Note: Descriptions are shown in the official language in which they were submitted.
- 1 - Case 2790
MONITOR FOR SCR SYSTEM
This invention relates to a monitor for a
system using a plurality of semiconductor devices.
The invention is particularly suitable for
monitoring a plurality of SCRs (silicon controlled
rectifiers) and will be discussed hereinafter in
connection with SCRs. However it may be used to
monitor otber semiconductive devices such as diode
rectifiers or thyristors of other types.
There are systems which use equipment
having a number of SCRs, for example perhaps 18 -
1000 and more. A converter which is used in a high
voltage direct current system (i.e. HVDC system) to
convert AC to DC and DC to AC may use a considerable
number o SCRs and these will normally be operating
at high voltages. HVDC converters are often used in
the interties between two AC systems and a brief
explanation of the advantages can be found in the
Standard Handbook for Electrical Engineers, Donald ~.
Fink editor, eleventh edition, pages 15-58, section
133.
The SCRs are frequently connected in a
series-parallel arrangement and it is important to
know when there is a failure of one or more SCRs.
Several methods have been used to monitor the
condition of SCXs in converters or other equipment.
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One monitoring method uses individiaul neon
lamps, each lamp being connected in parallel with a
respective single SCR or in parallel with a group of
SCRs. The voltage developed across the SCR when it
is not conducting, that is when it is reverse-biased
or when it is blocking in the ~orward direction, is
used as a voltage source for the neon lamp. If an SCR
fails in the shorted condition, then the neon lamp
connected across that SCR, or across a group of
parallel SCRS including the failed SCR, will not light.
In practice, when this monitoring equipment
is operating normally, the voltage which causes the
neon lamp to light will be a ~luctuating voltage.
This fluctuating voltage fluctuates in synchronism
with the system AC voltage. When the SCX is turned on
(i~e. when it is conducting) by its control circuit
only a minimal voltage will be present across the SCR
and the neon lamp will not light. When the SCR blocks
(i.e. when it is not conducting) and the voltage
across it is at a level which exceeds the threshold
turn-on voltage for the neon lamp, then the neon lamp
will conduct and emit light. If the system frequency
is sufficiently high, as is usually the case, the neon
lamp will turn on and off and change brightness
rapidly enough so as to appear to provide uni~orm
illumination to the human eye.
secause the SCRs are at a high voltage, the
neon lamps (commonly light emitting diodes or LEDs~
are mounted near the SCRs and viewed remotely by an
operator in the monitoring method described. In
another alternative monitoring method the neon lamps
are connected by individual respective light guides to
equipment at ground potential which monitors the neon
lamps. A monitoring system using this method of
monitoring is descirbed, ~or example, in Canadian
Patent No. 868,007 - Cook et al, issued April 6, 1971
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This patent also describes a calibration circuit Eor
periodically transmitting both a zero level and a
positive or negative pulse of known amplitude over
the light guides to calibrate the detecting equipment
which receives the light.
The use of fibre optic light guides to
transmit information from the numerous light sources
at high potential to a monitoring system at or near
ground potential is not convenient. In equipment
employing in excess of one thousand SCRs the light
guides become cumbersome and inconvenient, and the
complexity of the associated equipment is not
attractive.
Advancing integrated circuitry and video
sensing technologv are now available which may be
used to provide for a video scanning and automatic
surveillance of the neon lamps. Large scale
integrated circuits which are sensitive to light can
be manufactured with a matrix of, for example, 60,000
elements. Each element is individually capable of
responding to light by storing electrical charge in
an amount which is a function of the time for whic~
light falls on the element and the intensity of the
light which falls on the element. The amount of
charge stored after a predetermined interval can be
recorded for each element in a separate memory
system. A description of such equipment may be found
in the following reference: TN2500 Solid State
Video-Digital Camera Operating Manual, General
30 Electric Company, 19~0, pages 14-21.
There are, however, some difficulties
involved in developing a monitoring system for
monitoring SCRs by using equipment of this type. One
of these difficulties or problems involves the
cyclical lamp intensity.
While the human eye may be unable to detect
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fluctuations in larnp intensity or to observe that the
lamp is not lit for a considerable portion of a
cycle, a solid state system is able to do so.
Consequently a solid state system could become
confused if the interpretation o~ the image is not
done correctly. In accordance with the invention the
difficulty may be overcome in two ways.
The first way is to ensure that each
individual light sensitive element is integrating
charge for at least one complete cycle of the system
frequency. For a system having a system frequency of
60 Hz the element should integrate charge for at
least 16.7 m sec. This will ensure that any change
in the level of the charge is due to a change in
light intensity and is not caused by a sampling
interval that is somewhat smaller and shifts. The
decision as to whether a neon lamp is operating at a
required level is based on a single sample each cycle.
The second way is to have a very much
faster sampling rate and where the rate is not a
multiple of system frequency. The samples are
gathered for a period at last as long as the period
associated with system frequency. In this way the
decision as to the operation of a neon lamp may be
made on an average of all samples over the sampling
period or on a sample by sample analysis.
The neon lamps which are connected across
single SCRs or groups of SCRs in parallel are placed
on one or more panels in an array and the lamp array
is focused, using a lens, onto a multi-element
sensor. Even though there are many thousands of
elements in the light sensor, a neon lamp image will
fall on only a few elements because the image is
relatively small. For example, suppose the neon
lamps to be monitored are distributed over a flat
panel that is 5 m x 5 m in size. Also suppose there
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are in the solid state sensor an array of light
sensitive elements 250 elements x 25U elements. Each
element will have focused upon it a portion of the
panel which is 20 mm x 20 mm. Since a typical neon
lamp has a diameter of about 10 mm, only a quarter of
an element would be illuminated by a lamp or a
smaller fraction of two or more elements.
It is desirable, for reasons of reliability
that several elements be illuminated by a given
lamp. ThiS can be done in several ways as follows:
(a) Several neon lamps can be clustered in
each location to increase image size. This is also
convenient for detecting lamp failure as will be
described subsequently.
(b) A single lamp or a multiple lamp set (a
cluster) can be incorporated into an assembly which
is covered by a lens or by a fresnel lens which will
increase the si2e of the image.
(c) The lens which focuses the neon lamp
or lamps onto the array of light sensitive elements
can be designed to be out of focus. This will cause
a blurring of the image which will tend to increase
the apparent size of the lamp.
(d) The effect obtained in (c) above can be
improved through the design of special lens systems
to multiply each lamp image or provide a much larger
blurred image.
Equipment for monitoring a plurality of
SCRs should be able to make a decision that there
has been a failure of one SCR or more than one SCR
and that the power handled by the system should be
reduced or the system shut down. since the costs
associated with reducing power or shutting down a
system may be quite large, it is desirable that the
monitoring system have a high degree of accuracy.
Thus there must be in the monitoring system some
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means to distinguish between failure of a neon lamp
and failure of an SCR~
Lamp failure can be determined provided
that each cell being monitored has more than one neon
lamp across it. If there is more than one neon lamp
per SC~ or SCR group, then lamp failure is determined
in one of the following ways.
If a lamp for a given SCR can be
individually detected, then obviously it would be
possible to tell how many of the plurality of lamps
associated with that SCR were lit and how many were
not lit. Lamp failure would be readily determined.
However, as was previously discussed, an individual
lamp may be too small to be clearly distinguised.
If a cluster of neon lamps is used (i.e.
two and preferably three or more lamps grouped
closely together) to represent a respective ~C~ or
single group of SCRs in parallel, then a number of
light sensitive elements will detect light from the
cluster. Some elements will detect more light than
others depending on their position relative to the
image of tbe cluster, but tbere will normally be
several elements involved per cluster. By storing
information on the amount of light detected by each
element that is receiving from a given cluster, the
failure of one lamp will be detected by a sudden
change in level from one set of fairly uniform data
to a new and different level also fairly uniform.
The procedure for detecting lamp failure
from the changing levels of light is accomplished by
changing the threshold sensitivity used in the
monitoring electronics on se~uential scans. That is,
the threshold is incremented, for example, from 0 to
255 representing a minimum and maximum tbreshold. It
will be apparent tbat the threshold level could be
decremented rather than incremented. Thus the
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threshold is changed Erom dark to fully lighted
values or vice versa. It is thus possible to monitor
and record the apparent brightness of each source of
light whether it is a single lamp or a cluster of
lamps, and to determine a step reduction due to
failure of a lamp. Lamp failure will result in the
reduction of light sensed by some elements, and
perhaps other elements may suddenly detect an absence
of light.
Over a long period of time the light source
will get dimmer due to lamp ageing. This means that
using a fixed threshold to indicate lamp failure
would not be desirable. An indication of overall
lamp ageing may be determined by monitoring the long
term gradual reduction in light, and by using a
changing threshold as described~ this is possible.
The monitoring system should be able to
distinguish the light from the neon lamps from
ambient light. The ability to sense the light from
the lamps depends on the brightness of the lamps
relative to the background light where the lamps are
located. There is a background panel or surface
behind the neon lamps and the panel surface will
reflect ambient light depending on the surface
characteristics. Polished surfaces, particularly if
they are convex, will tend to produce images of the
building lighting ~ixtures and consequently the panel
should be designed to avoid this. In addition,
building lighting should preferably be selected to
give off light which is predominantly in a spectral
range that can be filtered out in the optical system
of the light receiver. For example, if the neon
lamps or light emitting diodes (LEDs) are used as
indicator light sources, then the use of mercury
vapour lamps for illumination in the building where
the monitor is located would be suitable. Thus,
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optical Eilters could be used to be substantially
transparent to infra-red and red light, but
substantially opaque to higher frequencies of light
radiation. Mercury lamps produce little red and
infra-red radiation and would cause relatively little
interEerence. Also, the system which analyses the
light sensed by the light elements can store data
representing the background illumination with all the
neon lamps off and this value can be subtracted from
the values detected during monitoring.
It is therefore a feature of the present
invention to provide an improved system for
monitoring the operation of a plurality of thyristors
or SCRs.
It is another feature of the invention to
provide for warning signals and shutdown of e~uipment
in response to failure of one or more semiconductive
devices whose operation is represented by cluster of
neon lamps and where failure of a neon lamp can be
distinguished.
Accordingly there is provided a monitor
system for a plurality of semiconductive devices
arranged for operating purposes in groups of one or
more, comprising a plurality of indicating lamps for
each group connected to show operation of a
respective group, each said plurality of lamps being
mounted in a cluster, the clusters being spaced apart
by a distance greater than the spacing between
indicating devices in the clusters, a video camera
means directed at said clusters for providing
information on the location of each said cluster and
the intensity of light received from each cluster, a
memory for receiving said information from said video
camera means and for storing information on the
location and intensity of light of each said cluster,
and a decision processor connected to said memory to
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provide an output in response to one or more of said
clusters indicating a failure of a group.
Also there is providing a monitor system
for monitoring operational parameters of high voltage
thyristors connected in a series/parallel arrangement
across an AC source, said system monitoriny said
characteristics of said thyristors in groups having
at least one thyristor in a group, comprising a
plurality of indicating neon lamps for each group
connected in parallel across a respective group, each
said plurality of lamps being mounted on a panel and
arranged in a cluster, the clusters being spaced
apart by a distance greater than the spacing between
indicating lamps in a cluster, a video camera means
having light sensitive elements mounted to receive
light from the clusters on a panel, means to derive
information from said light sensitive elements
representing the location of each said cluster and
the light from each said cluster, memory means for
storing information derived from said light sensitive
elements, and a decision processor connected to said
memory means for processing the stored information
and providing an output signal in response to at
least one of said clusters providing no light output
for a predetermined length of time.
The invention will be described in more
detail with reference to the accompanying drawings~
in which
Figure 1 is a schematic front view
representation of a board for mounting panels which
carry indicator lamps~
Figure 2 is a schematic front view
representation of a panel which has clusters of
indicator lamps,
Figure 3 is a partial view of the panel of
Figure 3 showing an added lens,
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Figure 4 is a sectional view taken along
line 4-4 of Figure 3, and
Figure 5 is a block schematic diagram of
circuitry according to the invention.
Referring to Figure 1, there is shown a
schematic front view representation of a board lU for
mounting a plurality of panels 11. The surfaces of
both board 10 and panels 11 are preferably poor
reflecting surfaces. The board 10 is mounted in
proximity to a plurality of SCRs (or more generally~
thyristors) which are to have their operation
monitored and which are not shown. There may be
several panels required and this will depend on the
number of SCRs in the system which are to be
monitored.
Because the SCRs are operating at high
voltages and the board 10 is in proximity and the
indicator lamps as hereinafter referred to are
mounted on the panels on the board, it may be
desirable to including a corona ring around each
panel. One corona ring 13 is indicated in Figure 1.
If corona rings 13 are used, the~ would of course be
around each panel.
Referring now to Figure 2, panel 11 is
shown in more detail. Indicating lamps 12, which are
preferably neon lamps or light emitting diodes, are
formed in clusters 14 of four lamps. As shown, each
panel 11 has eight clusters 14 for monitoring
conduction of associated SCRs. Thus, each panel, as
sbown, monitors eight SCRs or eight groups of SCRs.
Eacb panel 11 also includes four clusters 15 for
monitoring the control signals applied to the same
SCRs or groups of SCRs as will be described
hereinafter.
Referring for the moment to Figure 3, there
is shown a partial front view of panel 11 showing lamp
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clusters 1~ covered by a lens 16 which defocuses or
blurs the image slightly so that the clusters 14
appear as single larger light sources rather than as
four distinct light sources per cluster. PreEerably
lens 16 is a fresnel lens. Figure 4 is a view taken
along lines 4-4 of Figure 3 representing a side view
of a light cluster 1~ covered by a lens 16.
Referring now to Figure 5, there is shown a
block schematic circuit diagram of a monitoring
system according to one form of the invention. One
or more boards 10 are mounted in proximity to the
SCRs (not shown) being monitored. Two boards
designated 10 and 10' are indicated, however the
number of boards is related to the number of SCI~s or
groups oE SCRs being monitored, to the number of
panels on each board and the number of clusters per
panel. It is convenient to select a camera and le~s
system such that the board image covers the screen of
light sensitive elements in a camera. Thus, there is
preferably a camera 11 for each board 10. The
cameras 11 are preferably of the type described in
the aforementioned reference "TN2500 Solid ~tate
Video-Digital Camera Operating Manual". A
description oE such video cameras and apparatus for
capturing and reading out digital signals may be
found in U.S. Patent No. 3,805,062 - Michon et al,
issued April 16, 1974, and in U.S. Patent No.
3,993,897 - Burket et al, issued November ~3, 1976.
Each camera 17 is connected to a camer~
interface 13 and each camera interface is connected
to multiplexer 20. It will be apparent that
multiplexer 20 is not required for an installation
having only a single camera 17 as it is used to
multiplex or combine signals from several cameras.
Multiplexer 20 is connected to an image processor ~1
which determines the boundaries and the location oE
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each light source ancl measures the intensity of the
light. The image processor 21 is a complex and a
known piece of equipment. A suitable image processor
is described, for example, in a publication entitled
"Optomation Instrument System - Electronic Vision
for Process Control", published in 1980 by the
General Electric Company and available under
publication designation EHM-12844.
The image processor 21 also receives over
conductor 22 a signal from a decision processor 23,
which also serves as the control for the system, a
signal indicating when the threshold is to change.
It will be recalled that the output from camera 17 is
compared to a threshold level to obtain an output
when the signal e~ceeds the threshold, and that the
threshold is stepped through a series of levels
between a level representing no light Erom a cluster
and maximum light from a cluster. The threshold
level is preferably stepped through 0-255 levels
although fewer levels may be used. The signal on
conductor 22 provides image processor 21 with the
timing for the threshold changes and the image
processor 21, in turn, provides a signal via
multiplexer 20 to camera interface 13 which changes
the threshold level.
The image processor 21 is connected to
memory 24 and the memory 24 stores data relating to
the location and intensity of the various clusters of
lamps. Memory 24 is connected to decision processor
23 and may be accessed by decision processor 23. The
data on location and intensity of the lights is thus
available to decision processor 23.
As shown, decision processor 23 has four
outputs 25-28. The output 25 is to connect to a
warning system, for example a light warning; the
output 26 is to connect to an alarm, for example an
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audible alarm; the output 27 is to connect to a
system for shutting down the apparatus in w~ich the
monitored SCRs are connected; and output 28 is to
indicate failure of the neon lamps. ~11 these
outputs may not be required in a particular
monitoring system. As an example, these outputs
25-28 are convenient in an HVDC system where a large
number of SCRs are connected in a series-parallel
arrangement. The system was designed so that in any
particular series chain of parallel connected SCRs,
the system would operate with one SCR failed. TwO
failed SCRs in a series chain were critical and three
failed SCRs in a series chain required immediate
shutdown. Consequently the failure of one SCR caused
an output at output 25 giving a warning; the failure
of two SCRs in a series chain caused an output at
output 26 giving an audible alarm; and the failure of
three SC~s in a series chain caused an output on
output 27 shutting down the system. The decision
processor can be programmed by inputs at control
status inputs 30 to provide various decisions which
give a desired output.
While the monitoring of the operation of
the SCRs is based on a loss of light from a cluster
over a time interval representing a cycle, the
monitoring of lamp failure depends on an abrupt minor
reduction of light from a cluster that is determined
from the sequencing through the threshold levels.
The failure of a lamp causes an output at output 2~
which may cause the illumination of a warning lightO
The position of the failed lamp is in memory and the
position can be provided at printer 31 or on the
screen display 32. Any relevant data on the
condition of SCRs or lamps can be requested via
keyboard 33 to be displayed on printer 31 and/or
screen display 32.
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It will be recalled that clusters 15
(Figure 2) were provided to indicate the presence of
control signals at the gates of the SCRs. These
would, of course, not be reauired if diodes were used
instead of SCRs. A control signal is normally
applied to a group of SCRs at the same time and
consequently fewer clusters 15 are shown in Figure 2
than clusters 14. The clusters 15, when lit,
indicate a control signal has been applied to a group
of SCRs. If there is a failure of the control signal
for some reason, then the failure of SCRs to conduct
does not represent failed SCRs and as the signals
representing clusters 14 and 15 (Figure 2) are
available to decision processor 23 (Figure 5),
allowance can be made for this. In addition, it is
desirable to be able to provide an input to decision
processor 23, via control status inputs 30 or at
another input, which will disable decision processor
23 for a short interval of time when, for example,
switching transients are expected which might affect
at least some SCRs briefly.
It is believed that the preceding
description will provide a clear understanding of the
invention in its various forms.
