Title of Invention

METHOD FOR GENERATING A FAULT SIGNAL INDICATING A FAULT PRESENT IN A SECONDARY CURRENT TRANSFORMER CIRCUIT, AND DIFFERENTIAL PROTECTIVE DEVICE

Abstract

The invention relates to a method for producing a fault signal (F), which indicates a fault in the secondary circuit (for example 32a, 35a) of a current transformer (for example 31a, 34a), which interacts w with a local differential protective device (33), which monitors a section end (30) of an electrical energy supply system, wherein in the method, measured current values which are detected by the current transformer (for example 41a, 34a) and indicate a current flowing through the section end (30) are monitored by the local differential protective device (33), and a suspicion signal (V) is produced if the absolute values of successive measured current vaalues drop suddently, and the fault signal (F) is produced if the suspicion signal (V) is present. In order to be able to identify a fault in a secondary current transformer circuit even more reliably with such a method the invention proposes that a first rest signal (R1) is produced by the local differential protective device (33) if comparisson measured current values, which are detected at the time at which the suspicion signal (V) is produced in at least one remote differential pro otective device which monitors a further section end of the electrical energy supply system, likewise drop suddenly in terms of their absolute values, and the fault signal (F) is blocked if the first reset signal (R1) is present. The invention also relates to a corres spondly designed differential protective device.

Full Text

Method for generating a fault signal indicating a fault present in a secondary current transformer circuit, and differential protective device The invention relates to a method for generating a fault signal indicating a fault in the secondary circuit of a current transformer which interacts with a local differential protective device, which monitors a section end of an electrical power supply system, wherein in the method current measurement values which are detected by the current transformer and indicate a current flowing through the section end are monitored by the local differential' protective device and a suspicion signal is generated if the magnitudes of successive current measurement values fall abruptly; the fault signal is generated if the suspicion signal is present. Moreover, the invention relates to a differential protective device for monitoring a section end of an electrical power supply system, which interacts with at least one current transformer, by means of which current measurement values which characterize a current, flowing in the section end of the power supply system are detected by the differential protective device. The differential protective device has a computation device, which carries out the monitoring of the section end on the basis of the current measurement values and comparison current measurement values communicated to the local differential protective device from at least one remote differential protective device, wherein the computation device has a monitoring unit, which monitors a secondary circuit of the current transformer for faults. Electrical differential protective devices are used in electrical power supply systems for monitoring selected sections for faults, such as e.g. short circuits or ground faults. Typical sections of an electrical power supply system that are monitored by means of differential protective devices are for example electrical power supply lines or transformers. A number of differential protective devices corresponding to the number of ends of the respective section are required for monitoring the respective section. Thus, by way of example, in the case of a power supply line having three section ends - that is to say for example a main line with a branch line issuing therefrom - three differential protective devices are correspondingly required, wherein one of the differential protective devices is provided at each section end. In this case, the differential protective devices operate according to the following protection principle: each differential protective device detects current measurement values for each phase conductor of the section end monitored by said device, said current measurement values indicating a current respectively flowing through said phase conductor. The current measurement values detected at all ends of the monitored section are thereupon added in a manner observing their respective signs. In this case, the summation can be effected either in a selected differential protective device or else in all the differential protective devices. For this purpose, current measurement values detected at the same time respectively by the remote differential protective devices are communicated to the local differential protective device via data transmission lines running between the differential protective devices, such that said local differential protective device can form the sum of its own detected current measurement values and the communicated current measurement values of the remote differential protective devices. In the fault-free case, the calculated current sum should yield approximately the value zero, that is to say that the current that flowed into the section of the electrical power supply system also flows out of this section again. In the faulty case, by contrast, the result is a current sum that is significantly different than zero. In order to carry out the differential protection principle therefore, the calculated current sum has to be compared with a predetermined threshold value. In the case where the threshold value is exceeded, the respective local differential protective device generates a trigger signal that causes electrical circuit-breakers provided at the section ends of the faulty section to open their switching contacts, whereby the faulty section is disconnected from the rest of the electrical power supply system. Usually, the current measurement values required for carrying out the differential protection principle explained are firstly tapped off via current transformers - usually inductive current transformers - installed directly at each section end of the electrical power supply system and are communicated via electrical lines to the respective electrical differential protective device. Since the current intensity of the currents detected by these first current transformers is usually too high for internal processing in the differential protective device, the electrical differential protective device has device-internal current transformers again on the input side, which transform the communicated currents again to a lower current intensity level. Afterward, the currents thus detected are usually fed to an analog-to-digital converter, which assigns corresponding digital current measurement values to the analog currents. Said current measurement values are used in a computation device of the respective differential protective device for carrying out the differential protection principle. Since the current measurement values form the basis for carrying out the differential protection for the monitored section of the power supply system, their route from detection to processing in the differential protective device has to be monitored throughout. This is because there is the possibility of a current transformer that is provided for detecting the current measurement values of a phase being affected on its secondary side by a fault, for example an interruption in one of the coil windings or one of the lines (a so-called wire break), with the result that the electrical differential protective device measures apparent current measurement values having the value zero, even though currents indeed flow through the corresponding phase conductor of the section end. The current sum would be crucIaLly altered by such current measurement values detected in a faulty manner, such that in this case the differential protective devices would respond in an undesired manner and open their corresponding circuit-breakers. Since such an excess function, that is to say turning off an actually fault-free section of the power supply system, is associated with high costs for the system operator, it should be avoided if possible. For this purpose, methods are known which monitor the secondary circuits of current transformers for faults, such as wire breaks, for example, and generate a corresponding fault signal if they have ascertained a fault in the secondary circuit of a current transformer. Said fault signal is used to block the differential protection function for the corresponding phase of the section of the elegtrical power supply system, such that the section is not turned off in an undesired manner. By way of example, such a method is known from the device manual from the applicant "SIPROTEC, Leitungsdifferentialschutz mit Distanzschutz 7SD5, V4.3" ["SIPROTEC, line differential protection with distance protection 7SD5, V4.3"] (order no. C53000-G1100-C169-1) , which discloses in the section "monitoring functions" a method for monitoring wire breaks in which the current measurement values of each phase are monitored. If the profile of the current measurement values drops abruptly to zero, a fault in the secondary current transformer circuit is inferred. A fault signal indicating the fault in the secondary circuit of the current transformer is blocked if at the same time a jump also occurs in the profile of detected ground current measurement values, since such a jump in the ground current profile indicates an actual fault in the section of the electrical power supply system. It is an object of the invention to specify a method and a differential protective device of the type mentioned in the introduction which make it possible to achieve an even more reliable identification of faults in secondary current transformer circuits. In order to achieve this object, with regard to the method it is proposed according to the invention that a first reset signal is generated by the local differential protective device if comparison current measurement values detected at the time of the generation of the suspicion signal in at least one remote differential protective device, which monitors a further section end of the electrical power supply system, likewise fall abruptly with regard to their magnitudes, and the fault signal is blocked if the first reset signal is present. Owing to the use of the comparison current measurement values detected by the at least one remote differential protective device at a different section end, the decision as to whether a fault in the secondary circuit of a current transformer is involved can be made even more reliably since the probability of the simultaneous occurrence of faults in secondary current transformer circuits of differential protective devices that are remote from one another is very low. In accordance with one advantageous embodiment of the method according to the invention it is proposed that in the case of a polyphase power supply system, the current measurement values detected by current transformers of all the phases are respectively monitored for an abrupt fall with regard to their magnitudes, the suspicion signal is generated if an abrupt fall is identified for at least one phase in the case of the magnitudes of successive current measurement values of a current transformer of the local differential protective device, and the first reset signal is generated if an abrupt fall is likewise identified for respectively the same phase in the case of the magnitudes of successive comparison current measurement values of a current transformer of the at least one remote differential protective device. In this way, with the method according to the invention, a fault in a secondary current transformer circuit can also be identified reliably in the case of a polyphase power supply system. Moreover, faults in secondary current transformer circuits which occur simultaneously in a plurality of phases are also identified; even faults which occur simultaneously in the secondary current transformer circuits of all the phases are identified reliably. A further advantageous embodiment is seen in the fact that in the local differential protective device, the secondary circuits of the current transformers of all the phases are monitored for current flow, a second reset signal is generated if the magnitudes of successive current measurement values fall abruptly in the case of at least one current transformer where current flow is present, and the fault signal is also blocked if at least the second reset signal is present. In this way, an excess function of the differential protective device can be prevented even more dependably since the fault signal is also blocked when a current flow is present in the corresponding secondary current transformer circuit, which indicates an intact secondary current transformer circuit. In accordance with a further advantageous embodiment of the method according to the invention, it is proposed that in the local differential protective device, summation or ground current measurement values indicating a summation or ground current in the section end are detected, a third reset signal is generated if, at the time of the generation of the suspicion signal, the profile of successive summation or ground current measurement values has an abrupt change, and the fault signal is also blocked if at least the third reset signal is present. By means of the evaluation of the summation or ground current, an excess function of the differential protective device can be prevented even more reliably since a jump in the summation or ground current indicates an actual fault in the section of the electrical power supply system. In order additionally to prevent a blocking of the differential protection in the case of very high currents in the phases of the electrical power supply system, it is additionally proposed according to the invention that in the local differential protective device, the magnitudes of the current measurement values of all the phases are monitored for exceeding a predetermined threshold value, and a fourth reset signal is generated if the magnitude of the current measurement values of at least one phase exceeds the threshold value, and the fault signal is also blocked if at least the fourth reset signal is present. A further advantageous embodiment of the method according to the invention additionally provides that a circuit-breaker which can be driven by the local differential protective device is checked with regard to the position of its switching contacts, and a fifth reset signal is generated if the switching contacts of the circuit-breaker are open, and the fault signal is also blocked if at least the fifth reset signal is present. The blocking of the differential protection is thereby prevented for the case where the section of the electrical power supply system is in a turned-off state. In order to be able even more reliably to make a statement about whether a fault is present in a secondary current transformer circuit, it is further proposed according to the invention that voltage measurement values indicating voltages present at the section end are detected by the local differential protective device for all the phases, and a sixth reset signal is generated if, at the time of the generation of the suspicion signal, the profile of successive voltage measurement values of at least one phase has an abrupt change, and the fault signal is also blocked if at least the sixth reset signal is present. This is because a jump in the profile of the detected voltages likewise indicates a fault that has actually occurred on the section of the power supply system. In addition, in accordance with a further advantageous embodiment of the method according to the invention, it can be provided that with regard to a phase with respect to which the suspicion signal was generated, further current measurement values following the time of the generation of the suspicion signal are monitored, and a seventh reset signal is generated if the magnitudes of said further current measurement values are greater than the magnitude of that current measurement value which led to the generation of the suspicion signal, and the fault signal is also blocked if at least the seventh reset signal is present. The reliability of the identification of a fault in the secondary current transformer circuit can be increased further by this means, too. A further advantageous embodiment of the method according to the invention additionally provides that those current measurement values which are also transmitted to the local differential protective device from the at least one remote differential protective device for the purpose of carrying out the differential protection function are used as comparison current measurement values of the at least one remote differential protective device. This has the advantage that via the communication lines existing between the differential protective devices no additional data have to be transmitted in order to be able to make a reliable decision about a fault in the secondary current transformer circuit. Rather, the method according to the invention can be carried out with the data which are required for carrying out the differential protection and are therefore interchanged anyway between the differential protective devices, such as, for example, the current measurement values of the individual section ends, current intermediate sums or stabilization current values. In accordance with a further advantageous embodiment of the method according to the invention, it is additionally provided that when the fault signal is present, the differential protection functions of the local differential protective device and of the at least one remote differential protective device are blocked with regard to the phase affected by the fault in the secondary circuit of the corresponding current transformer. By this means an excess function of all the differential protective devices monitoring the corresponding section of the power supply system can be prevented simultaneously. In order that information about a fault that has possIbLy occurred in a secondary current transformer circuit is additionally conveyed to the operating personnel of the electrical power supply system, in accordance with a further advantageous embodiment of the method according to the invention it is additionally provided that the presence of the fault signal is indicated optically by the local differential protective device and/or the at least one remote differential protective device and/or a control center computer. With regard to the differential protective device, the object mentioned above is achieved by means of a differential protective device of the type specified in the introduction in which the monitoring unit is set up for carrying out a method as claimed in any of claims 1 to 11. The invention is explained in more detail below on the basis of exemplary embodiments. In this respect, figure 1 shows a schematic view of a differential protective system for the protection of a section of a power supply system that has two section ends, figure 2 shows schematic view of a differential protective system for the protection of a section of a power supply system that has three section ends, figure 3 shows a block diagram with a differential protective device arranged at one section end, figure 4 shows a logical sequentIaL flow diagram for elucidating a first exemplary embodiment of a method for generating a fault signal, figure 5 shows a further logical sequentIaL flow diagram for elucidating a second exemplary embodiment of a method for generating a fault signal, and figure 6 shows a further logical sequentIaL flow diagram for elucidating a third exemplary embodiment of a method for generating a fault signal. Figure 1 shows a section 10 of an electrical power supply system, the rest of which is not illustrated in further detail. The section 10 is illustrated as part of a power transmission line in figure 1. However, the section 10 of the power supply system can equally also be a transformer or some other component to be protected in an electrical power supply system. In order to monitor the section 10 for faults, such as short circuits or ground faults, for example, a differential protective device is provided at each end of the section 10. Thus, a first differential protective device 12a is arranged at a first section end 11a and a second differential protective device 12b is arranged at a second section end lib. The differential protective devices 12a and 12b pick up current measurement values by means of current transformers 13a, 13b arranged at the respective section ends 11a, 11b, said values indicating the current flowing through the respective section end 11a, 11b. If the electrical power supply system is a polyphase, for example three-phase, electrical power supply system, then corresponding current measurement values are picked up correspondingly for each phase of the section 10 at the section ends 11a, 11b and fed to the respective differential protective device 12a, 12b. The differential protective devices 12a, 12b calculate a current sum from their own current measurement values and simultaneously detected current measurement values of the respectively remote differential protective device, with the respective signs being taken into account. For this purpose, the current measurement values can be interchanged between the individual differential protective devices 12a, 12b via a communication line 14. In the fault-free case of the section 10, the calculated current sum should assume a value of approximately zero. However, if a fault, such as a ground fault, for example, is present on the section 10, then at a point in time at the section ends 11a, 11b current that flowed into a section 10 and current that emerged therefrom no longer correspond and the calculated current sum assumes a value not equal to zero. The calculation of the current sum can take place in both differential protective devices 12a, 12b or in just one of the two devices. If the calculated current sum therefore exceeds a preset threshold value, then a fault on the section 10 is inferred and the differential protective devices 12a, 12b generate a trigger signal A, which is fed to circuit-breakers 15a and 15b arranged at the respective section ends 11a, 11b, whereby said circuit-breakers are caused to open their switching contacts. In this way, the faulty section 10 is disconnected from the rest of the power supply system. Figure 2 shows a further differential protective system. The illustration in accordance with figure 2 substantIaLly corresponds to that in figure 1. It is just that with the differential protective system in accordance with figure 2 a section 20 of an electrical power supply system is monitored which now has three section ends 21a to 21c. The number of section ends is not restricted to three, but rather a section having any desired number of section ends can be monitored. The number of differential protective devices used for this purpose corresponds to the number of section ends. Correspondingly, in the case of the example in accordance with figure 2, an electrical differential protective device 22a to 22c is provided at each section end 21a to 21c and detects current measurement values by means of correspondingly connected current transformers 23a to 23c. Via communication lines 24 connecting all three differential protective devices 22a to 22c, the current measurement values are interchanged between the differential protective devices 22a to 22c and can thus be used for forming a current sum that takes account of all three section ends 21a to 21c. In this case, too, the calculation of the current sum can take place respectively in all the differential protective devices 22a to 22c or in a selected differential protective device. In a manner corresponding to the explanation concerning figure 1, the differential protective devices 22a to 22c generate a respective trigger signal A if the calculated current sum exceeds a preset threshold value. The trigger signals A in turn cause circuit-breakers 25a to 25c at the respective section ends 21a to 21c to open their switching contacts, whereby the faulty section 20 is disconnected from the rest of the power supply system. Since the current measurement values picked up at the individual section ends represent the basis for calculating the current sum and are therefore absolutely fundamental for the decision as to whether a fault is present on the section of the electrical power supply system, it is necessary to check the path of the current measurement values from their detection to use in the respective differential protective device for possIbLy occurring faults. This is explained below in greater detail by way of example with reference to figures 3 to 6. In this respect, figure 3 shows a section end 30 of a section of a three-phase power supply system, the rest of said section not being illustrated in further detail. Accordingly, the section end 30 also has three phase conductors LI, L2 and L3. First current transformers 31a, 31b, 31c, which can be conventional inductive current transformers, for example, are arranged at the individual phase conductors L1, L2 and L3. At their secondary side, the first current transformers 31a to 31c output currents of lower current intensity which are proportional to the currents flowing in the individual phase conductors LI, L2 and L3 and which are transmitted via measurement lines of a respective secondary current transformer circuit 32a, 32b, 32c to measurement inputs of a differential protective device 33. The differential protective device 33 has device-internal (second) current transformers 34a, 34b, 34c at its measurement inputs, which current transformers transform the currents transmitted via the secondary current transformer circuits of the first current transformers 31a, 31b, 31c once again to a lower level in order that said currents can be processed by the sensitive electronic circuits of the differential protective device 33. The device-internal current transformers 34a to 34c are likewise inductive current transformers, for example. On their secondary side, they in turn output currents proportional to the currents transmitted to the differential protective device 33 via the secondary current transformer circuits 32a to 32c of the first current transformers 31a to 31c. Consequently, the device-internal current transformers 34a to 34c also have secondary current transformer circuits 35a, 35b, 35c. The currents flowing in these secondary current transformer circuits 35a to 35c of the device-internal current transformers 34a to 34c are fed within the differential protective device 33 to analog/digital converters 36a, 36b, 36c, which convert the analog currents into digital current measurement values. The current measurement values respectively generated are fed to a computation device 37 of the differential protective device 33. As already explained with regard to figures 1 and 2, the computation device 37 of the differential protective device 33 carries out the formation of a current sum on the basis of firstly the device's own current measurement values detected with regard to the individual phase conductors LI, L2 and L3 and secondly comparison current measurement values detected by at least one remote differential protective device simultaneously for the individual phase conductors LI, L2, L3 at another section end. For this purpose, the computation device 37 has a communication unit COM, which is connected to a data transmission line 38. Via the data transmission line 38 and the communication unit COM, comparison current measurement values can be communicated from at least one other remote differential protective device to the local differential protective device 33. Correspondingly, the local differential protective device 33 can also transmit its own current measurement values to the at least one remote differential protective device via the communication unit COM and the data transmission line 38. In the case of a fault on the section of the electrical power supply system, the current sum will assume - as already explained - a value not equal to zero. In this respect, the differential protective device 33 checks, by means of its computation device 37, whether the calculated current sum exceeds a preset current threshold value, and generates a trigger signal A at a command output if a threshold value violation is present. The trigger signal A is used to cause an electrical circuit-breaker 39 to open its switching contacts. If the fault on the section of the electrical power supply system is a single-phase fault, for example a ground fault of the phase conductor LI, then it suffices if the circuit-breaker 39 opens only those switching contacts which are assigned to the phase conductor LI. In the case of a polyphase fault, the respective switching contacts of the affected phase conductors LI, L2, L3 of the circuit-breaker 39 are correspondingly opened. This is done both at the section end 30 and at the at least one further section end of the section of the electrical power supply system. A ground current or a summation current is often also detected for the individual phase conductors L1, L2, L3 of the section 30 of the electrical power supply system. In the case of a grounded three-phase section of an electrical power supply system, a ground current can be tapped off at the connection between star point and ground, for example. A summation current, by contrast, can be detected as indicated in figure 3, for example, by means of a core-balance transformer that is embodied as a surrounding transformer 31d and encompasses all the phase conductors Ll to L3 of the section 30 of the electrical power supply system. The detected summation current is in turn fed via a secondary current transformer circuit 32d of the surrounding transformer 31d, a device-internal current transformer 34d and a device-internal secondary current transformer circuit 35d to a further analog/digital converter 36d, which converts the analog summation current into digital summation current measurement values and outputs them to the computation device 37 of the differential protective device 33. Should a fault occur, then, on one of the secondary current transformer circuits (that is to say either one of the secondary current transformer circuits 32a to 32c of the first current transformers 31a to 31c or one of the device-internal current transformer circuits 35a to 35c of the device-internal current transformers 34a to 34c), faulty current measurement values are transmitted to the computation device 37 of the differential protective device 33. In the most frequent case, faults in secondary current transformer circuits are so-called wire breaks, that is to say an interruption for example of the secondary winding of the respective current transformer or of the measurement lines of the secondary current transformer circuit. This involves e.g. interruptions of the current transformer circuits 32a to 32c of the first current transformers 31a to 31c which may be brought about undesirably by construction machines on account of construction activities in the vicinity of the section end. In the case of such an interruption of one (or more) secondary current transformer circuit(s), no correct transformer measurement values are fed to the computation device 37 of the differential protective device 33, which will lead to a faulty calculation of the current sum and thus to an undesired triggering of the electrical circuit-breaker 39. In order to prevent such an excess function of the differential protective device 33, which is usually associated with high costs for the operator of the electrical power supply system, the computation device 37 of the differential protective device 33 has a monitoring unit 40, which monitors the secondary current transformer circuits 32a, 32b, 32c of the first current transformers 31a, 31b, 31c and/or the device-internal secondary current transformer circuits 35a to 35c for interruptions and outputs a fault signal in the case of an ascertained interruption. When a fault signal is present, the computation device 37 is caused to block the differential protection functions for the phase conductors LI, L2, L3 correspondingly affected by the fault in the secondary current transformer circuit. In this way, the outputting of a trigger signal A on the basis of a current sum calculated with faulty current measurement values is avoided and the circuit-breaker contacts remained closed. The method carried out by the monitoring unit 4 0 will be explained in greater detail below with reference to figures 4 to 6. Firstly, for the sake of simplicity, the case of a section of a single-phase electrical power supply system shall be assumed in figure 4. Current measurement values IL picked up at the one phase conductor are fed to the monitoring unit 4 0a of this exemplary embodiment at a first input 41a. The monitoring unit 40a monitors - as indicated in accordance with block 42a - said current measurement values in respect of whether the temporal profile of their magnitudes has an abrupt fall. Such an abrupt fall in the magnitudes of the current measurement values can indicate an interruption of a secondary current transformer circuit, but also an actual fault in the monitored section of the electrical power supply system. Therefore, firstly the monitoring unit 40a does not yet output a fault signal indicating a fault in a secondary current transformer circuit, but rather only generates a suspicion signal V, as indicated by block 43a. If the suspicion signal V is present, it is present at an input of a block 44 for generating the fault signal F. In order to verify the presence of a fault in a secondary current transformer circuit, the monitoring unit 40a consults comparison current measurement values IaL, IbL detected for the phase conductor by remote differential protective devices at other section ends of the monitored section. As mentioned, these comparison current measurement values are communicated to the local differential protective device from the remote differential protective devices via data transmission lines. The comparison current measurement values are fed to the monitoring unit at second inputs 41b, 41c, which are highlighted by a dashed frame 41b in figure 4. In a manner corresponding to the monitoring of its own current measurement values, the monitoring unit 4 0a also examines the magnitudes of the comparison current measurement values with regard to an abrupt fall, as is indicated by blocks 42b and 42c. If an abrupt fall is present in the magnitudes of the comparison current measurement values at the same time as the local current measurement values, then this indicates an actual fault in the monitored section of the electrical power supply system since the probability of a simultaneous fault occurring in the secondary current transformer circuits at different section ends is extremely improbable. If no abrupt fall in the magnitudes can be identified in the case of the comparison current measurement values, however, then this indicates a fault in the secondary current transformer circuit of a current transformer interacting with the local differential protective device. Correspondingly, in a block 43b, a first reset signal Rl is generated if the occurrence of an abrupt fall is also identified with regard to the magnitudes of the comparison current measurement values at the same time as the abrupt fall in the magnitudes of the local current measurement values. Said first reset signal Rl is fed to a blocking input of the block 44 for generating the fault signal F and blocks the outputting of the fault signal F. In other words, a fault signal F is consequently generated by the block 44 precisely when an abrupt fall has been identified in the magnitudes of the local current measurement values but no abrupt fall has been identified in the comparison current measurement values of the remote differential protective devices. If, by contrast, an abrupt fall can also be noted simultaneously in the case of the comparison current measurement values, then this indicates an actual fault on the section of the electrical power supply system and the monitoring unit 40a correspondingly does not output a fault signal F which blocks the differential protection functions of the differential protective device. The method - explained with regard to a single-phase power supply system in accordance with figure 4 - for generating a fault signal F indicating a fault in a secondary current transformer circuit is extended to a three-phase power supply system in accordance with figure 5. For this purpose, figure 5 shows a monitoring unit 4 0b, to which the current measurement values IL1, IL2, IL3 of the three phase conductors that are detected at the local differential protective device are fed at a first input 51a. Said values are thereupon checked as to whether the profile of their magnitudes has an abrupt fall. If such an abrupt fall is identified in at least one phase of the current measurement values, then the first suspicion signal V is generated in block 53a. If the suspicion signal is present in block 53a, then this signal is forwarded to a block 54 for generating a fault signal. In addition, the comparison current measurement values IaLl to IaL3 and IbLl to IbL3 of in each case all three phases of the remote differential protective devices are also fed to the monitoring unit 40b at inputs 51b and 51c. In blocks 52b and 52c, in a manner corresponding' to the procedure in the case of the single-phase system in accordance with figure 4, a check is made to determine whether the magnitudes of the comparison current measurement values of the remote differential protective devices have a simultaneously occurring abrupt fall. In a block 53b, a first reset signal Rl is correspondingly generated precisely when, relative to the same phase conductor in which the jump occurred in the case of the local current measurement values, an abrupt fall is also identified in at least one profile of the other comparison current measurement values. For this purpose, block 53b requires the information about the phase conductor with regard to which the abrupt fall occurred in the local current measurement values. The communication of this information is indicated by a dashed line 56 in figure 5. If the first reset signal Rl is also present with regard to the same phase with respect to which the suspicion signal V was generated, then this reset signal is communicated to the blocking input of the block 54 for generating the fault signal F and blocks the outputting of the fault signal F since an actual fault in the section of the power supply system is identified in this case. The particular advantage of the method explained is that it is thereby also possible to identify interruptions in all three secondary current transformer circuits, such as may arise for example as a result of an external effect, e.g. as a result of construction machines, on the measurement lines between the first current transformers and the differential protective device (cf. figure 3) . In this case, namely, an abrupt fall is present with regard to all three phases in the local current measurement values, while the comparison current measurement values communicated by the remote differential protective devices are uninfluenced thereby. In this way, it is possible in a simple manner to make a reliable decision for generating the fault signal F also in the case of an interruption of the secondary current transformer circuits of all three phases. Finally, figure 6 illustrates a further exemplary embodiment of a monitoring unit. The monitoring unit 40c in accordance with figure 6 carries out some additional checks which make it possible, in an even more reliable manner, to make a decision about whether a fault has occurred in a secondary current transformer circuit or whether an actual fault is present on the section of the electrical power supply system. Firstly, in a manner corresponding to the function of the monitoring unit 40b in accordance with figure 5, the local current measurement values are detected with regard to all the phase conductors at input 61a and checked for an abrupt fall in blocks 62a. A suspicion signal V is generated in block 63a if such a jump has been identified in the current measurement values of at least one phase. The suspicion V is forwarded to block 64 for generating the fault signal F. Likewise in a manner corresponding to the exemplary embodiment in accordance with figure 5, the comparison current measurement values, which are present at inputs 61b and 61c, are monitored in blocks 62b and 62c with regard to a simultaneously occurring abrupt fall in their magnitudes. Provided that an abrupt fall can be identified relative to the same phase in the comparison current measurement values of at least one remote differential protective device, the first reset signal Rl is generated in block 63b. The first reset signal Rl is fed to an input of an OR component 65, which, on the output side, is connected to the blocking input of the block 64 for generating the fault signal. In order to increase still further the reliability of the decision about generating the fault signal, the secondary current transformer circuits are additionally monitored for current flow. A current flow can be identified for example by current sensors, for example Hall sensors, correspondingly used at the secondary current transformer circuits. This information is fed in at input 61d of the monitoring unit 40c. In blocks 62d, a check is made to determine whether a corresponding current flow is present and a second reset signal R2 is generated if, with regard to that phase with respect to which the suspicion signal V was generated, a current flow is present in a secondary current transformer circuit, since a current flow indicates that the secondary current transformer circuit is not interrupted. The second reset signal is likewise fed to the OR component 65 on the input side. The monitoring unit 40c additionally detects, at a further input 61e, the summation or ground current that has been detected by means of a corresponding transformer (cf. figure 3) at the respective section end. By way of example, the summation current Isum is intended to be detected in accordance with figure 6. In a block 62e, a check is made to determine whether a jump in the profile of the summation or ground current also occurs at the same time as the abrupt fall in the magnitudes of the local current measurement values. If this is the case, then a third reset signal is generated since a jump in the profile of the summation or ground current measurement values indicates an actual fault on the section of the electrical power supply system. The third reset signal R3 is likewise fed to the OR component 65. In a further block 62f, moreover, the locally detected current measurement values are checked in respect of whether they exceed a predetermined threshold. If this is the case, then a fourth reset signal R4 is generated, which is fed to the OR component 65. A blocking of the differential protection functions for the case of very high currents on the section of the power supply system is thereby intended to be avoided. This is because in such a case, for example, short-circuit currents flowing in the section of the electrical power supply system can be involved, such that the differential protection functions must on no account be blocked, for safety reasons. At a further input 61g, the monitoring unit 40c obtains information about the state (open/closed) of the switching contacts of the circuit-breaker assigned to the local differential protective device. In block 62g, a check is made to determine whether the switching contacts are in the open position, that is to say whether the monitored section has already been disconnected from the power supply system. A fifth reset signal R5 is generated if the switching contacts of the circuit-breaker are in the open state. Said fifth reset signal R5 is likewise fed to the OR component 65. What is intended to be achieved by this means is that the differential protection functions are not interrupted if the section of the power supply system has been turned off. This is because that could lead to an undesirable blocking of the differential protection upon the section of the power supply system being started up again. At a further input 61h, the monitoring unit 40c is communicated voltage measurement values with regard to all the phases of the section of the power supply system. For the sake of simplicity, however, figure 6 shows only one voltage measurement value input; the latter is representative of voltage value measurement inputs of all three phases. The profile of the voltage measurement values is thereupon checked in block 62h to determine whether it has an abrupt change and a sixth reset signal R6 is generated in box 63h provided that an abrupt change in the profile of the voltage measurement values occurs at the same time as the generation of the suspicion signal V. This is because such an abrupt profile of the voltage measurement values would likewise indicate a fault on the section of the electrical power supply system and not a fault in the secondary current transformer circuit. Finally, in block 62i, the locally detected current measurement values are thereupon checked to determine whether the profile of their magnitudes also decreases further after the suspicion signal V has been generated. This is because usually when an interruption is present in a secondary current transformer circuit, the magnitude of the current measurement values, after a first abrupt fall, will with time approximate to the value zero, that is to say decrease monotonically. Should this monotonic decrease in the magnitudes of the current measurement values not be present, however, then this indicates that no fault is present in a secondary current transformer circuit. A seventh reset signal R7 is logically consistently generated in block 63i provided that the monotonic condition is not met, that is to say if the current measurement values following the generation of the suspicion signal V do not approximate to the value zero. The seventh reset signal R7 is also fed to the OR component 65. The OR component outputs a signal at its output to the blocking input of the component 64 for generating the fault signal F precisely when at least one of the reset signals Rl to R7 is present. In such a case, the generation of the fault signal F is intended to be blocked, such that the differential protection functions of the differential protective device are not impaired. It should be emphasized that all of the checks discussed with respect to figure 6 for generating the reset signals Rl to R7 do not necessarily have to be carried out in a monitoring unit within the meaning of the invention. A corresponding selection can also be effected. All that is essentIaL is that, in accordance with the illustration in figure 5, in addition to the local current measurement values, the comparison current measurement values of the other differential protective devices are also included in the check. The further checks that were discussed in accordance with figure 6 serve for verifying the decision about a fault in the secondary current transformer circuit and can optionally be added individually or jointly. After the generation of a fault signal F indicating a fault in the secondary current transformer circuit, the differential protection functions are blocked for the affected phase of the electrical power supply system in the local differential protective device and the remote differential protective devices. The blocking is cancelled again as soon as the fault signal F is no longer generated, that is to say if the fault in the secondary current transformer circuit has been rectified. At the same time, by means of the local and/or remote differential protective devices, an indication can be given optically about the fact that a fault has been identified in a secondary current transformer circuit. This fault can be specified in greater detail by the indication of the corresponding phase and location of the current transformer. Such an indication can alternatively or additionally also be indicated to the operating personnel of the electrical power supply system in a control center by means of a control computer. In this way, the operating personnel can immediately instigate actions for rectifying the fault in the secondary current transformer circuit. In the exemplary embodiments discussed here, in each case the comparison current measurement values measured at the respectively remote differential protective devices were checked for abrupt changes. It is likewise possible for example also to use the profile of the current sum or of a current intermediate sum or the profile of so-called stabilization current values that can be used for stabilizing the differential protective system. The type of used information about the current measurement values picked up at the remote differential protective devices should advantageously be chosen in such a way that values interchanged between the differential protective devices anyway in the course of the differential protection method are consulted for the function of the monitoring unit. This means that there is no need for additional transmission bandwidth on the data transmission line between the differential protective devices for transmitting additional information. Patent claims 1. A method for generating a fault signal (F) indicating a fault in the secondary circuit (e.g. 32a, 35a) of a current transformer (e.g. 31a, 34a) which interacts with a local differential protective device (33), which monitors a section end (30) of an electrical power supply system, wherein in the method current measurement values which are detected by the current transformer (e.g. 41a, 34a) and indicate a current flowing through the section end (30) are monitored by the local differential protective device (33) and a suspicion signal (V) is generated if the magnitudes of successive current measurement values fall abruptly, and the fault signal (F) is generated if the suspicion signal (V) is present, characterized in that a first reset signal (R1) is generated by the local differential protective device (33) if comparison current measurement values detected at the time of the generation of the suspicion signal (V) in at least one remote differential protective device, which monitors a further section end of the electrical power supply system, likewise fall abruptly with regard to their magnitudes, and the fault signal (F) is blocked if the first reset signal (Rl) is present. 2. The method as claimed in claim 1, characterized in that in the case of a polyphase power supply system, the current measurement values detected by current transformers (e.g. 31a, 31b, 31c) of all the phases (L1, L2, L3) are respectively monitored for an abrupt fall with regard to their magnitudes, the suspicion signal (V) is generated if an abrupt fall is identified for at least one phase (e.g. L1) in the case of the magnitudes of successive current measurement values of a current transformer (e.g. 31a) of the local differential protective device (33), and the first reset signal (R1) is generated if an abrupt fall is likewise identified for respectively the same phase (e.g. L1) in the case of the magnitudes of successive comparison current measurement values of a current transformer of the at least one remote differential protective device. 3. The method as claimed in claim 2, characterized in that in the local differential protective device (33) , the secondary circuits (e.g. 32a, 32b, 32c) of the current transformers (e.g. 31a, 31b, 31c) of all the phases (LI, L2, L3) are monitored for current flow, a second reset signal (R2) is generated if the magnitudes of successive current measurement values fall abruptly in the case of at least one current transformer (e.g. 31a) where current flow is present, and the fault signal (F) is also blocked if at least the second reset signal (R2) is present. 4. The method as claimed in claim 2 or 3, characterized in that in the local differential protective device (33) , summation or ground current measurement values indicating a summation or ground current in the section end (30) are detected, a third reset signal (R3) is generated if, at the time of the generation of the suspicion signal (V) , the profile of successive summation or ground current measurement values has an abrupt change, and the fault signal (F) is also blocked if at least the third reset signal (R3) is present. 5. The method as claimed in any of claims 2 to 4, characterized in that in the local differential protective device (33), the magnitudes of the current measurement values of all the phases (L1, L2, L3) are monitored for exceeding a predetermined threshold value, and a fourth reset signal (R4) is generated if the magnitude of the current measurement values of at least one phase (e.g. L1) exceeds the threshold value, and the fault signal (F) is also blocked if at least the fourth reset signal (R4) is present. 6. The method as claimed in any of claims 2 to 5, characterized in that a circuit-breaker (39) which can be driven by the local differential protective device (33) is checked with regard to the position of its switching contacts, and a fifth reset signal (R5) is generated if the switching contacts of the circuit-breaker (39) are open, and the fault signal (F) is also blocked if at least the fifth reset signal (R5) is present. 7. The method as claimed in any of claims 2 to 6, characterized in that voltage measurement values indicating voltages present at the section end (30) are detected by the local differential protective device (33) for all the phases (L1, L2, L3) , and a sixth reset signal (R6) is generated if, at the time of the generation of the suspicion signal (V), the profile of successive voltage measurement values of at least one phase (e.g. L1) has an abrupt change, and the fault signal (F) is also blocked if at least the sixth reset signal (R6) is present. 8. The method as claimed in any of claims 2 to 7, characterized in that with regard to a phase (e.g. L1) with respect to which the suspicion signal (V) was generated, further current measurement values following the time of the generation of the suspicion signal (V) are monitored, and a seventh reset signal (R7) is generated if the magnitudes of said further current measurement values are greater than the magnitude of that current measurement value which led to the generation of the suspicion signal (V), and the fault signal (F) is also blocked if at least the seventh reset signal (R7) is present. 9. The method as claimed in any of the preceding claims, characterized in that those current measurement values which are also transmitted to the local differential protective device (33) from the at least one remote differential protective device for the purpose of carrying out the differential protective function are used as comparison current measurement values of the at least one remote differential protective device. 10. The method as claimed in any of the preceding claims, characterized in that when the fault signal (F) is present, the differential protective functions of the local differential protective device (33) and of the at least one remote differential protective device are blocked with regard to the phase (e.g. L1) affected by the fault in the secondary circuit (e.g. 32a, 35a) of the corresponding current transformer (e.g. 31a, 34a). 11. The method as claimed in any of the preceding claims, characterized in that the presence of the fault signal (F) is indicated optically by the local differential protective device (33) and/or the at least one remote differential protective device and/or a control center computer. 12. A differential protective device (33) for monitoring a section end (30) of an electrical power supply system, which interacts with at least one current transformer (e.g. 31a, 34a), by means of which current measurement values which characterize a current flowing in the section end (30) of the power supply system are detected by the differential protective device (33), and which has a computation device (37), which carries out the monitoring of the section end (30) on the basis of the current measurement values and comparison current measurement values communicated to the local differential protective device (33) from at least one remote differential protective device, wherein the computation device (37) has a monitoring unit (40), which monitors a secondary circuit (e.g. 32a, 35a) of the current transformer (e.g. 31a, 34a) for faults, characterized in that the monitoring unit (40) is set up for carrying out a method as claimed in any of claims 1 to 11. The invention relates to a method for producing a fault signal (F), which indicates a fault in the secondary circuit (for example 32a, 35a) of a current transformer (for example 31a, 34a), which interacts w with a local differential protective device (33), which monitors a section end (30) of an electrical energy supply system, wherein in the method, measured current values which are detected by the current transformer (for example 41a, 34a) and indicate a current flowing through the section end (30) are monitored by the local differential protective device (33), and a suspicion signal (V) is produced if the absolute values of successive measured current vaalues drop suddently, and the fault signal (F) is produced if the suspicion signal (V) is present. In order to be able to identify a fault in a secondary current transformer circuit even more reliably with such a method the invention proposes that a first rest signal (R1) is produced by the local differential protective device (33) if comparisson measured current values, which are detected at the time at which the suspicion signal (V) is produced in at least one remote differential pro otective device which monitors a further section end of the electrical energy supply system, likewise drop suddenly in terms of their absolute values, and the fault signal (F) is blocked if the first reset signal (R1) is present. The invention also relates to a corres spondly designed differential protective device.

Documents:

939-KOLNP-2009-(10-06-2014)-ABSTRACT.pdf

939-KOLNP-2009-(10-06-2014)-ANNEXURE TO FORM 3.pdf

939-KOLNP-2009-(10-06-2014)-CLAIMS.pdf

939-KOLNP-2009-(10-06-2014)-CORRESPONDENCE.pdf

939-KOLNP-2009-(10-06-2014)-DESCRIPTION (COMPLETE).pdf

939-KOLNP-2009-(10-06-2014)-DRAWINGS.pdf

939-KOLNP-2009-(10-06-2014)-FORM-1.pdf

939-KOLNP-2009-(10-06-2014)-FORM-2.pdf

939-KOLNP-2009-(10-06-2014)-OTHERS.pdf

939-KOLNP-2009-(10-06-2014)-PA.pdf

939-KOLNP-2009-(10-06-2014)-PETITION UNDER RULE 137.pdf

939-kolnp-2009-abstract.pdf

939-kolnp-2009-claims.pdf

939-KOLNP-2009-CORRESPONDENCE-1.1.pdf

939-kolnp-2009-correspondence.pdf

939-kolnp-2009-description (complete).pdf

939-kolnp-2009-drawings.pdf

939-kolnp-2009-form 1.pdf

939-kolnp-2009-form 18.pdf

939-kolnp-2009-form 2.pdf

939-kolnp-2009-form 3.pdf

939-kolnp-2009-form 5.pdf

939-kolnp-2009-gpa.pdf

939-kolnp-2009-international publication.pdf

939-kolnp-2009-international search report.pdf

939-KOLNP-2009-OTHERS.pdf

939-kolnp-2009-pct request form.pdf

939-kolnp-2009-specification.pdf

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