Note: Descriptions are shown in the official language in which they were submitted.
.~ 7
48, 149
RAPID TR~NSIT SYSTEM TRANSIENT
VOLTAGE SUPPRESSION APPARATUS
BACXGROUND OF THE INVENTIO~
It is known in the prior art to use an electro-
lytic capacitor with a power supply line to suppress tran-
sient energy voltages by storing the energy within the
: capacitor, with the standard practice being to limit the so
applied voltage to the surge voltage rating of the capaci-
tor as specified by the capacitor manufacturer.
It is also known to suppress transien-t energy
voltages by the use of a copper oxide rectifier above its
- 10 voltage blocking limit so that transient energy is conduc-
ted through the device in the reverse direction to dissi-
pate the transient in the form of heat. The copper oxide
rectifier uses an oxide layer, which conducts in one direc-
tion and blocks in the other direction up to the breakdown
voltage capability of the oxide layer, whereon the oxide
layer behaves like a zener diode and conducts current above
that breakdown voltage.
It is known in the prior art to employ thyristor
switch devices in a ch.opper apparatus to control the cur-
; 20 rent supplied to transit vehicle propulsion motors, as
described in an article entitled, "Alternative Systems For
. , ~
, .
2 48,149
Rapid Transit Propulsion And Electrical Braking" that was
published by B. J. Krings at pages 34 to 41 of the Westing-
house Engineer for March 1973.
SUMMARY OF THE INVENTION
.
The present invention relates to the provision of
electrolytic capacitors in the filter capacitor apparatus
associated with a power supply line for a transit vehicle
propulsion motor current regulating thyristor chopper
apparatus for providing transient voltage protection for
that thyristor chopper apparatus. Each electrolytic capaci-
tor is selected to have a predetermined zener breakdown
voltage and arranged in a filter circuit such that it
becomes operative in its zener breakdown mode -to absorb
energy for preventing the occurrence of a transient voltage
on the power supply line that is greater than the known
voltage rating of the protected thyristor device within the
chopper apparatus associated with that filter circuit.
BRIEF DES~RIPTION OF THE DRAWINGS
Figure 1 shows two -transit vehicles operated with
the same third rail line of a power supply station;
Fig. 2 schematically shows a motor current regu-
lating chopper apparatus, such as used with transit vehicle
propulsion motors;
Fig. 3 illustrates a prior art line filter capaci-
tor circuit arrangement operative with a two-thyristor
chopper appara-tus;
Fig. 4 illlustrates another prior art line fil-ter
capacitor circuit arrangement operative with a two-
thyristor chopper apparatus;
Fig. 5 illustrates the line filter capacitor cir-
3 48,149
cuit arrangement of the present invention which permits a
single thyristor chopper apparatus;
Fig. 6 illustrates the typical zener breakdown
mode of operation of an electrolytic capacitor;
Fig. 7 illustrates the relationship of the induc-
tive stored energy that is absorbed in the capacitor mode
of the capacitor device and that absorbed in the zener mode
of the capacitor device; and
Fig. 8 illustrates the operation of the filter
capacitor bank to protect the motor current control thyris-
tor device in accordance with the present invention.
PREFERRED EMBODIMENT OF THE INVENTION
In Figure 1 there is shown the well known opera-
tion of two vehicles 10 and 12 of a rapid transit system,
which vehicles are supplied direct current power from a
common power supply source, such as third rail 14, that
would occur when a first vehicle is followed by a second
vehicle through the same power station. The first vehicle
:~ includes propulsion motors and motor current control appar-
:'
-~ 20 atus 16 and the second vehicle includes propulsion motors
and motor current control apparatus 18. If, for some
reason, a ground fault, such as caused by a motor flashover
- or the like, should occur within a first one of those
vehicles, a transient line current build-up in the order of
5,000 or more amperes could pass through the second vehicle
due to the stored energy of the third rail inductance. The
- ground fault current would blow the line fuse of the first
vehicle and the filter capacitor of the first vehicle would
discharge stored energy back through the fault and into the
supply line in the relationship of 1/2 LI2, which fault
,. '` ' :
4 ll8,149
energy could amount to 30,000 or more joules of stored
energy. The filter capacitor bank of the second vehicle
now has to absorb that stored transien-t energy if the
chopper thyristors of the second vehicle are to be protec-
ted against a damaging too high applied voltage greater
than the known maximum voltage rating of those thyristors.
The worst case situation for such a fault condi-
tion is shown in Figure 1, where only one additional second
vehicle is available to absorb all of the stored energy
0 when a fault condition occurs on a first vehicle. If
instead, several additional vehicles are operating with
commonly supplied energy from the same power supply station
when a fault condition occurs in one of the vehicles, there
are then more additional vehicles available to absorb and
share this provided sudden burst of transient energy.
In Fig. 2 there is shown a simplified and well-
known propulsion motor current control chopper apparatus 22
connected in the motoring mode as described in the above
published article. The chopper feeds two motor circuits 24
and 26 of a vehicle 28. A well known thyristor firing
control 30 provides an OFF pulse to turn on the turn-off
thyristor T2, such -that the commutating capacitor Cc
charges to the same level as the line voltage. The commu-
tating capacitor Cc would charge to twice line voltage due
to its combination with the smoothing reactor L2 if it were
not for the free wheeling diode FWD. When the vol-tage on
the commutating capacitor Cc reaches line voltage level,
the current through the capacitor Cc and the thyristor T2
goes to zero and the thyristor T2 turns off. An ON pulse
is now provided by the firing control 30 which turns on the
48,149
turn-on thyristor Tl and the reversing loop thyristor T3.
The load is thus connected directly to the supply voltage,
causing the motor current to build up. Also, the voltage
on the capacitor Cc begins to decay as current flows
through the thyristor T3, the reversing loop reactor L3 and
the thyristor Tl. The thyristor T3 turns off when this
current has reached zero and the voltage on capacitor Cc
has reversed completely. Current now flows in the load
only and the circuit is ready for turn-off. Turn-off is
accomplished by the firing control 30 turning the thyristor
T2 on. The load current now flows through the thyristor T2
and the capacitor Cc. After a short delay due to the
reactor L2, the thyristor Tl turns off and the diode D4
conducts, helping speed the charging of the capacitor Cc.
The reactor L4 limits the rate of rise of current in the
diode D4, and the diode D4 stops conducting before the
capacitor Cc charges to line voltage. When the capacitor
Cc has charged to line voltage, the free wheeling diode
, ~
conducts current and the thyristor T2 turns off, leaving
the circuit ready for another ON pulse and the start of
another cycle. The current from the third rail 32 goes
through the line fuse 34 and the line filter reactor 36 to
the chopper apparatus 22. The line filter capacitor 38 is
connected in parallel with the chopper apparatus 22.
In Fig. 3 there is shown the line filter capaci-
tor break circuit 40 used with a previously supplied chop-
per system for transit vehicles. The individual capacitors
in the line filter capacitor bank are arranged in twelve
parallel branches with six in series in each of the twelve
3o parallel branches. The capacitors were selected to have a
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6 48,149
high enough surge voltage capability such that 1/2 CV2
would equal the power supply line fault condition transient
stored energy intended to be absorbed. The voltage surges
are caused because for any large DC power supply system
where there is a substantial line inductance, if a large
load such as transit vehicle propulsion motors is turned
off due to a fault condition, the resulting current can
build up to in the order of 5,000 amperes or more in a
particular power supply station section of track. A motor
flashover on one vehicle can cause such a fault condition,
which fault will blow the fuse or trip the line switch on
the involved vehicle. With for example a current of 4,000
amperes coming down the supply line, which line may have
1.5 millihenry of inductance, the transient energy is 1/2
LI or about 12,000 joules, and this energy is applied to
the capacitor bank of the second vehicle. The capacitor
bank shown in Fig. 3 was operative with a power supply line
of 1,000 volts, and each capacitor was rated at 300 volts
: with a 350 volt surge rating and a zener breakdown of about
` 20 400 to 450 volts. Therefore, each serial branch of six
capacitors would act as a capacitor up to about 2600 volts.
The transient energy that could be absorbed as capacitors
would be 1/2 CV2, with C being 12,000 microfarad so at 2600
volts and acting as capacitors, they could absorb about
40,560 joules of energy. Since the transient energy pro-
vided in the above example is only about 12,000 joules,
there is no problem concerning the two thyristors 42 and 44
provided in the current regulating chopper 46 operative
with the propulsion motors 48 and which thyristors were
3o 2400 volt rated. However, should the line inductance
7 48,149
increase for some reason and should the fault current
increase such that the voltage across the two series con-
nected thyristors goes above 2400 volts, the capacitors
would continue to act as capacitors until the -thyristors
failed due to an applied voltage above their rated voltage.
In Fig. 4 there is shown another line filter
capacitor arrangement for a transit vehicle chopper system.
The third rail voltage in this example was 750 volts, and
each capacitor was rated at 250 volts, with each capacitor
lo in the five capacitor series branch being applied at 75Q
divided by 1250 or 60% of its rated voltage.
In general, an electrolytic capacitor requires an
adequate voltage stress normally applied to it to maintain
the oxide layer thickness. Otherwise, a deforming of that
layer can occur. For the 300 volts rated capacitors of the
application illustrated in Fig. 3 with a line voltage of
1000 volts and six series connected capacitors in each
branch, each capacitor had a normally applied voltage of
166 volts or about 55% of its rated voltage. For the 250
volts rated capacitors in the application illustrated in
Fig. 4 with a line voltage of 750 volts and with five
series connected capacitors in each branch, each capacitor
had a normally applied voltage of about 150 volts or 60% of
its rated voltage. One reason for selecting the arrange-
ment shown in Fig. 4 was to increase the stress on each
capacitor. Another reason was since the line fil-ter capaci-
tor bank is provided to minimize the chopped motor current
from getting back into the power supply third rail line
because it can interfere with the vehicle track signal
currents in that line for wayside control and communication
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g 48,149
with the vehicles, by decreasing the number of capacitors
to five in each branch of the filter, as shown in Figure 4,
this decreases the high frequency signal impedance of each
branch by increasing the microfarads per branch; and this
is effective to filter more of the chopped motor current
away from -the third rail power supply line.
In Fig. 5 there is shown the line filter capaci-
tor arrangement 60 in accordance with the present invention
for the example of the third rail power supply being 600
lo volts direct current and providing three series 300 volts
rated capacitors in each parallel ~ranch of the line filter
capacitor to give a 900 volts rated capacitor branch. This
would provide a voltage stress level of 66% of rated volt-
age on each capacitor in every branch. The zener breakdown
conducting voltage characteristic of each capacitor is
about 495 volts to give a transient stored energy voltage
protection clamp per branch of just under 1500 volts. This
now assures a known maximum line transient energy caused
voltage that enables the selection of a single thyristor
device 62 in the chopper apparatus 64 operative with the
propulsion motors 66 of the vehicle 68, which thyristor
~ /t~-s~
device 62 can have a volt~-g~e rating above 1500 volts. The
supply line transient energy caused voltage build-up is
determined by the energy stored in the supply line induc-
tance when a fault condition occurs and depends upon the
fault current before it was interrupted. The arrangement
of Fig. 5 employs the zener breakdown conductivity charac-
teristic of each capacitor in a parallel branch to absorb
transient stored energy and convert that energy into heat
3o as a zener device. The 1500 transient energy caused volt-
9 48,149age clamp provided by the capacitor arrangement of Fig. 5
permits using a single thyristor 62 having a voltage rating
in the order of 1600 volts.
For a power supply line voltage of 750 volts and
with three series connected 300 volts rated capacitors in
each parallel branch of the line filter capacitor bank 60
this would provide a voltage stress per capacitor of 83% of
rated voltage. It is generally recommended by the manufac-
turers of electrolytic capacitor cans of the type employed
0 for filtering a transient propulsion motor chopper appara-
tus that a voltage stress between 60% and 80% of rated
voltage is desirable to maintain and prevent deforming of
the oxide layer. If it is desired to get this voltage
stress to Delow 80% of the capacitor voltage rating, then
three series connected 320 volts rated capacitors in each
parallel branch would provide a voltage stress per capaci-
tor of 78% of rated voltage. As compared to the 300 volts
rated capacitors, the zener breakdown voltage of the latter
320 volts rated capacitors would increase some also; but
the series branch would still be below the 1600 volts
rating of the thyristor device to be protected. If desired,
a single thyristor device can be presently obtained in the
open market at a voltage rating up to about 2000 volts, so
a thyristor device having an adequate voltage rating can be
utilized with the three series connected 320 volts rated
capacitors in each parallel branch of the line filter
capacitor bank, such as shown in Fig. 5.
~ he present invention utilizes the electrolytic
capacitors used in the line filter capacitor bank to pro-
vide transient stored energy caused voltage protection for
48,149the thyristor switch device or devices in the propulsion
motor current chopper apparatus. Each capacitor, first as
a capacitor and then as a zener device, can absorb a known
wattseconds or joules of energy. If all the capacitors are
arranged in a single series branch across the power supply
line, then the same amount of wattseconds energy can be
absorbed at a very high voltage. If all the capacitors are
arranged in respective parallel branches, the same amount
of wattseconds energy can be absorbed at a much lower
0 voltage. By a suitable selectlon of capacitor voltage
rating and a suitable circuit arrangement, for example as
shown in Fig. 5, the required absorption of wattseconds
energy is provided and a thyristor switch device selection
is permitted having a lower voltage rating and having an
improved transient energy caused voltage protection. In
the present market, a lower voltage rating thyristor is
less costly and the illustrated capacitor arrangement
provides a known protection of the single thyristor device
that provides greater reliability of operation, which is
important to customers of this apparatus. The number of
series connected capacitor cans in each parallel branch
- depends upon the known inductive stored energy characteris-
tics of the third rail power line, which inductive stored
energy does tend to decrease as the line voltage is reduced
The number of parallel branches depends upon the determined
ripple current provided by the chopper when regulating the
propulsion motor current during operation of the transient
vehicle.
An addi-tional consideration in relation to the
selection of thyristor switches at the present time is that
-
G'~
. 48, 149
V~: I t~, J ,:
the 1000 volts family product in general has a ~t~gr-e
rating between 700 volts and 1400 volts and the top of -the
family at 1400 volts would provide a thyristor having fast
turn-off, low forward drop and excellent characteristics
for the application here intended. At the present time,
the 2000 volts family product would be made by the high
voltage process and the latter product has a slower turn-
off and higher forward drop. The present invention enables
using a single thyristor from the 1000 volts family product
0 in the motor current regulating chopper for a typical
transit vehicle propulsion motor chopper application, which
is less expensive than two such devices and includes one
less gating control and is better protected against stored
energy transient voltages. In the zener mode of opera-tion,
a capacitor can in general absorb up to about ten times as
much energy as it is able to absorb in the capacitor mode
of operation.
The electrolytic capacitor is more attractive as
a transient voltage suppression device for direct current
service as compared to the copper oxide rectifier, since
the capacitor can absorb and store energy as a capacitor
before the capacitor then absorbs energy in the zener mode
- of operation similar to -the copper oxide rectifier. Be-
cause the electrolytic capacitor has a large area of high
purity aluminum and has a consistent thickness of oxide,
the direct current blocking level of the device to the
clamping level is much lower than for the copper oxide
rectifier; and this blocking to discharge ratio is in the
order of 1.67:1 as compared with a ratio of 2.~:1 for the
copper oxide rectifier. The capacitor has a large mass and
12 48,149
large foil area to permit the capacitor to absorb very
large amounts of energy, which in the case of a large
electrolytic filter capacitor such as employed with transit
vehicle propulsion motor control apparatus, is typically
about 10,000 wattseconds for a single can as compared to
250 wattseconds for a large copper oxide rectifier, which
latter rectifier costs several times more than the capaci-
tor. Actual tests have been run to show that each capaci-
tor, similar to those typically used in the main propulsion
0 chopper filter for a transit car can absorb 600 amps per
capacitor path and if twenty paths were used per car, then
a single car could absorb a 12,000 ampere surge. If the
zener clamping level were chosen to be 1800 volts, the
energy capability would be 648,000 joules. This level of
suppression capability is sufficien-t to limit all known
: stored energy transient voltages with a 750 volt DC third
rail system to below 1800 volts.
One advantage of using the capacitor suppression
as herein described is the resulting cost reduction and
higher reliability that can be achieved in the chopper
propulsion system by using a single thyristor device, as
shown in Fig. 5 as compared with the practice of using two
devices in series, as shown in Figs. 3 and 4. The use of
the capacitor as a suppression device in the zener mode can
protect the thyristor semiconductors, which are used in
conjunction with the capacitor arrangement to have a higher
voltage rating than the capacitor zener conducting voltage
clamp; therefore, the capacitors can protect the thyristor
semiconductor devices using zener discharge of the excess
energy. It was the prior art practice for the capacitor
13 48,149
surge voltage rating to be above the upper voltage rating
of the thyristor semiconductor devices used; and the semi-
conductor devices would fail before a stored energy tran-
sient voltage was raised to the zener discharge level of
the capacitors such that the zener mode of operation of the
capacitors was not used.
In Fig. 6 there is shown a voltage breakdown
characteristic for a typical electrolytic capacitor wherein
at 500 volts the oxide layer breaks down and a zener conduc-
o tivity mode of operation takes place.
The curve of Figure 7 shows the capacitor mode ofoperation where the current is charging the capacitor, and
after that the zener mode of the capacitor device becomes
operative. The energy absorbed in each mode is the inte-
gral of the volts times the amperes for the respective mode
time periods.
Figure 8 shows an illustrative circuit apparatus
to functionally illustrate the operation of the present
invention. The line inductance is shown to have a represen-
tative inductance of two millihenries. When the line fuse100 of a first vehicle 102 is blown due to a ground fault
condition or the like, this can result with a power supply
voltage of 750 volts in a line current of about 5000 am-
peres or more being applied across the filter capacitor
bank 104 of a second vehicle 106. The capacitor bank 104
has to absorb the resulting transient stored energy before
the voltage across the capacitor 104 goes too high for the
motor control thyristor device 108. The thyristor device
108 is connected in parallel with the capacitor 104, such
3o as generally shown in Figure 2. It is important that the
- .
14 48,1~9
voltage across the capacitor 104, such as shown by curve 70
in Figure 7, not go too high for the thyristor device 108
used in conjunction with the capacitor 104. When the
capacitor 104 is charging in the capacitor mode the voltage
is rising across the capacitor 104 and across the thyristor
device 108. Then when the zener breakdown conductivity
voltage of the capacitor 104 is reached, this provides an
upper limit on the voltage across the capacitor 104 and the
thyristor device 108, such as shown by curve 70 of Figure
0 7. The current then will ramp down to zero in a reverse
relationship as shown by curve 72. The zener mode time
period can typically cover a time period of several times
the capacitor mode charging time period. Beyond the zener
mode time period, the self-discharge of the capacitor will
put current back into the line and the voltage across the
capacitor will reduce to in the order of the supply source
voltage, as shown in Figure 7. The filter capacitor bank
can be selected to provide all the stored transient energy
absorption required to protect the associated thyristor
.- 20 devices, without going above the critical upper voltage
~ rating of the thyristor devices.
