Abstract:
A monitoring system for detecting a short circuit condition in a switching subsystem of a bypass input of a power supply. The system may comprise a voltage monitoring subsystem for monitoring a voltage across at least one component of the switching subsystem and generating a first output signal in accordance therewith. A processor, responsive to the first output signal from the voltage monitoring subsystem, may be used to compare the first output signal with a predetermined threshold signal and to generate a second output signal based on the comparison. The processor may also be used to consider a state of a circuit breaker in communication with the switching subsystem and to use the state of the circuit breaker and the second output signal to determine whether at least one of a short circuit condition and an open circuit condition exists with respect to the switching subsystem.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/089,280, filed on Feb. 20, 2009. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to voltage monitoring systems used with power supplies, and more particularly to a voltage monitoring system and method that may be used with an uninterruptible power supply for detecting both short circuit and open circuit conditions that may arise for one or more switching circuits used in a bypass input of the uninterruptible power supply. 
     BACKGROUND 
     Both Underwriters Laboratories (“UL”) and the International Electrotechnical Commission (“IEC”) have requirements that state that hazardous voltage shall not appear on the input terminals of an uninterruptible power supply (“UPS”) when the UPS is operating in a stored energy mode. The hazardous voltage is presently designated as being any voltage above 30V RMS. Additional requirements are that the hazardous voltage condition shall be cleared within fifteen seconds given a single component failure within the detection/prevention circuit being used (UL 1778 and IEC 62040-1-1). 
     Ideally, any monitoring/detecting system used to detect open and short circuit conditions in a bypass input of a UPS should be cost effective to implement and not require major modifications to the construction of the UPS itself. 
     SUMMARY 
     In one aspect the present disclosure relates to a monitoring system for detecting a short circuit condition in a switching subsystem of a bypass input of a power supply. The system may comprise a voltage monitoring subsystem for monitoring a voltage across at least one component of the switching subsystem and generating a first output signal in accordance therewith. A processor, responsive to the first output signal from the voltage monitoring subsystem, may be used to compare the first output signal with a predetermined threshold signal and to generate a second output signal based on the comparison. The processor may also be used to consider a state of a circuit breaker in communication with the switching subsystem and to use the state of the circuit breaker and the second output signal to determine whether at least one of a short circuit condition and an open circuit condition exists with respect to the switching subsystem. 
     In another aspect the present disclosure relates to a monitoring system for monitoring operation of a static switching circuit of a bypass input of an uninterruptible power supply (UPS). The system may comprise: a voltage monitoring subsystem for monitoring a voltage across at least one component of the static switching circuit and generating a first output signal. The circuit breaker may be coupled in series with the static switching circuit. A processor, responsive to the first output signal and to a condition of the circuit breaker, may be used to compare the first output signal with a threshold signal and to generate a second output signal based on the comparison. The processor may be adapted to use the second output signal and the condition of the circuit breaker to determine if at least one of a short circuit condition and an open circuit condition exists in the static switching circuit. 
     In still another aspect the present disclosure relates to a method for monitoring a switching subsystem of a bypass input of an uninterruptible power supply (UPS). The method may comprise monitoring a voltage across at least one component of the switching subsystem and generating a first output signal indicative of a voltage across the one component. A comparison may be made between the first output signal and a threshold signal, and a second output signal generated in response to the comparison. A state of a circuit breaker in communication with the switching subsystem may be detected, and the second output signal and the state of the circuit breaker may be used to determine if at least one of a short circuit condition and an open exists in the switching subsystem. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a block diagram of one embodiment of a monitoring system of the present disclosure, with the monitoring system being located within an uninterruptible power supply (UPS); and 
         FIG. 2  is a flowchart of major operations that are performed in monitoring the status of the static switching system for each of an open circuit condition or a short circuit condition. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     Referring to  FIG. 1 , there is shown a monitoring system  10  that is implemented within an uninterruptible power supply (“UPS”)  12 . The monitoring system  10  is able to monitor and detect when an open circuit condition or a short circuit condition arises between any phase of the bypass input power lines  14  and the same phase on the output of the UPS  12 . 
     The monitoring system  10  is in communication with a static switching system  16  of the UPS  12 . The static switching system  16  in this example includes three distinct static switching circuits  18 ,  20  and  22  that each include a pair of back to back silicon controlled rectifiers (SCRs) that each function as a static switch. Static switching circuit  18  includes a snubber circuit  18   a  comprised of a resistor  18   a   1  and a snubber capacitor  18   a   2 . Similarly, static switching circuit  20  has its own snubber circuit  20   a  having resistor  20   a   1  and snubber capacitor  20   a   2 . Likewise, static switching circuit  22  has its own snubber circuit  22   a  having resistor  22   a   1  and snubber capacitor  22   a   2 . A back feed breaker  24  is also typically placed in the power lines that run to the bypass input terminals  14 . However, circuit breakers may be disposed at various other locations within the bypass input power lines  14  running to the static switching circuits  18 ,  20  and  22 . 
     The monitoring system  10  may include a voltage monitoring subsystem  26  made up of three independent voltage monitoring circuits  26   a ,  26   b  and  26   c . Voltage monitoring circuit  26   a  is in communication with the snubber circuit  18   a  of static switching circuit  18 , while voltage monitoring circuit  26   b  is in communication with the snubber circuit  20   a  of static switching circuit  20 , and voltage monitoring circuit  26   c  is in communication with the snubber circuit  22   a  of static switching circuit  22 . This enables the differential voltage across the snubber capacitor  18   a   2 ,  20   a   2  and  22   a   2  of each snubber circuit  18   a ,  20   a  and  22   a , respectively, to be detected. As will be explained in the following paragraphs, this voltage detection enables both a short circuit condition and an open circuit condition for any of the static switching circuits  18 ,  20  and  22  to be detected by a computer programmable logic device (CPLD)  28 . The CPLD receives voltage sense signals from each of the voltage monitoring circuits  26   a ,  26   b  and  26   c . It will also be appreciated, however, that the precise sense point where the voltage monitoring circuits  26   a ,  26   b  and  26   c  are connected, relative to the switching circuits  18 ,  20  and  22 , will depend on the precise location of the circuit breakers that are used in connection with the bypass input lines. Thus, it will be understood that the precise sensing points shown in  FIG. 1  within each static switching circuit  18 ,  20  and  22  are purely exemplary and may be varied to accommodate the placement of various circuit breakers. 
     With further reference to  FIG. 1 , each of the voltage monitoring circuits  26   a ,  26   b  and  26   c  may include conventional rectification circuits to provide a direct current voltage (vdc) signal, which may be termed a “first output signal”, that can be analyzed by the CPLD  28  against a predetermined voltage reference threshold. In one example this voltage reference threshold may be 2.5 vdc, although it will be appreciated that this value may vary significantly to meet the needs of a specific application. The CPLD  28  makes a comparison of the sensed voltage across the snubber capacitor  18   a   2 , 20   a   2 , 22   a   2  of each snubber circuit  18   a ,  20   a  and  22   a  and generates what may be termed a “second output signal” in response to each comparison. The second output signals may be used to assist in determining if an open circuit or a short circuit condition is present in each of the static switching circuits  18 ,  20  and  22  while the static switching circuits are inactive. The CPLD  28  may also analyze information concerning the location and state of each breaker that is used with the system  10  in making the short circuit and open circuit determinations. In this example only the back feed breaker  24  is being used, and such information, together with the state of the back feed breaker  24 , is provided to the CPLD  28 . The CPLD  28  uses the second output signals and the state of the back feed breaker  24  to determine whether an open circuit condition or a short circuit condition exists with any of the static switching circuits  18 ,  20  and  22 . If a short circuit condition is detected by the CPLD  28 , the CPLD may send a signal on line  28   b  to place the back feed breaker  24  in an open condition. If an open circuit condition is detected in any of the static switching circuits  18 ,  20  or  22 , the CPLD  28  may send a signal on line  28   a  to an external controller to notify the external controller of the open circuit condition. 
     The operation of the monitoring system  10  will now be explained with reference to the flowchart  100  of  FIG. 2 . Initially at operation  102 , the voltage across the snubber capacitor  18   a   2 , 20   a   2 , 22   a   2  of each snubber circuit  18   a ,  20   a  and  22   a  is sensed by each of the voltage monitoring circuits  26   a ,  26   b  and  26   c . The sensed voltages form the first output signals that are all provided to the CPLD  28 . At operation  104  the position of the back feed breaker  24  is sensed by the CPLD  28 . At operation  106  the CPLD  28  compares the sensed voltage for each of the snubber circuits  18   a ,  20   a  and  22   a  with the predetermined threshold voltage, in this example +2.5 vdc, and generates the second output signal in response to each comparison performed. If any of the second output signals indicate that one or more of the sensed voltages across the snubber capacitors  18   a   2 , 20   a   2 , 22   a   2  of snubber circuits  18   a ,  20   a  or  22   a  is greater than the 2.5 vdc threshold value, then a check is made by the CPLD  28  to sense if the back feed breaker  24  is closed, as indicated at operation  108 . If the back feed breaker  24  is detected as being closed, then the CPLD  28  determines that an open circuit condition exists, as indicated at operation  110 . At operation  112  the CPLD  28  may then notify an external (not shown) UPS controller of this condition. If the check at operation  108  indicates that the back feed breaker  24  is open, then monitoring by the voltage monitoring circuits  26   a ,  26   b  and,  26   c  continues at operation  102 . 
     If the check at operation  106  by the CPLD  28  produces a “No” answer, meaning that the sensed voltages from all of the voltage detection circuits  26   a ,  26   b  or  26   c  are each less than or equal to the 2.5 vdc threshold voltage, then the CPLD  28  generates the second output signals and makes a check at operation  114  to determine if the back feed breaker  24  is closed. If the check of the back feed breaker  24  at operation  114  reveals that it is closed, then the CPLD  28  concludes that a short circuit condition exists with one or more of the static switching circuits  18 ,  20  and  22 , as indicated at operation  116 . At operation  118  the CPLD  28  will then trip (i.e., open) the back feed breaker  24  via a signal on line  28   b  to interrupt power on the bypass input lines  14  to the static switching circuits  18 ,  20  and  22 . The monitoring of the voltages across each of the static switching circuits  18   a ,  20   a  and  22   a , as well as the evaluation performed by the CPLD  28  and the circuit breaker tripping signal generated by the CPLD  28 , all occur in real time. If the check at operation  114  shows that the back feed breaker is open, then monitoring by the voltage monitoring circuits  26   a ,  26   b , and  26   c  continues at operation  102 . 
     A particular advantage of the system  10  is that both short circuit and open circuit conditions can be detected by the CPLD  28  in real time. Thus, the analysis of the CPLD  28  takes into account the sensed voltages across all of the snubber capacitors  18   a   2 ,  20   a   2  and  22   a   2  as well as the location and state of the breaker(s) being used with the system  10 . The system  10  detects a short circuit condition whether it is one of the snubber capacitors  18   a   2 , 20   a   2 , 22   a   2  or one of the SCRs of each static switching circuit  18 ,  20  or  22  that becomes shorted. Thus, the system  10  can detect the presence of a hazardous voltage in real time on the bypass input power lines  14 . 
     The monitoring system  10  thus forms a cost effective and easily implemented system within a UPC. While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.