CA1218706A - Monitor for scr system - Google Patents

Abstract

Case 2790 MONITOR FOR SCR SYSTEM

Abstract of the Disclosure A monitor for a system having a number of SCRs connected across an AC power source has several neon lamps connected across each SCR or each group of SCRs. The SCRs are at a high voltage and the neon lamp display is in proximity to the SCRs. The several neon lamps associated with one SCR or group of SCRs are formed in a cluster with the distance between clusters being greater than the distance between the lamps in a cluster. A video camera, spaced from the SCRs and substantially at ground potential provides an image of clusters of neon lamps. An image processor derives from the image, information on the location and light intensity of each cluster and this information is stored in a memory. In one form of the invention a lens over each cluster blurs the image of the cluster to make each cluster appears as a larger single light source rather than several individual light sources. If a cluster fails to light over a cycle of the AC power source, a decision processor provides an output indicating this. The decision processor can be programmed to shut down the system when a critical combination of SCRS fail. The condition of the lamps in each cluster is also monitored to determine lamp failure by comparing the light intensity from a cluster with a threshold that is stepped between a level representing no light and a level representing maximum light. A sudden decrease in the level of light from a cluster indicates lamp failure and a warning signal is provided.

Description

- 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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- 2 - Case 2790 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 37(1~

- 3 - Case 27gU

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 7(?~

- ~ ~ Case 2790 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 i2~ 7~!6 - 5 - Case ~790 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 8~ 6 - 6 - Case 279U

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 ~8~

- 7 ~ Case ~790 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, L8~

- 8 - Case ~790 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 37t~

- 9 - Case ~79~

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, 37(~6 - 10 - Case 2790 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 871~6 ~ 11 Case 2790 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

- 12 - Case ~7gO

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 7~6 - 13 - Case ~.790 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.

~87(~;
- 14 - Case 2790 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.

Claims (10)

- 15 - Case 2790 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 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 intensity of light received from each said 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 provide an output in response to one or more of said clusters indicating a failure of a group.

2. A monitor system as defined in claim 1 in which said semiconductive devices are silicon controlled rectifiers.

3. A monitor system as defined in claim 1 in which said indicating lamps are neon lamps.

4. A monitor system as defined in claim 1 in which each cluster comprises four neon lamps.

5. A monitor system as defined in claim 1 and further comprising a lens over each cluster to cause the image of the cluster to appear as a single light source.

6. A monitor system as defined in claim 1 and further comprising in the video camera means a - 16 - Case 2790 threshold changing means for changing a threshold in steps between a level representing no light from a cluster and a level representing maximum light from a cluster, and means comparing the information received from the video camera means representing light intensity with each threshold level in sequence to determine light intensity with respect to a particular threshold level and to provide a signal representing a cluster light level, said decision processor monitoring said signals representing cluster light level for each said cluster to provide a warning output signal indicating a lamp failure when a signal representing light level for a cluster decreases abruptly.

7. A monitor system for monitoring operational parameters of high voltage thyristors connected in a series/parallel arrangement across an AC source, said system monitoring 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 - 17 - Case 2790 providing an output signal in response to at least one of said clusters providing no light output for a predeter-mined length of time.

8. A monitor system as defined in claim 7 and further comprising a lens covering each cluster to blur the image of each cluster.

9. A monitor system as dunned in claim 8 in which the lens is a fresnel lens.

10. A monitor system as defined in claim 8 in which said output signal provided by said decision processor is a warning output signal in response to one cluster providing no light and representing the failure of a group of thyristors, and in which a further output signal is provided by said decision processor in response to two said clusters providing no light and representing failure of two groups of thyristors in a series connection, said further output signal representing a critical condition.