Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COMPUTERIZED METHOD AND SYSTEM
FOR DETERMINING DEGRADATION
OF DC LINK CAPACITORS
BACKGROUND OF THE INVENTION
The present invention relates generally to power conversion systems, such as
inverters used in locomotives, electric or hybrid buses, power generation
systems,
etc., having one or more capacitors in a DC link, and, more particularly, to
system and
method for determining degradation of the DC link capacitors.
The description below is given for purposes of illustration and not of
limitation in the context of a locomotive. As will be appreciated by those
skilled in
the art, a locomotive is a complex electromechanical system comprised of
several
complex subsystems. Each of these subsystems, such as the DC link capacitors,
is
built from components which over time fail. The ability to automatically
predict
failures before they occur in the locomotive subsystems is desirable for
several
reasons. For example, in the case of the DC link capacitors, that ability is
important
for reducing the occurrence of primary failures which result in stoppage of
cargo and
passenger transportation. These failures can be very expensive in terms of
lost
revenue due to delayed caxgo delivery, lost productivity of passengers, other
trains
delayed due to the failed one, and expensive on-site repair of the failed
locomotive.
Further, some of those primary failures could result in secondary failures
that in turn
damage other subsystems and/or components.
It will be further appreciated that the ability to predict failures before
they
occur in the DC link capacitors would allow for conducting condition-based
maintenance, that is, maintenance conveniently scheduled at the most
appropriate
time based on statistically and probabilistically meaningful information, as
opposed to
maintenance performed regardless of the actual condition of the subsystems,
such as
would be the case if the maintenance is routinely performed independently of
whether
the subsystem actually needs the maintenance or not. Needless to say, a
condition-
based maintenance is believed to result in a more economically efficient
operation and
maintenance of the locomotive due to substantially large savings in cost.
Further,
such type of proactive and high-quality maintenance will create an
immeasurable, but
very real, good will generated due to increased customer satisfaction. For
example,
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each customer is likely to experience improved transportation and maintenance
operations that are even more efficiently and reliably conducted while keeping
costs
affordable since a condition-based maintenance of the locomotive will
simultaneously
result in lowering maintenance cost and improving locomotive reliability.
As suggested above, it is desired to develop a predictive diagnostic strategy
that is suitable to predict incipient failures in the DC link capacitors.
Previous
attempts to overcome the above-mentioned issues have been generally limited to
diagnostics after a problem has occurred, as opposed to prognostics, that is,
predicting
a failure prior to its occurrence. For example, previous attempts to diagnose
problems
occurnng in a locomotive have been performed by experienced personnel who have
in-depth individual training and experience in working with locomotives.
Typically,
these experienced individuals use available information that has been recorded
in a
log. Looking through the log, the experienced individuals use their
accumulated
experience and training in mapping incidents occurring in locomotive
subsystems to
problems that may be causing the incidents. If the incident-problem scenario
is
simple, then this approach works fairly well for diagnosing problems. However,
if the
incident-problem scenario is complex, then it is very difficult to diagnose
and correct
any failures associated with the incident and much less to prognosticate the
problems
before they occur.
Presently, some computer-based systems are being used to automatically
diagnose problems in a locomotive in order to overcome some of the
disadvantages
associated with completely relying on experienced personnel. Once again, the
emphasis on such computer-based systems is to diagnose problems upon their
occurrence, as opposed to prognosticating the problems before they occur.
Typically,
such computer-based systems have utilized a mapping between the observed
symptoms of the failures and the equipment problems using techniques such as a
table
look up, a symptom-problem matrix, and production rules. These techniques may
work well for simplified systems having simple mappings between symptoms and
problems. However, complex equipment and process diagnostics seldom have
simple
correspondences between the symptoms and the problems. Unfortunately, as
suggested above, the usefulness of these techniques have been generally
limited to
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diagnostics and thus even such computer-based systems have not been able to
provide
any effective solution to being able to predict failures before they occur.
In view of the above-mentioned considerations, there is a general need to be
able to quickly and efficiently prognosticate any failures before such
failures occur in
the DC link capacitors of the locomotive, while minimizing the need for human
interaction and optimizing the repair and maintenance needs of the subsystem
so as to
able to take corrective action before any actual failure occurs.
SUMMARY OF THE INVENTION
Generally speaking, the foregoing needs are fulfilled by providing a
computerized method for determining degradation of one or more respective
capacitors in a DC link of a power converter system. The method allows for
monitoring a signal indicative of an estimated capacitor value based on a
first set of
operating and environmental conditions. The method further allows for
providing a
nominal capacitor value based on a second set of operating and environmental
conditions and for storing the adjusted capacitor value to generate historical
data of
the respective capacitor value. Analysis is executed on the historical data to
determine the presence of incipient faults in the respective DC link
capacitor.
The present invention generally fulfills the foregoing needs by providing in
another aspect thereof a system for predicting faults of one or more
respective
capacitors in a DC link of a power converter system. The system comprises a
signal
monitor configured to monitor a respective signals indicative of an estimated
capacitor value based on a first set of operating and environmental
conditions.
Memory is configured to store a nominal capacitor value based on a second set
of
operating and environmental conditions. A database is configured to store the
adjusted capacitor value to generate historical data of the respective
capacitor value.
A diagnostics module is configured to execute analysis on the historical data
to
determine the presence of incipient faults in the respective DC link
capacitor.
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DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an exemplary power conversion system
embodying one aspect of the present invention;
FIG. 2 is a block diagram of an exemplary processor including
respective modules that may be used for detecting incipient failures of DC
link
capacitors, either onboard the vehicle, at a remote diagnostic service center,
or both;
FIG. 3, made up of FIGS. 3A and 3B, is a flow charts illustrating
exemplary actions that may be executed by the processor of FIG. 2;
FIG. 4 is a plot illustrating an exemplary voltage discharge signal
across a respective DC link capacitor;
Fig. 5 is a plot illustrating an exemplary trend over time in the value of
a respective DC link capacitor, which trend may be indicative of an incipient
fault in
the capacitor; and
FIG. 6 is a plot illustrating an exemplary shift in the value of a
respective DC link capacitor, which shift may be indicative of a fault
condition in the
capacitor.
Before any embodiment of the invention is explained in detail, it is to
be understood that the invention is not limited in its application to the
details of
construction and the arrangements of components set forth in the following
description or illustrated in the drawings. The invention is capable of other
embodiments and of being practiced or being carried out in various ways. Also,
it is
to be understood that the phraseology and terminology used herein is for the
purpose
of description and should not be regarded as limiting.
DETAILED DESCRIPTION OF THE INVENTION
To generate an understanding of the present invention, reference is first made
to FIG. l, which shows an exemplary power conversion system 10 for conveying
power between a DC power source 12 and an electric load comprising first and
second motors 16 and 1 ~ electrically connected in parallel. By way of
example,
motors 16 and 1 ~ may be three-phase AC induction-type traction motors used
for
propelling a vehicle, such as a transit vehicle or locomotive (not shown), and
the DC
source 12, in the case of a transit vehicle, may comprise a wayside power
distribution
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system including either a third rail or an overhead catenary with which a
current
collector on the vehicle makes sliding or rolling contact. It will be
appreciated that
the power source in a locomotive could be the output of a rectifier device
that receives
power from a suitable alternator onboard the locomotive. In FIG. 1, the
relatively
positive line 17 represents such a current collector, and the negative line 19
represents
a conductor in contact with a grounded rail serving as the other terminal of
the DC
source.
The power conversion system 10 includes a controllable DC-to-AC converter,
such as an inverter 20 having a pair of DC terminals 22 and 24 on its source
side and
a set of three AC terminals 26, 28, and 30 on its motor side. The DC terminal
22 is
connected via a conductor 40 to the line 17 of the positive potential, and the
terminal
24 is connected via relatively negative conductors 41 and 42 to the other line
19 of the
DC power source 12. The conductors 40-42 thus serve as a DC link between the
source 12 and the inverter 20. The AC terminals 26, 28, and 30 are
respectively
connected to the three different phases of each of the AC motors 16 and 18.
During motoring, i.e., when electrical power is being conveyed from the
source to the motors, direct current is supplied to the inverter through its
DC terminals
22 and 24, and the inverter operates to convert this direct current into
alternating
current supplied through AC terminals 26, 28, and 30 to the motors 16 and 18.
The
inverter is controlled by suitable controls which may be internal or external
(such as
shown by a controller 70 in FIG. 1) for varying the amplitude and frequency of
the
alternating voltage at its AC terminals to provide the needed acceleration or
deceleration of the vehicle driven by the motors 16 and 18. As will be
appreciated by
those skilled in the art, any well known pulse width modulation (PWM) control
technique, such as sine-triangle comparison, space vector modulation or duty
cycle
area modulation, can be used for inverter operation. The controller may
comprise a
computer or a microprocessor, for example.
The power conversion system 10 has alternative motoring and electrical
braking modes of operation. During electrical braking, each of the motors 16
and 18
operates as an electrical generator driven by the inertia of the transit
vehicle, returning
power to the system 10. This return power flows through the inverter 20 in a
reverse
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direction from the direction of flow during motoring and appears as a
unipolarity
voltage and direct current at the DC terminals 22 and 24.
The conversion system 10 is designed to provide for both dynamic braking
and regenerative braking. Dynamic braking is effected by connecting across the
conductors 40 and 42 of the DC link a dynamic braking resistor 34 through
which at
least some of the braking current can be made to flow, thus dissipating
electric energy
in the form of heat. For controlling current in the resistor 34, a DC-to DC
converter,
such as a DC power chopper 36 is connected in series therewith. As is well
known to
persons skilled in the art, the chopper 36 is a solid-state switch that can be
repetitively
turned on and off by controller 70, for example, which, in one form, controls
the ratio
of the "on time" to the "off time" during successive intervals each of fixed
duration.
The average magnitude of current in the resistor varies directly with this
ratio.
For attenuating harmonics generated by operation of the power conversion
system 10 and for effectively isolating the system from any undesirable
electrical
transients in the DC power source 12, a single-stage electrical filter of the
L-C type is
included in the connections between the source 12 and the inverter 20. The
filter may
comprise a series line-filter inductor 62 connected in the path of current
between the
line 17 and the positive conductor 40 of the DC link, and shunt capacitors 54
and 56.
The first capacitor 54 (referred to as the DC link capacitor) spans the
conductors 40
and 41 and thus is directly connected between the two DC terminals 22 and 24
of the
inverter. The second capacitor 56 (referred to as the line capacitor) spans
the
conductors 40 and 42 and thus is interconnected in parallel with the capacitor
54. The
filter serves to attenuate harmonics generated by operation of the inverter 20
so that
such harmonics are isolated from the DC source 12 and will not interfere with
the
usual wayside signaling system. During motoring, the DC link capacitor 54
serves
mainly as the required "stiff' voltage source for the inverter 20. In the
electrical
braking mode of operation, the line capacitor 56 serves mainly as a filter for
the
chopper 36, providing a temporary path for braking current during the off
periods of
the chopper in the dynamic braking circuit (which comprises resistor 34 and
chopper
36) which, as can be seen in FIG. 1, is connected across this capacitor.
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For disconnecting the power conversion system 10, an electric circuit breaker
60, applied in a conventional manner, is provided between the system and the
DC
power source. This circuit breaker 60 is operated by the controller 70 in
response to
an operator's command or to fault conditions forcing the circuit breaker to an
open
condition. In the illustrative system of FIG. 1, a closed contactor 66 may
represent a
current collector in sliding contact with a wayside conductor. The contactor
66 may
be a pantograph for an overhead conductor or a spring biased shoe for
contacting a
third rail.
Current to the propulsion system is monitored by a current monitor 68 of a
type well known in the art. Monitor 68 generates a signal IL representative of
the
magnitude and frequency of current in the DC conductor 40. The voltage at DC
link
conductor 40 is indicated by signal VL obtained through buffer resistor 76
connected
to conductor 40. The filter capacitors 54 and 56 can be discharged through
discharge
resistor 78 via discharge contactor 80. As shown in FIG. 4, and further
described
below, a voltage signal across each respective DC link capacitor may be
monitored
during respective discharge conditions to record the discharge time i of the
DC link
capacitor. The recorded data may be accumulated and monitored over time,
locally or
remotely, to detect trends and/or shifts indicative of incipient faults in the
DC link
capacitor since the value of the time constant for discharging the capacitor
from a
known starting value to a predefined lesser value is directly proportional to
the actual
value of the capacitor.
In a typical transit vehicle, there may be a second voltage source inverter,
in
addition to the inverter described above, for supplying alternating current to
two more
traction motors for propelling the vehicle. FIG. 1 illustrates a power
conversion
system including such an additional inverter and with third and fourth AC
motors
being connected to the set of the AC terminals on its motor side. Components
common to those described above are designated by the same reference numerals
plus
100.
The positive DC terminal 122 on the source side of the second inverter 120 is
connected, via the conductor 40 of the DC link, to the line 17 of positive
potential,
and the relatively negative DC terminal 124 is connected, via a separate
conductor
141 and the common conductor 42, to the other line 19 of the DC power source
12.
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The AC terminals 126, 128, and 130 of the inverter 120 are respectively
connected to
three different phases of each of the AC motors 116 and 118. A second DC link
capacitor 154 individually associated with the inverter 120, is directly
connected
between the DC terminals 122 and 124, and a line capacitor 56 shared by both
of the
inverters 20 and 120 and both of the choppers 36 and 136 span the conductors
40 and
42 of the DC link capacitors 54 and 154 during the motoring mode of operation
of the
conversion system.
As is shown FIG. 1, a second dynamic braking circuit, comprising the series
combination of another dynamic braking resistor 134 and a second electric
power
chopper 136, is connected between the DC link conductors 40 and 42 and hence
across the line capacitor 56. In addition to sharing the common shunt line
capacitor
56, the two inverters 20 and 120 utilize the same series line-filter inductor
62 which is
connected on the DC power source side of the capacitor 56 between the DC link
conductor 40 and the line 17.
The two inverters 20 and 120 can be controlled by controller 70 which
responds to alternative command signals from interlocked throttle and brake
controllers 72 and 74, respectively. The controller 70 also receives feedback
signals
representative of sensed values of voltage, current, and other selected
variables in
each of the inverters 20 and 120. To operate in a dynamic braking mode, the
controller 70 derives a train of suitably timed periodic signals that
determine the
repetitive on and off intervals of the choppers 36 and 136, and it varies the
ratio of
these intervals as desired. As suggested above, it is desirable to detect
degradation
and incipient faults of the DC link capacitors.
A processor system 200 may be coupled to the respective circuit made up of
DC link capacitor 54 and discharge resistor 78 and to the circuit made up of
DC link
capacitor 154 and discharge resistor 178 to monitor and collect a voltage
signal that
would allow the processor to assess the condition of a respective DC link
capacitor.
The monitored signal is indicative of an estimated capacitor value based on a
first set
of operating and environmental conditions, such as ambient temperature,
locomotive-
to-locomotive variation, discharge resistor variation, etc. As suggested
above,
processor 200 may include a timer that allows for measuring, at times when the
DC
power supply, inverter and motors are inactive, the discharge time of the
capacitor, or
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more specifically allows for measuring the amount of time it takes the
monitored
voltage signal to reach a predefined level from a known starting voltage
level. The
predefined level may vary as a function of the starting voltage level. Using
basic RC
circuit principles, one can readily calculate the value of the DC link
capacitor and
populate a database 214 (FIG. 2) containing historical data values of each
respective
DC link capacitor. It will be appreciated that processor system 200 may be
installed
on-board, that is, local relative to system 10, or could be installed at a
remote
diagnostics service center that would allow a service provider to monitor a
fleet of
locomotives using suitable diagnostic tools 216 that when run on the database
of
historical data of respective data link capacitors would allow a trend
detection module
218 for detecting trends, such as shown in FIG. 5, or for detecting sudden
shifts in the
value of the respective DC link capacitor, such as shown in FIG. 6. A fault
prediction
module 220 would allow for declaring an incipient fault condition in the DC
link
capacitor, if, for example, the trend in the historical values of the
capacitors exceeds a
predefined rate of change, or, if, for example, the shift in the capacitor
value exceeds
a predefined level. The fault prediction module could be configured to provide
based
on the historical data, a probabilistic determination of when a DC link
capacitor
failure is likely to occur. By way of example, signal transmission from the
locomotive to the diagnostics site could be implemented using a suitable
transmitter
222 (FIG. 2) that may be part of a wireless data communication system (not
shown).
FIG. 2 shows further details regarding processor system 200 that includes a
signal monitor 202 that receives the voltage signal indicative of the
estimated
capacitor value based on the first set of operating and environmental
conditions, that
is, conditions affecting the capacitor being evaluated. As shown in FIG. 2, an
adjuster
module 204, drawn in dashed lines, may be optionally coupled to signal monitor
202
to adjust the monitored signal for deviations from a nominal capacitance value
that
may be based on a second set operating and environmental conditions to
generate an
adjusted capacitor value, that is, the second set of operating and
environmental
conditions may correspond to ideal operating and environmental conditions as
opposed to the actual conditions being experienced by a given capacitor. The
adjusted capacitor value may be derived through suitable adjusting factors
that may be
analytically and/or experimentally derived by collecting actual data and/or
simulation
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data that takes into account multiple scenarios of locomotive operation, and
should
preferably include a sufficiently large sample of locomotives and/or
propulsion
subsystems so as to statistically demonstrate the validity and accuracy of the
correcting factors. A submodule 206 in adjuster module 204 allows for
retrieving
and/or generating the respective adjusting factors.
A comparator module 208 may be coupled to signal monitor module 202 to
receive the estimated capacitor value. In the event adjuster module 204 is
used, then
comparator module 208 may be coupled to adjuster module 204 to receive the
adjusted capacitor value. In either case, comparator module 208 allows for
comparing
the value of the capacitor value against the nominal capacitor value to
determine the
condition of the respective DC link capacitor. As suggested above, the
estimated
value of the capacitor, as estimated by measuring its discharge time
characteristics,
may be adjusted for deviations due to the various external factors or rnay be
the
unadjusted estimated capacitor value. A memory 210 may be used for storing a
programmable look-up table (LUT) for storing a first range of capacitor values
so that
estimated capacitor values within that first range are indicative of
acceptable DC link
capacitor performance. Memory 210 may further be used for storing a second
range
of capacitor values so that estimated capacitor values within the second range
are
indicative of degraded DC link capacitor performance. Memory 210 may be
further
used to store the nominal capacitor value. It will be appreciated that the
techniques of
the present invention need not be limited to first and second ranges being
that
additional ranges may be employed if finer gradation is desired.
A status module 212 may be used for generating and issuing a signal
indicative of a degraded DC link capacitor performance when the capacitor
value is
beyond the first range of stored capacitor values and within the second range
of
capacitor values, that is, a cautionary signal that could be analogized to a
yellow light
in a traffic light. Similarly, module 212 may be used for generating and
issuing a
signal indicative of unacceptable DC link capacitor performance when the
capacitor
value is outside the second range of capacitor values, that is, a warning
signal that
could be analogized to a red light in a traffic light that requires immediate
action by
the operator, such as performance reduction, scheduling a repair, or
replacement of a
given DC link capacitor. In one exemplary implementation, capacitor values
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about 16% of the adjusted nominal value of the capacitor may be considered
normal
values. Capacitor values within about 33% to about 50% relative to the
adjusted
nominal value of the capacitor may result in degraded operation of the
locomotive
while capacitor values below 50% of the nominal value of the capacitor will
result in
locomotive shut down. It will be appreciated that the foregoing numerical
ranges are
merely illustrative of one exemplary embodiment and axe not meant to restrict
this
aspect of the present invention.
FIG. 4 is an exemplary flow chart of one aspect of the method of the
present invention for determining degradation in the condition of the DC link
capacitors of the locomotive. Upon start of operations in step 300, step 302
allows for
monitoring a signal indicative of an estimated capacitor value. Optional step
304,
represented in a block drawn with dashed lines, allows for adjusting the value
of the
monitored signal for deviations from a nominal capacitor value due to
predetermined
external variables or conditions to generate an adjusted capacitor value. Step
306
allows for comparing the value of the, adjusted or unadjusted, capacitor value
against
the nominal capacitor value to determine the condition of the respective DC
link
capacitor. Step 308 allows for determining whether the estimated capacitor
value is
within the first range of capacitor values stored in the LUT. If the answer is
yes, then
step 310 allows for declaring that the respective DC link capacitor has
acceptable
performance and the process is ready to start another iteration through
connecting
node B. If in step 308 the answer is no, then through connecting node A, step
312
allows for determining whether the estimated capacitor value is within a
second range
of capacitor values stored in the LUT. If the answer is yes, then step 314
allows for
declaring that the respective DC link capacitor has degraded. If the answer is
no, then
step 318, prior to return step 324, allows for determining whether the
capacitor value
is outside the second range of capacitor values stored in the LUT. If the
answer is yes,
then step 320 allows for declaring or indicating an unacceptable DC link
capacitor
performance. This indication will generally require suitable corrective action
by the
user. As discussed in the context of FIG. 3, step 316 allows for issuing the
yellow
cautionary signal, that is, a signal indicative of DC link capacitor
degradation.
Similarly, step 322 allows for issuing the high-level warning signal that
could be
analogized to a red light in a traffic light that requires immediate action by
the
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operator. It will be appreciated that the detection technique of the present
invention
can be conveniently "fme-tuned" or optimized by collecting actual locomotive
or
simulation data that allows for measuring the predicting accuracy of the
detection
algorithm by using well-understood statistical tools that enable to compute
the
probability that in fact an actual degradation condition will be detected.
It will be understood that the specific embodiment of the invention shown and
described herein is exemplary only. Numerous variations, changes,
substitutions and
equivalents will now occur to those skilled in the art without departing from
the spirit
and scope of the present invention. Accordingly, it is intended that all
subject matter
described herein and shown in the accompanying drawings be regarded as
illustrative
only and not in a limiting sense and that the scope of the invention be solely
determined by the appended claims.
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