. The protective means of claim 7 wherein said predetermined frequency is one of about 360 Hz and about 720 Hz.