Abstract:
The invention relates to a method, a computer program and a device ( 20 ) for the plausibility checking of current transformers ( 7 ) in an electrical switchgear assembly ( 1 ) and to a switchgear assembly ( 1 ) having such a device ( 20 ). According to the invention, zones ( 1   a   , 1   b   , 1   c ) bounded by current transformers ( 7 ) and possibly by open switches ( 3-5 ) are detected for an instantaneous topology of the switchgear ( 1 ), in each zone ( 1   a   , 1   b   , 1   c ) the signed current measurement signals are added and in the case of significant deviations of the current sum from zero, all current transformers ( 7 ) of the associated zone ( 1   a   , 1   b   , 1   c ) are identified as being problematic. Exemplary embodiments relate to, among other things: a warning counter ( 2   e ) for problematic current transformers ( 7 ); in the case of a defective current transformer ( 7 ), an operation with calculated currents or an automatic combining of zones ( 1   a   , 1   b   , 1   c ); and coordinating the plausibility test with switching actions. Advantages are, among other things: the method is independent of the complexity and operating state of the switchgear ( 1 ); dynamic topology tracking; high information content of the plausibility test; and access to current measuring values already available in the substation control system ( 2 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of system control technology, in particular to the substation control technology for high, medium or low voltage switchgear assemblies. 
     BACKGROUND OF THE INVENTION 
     A power supply network comprises substations or electrical switchgears, in particular high or medium voltage switchgears which are controlled by a distributed substation control system. The substations comprise primary or field devices, e.g. switches, drives, generators, transformers, voltage and current transformers. The control system has, for example, field control devices and a control station which are connected to one another by means of various communication buses and bus couplers. The field control devices control, monitor and protect the field devices of the system. 
     Current transformers in electrical substations measure the system currents at predetermined measuring points of the switchgear with a certain measurement inaccuracy. The measuring points are typically located at all incoming and outgoing lines and possibly also within the system, e.g. for the busbar protection. The current measurement values are filtered, scaled to primary current values of the system, digitized, if necessary, and detected as current measurement signals by the control system. For communication purposes, additional rate-of-change filtering can be provided which e.g. comprises temporal averaging by integration or an algorithm for deciding about updating or retaining and transmitting or not transmitting the current measurement value. The current measurement signals are used for protective functions, for monitoring the substation, for calculating performance data for operating purposes or for consumption billing and for the representation on a display. Both the measurement values detected by the current transformer and the parameters of the filtering and scaling, i.e. the so-called parametrisation of the measurement values can have errors. Primary errors in the current transformer itself are produced e.g. by defective parts or material fatigue. Errors in the parametrisation or in the power calculation can be caused in the electronics by external influences, aging, drift, errors by the operating personnel or the like. Errors in the transmission of measurement values or measurement signals can occur in the device and function chain from the current transformer to the screen display or power system control center. 
     For protection purposes, it is known to monitor the current transformers by means of local plausibility tests at an isolated point of the facility. During the phase balance supervision test, all three phase currents and the neutral current are measured at one point in the line and deviations from an assumed maximum asymmetry between the phases are detected. During the comparison test of current and voltage values, coarse inconsistencies between voltage and current values can be discovered at the measuring point. These local plausibility tests related to a single measuring point are very coarse and only permit a yes/no decision, whether a current transformer is operating or not. Wrong scaling or a loss of accuracy, in contrast, cannot be detected, particularly since the tests in the protective device are performed, as a rule, with relative values to the nominal voltage. 
     In differential protection for transformers, the currents of the high-voltage end and the low-voltage end are measured, provided with a scaling factor given by the transformation ratio of the transformer and compared with one another. This makes it possible to detect and correct scaling errors for the differential protection. 
     Such tests are performed in protective devices for protective current transformers. In general, such algorithms cannot be used in control devices for measuring-type current transformers, because, as a rule, transducers are used for digitizing the current measurement values and these supply, instead of the instantaneous values needed, time-averaged RMS or effective values of the current, the voltage or the power, the frequency or the phase angle. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to specify a method, a computer program, a device and a switchgear with such a device for the improved and simplified monitoring of current transformers in electrical switchgears. 
     In a first aspect, an exemplary embodiment of invention includes a method for the plausibility checking of current transformers in an electrical switchgear, in particular a high-voltage or medium-voltage switchgear, the switchgear being controlled by a control system and measuring signals from current transformers arranged at at least two different measuring points being processed by the control system, the following steps being performed for the plausibility testing of the current transformers: (i) recording an instantaneous topology of the switchgear by the control system on the basis of the existing electrical connections of primary devices and of instantaneous positions of switches; (ii) based on the instantaneous topology, identifying at least one zone of the switchgear, which zone represents a conductively connected area which is bounded by at least one current transformer and possibly by open switches; (iii) detecting the measuring signals of the current transformers in the zone with a current-direction-dependent sign and adding the measurement signals to form a current sum of the zone; and (iv) testing the current sum for a value zero within a permissible current transformer measurement accuracy and marking the current transformers of the zone as problematic, if the current sum is not equal to zero. 
     Thus, each zone represents a current node, the current sum of which is ideally zero. Open switches can be assigned a current measuring value of zero since they correspond to current transformers with a nominal current of zero. The test of the current sum for deviations from the ideal value of zero represents a simple, efficient method for checking, by means of a relative comparison of the current transformers of a zone, their scaling within permissible measurement signal deviations. The method requires little computing effort and no additional measuring expenditure in the current transformers. In particular, it is not necessary to know or estimate a correct or absolute current value. The identification of current transformers as problematic represents a first warning or alarm stage and signals that further monitoring of the current transformers of this zone is required. The method is independent of the configuration or the network arrangement of the primary devices of the switchgear and, in particular, independent of the complexity of their networking and, therefore, can be implemented for any desired arrangements without significant adaptations. The method is also independent from the instantaneous operating state of the switchgear and, in particular, can also be performed with fluctuating current values. Tracking the topology is done dynamically, i.e. the topology or the division into zones is updated continuously or on demand. In particular, it can change due to switching actions. It is also advantageous that measuring signals of the current transformers already available in the control system can be accessed. The method is also compatible with conventional local plausibility tests for current transformers. Going beyond these, however, it allows slight deviations or errors in the scaling of current transformers to be detected. More severe faults or defects on the primary side of current transformers can also be detected. 
     A preferred exemplary embodiment comprises the following features: in each zone, the same number of tests for the presence of a current sum not equal to zero is performed; in each test for the presence of a current sum not equal to zero, a warning counter for the current transformers identified as being problematic in this zone is incremented; and, finally, current transformers having a higher warning count than other current transformers, particularly having a highest warning count, are identified as being defective. This is because, if a defective current transformer belongs to two or more zones, its warning counter is incremented with each formation of a current sum in its zones, whereas the other warning counters are only incremented by one unit. The higher or highest warning count is, therefore, a reliable measure for detecting the defective current transformer. 
     An exemplary embodiment wherein at least one zone includes a number of current transformers, and wherein the warning counters of the current transformers of this zone are reset to zero if a current sum equal to zero is measured in the zone and a measurement signal not equal to zero is measured on at least two current transformers of the zone, has the advantage that the warning signal of a current transformer which is arranged within the switch gear assembly and is not defective can be reset to zero immediately in dependence on the order of the formation of sums in the zones and the current transformer can be detected as intact. 
     An exemplary embodiment wherein, in a zone having one and only one current transformer, the current transformer is identified as being defective if its measurement signal is repeatedly not equal to zero, has the advantage that zones with one and only one current transformer can be monitored in a very simple and reliable manner. 
     An exemplary embodiment wherein each current transformer of the switch gear assembly is divided into at least one zone, and wherein in an overall test of the switch gear assembly one and only one test for the presence of a current sum not equal to zero is performed in each zone, has the advantage that the current transformers of the entire switch gear assembly can be checked by exemplary methods according to the invention. 
     An exemplary embodiment wherein the identification of the zones is based on a busbar of the switch gear assembly and all current transformers conductively connected thereto are searched out, and/or wherein two zones of the switch gear assembly are identified which are adjacent to one another and which adjoin one another via a common current transformer and if the common current transformer fails or if the common current transformer is detected as being defective, the two zones are automatically combined to form a single zone, has the advantage of particularly simple zone detection and fast combining of zones in the case of defective current transformers. 
     An exemplary embodiment wherein, when exactly one current transformer fails in one zone, a current measurement signal to be detected by this current transformer is calculated from the remaining current measurement signals of the zone, assuming that the current sum is equal to zero, has the advantage that it is possible to operate the switch gear assembly with calculated currents. 
     In other aspects, exemplary embodiments of the invention relate to a computer program for the plausibility checking of current transformers, the method steps being implemented by a program code, furthermore to a device for carrying out the method and to a switch gear assembly comprising the device. 
     Further statements, advantages and applications of the invention will be apparent from the description now following and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a single-pole diagram in a first topological state defined by switch positions, 
         FIG. 1   b  shows the single-pole diagram in a second topological state. 
     
    
    
     In the figures, identical parts are provided with the same reference symbols. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1   a  and  1   b  show a single-pole scheme of an electrical high-voltage switchgear assembly  1 . A control system  2  of the switchgear  1  is shown diagrammatically and without the connections to the primary devices  3 - 9  and without reference to its spatial arrangement.  1   a ,  1   b ,  1   c  designate topological sections or current areas or zones  1   a ,  1   b ,  1   c  which result from the instantaneously assumed position of circuit breakers  3 , isolators  4  and earthing isolators  5 . The busbars are designated by  6 , the current transformers by  7 , the voltage transformers by  8  and the outgoing lines or outgoers by  9 . 
     In  FIG. 1   a , a first instantaneous topology with a first, second and third zone  1   a ,  1   b  and  1   c  is shown. The first zone  1   a  comprises a first current transformer  7  at the left-hand outgoing line  9  and a second current transformer  7  in the coupling part between the busbars  6 . The adjoining zone  1   b  contains the second current transformer  7  and the third and fourth current transformer  7  at the right-hand outgoing line  9 . The other adjoining zone  1   c  contains the first current transformer  7 . All current transformers  7  are arranged at spatially separate positions  7   a . According to an exemplary embodiment of the invention, the measurement signals of the current transformer  7  are detected separately with a current-direction-dependent sign in each zone  1   a ,  1   b ,  1   c  to be monitored and are added to form a current sum of the respective zone  1   a, b ,  1   c  and, if a current sum which is not equal to zero within a permissible current transformer measurement accuracy is present, all current transformers  7  of the respective associated zone  1   a ,  1   b ,  1   c  are identified as being problematic. The current sum is checked for zero within a permissible measurement error or on the basis of measurement inaccuracies of the current transformer  7  which are allowed and normally known a priori. If the current sums deviate from zero by typically more than the measurement inaccuracy, at least one current transformer  7  is presumably defective. 
     The transition from  FIG. 1   a  to  FIG. 1   b  occurs in that the substation control system  2 , by tracing the topology, detects which positions of the switches  3 - 5  are instantaneously current.  FIG. 1   b  shows a second topological operating state of the switchgear  1  in which zone  1   a  contains the first and fourth current transformer  7 , zone  1   b  contains the second and third current transformer  7  and the new zone  1   c  contains the second current transformer  7 . Adjacent zones, i.e. zones with a common current transformer  7 , are zones  1   a ,  1   b  and  1   a ,  1   c  in  FIG. 1   a  but not  1   b ,  1   c  and zones  1   b ,  1   c  in  FIG. 1   b . In the coupling part, there is also a line area which is not divided into zones and which is exclusively bounded by open switches  3  and isolators  4 . 
     Thus, in the method, the current transformer measurement signals detected at various measuring points  7   a  within each zone  1   a ,  1   b ,  1   c  are compared with one another for consistency in that, when their current sum deviates from the ideal value of zero by more than a predetermined difference value, the current transformers  7  of the associated zone  1   a ,  1   b ,  1   c  are marked as being problematic. The difference value is typically greater than or equal to the measurement inaccuracy of the transformers  7 . 
     In an advantageous embodiment, in each test of a zone  1   a ,  1   b ,  1   c  for the presence of a current sum not equal to zero, a warning counter  2   e  is incremented for the current transformers  7  graded as being problematic. If the zones  1   a ,  1   b ,  1   c  to be monitored are tested with the same incidence, current transformers  7  having a higher or having the highest warning count can be marked as being defective. 
     A higher or highest warning count is a reliable measure of defective current transformers  7 : if in a zone  1   c  having exactly one current transformer  7 , this transformer is defective, its warning counter  2   e  is incremented and all other warning counters  2   e  remain at zero. If the defective current counter  7  is a member in two adjacent zones  1   a ,  1   b ;  1   a ,  1   c  ( FIG. 1   a ) or  1   b ,  1   c  ( FIG. 1   b ), its warning counter  2   e  is incremented to twice the value of the intact current transformers  7  of these zones. If the defective current counter  7  is a member in a disjoint zone  1   a  ( FIG. 1   b ) which does not overlap other zones  1   b ,  1   c  ( FIG. 1   b ) and thus does not have a common current transformer  7 , the warning counters  2   e  of all current transformers  7  of this zone  1   a  are uniformly incremented and the defective current transformer  7  can only be detected if another topology or zone division exists on the basis of switching actions. To provide identification, suitable switching actions can be carried out intentionally, if necessary, as long as these switching actions are compatible with the other requirements for the operation of the substations. In the text which follows, further exemplary embodiments are specified. 
     If there is at least one zone  1   a ,  1   b  having a plurality of current transformers  7 , the warning counters  2   e  of the current transformers  7  of this zone  1   a ,  1   b  are reset to zero, if in the zone  1   a ,  1   b  a current sum equal to zero and by at least two current transformers  7  of the zone  1   a ,  1   b  a measurement signal not equal to zero is measured. In a zone  1   a ,  1   b ,  1   c  having exactly one current transformer  7 , the current transformer  7  is identified as being defective, if its measuring signal is repeatedly not equal to zero. 
     Advantageously, each current transformer  7  of the switchgear  1  is divided into at least one zone  1   a ,  1   b ,  1   c  and in an overall test of the switchgear  1 , exactly one test for the presence of a current sum not equal to zero is performed in each zone  1   a ,  1   b ,  1   c . For simple identification of the zones one starts from a busbar  6  of the switchgear  1  and looks for all current transformers  7  conductively connected thereto. 
     If two zones  1   a ,  1   b ;  1   b ,  1   c  of the switchgear  1  are adjacent to one another and thus adjoin one another via a common current transformer  7 , the two zones  1   a ,  1   b ;  1   b ,  1   c  can be automatically combined to form a single zone in the case of a failure of the common current transformer  7  or when the common current transformer  7  is detected as being defective. 
     In the case of a failure of exactly one current transformer  7  in a zone  1   a ,  1   b ,  1   c , a current measuring signal to be detected from this current transformer  7  can be calculated from the remaining current measuring signals of the zone  1   a ,  1   b ,  1   c , assuming a current sum equal to zero. The calculated current value can be used like a measured current measuring signal for monitoring operation or for billing. 
     The plausibility test of the current transformer  7  can be performed separately for each phase or alternately for various phases, in particularly cyclically alternately, or for average values of at least two phases, particularly all phases. In the operating state of the switchgear  1 , tests for the presence of a current sum not equal to zero and, in particular, overall tests of the switchgear  1  can be repeated periodically and/or after switching actions, i.e. after each change in the instantaneous topology. The plausibility test of the current transformers  7  can be carried out regardless of any switching actions and can be repeated when inconsistencies of the measurement signals caused by switching actions occur. As an alternative, the plausibility test of the current transformers  7  is only carried out or evaluated when a previous checking for instantaneous switching actions has proven to be negative. The plausibility test can also be carried out when current transformers  7  are taken into operation, to provide early detection of inconsistencies in their measurement value parametrisation. 
     An asynchronism of the measurement signals of a zone  1   a ,  1   b ,  1   c  can be measured or estimated and the greater the asynchronism, the greater the permitted current transformer inaccuracy which can be selected. 
     The results of the plausibility test can be represented in the form of a list of the current transformers  7  with their zone allocation, as statistics of the frequency with which each current transformer  7  has been identified as being problematic and/or as graphical, particularly colored information in a single-pole diagram of the switchgear  1 . Preferably, the current transformers  7  are measuring-type current transformers  7 , the measuring signals are effective current measurement values of the current transformers  7 , in particular RMS values, and/or the current-direction-dependent sign is determined from a phase angle between voltage and current of a phase, between currents of different phases or in other ways. 
     In another aspect, an exemplary embodiment of the invention relates to a computer program product for the plausibility checking of voltage transformers  8  in an electrical switchgear  1 , comprising a computer-readable medium and computer-readable program code means which, when executed on a data processing means of a control system  2  of the electrical switchgear  1  cause the data processing means to execute the method described above. Furthermore, a computer program for the plausibility checking of current transformers  7  in an electrical switchgear  1 , which can be loaded and executed on a data processing unit of a control system  2  of the switchgear  1  is claimed, wherein the computer program, when executed, executes the steps of the method represented above. 
     In a further aspect, an exemplary embodiment of the invention relates to a device  20  for carrying out the method for the plausibility checking of current transformer  7 . The device comprises means  2   a  for recording the instantaneous switchgear topology, means  2   b  for detecting at least one switchgear zone  1   a ,  1   b ,  1   c , defined as conductive area  1   a ,  1   b ,  1   c  bounded by at least one current transformer  7  and possibly open switches  3 - 5 , means  2   c  for detecting the measuring signals of the current transformers  7  in the zone  1   a ,  1   b ,  1   c  with a current-direction-dependent sign and for adding the measuring signals to form a current sum of the zone  1   a ,  1   b ,  1   c , and means  2   d  for testing the current sum for a value of zero within a permitted current transformer measurement accuracy and for marking the current transformers  7  of the zone  1   a ,  1   b ,  1   c  as problematic if the current sum is not equal to zero. Preferably, the device  20  comprises a warning counter  2   e  for monitoring the frequency with which a current transformer  7  is identified as being problematic, and/or means  2   f  for carrying out the method represented above. 
     The device  20  can be a station monitoring device  20 , which can be connected to the substation control system  2 , or it can be integrated in an operating interface of the control system  2 . Furthermore, all device means  2   a - 2   f  mentioned can be implemented in hardware and/or software. 
     Exemplary embodiments of the invention also extend to an electrical switchgear  1  which comprises such a device  20 . 
     LIST OF REFERENCE SYMBOLS 
     
         
         
           
             Electrical switchgear 
               1   a ,  1   b ,  1   c  Sections of the topology, zones 
               2  control system 
               20  Device for the plausibility checking 
               2   a  Means for topology recording 
               2   b  Means for detecting a zone 
               2   c  Means for measurement signal detection 
               2   d  Means for current sum testing 
               2   e  Warning counter 
               2   f  Execution means 
               3  Circuit breaker 
               4  Isolator 
               5  Earthing isolator 
               6  Busbar 
               7  Current transformer 
               7   a  Measuring point of a current transformer 
               8  Voltage transformer 
               9  Outgoing lines.