Differential protection method, differential protection device and differential protection system

Abstract

In a differential protection method for monitoring a line of a power grid, current indicator measured values are measured at the ends of the line and are transmitted to an evaluation device. A differential current value is formed with current indicator measured values temporally allocated to one another. The time delay between local timers of the measuring devices is used for the temporal allocation of the current indicator measured values measured at different ends. A fault signal indicating a fault affecting the line is generated if the differential current value exceeds a predefined threshold value. A check is carried out using electrical measured quantities temporally allocated to one another and a line-specific parameter to determine whether the time delay information indicates the actual time delay between the respective local timers. A time error signal is generated if erroneous time delay information is detected.

Claims

1. A differential protection method for monitoring a line of a power grid, the method comprising: measuring current phasor values in each case with measuring devices at ends of the line, the current phasor values indicating an amplitude and a phase angle of a phase current flowing at a respective end of the line, the measuring devices having local timers and allocating a timestamp to the current phasor values indicating a time of a measurement thereof; transmitting at least the current phasor values measured at one end via a communication connection to an evaluation device; forming a differential current value through vectorial addition in the evaluation device with current phasor values from the end of the line temporally allocated to one another, using time delay information that is being calculated continuously indicating a time delay between the local timers of the measuring devices for the temporal allocation of the current phasor values measured at different ends of the line; generating a fault signal indicating a fault affecting the line when the differential current value exceeds a predefined threshold value; carrying out a check using measured electrical quantities temporally allocated to one another which have been measured at the different ends of the line and a line-specific parameter relating to the line running between the ends in order to determine whether the time delay information indicates an actual time delay between the respective local timers, wherein the measured electrical quantities are selected from the group consisting of: amplitudes of current phasor values temporally allocated to one another and voltage phasor values temporally allocated to one another; determining a comparative phase angle in order to check the time delay information using the measured electrical quantities and the line-specific parameter, wherein the comparative phase angle is determined by extending the measured electrical quantities temporally allocated to one another by an amount of the line specific parameter and using an angle between extended amplitudes as the comparative phase angle; using the comparative phase angle to check the time delay information; when erroneous time delay information is detected in the check, generating a time error signal; and in response to generating the time error signal, blocking the fault signal and thereby, preventing shutdown of the line.

2. The differential protection method according to claim 1, which comprises: determining a time delay phase angle derived from the time delay information in order to check the time delay information; comparing the time delay phase angle with the comparative phase angle; and generating the time error signal if a deviation occurs between the time delay phase angle and the comparative phase angle.

3. The differential protection method according to claim 1, which comprises: determining comparative time delay information derived from the comparative phase angle in order to check the time delay information; comparing the comparative time delay information with the time delay information; and generating the time error signal if a deviation occurs between the comparative time delay information and the time delay information.

4. The differential protection method according to claim 1, which comprises: using the amplitudes of current phasor values temporally allocated to one another as the electrical measured quantities which have been measured at the different ends of the line; and using a charging current value indicating a charging current relating to the line running between the ends as the line-specific parameter.

5. The differential protection method according to claim 1, which comprises: using the voltage phasor values temporally allocated to one another as the electrical measured quantities which have been measured at the different ends of the line; and using a line length and/or a propagation constant relating to the line running between the ends as the line-specific parameter.

6. The differential protection method according to claim 1, which comprises: using products of current and voltage phasor values temporally allocated to one another as electrical measured quantities which have been measured at the different ends of the line; and using a line length and/or a propagation constant relating to the line running between the ends as the line-specific parameter.

7. The differential protection method according to claim 6, which comprises: determining a time delay phase angle derived from the time delay information in order to check the time delay information; comparing the time delay phase angle with the comparative phase angle; and generating the time error signal if a deviation occurs between the time delay phase angle and the comparative phase angle.

8. The differential protection method according to claim 7, which comprises generating the time error signal only if the deviation exceeds a predefined tolerance threshold value.

9. The differential protection method according to claim 6, which comprises: determining comparative time delay information derived from the comparative phase angle in order to check the time delay information; comparing the comparative time delay information with the time delay information; and generating the time error signal if a deviation occurs between the comparative time delay information and the time delay information.

10. The differential protection method according to claim 9, which comprises generating the time error signal only if the deviation exceeds a predefined tolerance threshold value.

11. The differential protection method according to claim 1, which comprises blocking an emission of the fault signal if the time error signal is present.

12. The differential protection method according to claim 1, which comprises increasing a value of the threshold value used to evaluate the differential current value if the time error signal is generated.

13. The differential protection method according to claim 1, wherein the time delay information is determined by transmitting messages via the communication connection and using one half a value of a time duration of the transmission of a first message in one transmission direction and of a transmission of a second message in the other transmission direction to determine the time delay information.

14. The differential protection method according to claim 1, which comprises, if the line has more than two ends, carrying out the check to determine whether the time delay information indicates the actual time delay between the respective local timers in each case for two given ends using the line-specific parameter relating to the line section between the two given ends.

15. A differential protection device for monitoring a line of a power grid, the device comprising: a measuring device having a local timer configured to measure current phasor values at one end of the line, the current phasor values indicating an amplitude and a phase angle of a phase current flowing at the one end of the line, and to allocate a timestamp to the current indicator measured values indicating a time of a measurement thereof; a communication device configured to exchange the current measured current phasor values via a communication connection with another differential protection device; and an evaluation device configured: to form a differential current value with locally measured current phasor values and current phasor values received from the other differential protection device that are temporally allocated to one another through addition with a correct algebraic sign, wherein a continuously calculated time delay information indicating a time delay between the local timers of the measuring devices of the differential protection devices is used for a temporal allocation of the current phasor values measured at different ends, and to generate a fault signal indicating a fault affecting the line if the differential current value exceeds a predefined threshold value; to carry out a check using locally measured electrical quantities and measured electrical quantities received from the other differential protection device that are temporally allocated to one another together with a line-specific parameter in order to determine whether the time delay information indicates an actual time delay between the respective local timers; to determine a comparative phase angle in order to check the time delay information using the measured electrical quantities and the line-specific parameter; wherein the comparative phase angle is determined by extending the measured electrical quantities temporally allocated to one another by a amount of the line-specific parameter and using an angle between extended amplitudes as the comparative phase angle; to use the comparative phase angle to check the time delay information; to generate a time error signal if erroneous time delay information is detected in the check: and to block the fault signal and thereby prevent shutdown of the line in response to generating the time error signal.

16. A differential protection system for monitoring a line of a power grid, the system comprising: at least two differential protection devices respectively disposed at a first end of the line and at a second end of the line; a communication connection connected between said differential protection devices in order to transmit current phasor values; each of said differential protection devices having: a measuring device having a local timer configured to measure current phasor values at a respective end of the line, the values indicating an amplitude and a phase angle of a phase current flowing at the end of the line, and to allocate a timestamp to the current phasor values indicating the time of their measurement; a communication device configured to exchange current phasor values via the communication connection with the respective other said differential protection device; at least one of said differential protection devices having an evaluation device configured: to form a differential current value with locally measured current phasor values and current phasor values received from the respectively other said differential protection device that are temporally aligned with one another through addition with a correct algebraic sign, wherein a continuously calculated time delay information indicating a time delay between the local timers of said measuring devices of said differential protection devices is used for a temporal allocation of the current phasor values measured at different ends, and to generate a fault signal indicating a fault affecting the line if the differential current value exceeds a predefined threshold value; and wherein said at least one of said differential protection devices is configured: to carry out a check using locally measured electrical quantities and measured electrical quantities received from the other said differential protection device that are temporally allocated to one another together with a line-specific parameter to determine whether the time delay information indicates an actual time delay between the respective local timers; to determine a comparative phase angle in order to check the time delay information using the measured electrical quantities and the line-specific parameter; wherein the comparative phase angle is determined by extending the measured electrical quantities temporally allocated to one another by an amount of the line-specific parameter and using an angle between extended amplitudes as the comparative phase angle; and to use the comparative phase angle to check the time delay information; to generate a time error signal if erroneous time delay information is detected in the check: and to block the fault signal and thereby prevent shutdown of the line in response to generating the time error signal.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 shows a schematic view of a differential protection system for monitoring a line of a power grid;

(2) FIG. 2 shows a diagram to explain the determination of a comparative phase angle; and

(3) FIG. 3 shows a diagram to explain the use of a tolerance range around the comparative phase angle.

DETAILED DESCRIPTION OF THE INVENTION

(4) Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a part 10 of a power grid. The latter will not be described in an further detail. The part 10 comprises a three-phase line 11 which may be designed, for example, as an overhead line or as a cable. The line 11 is monitored at its first end 11a by way of a first differential protection device 12a and at its second end 11b by way of a second differential protection device 12b for faults occurring on the line, such as, for example, short circuits. For this purpose, current signals are measured for each phase 13a, 13b, 13c of the line 11 with first current transformers 14a-14c at a first measuring point at the first end 11a of the line 11 and second current transformers 15a-15c at a second measuring point at the second end 11b of the primary component 11. The current measurements are fed to a respective measuring device of the differential protection devices 12a, 12b. Current indicator measured values which provide an indication of the amplitude and phase angle of the current signal at the time of measurement are generated from the analogue current signals. The current indicator measured values, including an A/D conversion, can be generated in the measuring device of the respective differential protection device 12a, 12b, in the current transformers themselves or in a suitable measuring device, e.g. a Phasor Measurement Unit (PMU), a Remote Terminal Unit (RTU) or a Merging Unit. Finally, the generated current indicator measured values are fed to an evaluation device, e.g. a CPU or a signal processor, of the respective differential protection device 12a, 12b.

(5) The differential protection devices 12a and 12b are interconnected by means of a communication connection 16 shown only schematically in FIG. 1, which may be, for example, an IP-based communication network or a telecommunication network. However, any other communication connection of any type can be used to connect the differential protection devices 12a and 12b. The respective differential protection device 12a or 12b can be supplied via this communication connection 16 with the current indicator measured values from the respective other end 11a, 11b of the line 11, i.e. pairs of current indicator measured values recorded at both ends 11a and 11b can be formed in each case in each differential protection device 12a and 12b for each phase 13a, 13b, 13c of the line 11.

(6) Using the current measured values from both ends 11a and 11b of the primary component 11 available in both differential protection devices 12a and 12b, a differential current value can be formed in one or both differential protection devices 12a and 12b by means of the evaluation device through vectorial addition of the current indicator values and subsequent amount formation for each phase, and can be compared with a threshold value.

(7) In the case of a fault-free line 11, the current entering the line 11 for each phase is more or less equal to the current flowing from the line 11, so that an indicator with the amount of around zero should be obtained through vectorial addition of the current indicator measured values. However, due to the charging current on the monitored line, the differential current indicator value virtually never permanently assumes exactly the value zero, even in the fault-free case, but instead lies below a predefined threshold value. In addition, transformer inaccuracies and measurement errors, for example, can also contribute to this effect. The predefined threshold value can be specified as either static or dynamic, for example adapted to the level of the respective phase currents.

(8) The threshold value can be specified as a separate parameter. However, it can also be provided to check whether the threshold value has been exceeded by evaluating the position of a measured value pair consisting of the differential current value and an associated stabilization value in a tripping diagram. For this purpose, differential current values and associated stabilization values are formed from associated, i.e. simultaneously measured, current indicator measured values and the position of the measured value pair consisting of a differential current value and a stabilization value is checked in the tripping diagram. If the measured value pair is located within a tripping range, a fault affecting the monitored line is inferred and the fault signal is generated.

(9) If the differential current value exceeds the predefined threshold value for a specific phase, this indicates a fault affecting the relevant phase of the line 11, which may, for example, be a short circuit to ground or a two-phase or multi-phase short circuit, i.e. a short circuit between two or more phases of the primary component. For the phase in which the fault has been detected, the differential protection devices 12a and 12b generate a fault signal, as a result of which the emission of a tripping signal is effected via control lines 17a, 17b to phase-selectively switchable power switches 18 and 19. The tripping signal causes the corresponding phase-related power switch 18a, 18b, 18c or 19a, 19b, 19c to open its switching contacts, so that the phase 13a, 13b, 13c affected by the fault is disconnected from the remainder of the power grid.

(10) If, for example, a short circuit to ground occurs on the phase 13b, the differential protection devices 12a and 12b detect this on the basis of a differential current value exceeding the respective threshold value and transmit tripping signals to the phase-related power switches 18b and 19b in order to disconnect the phase 13b of the line 11 from the power grid.

(11) Although a three-phase line 11 with only two ends 11a and 11b is shown in FIG. 1, the method according to the invention can also be used with any single-phase or multi-phase lines with two or more ends, for example electrical busbars with a plurality of branches.

(12) Furthermore, notwithstanding the representation according to FIG. 1, it can also be provided that the current indicator measured values are transmitted to a single differential protection device and are evaluated there. In this case, it suffices to place measuring devices at the ends 11a, 11b of the line 11 to measure the current indicator measured values and transmit them to the differential protection device. This differential protection device could be disposed at one of the line ends, but also at any other position, for example as a central differential protection device in a switching station or control station.

(13) In order to be able to determine the differential current value correctly, it is necessary that the current indicator measured values used for its formation have actually been simultaneously measured at the ends 11a, 11b of the line 11. However, a time delay normally occurs, particularly in the transmission of the current indicator measured values over a comparatively long communication route, so that the locally measured current indicator measured value cannot readily be linked to a current indicator measured value measured at a distant end and transmitted. If current indicator measured values that have not been simultaneously measured are used, differential current values may occur which exceed the threshold value and would therefore result in the emission of a fault signal, even in a line that is actually fault-free.

(14) For the temporal allocation of the current indicator measured values, the values are therefore normally provided with a marking in the form of a timestamp which indicates the time of their measurement. By selecting the current indicator measured values from different ends of the line which have a matching timestamp, it can be ensured that the differential current value is correctly calculated. However, a prerequisite for this procedure is that the measuring devices used to measure the current indicator measured values in each case have local clocks or timers (CLK) which are synchronized with one another or at least have a known time delay. In order to achieve this, a continuous determination of any time delay between the timers (CLK) of the respective measuring devices takes place which is either used to readjust a timer (CLK) or is used by the evaluation device of the differential protection device for the temporal allocation of the current indicator measured values. In the last-mentioned case, for the temporal allocation of the current indicator measured values, the determined time delay must be subtracted from the timestamp of the current indicator measured value of the measuring device which has the timer with the time that is ahead of the other timer (CLK).

(15) The ping-pong method, for example, can be used to determine the time delay between the clocks (CLK) of the measuring devices. The time duration which is required to transmit a first message in one direction and then a second message in the other direction via the transmission route between the two measuring devices is measured here. In each case, the transmitted messages have a timestamp which indicates the time of their dispatch. The measuring devices furthermore record the reception time of the respective message. The time duration for the pure transmission of the messages (without any time delays between the reception of the first message and the dispatch of the second message) can be determined by means of the timestamp. The determined time duration is halved and provides the transit delay on the communication route for a message transmitted in one transmission direction. The measuring devices can determine the time delay between the timers (CLK) of the measuring devices by means of the timestamp transmitted with the messages and the reception times and the transit delay which is now known. Further details of the ping-pong method can be found in the aforementioned U.S. Pat. No. 8,154,836 B2.

(16) However, the determination of the time delay can supply reliable results only if the communication route between the measuring devices is symmetrical, i.e. if the transit delays of the messages for the forward path and the return path over the communication route are identical. In the case of an asymmetrical communication route, i.e. non-identical transit delays for the forward path and the return path, the ping-pong method supplies an incorrect transit delay, so that the time delay determined using the transit delay is also erroneous. In this case, current indicator measured values which are not simultaneously measured are erroneously used for the calculation of the differential current indicator value. In the worst case, this can result in the determination of a differential current indicator value which exceeds the threshold value despite an actually fault-free line.

(17) It must therefore be ensured that an immediate detection takes place if a communication route is asymmetrical from the outset or changes, gradually or abruptly, from a symmetrical communication route to an asymmetrical communication route. For example, a previously symmetrical communication route can assume an asymmetrical behavior due to switching operations of switches or routers which modify the communication route of a message. Ageing effects or topology changes can also impact on the behavior of a communication route.

(18) Alternatively, the transit delay can also be determined according to the Precision Time Protocol (PTP) defined in the IEEE 1588 standard.

(19) According to the proposed method, the occurrence of an asymmetrical behavior of the communication route or other circumstances distorting the transit delay measurement can be comparatively easily detected. Only electrical quantities and line-specific parameters which are in any case present in a differential protection device as a result of measurement or parameterization are required for this purpose.

(20) It is assumed for the following explanations that the amplitudes of current indicator measured values temporally allocated to one another via the determined time delay are used as electrical measured quantities and a charging current value indicating the charging current of the line between the two measuring devices is used as the line-specific parameter.

(21) The mode of operation of the method is explained in detail with reference to FIG. 2. The detection of the onset of an asymmetrical behavior in relation to the communication route or a different circumstance distorting the transit delay measurement is based on the realization that the current flowing into the line must correspond to the sum of the current flowing out of the line and the charging current caused by the capacitive effect of the line (indicated schematically in FIG. 1 by line capacitors 20). As shown in the equation presented above for determining the charging current value I.sub.C, this charging current is dependent, inter alia, on the length of the line is therefore significant, particularly in the case of lines with a length of several kilometers.

(22) A triangle, the angle of which enclosed between the current indicator measured values corresponds to the time delay between the two measuring devices must thus be constructed from current indicator measured values which are actually temporally associated with one another and from the charging current value indicating the charging current.

(23) This can be used to carry out a plausibility check on the time delay which is determined e.g. via the ping-pong method. To do this, only the amplitudes, but not the phase information, of the current indicator measured values .sub.1 and .sub.2 assumed to be associated with one another via the time delay which is to be checked are used for this purpose (cf. FIG. 2). These are extended using the amount of the charging current value I.sub.C so that an indicator with a length corresponding to the amount of the charging current value can be positioned between the indicator tips of the two amplitudes .sub.1 and .sub.2. This procedure is graphically illustrated in FIG. 2 by drawing a circle 21 with the radius of the amount of the charging current I.sub.C around the indicator tip of the amplitude .sub.2. The amplitude .sub.1 is then positioned so that its indicator tip lies on the circumference of the circle. This produces the vectorial sum from a current indicator measured value with the amplitude .sub.1 and the difference between a current indicator measured value with the amplitude .sub.2 and the charging current value I.sub.C. This procedure, shown here graphically for better understanding, is normally solved numerically.

(24) The angle located between the two amplitudes .sub.1 and .sub.2 can be determined from the triangle 22 constructed in this way and can be used as the comparative phase angle .sub.V.

(25) This comparative phase angle .sub.V is compared with a time delay phase angle .sub.Z derived from the assumed time delay. This time delay phase angle .sub.Z can be simply calculated from the time delay information indicating the time delay and the angular frequency specified by the frequency.

(26) If the time delay phase angle .sub.Z indicates the actual time delay between the timers of the two measuring devices, the values of the time delay phase angle .sub.Z and the comparative phase angle .sub.V match one another. In this case, the communication route is symmetrical and the indicator measured values are correctly allocated to one another and can be used to calculate the differential current value. If this lies above the threshold value, an actual fault on the line which requires a shutdown can be assumed.

(27) Conversely, if the time delay phase angle .sub.Z does not indicate the actual time delay between the timers of the two measuring devices, a deviation exists between the values of the time delay phase angle .sub.Z and the comparative phase angle .sub.V, as shown by way of example in FIG. 2. This means that the determined time delay and the amplitudes .sub.1, .sub.2 of the two current indicator measured values do not match one another. It is highly probable that the reason for this is that the transit delay used to determine the time delay could not be correctly defined, e.g. due to an asymmetrical behavior of the communication route.

(28) If a deviation between the time delay phase angle .sub.Z and the comparative phase angle .sub.V has been established, the current indicator measured values must not simply be used to determine the differential current value, since the risk otherwise exists that a fault on the line is detected which does not in reality exist at all. If a deviation of this type occurs, a time error signal is therefore generated.

(29) The time error signal can be used, for example, to block the emission of the fault signal indicating a fault on the line. As a result, an unwanted shutdown of the line can be reliably prevented. Alternatively, if the time error signal is present, the threshold value used to evaluate the differential current value can also be increased. Along with an increase in an individual threshold value, this can also be done, for example, through corresponding modification of a characteristic of a limit curve in a tripping diagram. As a result, the sensitivity of the differential protection method is, in a manner of speaking, reduced, and a fault signal is generated only in the event of comparatively high differential current values.

(30) Since further effects such as measurement or calculation inaccuracies in the determination of the current indicator measured values may occur in practice along with possible inaccuracies in the determination of the time delay, any deviation between the time delay phase angle .sub.Z and the comparative phase angle .sub.V should be compared with a tolerance threshold value. As a result, the method can be stabilized against minor fluctuations and inaccuracies. This is indicated by way of example in FIG. 3. A tolerance range .sub.V is produced here by broken-line indicators around the value of the comparative phase angle .sub.V determined in the manner described above. If the time delay phase angle .sub.Z lies within the tolerance range, the time delay indicated by it is regarded as correct and no time error signal is generated. Conversely, if the time delay phase angle .sub.Z lies outside the tolerance range, the time error signal is generated.

(31) The details set out above relate to the case where two angular quantities are compared with one another. According to an alternative embodiment, a comparison of two time quantities can also be carried out in a corresponding manner by calculating corresponding comparative time delay information from the comparative phase angle with knowledge of the angular frequency, the information corresponding to the time delay indicated by the comparative phase angle. This comparative time delay information is then compared with the time delay information known, for example, from the ping-pong method and the time error signal is generated if a deviation occurs. The remaining details set out above can be applied accordingly to this embodiment.

(32) The method described can also be applied in the case of a line having more than two ends. In this case, the time delay information check is carried out in each case on the basis of two current indicator measured values of different ends and the charging current relating to the line section located between these ends.

(33) The details set out above can essentially also be transferred to the time information check using other electrical measured quantities (e.g. voltage indicators or load flows as products from current and voltage indicators) and line-specific parameters (line length and/or propagation constant).

(34) Although the invention has been illustrated and described in detail above by means of preferred example embodiments, the invention is not limited by the disclosed examples and other variations may be derived herefrom by the person skilled in the art without exceeding the protective scope of the patent claims set out below.