Patent Publication Number: US-RE41343-E

Title: Operational monitoring for a converter

Description:
TECHNICAL FIELD 
     The invention relates to the field of protective technology, in particular for electrical switchgear assemblies. It relates to an apparatus and to a method for monitoring the operation of a converter in accordance with the preamble to patient claims  1  and  5 . 
     PRIOR ART 
     Electrical switchgear assemblies, for example medium, high and low voltage switchgear assemblies, have control and protective devices. Protective devices must reliably detect, identify and locate electrical faults, for example short circuits or line interruptions, and must initiate suitable countermeasures in order to prevent faults from being propagated and system parts from being destroyed. At the same time, however, a protective device should not cause any system parts to be turned off unnecessarily. A protective device must therefore itself also have a high level of reliability. To this end, individual units of the protective device, in particular converters, need to be monitored. Examples of such converters are test amplifiers, digital-to-analog or analog-to-digital converters. A defect in a converter generally results in corruption of measured data and in a faulty response from the protective device. 
     In a generally known manner, operational monitoring is performed using a redundant design for units in a device: such a unit is of duplicate or triplicate design, and a lack of any match between output signals from the units is used to infer that a unit is malfunctioning. Such redundancy results, in the costs of the unit being at least doubled or tripled, however. Another manner of monitoring the operation of an appliance is known from U.S. Pat. No. 4,215,412. In this case, control variables and output variables for an aircraft engine are measured while the engine is operating. A computer-based simulation model of the engine continuously simulates a response of the engine and of the output variables during operation using the measured control variables. To this end, a complex mathematical model with an appropriate level of computational complexity is used. If the continuously changing, calculated and measured output variables do not match, an indicator is activated for an operator. However, producing a simulation model, matching it to a given engine and performing continuous calculations during operation require a high level of complexity. 
     ILLUSTRATION OF THE INVENTION 
     It is therefore the object of the invention to provide an apparatus and a method for monitoring the operation of a converter of the type mentioned in the introduction which eliminates the aforementioned disadvantages. 
     This object is achieved by an apparatus and a method for monitoring the operation of a converter having the features of patent claims  1  and  5 . 
     The invention&#39;s method for monitoring the operation of a converter involves detecting whether a value of an input variable for the converter assumes a first prescribed input reference value, and checking whether an output variable from the converter likewise assumes a corresponding, second prescribed output reference value. 
     This means that the operation of the converter is tested only at occasional instants, specifically only using individual, prescribed values. The fact that only prescribed values are compared with instantaneous values of the input and output variables means that the apparatus according to the invention can be produced using very simple means. 
     In one preferred variant of the invention, the prescribed values correspond to a value zero for a current, or to a value zero for a voltage, that is to say to a ground potential, and/or to a supply voltage for an electrical system. 
     The method according to the invention is particularly suitable for monitoring the operation of a converter in a control or protective device for an electrical switchgear assembly. In this context, when a malfunction in the converter is detected, all protective functions which are dependent on this converter are preferably turned off. 
     Further preferred embodiments can be found in the dependent patent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below with the aid of preferred illustrative embodiments which are shown in the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a structure of a converter having a monitoring apparatus according to the invention; 
         FIG. 2  shows a waveform for signals in accordance with the invention; 
         FIGS. 3 and 4  show structures of converter units having monitoring in accordance with the invention; and 
         FIG. 5  is a schematic illustration of a structure of a converter having a plurality of converter stages and a monitoring apparatus in accordance with the invention. 
       The reference symbols used in the drawings are summarized in the list of reference symbols together with their meanings. In principle, identical parts are provided with identical reference symbols in the figures. 
     
    
    
     WAYS OF IMPLEMENTING THE INVENTION 
       FIG. 1  is a schematic illustration of a structure of a converter  1  having a monitoring apparatus  10  in accordance with the invention. An input variable  2  for the converter  1  and an input reference value  41  are passed to an input detector  4 . An output variable  3  from the converter  1  and an output reference value  61  are passed to an output detector  6 . A digital input flag  5  from the input detector  4  and a digital output flag  7  from the output detector  6  are passed to an evaluation unit  8  to produce a status signal  9 . Optionally, the input flag  5  is also passed to the output detector  6 . 
     The method according to the invention works in the following manner: the input detector  4  continuously monitors the input variable  2 . So long as a value of the input variable  2  lies outside a prescribed tolerance range about the input reference value  41 , the input detector  4  sets the input flag  5  to a prescribed value, for example to the value zero. If the value of the input variable  2  is approximately the same as the input reference value  41 , that is to say if it lies within the prescribed tolerance range about the input reference value  41 , the input detector  4  detects a match between the input variable  2  and the input reference value  41 . The input detector  4  thus responds and sets the input flag  5  to another value, for example to the value one. In the same way, the output detector  6  continuously compares a value of the output variable  3  with the output reference value  61 . The output detector  6  forms the output flag  7  to indicate a match between the output variable  3  and the output reference value  61 , that is to say to indicate whether the output variable  3  lies within a prescribed tolerance range about the output reference value  61 . In one variant of the method according to the invention, the output variable  3  is compared only if the optional connection between the input flag  5  and the output detector  6  indicates a response from the input detector  4 . The evaluation unit  8  receives the input flag  5  and the output flag  7  and uses these to form the status signal  9  in the following manner: if the input flag  5  and the output flag  7  both indicate a response from the appropriate detector  4  or  6  then the status signal  9  conveys correct operation of the converter  1 . If there is no match between the input flag  5  and the output flag  7 , then the status signal  9  conveys a malfunction in the converter  1 . The status signal  9  can also be formed either continuously or else only if the input flag  5  indicates a response from the input detector  4 . 
     In one variant of the invention, the comparison in the output detector  6  is carried out continuously, and the comparison in the input detector  4  and/or the formation of the status signal  9  is carried out only if there is a response from the output detector  6 . 
     In a further variant of the invention, the evaluation unit  8  incorporates a converter delay time and compares the input flag  5  with an output flag  7  value which arises only after this delay time. This means that determination of whether both the input variable  2  and the output variable  3  lie within the respective prescribed tolerance range takes place on the basis of values of the input variable  2  and values of the output variable  3  which have been determined after having been shifted relative to one another by a delay time for the converter  1 . 
       FIG. 2  shows, along a horizontal time axis t, a curve s 1  for a first variable, a first reference value r 1 , a first tolerance range d 1  and a curve f 1  for a first flag, a curve s 2  for a second variable, a second reference value r 2 , a second tolerance range d 2  and a second flag f 2 , a curve s 3  for a third variable, a third reference value r 3 , a third tolerance range d 3  and a third flag f 3 , and also a curve s 4  for a fourth variable, a fourth reference value r 4 , a fourth tolerance range d 4  and a fourth flag f 4 . The first and second flags f 1 , f 2  are set to a value one, in the manner described above, only if the first and second variables s 1 , s 2  are situated within the first and second tolerance ranges d 1 , d 2  about the first and second reference values r 1 , r 2 . The same applies in a similar manner to the third and fourth flags f 3 , f 4 . The first variable has a continuous curve s 1  for its values, and the second variable has discrete values s 2 . The first and second variables correspond, by way of example, to an input variable  2  and an output variable  3 , respectively, of a working analog/digital converter  1 . In this case, the output variable from the converter  1  is in digitally coded form. In another variant of the invention, the converter  1  is an analog amplifier and, when the converter  1  is working, the curves for the input variable  2  and the output variable  3  both correspond approximately to the curve for the first variable. In a further variant of the invention, the converter  1  is a digital/analog converter and, when the converter  1  is working, the curves for the input variable  2  and the output variable  3  both correspond approximately to the curve for the second variable. 
     If the converter  1  is not working by reason of it driving/overriding the output variable  3  continuously to a maximum or a minimum after an instant t 1 , then a curve for the output variable  3  is produced in accordance with the curve s 3 . The corresponding curve f 3  for an output flag  7  shows that there is never any response from the output detector  6  after the instant t 1 . This means that no match arises between input flags  5  and output flags  7  either. Consequently, a malfunction in the converter  1  is detected and conveyed by setting the status signal  9 . 
     If the converter  1  is not working by reason of the output variable  3  having too great an offset, then, when an input variable  2  has a curve in accordance with s 1  or s 2 , a curve for the output variable  3  is produced in accordance with the curve s 4 . The corresponding curve f 4  for an output flag  7  shows that the output detector  6  responds at different instants than an input detector  4  for a curve in accordance with s 1 . This means that no match arises between input flags  5  and output flags  7  in this case either. Consequently, a malfunction in the converter  1  is detected and conveyed by setting the status signal  9 . 
     The reference values  41 ,  61  are determined by tuning the converter  1 , for example when equipment is brought, into service. If the converter  1  is an analog/digital converter, the input variable  2  preferably has a value zero connected as prescribed value, preferably a ground potential or a current zero. The defined input reference value  41  for which the input detector  4  responds is likewise a value zero. An offset in the converter  1  produces a particular value of the output variable  3  which is generally different than zero. This value of the output variable  3  is selected as the output reference value  61  for which the output detector  6  responds. The output reference value  61  corresponds to the input reference value  41  zero. If the converter  1  is a digital/analog converter, the input variable  2  is varied until the output variable  3  has a value zero. The corresponding value of the input variable  2  is selected as input reference value  41  and is assigned to an output reference value  61  zero. In a similar manner, tuning which uses a supply voltage or a rated voltage of a system as reference value for an analog side of a converter  1 , instead of a ground potential, is carried out. 
     The invention&#39;s monitoring apparatus  10  for monitoring the operation of a converter  1 , the converter  1  being an analog converter or an existing converter  1  in a system, thus has
         an input detector  4  for producing the input flag  5  on the basis of detection of the input reference value  41  for the input variable  2  for the converter  1 ,   an output detector  6 ,  6 ′,  6 ″,  6 ′″ for producing an output flag  7 ,  7 ′,  7 ″,  7 ′″ on the basis of detection of an output reference value  61 ,  61 ′,  61 ″,  61 ′″ for an output variable  3 ,  3 ′,  31 ′,  3 ′″, assigned to the output detector  6 ,  6 ′,  6 ″,  6 ′″, from the converter  1 , and   an evaluation unit  8  for comparing the results from the detectors, that is to say the input and outputs flags  5 ,  7 ,  7 ′,  7 ″,  7 ′″, and for producing a status signal  9  for conveying a match between the flags  5 ,  7 ,  7 ′,  7 ″,  7 ′″.       

       FIG. 3  shows a preferred variant of the invention: a monitoring apparatus  10  has a converter  1 , an output detector  6  and a digital unit  12 . The converter  1  is a digital/analog converter which receives the input variable  2  from the digital unit  12  in digitally coded form. The digital unit  12  receives the input variable  2  transmitted via an input (not shown), or produces the input variable  2  itself. The output detector  6  is an analog module which transmits the output flag  7  or analog flag to the digital unit  12 . The analog flag indicates a match between the output variable  3  and the output reference value  61 . The digital unit  12  has a programmable arithmetic unit, such as a microprocessor or an ASIC (Application-Specific Integrated Circuit) or FPGA (Field Programmable Gate Array) and/or a specially designed digital circuit. The above-described functions of the input detector  4  and of the evaluation unit  8  are realized in the digital unit  12 . To this end, the latter preferably forms a digital flag which indicates a match between the input variable  2  and the input reference value  41 . The digital flag is preferably produced by an internal variable or an internal signal in the digital unit  12 . 
       FIG. 4  shows a further preferred variant of the invention. A monitoring apparatus  10  has a converter  1 , an input detector  4  and a digital unit  12 . The converter  1  is an analog/digital converter which transmits the output variable  3  in digitally coded form to a digital unit  12 . The digital unit  12  transmits the output variable  3  via an output (not shown) and/or processes it itself. The input detector  4  is an analog module which transmits the input flag  5  or analog flag to the digital unit  12 . The analog flag indicates a match between the input variable  2  and the input reference value  41 . The digital unit  12  has a programmable arithmetic unit, such as a microprocessor or an ASIC or FPGA and/or a specially designed digital circuit. The above-described functions of the output detector  6  and of the evaluation unit  8  are realized in the digital unit  12 . To this end, the latter forms a digital flag which indicates a match between the output variable  3  and the output reference value  61 . The digital flag is preferably produced by an internal variable or an internal signal in the digital unit  12 . 
       FIG. 5  shows a further preferred variant of the invention. In this figure, the converter  1  comprises a plurality of cascaded converter sections  1 ′,  1 ″,  1 ′″ whose output variables  3 ′,  3 ″,  3 ′″ are passed to output detectors  6 ′,  6 ″,  6 ′″ to form output flags  7 ′,  7 ″,  7 ′″. The input flag  5  and the output flags  7 ′,  7 ″,  7 ′″ are evaluated by the evaluation unit  8 , which, if there is no match between the input flag  5  and/or the output flags  7 ′,  7 ″,  7 ′″, infers that a converter section  1 ′,  1 ″,  1 ′″ is malfunctioning. 
     In one preferred variant of the invention, the prescribed reference values correspond to a value zero for a current, or to a value zero for a voltage, that is to say to a ground potential, and/or to a supply voltage for an electrical system. Such reference values have the advantage that they occur throughout a system and are defined. If a reference value for an analog variable corresponds to a ground potential or to a value zero, then the analog detector is simply an analog comparator whose changeover threshold is the same as an upper limit of the prescribed tolerance range about the reference value zero. If an absolute value of the analog variable falls below the upper limit of the tolerance range, the comparator sets its output signal or analog flag to the value one and thus indicates that the analog variable has the prescribed reference value zero. 
     The method according to the invention is particularly suitable for monitoring the operation of a converter in a control or protective device for an electrical switchgear assembly. By way of example, digital/analog converters  1  are used in order to convert signals  2  transmitted digitally via glass fibers from protective appliances to analog signals  3 . These analog signals  3  are passed to analog inputs of existing prior protective appliances. Preferably, a monitoring apparatus  10  according to the invention based on the structure in  FIG. 3  is used for such a digital/analog converter  1 . It is preferably used to monitor voltages using reference values zero which correspond to ground potential. Currents are also preferably monitored using reference values zero. To tune monitoring for a converter  1  processing a current measurement, an appropriate section of line is switched to zero current. 
     In the case of a converter  1  processing a current measurement, a malfunction in a converter  1  which causes the output variable  3  from the converter  1  to be overridden cannot be distinguished from a short circuit current in the system without delay, that is to say in a time during which a protective device needs to react. In this case, the invention enables the malfunction to be detected. 
     When monitoring the operation of a converter  1  in a protective device for an electrical switchgear assembly, detection of a malfunction in the converter  1  preferably involves the output variable  3  being marked as invalid, for example by transmitting an appropriate status signal. In this case, protective functions which are dependent on the converter  1  are preferably turned off. This is permissible because faults in switchgear, assemblies occur very rarely. In this context, the risk of an operating fault caused by a defective converter  1  is advantageously greater than the risk of a fault not detected in optimum fashion. 
     The invention has the advantage that monitoring can be realized using very simple means. By way of example, only one additional analog module is required for a digital/analog converter or an analog/digital converter, while the rest of the functions are realized in a digital unit  12 , for example a programmable digital unit, which is present anyway. Monitoring according to the invention can also be implemented subsequently in an existing converter. 
     
       
         
           
               
             
               
                   
               
               
                 List of reference symbols 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 1 
                 Converter 
               
               
                 1′, 1″, 1′″ 
                 Converter section 
               
               
                 2 
                 Input variable 
               
               
                 3, 3′, 3″, 3′″ 
                 Output variable 
               
               
                 4 
                 Input detector 
               
               
                 5 
                 Input flag 
               
               
                 6, 6′, 6″, 6′″ 
                 Output detector 
               
               
                 7, 7′, 7″, 7′″ 
                 Output flag 
               
               
                 8 
                 Evaluation unit 
               
               
                 9 
                 Status signal 
               
               
                 10  
                 Monitoring apparatus 
               
               
                 12  
                 Digital unit 
               
               
                 41  
                 Input reference value 
               
               
                 61, 61′, 61″, 61′″ 
                 Output reference value 
               
               
                 s1, s2, s3, s4 
                 First, second, third and fourth curve 
               
               
                 r1, r2, r3, r4 
                 First, second, third and fourth reference value 
               
               
                 d1, d2, d3, d4 
                 First, second, third and fourth tolerance range 
               
               
                 f1, f2, f3, f4 
                 First, second, third and fourth flag