Abstract:
A system and method are described for detecting failures of switches in a switching network including a plurality of switches. The sensing circuit includes a plurality of detecting networks, the plurality of detecting networks being fewer than the plurality of switches, each detecting network providing signals indicative of a failure of at least one of the switches.

Description:
BACKGROUND 
       [0001]    This application relates to monitoring switching networks as used, for example, in high power regulation devices. 
         [0002]    Static VAR correctors, also referred to as static VAR compensators (SVCs), are electrical devices that provide reactance compensation to power transmission networks. SVCs are commonly used in various applications, including, for example, regulating utility line voltage, improving network steady-state stability, and establishing near unity power factor on transmission lines. 
         [0003]    Typically, an SVC includes a bank of controllable capacitors and reactors that can be individually switched into and out of a utility power network (e.g., a transmission or a distribution line) by a set of semiconductor switches (e.g., thyristors). Each switch is driven by electrical gating signals generated based on line conditions, allowing the corresponding capacitors or inductors to discharge or conduct in a controlled manner. When using thyristors that are capable of responding to gating signals within a sub-cycle (e.g., on the order of several milliseconds), an SVC is able to provide near-instantaneous reactance flow to compensate voltage or current fluctuations on utility networks. After extended use, thyristors can fail, rendering the SVC inoperable and leading to power service interruptions and costly replacements. For this reason, thyristors are monitored to prevent failure of the SVC. 
       SUMMARY 
       [0004]    In a general aspect of the invention, a sensing circuit is configured for use with a switching network including a plurality of switches. The sensing circuit includes a plurality of detecting networks, the plurality of detecting networks being fewer in number than the plurality of switches, and each detecting network providing signals indicative of a failure of at least one of the switches. 
         [0005]    Implementations of the sensing circuit may include one or more of the following features. 
         [0006]    The detecting networks are configured to send a warning signal if the failed switches are greater in number than zero and fewer than or equal to a number of redundant switches in the switching network. 
         [0007]    The detecting networks are configured to send a trip signal to disable the switching network if the failed switches are greater in number than the number of redundant switches. 
         [0008]    In a general aspect of the invention, a sensing circuit is configured for use with a switching network that includes a plurality of switches and has a number of redundant switches. The sensing circuit includes a plurality of detecting networks configured to send a warning signal indicating that a number of failed switches is greater than zero and is also fewer than or equal to the number of redundant switches. At least one of the detecting networks in the sensing circuit is configured to disable the switching network if a number of failed switches is greater than the number of redundant switches. 
         [0009]    Implementations of the sensing circuit may include one or more of the following features. 
         [0010]    The plurality of detecting networks is fewer than the number of switches. 
         [0011]    In a general aspect of the invention, a sensing circuit is configured for use with a switching network that includes a plurality of switches and has a number of redundant switches. The sensing circuit includes a plurality of detecting networks that are fewer than the plurality of switches. The detecting networks are configured to send a warning signal indicative of a number of failed switches greater than zero and fewer than or equal to the number of redundant switches. At least one of the detecting networks disables the switching network if a number of failed switches is greater than the number of redundant switches. 
         [0012]    Implementations of the sensing circuit may include one or more of the following features. 
         [0013]    Each of the plurality of detecting networks monitors, at most, a number of switches equaling all the switches in the switching network divided by the number of detecting networks. 
         [0014]    A number of detecting networks equals at least two more than the number of redundant switches. 
         [0015]    The switches of the switching network are in series. 
         [0016]    The number of redundant switches is two or more. 
         [0017]    The switches include one or more high-power semiconductor switch-diode pairs or one or more high-power semiconductor switch-switch pairs. 
         [0018]    The detecting networks detect voltage. 
         [0019]    The detecting networks include dropping networks, which may also include one or more of a resistor divider, a transformer, a set of reactors, or a set of capacitors. 
         [0020]    The plurality of detecting networks includes a differential amplifier. 
         [0021]    The plurality of detecting networks includes a processor that compares voltages across one or more of the plurality of switches. 
         [0022]    In a general aspect of the invention, a method of monitoring a switching network containing a plurality of switches, includes: obtaining signals from each of a plurality of detecting networks, wherein at least one detecting network monitors two or more switches; determining a number of failed switches in the switching network based on the received signal; and performing one or more actions depending on the number of failed switches in the switching network. 
         [0023]    Implementations of the method may include one or more of the following features. 
         [0024]    Obtaining signals includes measuring voltages across one or more switches using a dropping network or a differential amplifier. 
         [0025]    Determining includes comparing the received signals to stored signals representative of a known number of failed switches. 
         [0026]    Performing one or more action depending on the number of failed switches in the switching network includes sending a warning signal indicative of the number of failed switches if the number of failed switches is greater than zero and fewer than or equal to a number of redundant switches in the switching network. Performing one or more action depending on the number of failed switches in the switching network also includes disabling the switching network if the number of failed switches is greater than the number of redundant switches in the switching network. 
         [0027]    The above-described systems and methods may include one or more of the following advantages. 
         [0028]    Switches in a switching network can be monitored efficiently and effectively. A switching network having a redundant number of switches can be monitored so that the network continues normal operation even after a number of switches up to and including the redundant number of switches have failed. In this scenario, a warning is sent to alert that switches have failed so that maintenance can be scheduled. 
         [0029]    In the event that more than the redundant number of switches fails, the monitoring system and methods disable the switching network, preventing damage to the network. 
         [0030]    The switch monitoring systems and methods are efficient and cost-effective because the status of each switch is inferred without having to employ a separate monitor for each switch. 
         [0031]    Other features and advantages of the invention are apparent from the following description, and from the claims. 
     
    
     
       DESCRIPTIONS OF DRAWINGS 
         [0032]      FIG. 1  shows a static volt-ampere reactive compensator system connected to a portion of a utility power system. 
           [0033]      FIG. 2  is an example switch controlling and monitoring system. 
           [0034]      FIG. 3  is a flowchart listing an example measurement process to obtain data used in monitoring switches. 
           [0035]      FIGS. 4A and 4B  are tables listing measurement data used in monitoring switches. 
           [0036]      FIG. 5  is a flowchart listing an example process to monitor and control a device containing a plurality of switches. 
           [0037]      FIG. 6  shows an alternative implementation of a switch controlling and monitoring system. 
       
    
    
     DETAILED DESCRIPTION 
     System Overview 
       [0038]    Referring to  FIG. 1 , a portion of a utility power system  100  includes a static volt-ampere reactive (VAR) compensator (SVC)  104 , which is stationed at various points along transmission or distribution lines  102  to regulate transmission or distribution line voltage, improve network stability, control reactive power flow, and reduce energy losses. For convenience, SVC  104  is shown as connected to only one phase of the transmission line  102 . SVCs include switches, here thyristors  122 , which are integral to the proper functioning of SVCs. Thus, SVCs are often provided with redundant numbers of thyristors to ensure continuous SVC operation. When redundant thyristors are installed, that is, the SVC contains more than the minimum number of thyristors required for normal SVC operation, the SVC can still function properly even after a number of the thyristors fail (as long as the number of failed thyristors is fewer than or equal to the number of redundant thyristors). 
         [0039]    Generally, each of the monitors  120  receives signals related to associated groups of thyristors  122  and report to a controller  108  how many of the thyristors have failed. When the number of failed thyristors is fewer than or equal to the number of redundant thyristors, the controller  108  sends a warning. For example, the warning can be received by an operator who then schedules a replacement of the failed thyristors. 
         [0040]    When the monitor  120  reports that the number of failed thyristors is greater than the number of redundant thyristors, the controller  108  disables the SVC  104 . As will be described in greater detail below, an arrangement of monitors  120  and a method of operation of the monitors permit an efficient, effective means for monitoring the thyristors  122  within an SVC  104 . 
         [0041]    The SVC  104  regulates voltage by controlling the amount of reactive power injected into or absorbed from the power network. For example, when the network voltage is low, as can happen when customer usage increases during summer months, the SVC generates capacitive reactive power. On the other hand, when the system voltage is high, the SVC absorbs inductive reactive power. A controller  108  measures a stepped-down voltage and includes or excludes multi-phase banks of capacitors  110  and banks of inductors  112  in the utility power system  100  as needed. Valves  114  include a series of thyristors and control the capacitor banks  110 , which are referred to as thyristor-switched capacitors (TSCs)  116 , and inductor banks, which are referred to as thyristor-switched reactors (TSRs)  118 . Alternatively or in addition, inductors can be controlled by different phases, in which case they are referred to as thyristor-controlled reactors (TCRs, which are not shown in  FIG. 1 ). A monitor  120  is associated with one or more TSC  116  or TSR  118  and measures parameters related to the functionality of multiple thyristors  122  within the valves  114 . 
         [0042]    Referring to  FIG. 2 , an example valve  114  includes or excludes the capacitor  110  as part of the circuit. The valve  114  includes a number of thyristor-diode pairs  200  that function as switches. The thyristor  122  is included in the thyristor-diode pair  200 . The valve  114  also includes supporting hardware, such as heat sinks, gates, cooling equipment, and gating circuits (none of which are shown in  FIG. 2 ). The number of thyristor-diode pairs  200  required for usage of the valve  114  depends on the voltage across the valve and the rating of the thyristor-diode pairs. For example, a point  202  on one side of the valve is at a line voltage of 23,000 volts and there is a 13,200 volt line to neutral. The capacitor  110  will charge to the peak voltage because of the diodes in the thyristor-diode pairs  200 . A common design practice is for the TSR voltage rating of the valve  114  to be two times the peak line to neutral, or about 39,000 volts to withstand the peak voltage, and, for a TSC  116 , to increase the rating by a factor of four, or about 78,000 volts, in order to withstand peak-to-peak voltage. If the thyristor in the TSC thyristor-diode pair  200  is rated for 6,500 volts, 12 thyristors would be the minimum number required and two additional thyristors could be included for redundancy. Valve  114  contains two redundant thyristor-diode pairs  200 , for a total of 14 thyristor-diode pairs. The level of redundancy can be higher or lower. At higher voltages or different thyristor ratings, the number of thyristor-diode pairs  200  is changed as needed. 
         [0043]    Thyristors within the valve  114  can fail, for example, because of over-voltage or over-current operating conditions, inadequate cooling, or mechanical damage. When a thyristor fails, it often shorts as its failure mode, causing the voltages to change across the thyristor-diode pair  200  as well as across the entire series of thyristor-diode pairs in the valve  114 . To monitor for failure of the valve  114  and the SVC  104 , the monitor  120  (shown within a dotted line) measures parameters (e.g., voltages) related to the functionality of the thyristors within the valves  114 . 
         [0044]    The monitor  120  is integrated between the valve  114  and a thyristor bank controller  204  either during initial construction or by retrofitting. The monitor  120  contains four detection groups (e.g., detection groups  206   a - d ) that each monitors a group (e.g., groups  208   a - d ) of three or four thyristor-diode pairs  200 . In the example shown in  FIG. 2 , detection group  206   a  monitors four thyristor-diode pairs  200  in group  208   a , detection group  206   b  monitors three thyristor-diode pairs  200  in group  208   b , detection group  206   c  monitors three thyristor-diode pairs  200  in group  208   c , and detection group  206   d  monitors four thyristor-diode pairs  200  in group  208   d . Each of the detection groups  206   a - d  is connected to two taps  210  and measures the voltage difference between the two taps, for example, in hardware, such as a dropping network (e.g., a resistor divider, a transformer, a set of reactors, or a set of capacitors), or in software, by passing the measured voltages to a processor for further analysis. As shown in the implementation of  FIG. 2 , the taps  210  include a resistor divider network. More generally, a minimum number of the detection groups  206   a - d  is needed to detect patterns of failure among the groups  208  of thyristor-diode pairs  200 . The minimum number of detection groups  206   a - d  depends on the redundancy of the system and is typically equal to two more than the redundancy of thyristor-diode pairs  200  in the valve  114 . In  FIG. 2 , the four detection groups  206   a - d  are sufficient to monitor the 14 thyristor-diode pairs  200 . 
       Method of Use 
       [0045]    Referring to  FIG. 3 , a flowchart  300  illustrates a process of using detection groups  206   a - d  for measuring changes in voltage across groups  208   a - d  of thyristors  122  after various numbers of thyristors have failed. A device having S number of thyristors and a redundancy equal to R number of thyristors is obtained ( 302 ). N number of thyristors is placed ( 304 ) into a number G of groups such that each group contains at least two but no more than N/G thyristors. Voltages are measured ( 306 ) across each group of thyristors when all thyristors are off. A failure of F number of thyristors is simulated ( 308 ), in which F is initialized to equal 1. The F failed thyristors are distributed ( 310 ) among the G groups. For each possible unique combination of F failed thyristors in G groups, voltages across each of the G groups of thyristors are recorded ( 312 ). While F, the number of simulated failed thyristors, is fewer than or equal to R, the number of redundant thyristors, F is incremented ( 314 ) by 1. The additional failed switch is distributed ( 310 ) among the G groups and, for each possible unique combination of F failed thyristors in G groups, the voltages across each of the G groups of thyristors is again recorded ( 312 ). After F has been incremented to be greater than R, the process ends ( 316 ) and the measured voltages can be used by the controller  108  to determine how many thyristors  122  have failed. In some examples, symmetry of the thyristors  122  and the resulting symmetry in the patterns of voltage changes after thyristor failures will reduce the number of voltages that are recorded ( 312 ) after a failure of F thyristors in G groups. 
         [0046]    Referring to  FIG. 4A , a table  400  lists data obtained by the measurement process described in the flowchart  300  for TSC groups  208   a - d  of thyristors in the valve  114 . Listed at the bottom of the table  400  are the voltages measured across the four groups  208   a - d  of thyristor-diode pairs  200  when all thyristors are functioning properly. Voltages are expressed as a percentage of the voltage drop between the point  202  (which is typically at a voltage of 25,000 volts) and the capacitor  110 . Detection group  206   a  measures a voltage between two taps  210  on either side of group  208   a  to be 21.4%, detection group  206   b  measures a voltage of 28.6% across group  208   b , detection group  206   c  measures a voltage of 28.6% across group  208   c , and detection group  206   d  measures a voltage of 21.4% across group  208   d . Symmetries exist between groups  208   a  and  208   d  and also between groups  208   b  and  208   c . These symmetries reduce the number of separate measurements required for recording voltages across each of group  208   a - d  when a number of failed thyristors are distributed among the groups. 
         [0047]    Referring to the other rows of table  400 , measured voltages across the TSC groups  208   a - d  are listed, in which one, two, or three shorted thyristors are distributed among the groups. A negative voltage value indicates that the voltage across a group has decreased and the group contains one or more shorted thyristors. For example, the top row of table  400  lists voltages measured when one failed thyristor is distributed among groups  208   a - d . A voltage of 7.7% is measured across each of groups  208   a ,  208   c , and  208   d , and a voltage of −19.2% is measured across group  208   b . As such, the failed switch is localized to group  208   b . Because of the noted symmetry in the groups  208   b  and  208   c , if the failed switch were instead in group  208   c , the voltage across each of groups  208   a ,  208   b , and  208   d  would be 7.7% and the voltage across group  208   c  would be −19.2%. 
         [0048]    The sixth row of table  400  lists voltages measured when one failed thyristor is located in group  208   a . In this scenario, a voltage of 7.7% is measured across each of groups  208   b ,  208   c , and  208   d , and a voltage of −28.2% is measured across group  208   a . Because of the noted symmetry in the groups  208   a  and  208   d , if the failed switch were instead in group  208   d , the voltage across each of groups  208   a ,  208   b , and  208   c  would be 7.7% and the voltage across group  208   d  would be −28.2%. The remaining rows of table  400  list the measured voltages across the groups  208   a - d , in which two or three shorted thyristors are distributed among the groups. 
         [0049]    Referring to  FIG. 5 , a flowchart  500  describes a process to monitor and control a device (e.g., the SVC  104 ) containing a plurality of thyristors (thyristor-diode pairs  200 ). A device is obtained ( 502 ) having a plurality of thyristors and a redundancy of thyristors equal to R number. N number of thyristors is placed ( 504 ) into G number of groups such that each group contains at least two but no more than N/G thyristors. A voltage across each group of thyristors is measured ( 506 ). The measured voltages are compared ( 508 ) to previously-recorded voltages for zero failed thyristors. A decision is made ( 510 ) whether or not the measured voltages across the groups follow a similar pattern as the previously-recorded voltages for zero failed thyristors. If the measured voltages are similar, then a voltage across each group of thyristors is measured ( 506 ) again. If the measured voltages are not similar, then a number F of failed thyristors is estimated ( 512 ), for example, by matching the measured voltages to a pattern of previously-measured voltages for one, two, or three failed thyristors. A decision is made ( 514 ) if the number of failed thyristors F is fewer than or equal to the number of redundant thyristors R. If F is fewer than or equal to R, a warning is sent ( 516 ) and a voltage across each group of thyristors is measured ( 506 ) again. If F is greater than R, the device is disabled ( 518 ). 
       Alternative Embodiments 
       [0050]    It is to be understood that the configurations of the monitor  120  shown in  FIG. 2  is one example implementation. An alternative design is shown in  FIG. 6  and includes the monitor  120  interfacing an example valve  114  that includes or excludes the inductor  112  as part of the circuit. The valve  114  includes a number of thyristor-thyristor pairs  602  that function as switches. Thyristors  122  are included in the thyristor-thyristor pair  602 . The number of thyristor-thyristor pairs  602  required depends on the voltage across the valve  114  and the rating of the thyristor-thyristor pairs. For example, a point  604  on one side of the valve is at a voltage of 23,000 volts. Because there are thyristors in both directions, the inductor  112  has no voltage across it when the valve  114  is off, and there is a 13,200 volt line to neutral. 
         [0051]    A standard design practice is for the voltage rating across the valve to be two times the peak voltage rating, or 13,200×2×sqrt(2)˜37,336 volts. Using thyristors that are each rated at 6,500 volts, six thyristor-thyristor pairs  602  are needed. If a redundancy of two pairs is desired, eight thyristor-thyristor pairs  602  are needed. The level of redundancy can be higher or lower. At higher voltages or different thyristor ratings, the number of thyristor-thyristor pairs  602  is changed as needed. 
         [0052]    As in the previous implementation, the monitor  120  is integrated between the valve  114  and a thyristor bank controller  204 . The integration can be performed during initial construction or by retrofitting. The monitor  120  contains four detection groups (e.g., detection groups  606   a - d ) that each monitors a group (e.g., group  608   a - d ) of two thyristor-thyristor pairs  602 . In the example shown in  FIG. 2 , detection group  606   a  monitors two thyristor-thyristor pairs  602  in group  608   a , detection group  606   b  monitors two thyristor-thyristor pairs  602  in group  608   b , detection group  606   c  monitors two thyristor-thyristor pairs  602  in group  608   c , and detection group  606   d  monitors two thyristor-thyristor pairs  602  in group  608   d . Each of the detection groups  606   a - d  is connected to two taps  610  and measures the voltage difference between the two taps, for example, in hardware, such as a dropping network (e.g., a resistor divider, a transformer, a set of reactors, or a set of capacitors), or in software, by passing the measured voltages to a processor for further analysis. As shown in the implementation of  FIG. 6 , the taps  610  include a resistor divider network. More generally, a minimum number of the detection groups  606   a - d  is needed to detect patterns of failure among the groups  608  of thyristor-thyristor pairs  602 . The minimum number of detection groups  606   a - d  depends on the redundancy of the system and is typically equal to two more than the redundancy of thyristor-thyristor pairs  602  in the valve  114 . In the monitor  120  of  FIG. 6 , four detection groups  606   a - d  are sufficient to monitor the eight thyristor-thyristor pairs  602 . 
         [0053]    Referring to  FIG. 4B , a table  450  lists data obtained by the measurement process described in the flowchart  300  for groups  608   a - d  of thyristors in the example valve  114  shown in  FIG. 6 . Listed at the bottom of table  450  are the voltages measured across the four groups  608   a - d  of thyristor-thyristor pairs  602  when all thyristors are functioning properly. Voltages are expressed as a percentage of the voltage drop between the point  604  (which is typically at a voltage of 25,000 volts) and the inductor  112 . Because of the symmetry in each of the four groups, each detection group  606   a - d  measures a voltage between two taps  610  on either side of a group of two thyristor-thyristor pairs  602  to be 25%. These symmetries reduce the number of separate measurements required for recording voltages across each of group  608   a - d  when a number of failed thyristors are distributed among the groups. 
         [0054]    Referring to the other rows of table  450 , measured voltages across the groups  608   a - d  are listed, in which one, two, or three shorted thyristors are distributed among the groups. A negative voltage value indicates that the voltage across a group has decreased and the group contains one or two shorted thyristors. For example, the top row of table  450  lists voltages measured when one failed thyristor is distributed among groups  208   a - d . A voltage of 14.3% is measured across each of groups  608   a ,  608   c , and  608   d , and a voltage of −42.9% is measured across group  608   b . As such, the failed switch is localized to group  608   b . Because of the noted symmetry in the groups, the voltage across any group that contains one failed switch would be −42.9%, and the voltage across the remaining groups that each has two properly-functioning thyristors would be 14.3%. This is confirmed in the fifth row of table  450 , in which group  608   a  contains the failed switch. 
         [0055]    The remaining rows of table  450  list the measured voltages across the groups  608   a - d , in which two or three shorted thyristors are distributed among the groups. 
         [0056]    While the above examples have described monitoring thyristors within SVCs, the methods and systems described can also be applied to monitor other switches or switching devices, including but not limited to silicon controlled switches, rectifiers, transistors, and bi-directional triode thyristors (also called “triacs”). 
         [0057]    The techniques described herein can be implemented in one or more of digital electronic circuitry, computer hardware, firmware, or software. The techniques can be implemented as logic gates or a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
         [0058]    Method steps of the techniques described herein can be performed by one or more programmable processors executing a computer program to perform functions of the invention by operating on input data and generating output. Method steps can also be performed by and apparatus of the invention can be implemented as special purpose logic circuitry, e.g., a field programmable gate array (FPGA) or an application-specific integrated circuit (ASIC). Modules can refer to portions of the computer program and/or the processor/special circuitry that implements that functionality. 
         [0059]    Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., random access memory (RAM), magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry. 
         [0060]    To provide for interaction with a user (e.g., a warning that alerts of failed thyristors), the techniques described herein can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer (e.g., interact with a user interface element, for example, by clicking a button on such a pointing device). Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. 
         [0061]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.