Digital protective relay apparatus

Abstract: A digital protective relay apparatus comprising an analog input section, a correction value-calculating section, a storage section, a correction section, a protective calculation section. A correction value for correcting the error of the analog input section is predetermined and stored in the storage section. Digital data obtained through the analog input circuit is corrected by the correction section using the correction value. The protective calculation section performs predetermined calculation using the corrected digital data, thereby determining whether or not a trip instruction should be supplied to a breaker.

Description: This invention relates to a digital protective relay apparatus for outputting a trip instruction to a breaker provided in a power system for protecting the same when a fault occurs therein, and more particularly to an apparatus of this type equipped with a function of correcting an error caused by an analog input circuit incorporated therein. A conventional digital protective relay apparatus will be described referring to Fig. 4. In the digital protective relay apparatus, various analog amounts or electrical signals of a power system employed in the apparatus, such as current flowing through the power supply line and bus line of the power system and voltage applied thereto, are input in parallel into an analog input circuit 1. The analog input circuit 1 is constructed generally as shown in Fig. 5. Specifically, it has a plurality of input channels each comprising a transformer 2, a filter circuit 3, and a sample/hold circuit 4. The output terminal of each circuit 4 is connected to a multiplexer 5. The voltage or current input to each input channel of the circuit 1 is supplied to a corresponding transformer 2 where it is subjected to level transform, and is then supplied to the filter circuit 3 where those high frequency components contained in it which are unnecessary for protection calculation are removed. Thereafter, an output signal from the filter circuit 3 is input to the sample/hold circuit 4, where it is sampled at predetermined intervals of time and the resultant sampling value is kept for a predetermined period of time. The multiplexer 5 changes the outputs of the sample/hold circuits 4 from one to another, thereby selecting the analog values of the input channels. The analog signal selected by the multiplexer 5 is input to an A/D converter 6, where it is converted to a digital data signal. The digital data signal is input to a protective relay calculation circuit 7. The circuit 7 performs predetermined protective relay calculation using the input digital data signal, and supplies an output circuit 8 with a trip trigger signal when the calculation result indicates occurrence of a fault in the power system. Upon receipt of the trip trigger signal from the calculation circuit 7, the output circuit 8 outputs a trip instruction to a breaker provided in the power system. In the analog input circuit 1, there exist errors in the characteristics of an output with respect to an input (i.e., in the amplitude gain of the output and/or in the phase of the same) due to various constants of the structural elements therein. These errors should be eliminated since they adversely affect the protective function of the circuit. To this end, in the conventional digital protective relay apparatus, the analog input circuit 1 incorporates a correction circuit 113 comprising an adjustable resistor 111 and an adjustable capacitor 112 and disposed to perform error-correction of digital data, as is shown in Fig. 6. In this conventional apparatus, a calibration signal having a waveform similar to that of an analog amount of electricity is input to the analog input circuit 1 from the power system, and is detected after having passed the circuit 1. The variable resistor 111 and variable capacitor 112 are adjusted in accordance with a detection result. Since the constant of each component of the analog input circuit 1 varies with the lapse of time, it is necessary to periodically adjust the correction circuit 113. As described above, the conventional digital protective relay apparatus is disadvantageous in that its calibration procedure is complicated and hence requires much labor. It is the object of the invention to provide a reliable digital protective relay apparatus having an analog input circuit which can be calibrated in a simple manner. To attain the object, the digital protective relay apparatus of the invention, which comprises an analog input section for sampling an analog electrical signal of the power system and A/D converter means for receiving the analog electrical signal outputted from the analog input section to convert the same to digital data; is characterised by correction value-calculating means for receiving the digital data outputted from the A/D converter means when a predetermined calibration signal is applied to the analog input section to calculate a correction value for correcting an error in the digital data caused by the analog input section and the A/D converter means; storage means for storing the correction value obtained by the correction value-calculating means; correction means for correcting the digital data outputted from the A/D converter means by using the correction value stored in the storage means; and protective calculating means for receiving the corrected digital data and performing predetermined calculation using the corrected digital data, and outputting the trip instruction to the breaker when a calculation result indicates that a fault occurs in the power system. In the digital protective relay apparatus of the invention, the correction value is obtained by predetermined calculation performed using digital data in the correction value-calculating means, and is stored in the storage means. The digital data is corrected by the use of the correction value in the correction means, thereby providing data equivalent to those that would be obtained if there were no errors in the analog input section. By using this errorless data, the protective calculation means can perform errorless calculation. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram, showing a calculation circuit employed in a digital protective relay apparatus according to the invention; Fig. 2 is a block diagram, showing in detail a correction value-calculating section depicted in Fig. 1; Fig. 3 is a graph, useful in explaining a phase calculation by the use of a zero-crossing method; Fig. 4 is a block diagram, showing a conventional digital protective relay apparatus; Fig. 5 shows an analog input circuit employed in the apparatus of Fig. 4; and Fig. 6 shows a correction circuit employed in the apparatus of Fig. 4. The embodiment of the invention will be explained with reference to the accompanying drawings. The digital protective relay apparatus of the invention comprises an analog input circuit, an A/D converter, a correction/protection calculating section, and an output circuit. The analog input circuit, A/D converter, and output circuit have structures similar to those of the corresponding elements shown in Fig. 4. That is, the analog input circuit is constructed as shown in Fig. 5, and is supplied with a signal corresponding to an analog electricity amount or electrical signal of the power system. The A/D converter is connected to the output terminal of the analog input circuit, for converting to digital data the analog electricity amount selected by the multiplexer of the analog input circuit. The digital data is input to the correction/protection calculating section. Fig. 1 shows the correction/protection calculating section (hereinafter referred to as "calculating section"). The calculating section comprises the following: a correction value-calculating section 11, supplied with digital data from the A/D converter 6, for performing a calculation to obtain a correction value for removing a gain error caused by the analog input circuit 1 and A/D converter 6, a storage section 12 for storing the correction value obtained by the section 11, a correction section 13 for correcting digital data output from the A/D converter 6, using the correction value stored in the storage section 12, a protective relay calculating section 14 responsive to the corrected digital data for performing predetermined calculation, thereby outputting a trip trigger signal when the calculation result indicates that an abnormality occurs in the power system. Further, an operation-mode switching device 15 is provided for supplying the correction value-calculating section 11 with a signal instructing change of the mode of the apparatus from calibration mode to operation mode or vice versa. In the calibration mode, the correction value-calculating section 11 takes in digital data to obtain a correction value, and stores the correction value in the storage section 12. In the operation mode, the correction section 13 takes in the digital data and correction value, corrects the digital data, and outputs the corrected data to the protective relay calculating section 14. The correction section 13 corrects the digital data, supplied from the A/D converter 6, in accordance with the following equation (1): Xmo = Xm × K where symbol Xm represents digital data sampled at a certain point of time, and K a correction value stored in the storage section 12. The correction value-calculating section 11 obtains the correction value K by performing, in the calibration mode, calculation with respect of the digital data from the A/D converter 6, using the following equation (2): K = X DC /Xm where X DC represents ideal digital data output from the A/D converter 6 when an analog amount of electricity having a DC waveform is input to the analog input circuit 1, assuming that there are no gain errors in the circuit 1 and converter 6. This ideal digital data is predetermined at the stage of designing. The protective relay calculating section 14 performs a predetermined protective relay calculation using the digital data Xmo corrected in the correction section 13 with the use of the equation (1). The operation of the above-described apparatus will now be explained. Assuming that digital data to be obtained when there are no gain errors in the analog input circuit 1 and A/D converter 6 is Ym, the relationship between this digital data Ym and the digital data Xm output from the A/D converter 6 when gain errors exist in the analog digital circuit 1 can be expressed by the following equation: Xm × K = Ym The digital data X DC , which is to be obtained from the A/D converter 6 assuming that a calibration signal having a DC waveform is input to the analog input circuit 1, and that there are no gain errors in the circuit 1 and converter 6, is predetermined as explained above. On the other hand, the relationship between the digital data X DC and the digital data Xm obtained when a signal having the same predetermined DC waveform is input to the input circuit 1 in a case where gain errors actually exist therein will be given by the following equation in view of the above equation (3): Xmo × K = X DC Thus, the correction value calculating section 11 calculates X DC /Xm, thereby obtaining the correction value K. The value K is stored in the storage section 12. In the operation mode, the correction section 13 calculates Xmo by filling the correction value K in the equation (1). Xmo obtained using the correction value K is equal to Ym. The correction value K obtained from the equation (2) and stored in the correction value storage section 12 is renewed only when the operation mode switching device 15 is set in the calibration mode, and is kept as it is during normal operation. The input channels of the analog input circuit 1 have respective correction values K, and these values K are stored in the storage section 12. The correction section 13 reads from the storage section 12 a correction value K corresponding to the input channel of the digital data, thereby performing correction expressed by the equation (1). In the first embodiment, adjustment performed by the correction circuit 113 shown in Fig. 6 is unnecessary, and further gain errors can be removed from the analog input circuit 1 and A/D converter 6, thereby enabling protective relay calculation using digital data equivalent to gain-errorless digital data. As a result, processing required for correcting gain errors in the analog input circuit 1 and A/D converter 6 is omitted, thus enhancing the reliability of the apparatus. In the first embodiment, an analog amount of electricity having a DC waveform is subjected to the correction. Then, a digital protective relay apparatus equipped with a calculating section for correcting an analog amount of electricity having an AC waveform will be hereinafter explained as a second embodiment. The second embodiment is similar to the first embodiment except the correction section. Therefore, only the correction value calculating section will be described in detail. A correction value calculating section employed in the second embodiment performs calculation based on the following equations (5) and (6) or (7) so as to obtain a correction value K: K = X ACE /X E where symbol X ACE represents the root means square value of desired digital data output from the A/D converter 6, assuming that a calibration signal having an AC waveform is input to the analog input circuit 1, and that there are no gain errors in the circuit 1 and converter 6. Since the ideal digital data is predetermined, the root means square value X ACE is accordingly predetermined. Further, symbol X E represents the root means square value of the digital data output from the A/D converter 6 when a signal having the same AC waveform is input to the input circuit 1 in a case where gain errors are actually found therein. The value X E is given by the following equations (6) and (7): X E = {(Xm² + X(m-n)²)/2} 1/2 where symbols Xm and X(m-n) represent digital data items sampled at a time interval corresponding to 90 degrees of the AC waveform. X E = {(Xm² + X(m-1)² + ... + X(m-k)/(k-1)} 1/2 where symbols Xm - X(m-k) represent digital data items sampled each time a period corresponding to one cycle of the AC waveform elapses. According to the second embodiment, also in the analog input circuit 1 supplied with the AC waveform, gain errors contained in the digital data output from the circuit 1 can be removed, thereby providing an advantage similar to that of the first embodiment. The first and second embodiments are directed to the correction of gain errors caused in the analog input circuit. Apart from such gain errors, where the apparatus has a plurality of input channels, it is possible that phase errors are caused between the input channels. Such phase errors may adversely affect the calculation performed in the protective relay calculating section 14, thereby reducing the reliability of the apparatus. A third embodiment will be explained, which is directed to a digital protective relay apparatus capable of correcting such phase errors caused between input channels, as well as the gain errors caused in the analog input circuit 1 and A/D converter 6. This embodiment is similar to the second embodiment except the structures of the correction value calculating section 11, storage section 12, and correction section 13. Thus, only the correction value calculating section, storage section, and correction section will be explained in detail. As is shown in Fig. 2, the digital protective relay apparatus according to the third embodiment is provided with a correction value calculating section 20, a storage section 21, and a correction section 22. The correction value calculating section 20 has a phase-variable block 23, a phase-detecting section 24, a phase-error detecting section 25, an effective value-calculating section 26, and a gain correction value-calculating section 27. The phase-variable block 23 is supplied from the A/D converter 6 with the digital data Xm, which is provided for each input channel of the input circuit 1. Upon receipt of the digital data Xm, the block 23 performs calculation based on the received data, using the following equation: Xmo = K × (Xm + A × X(m-1)) where Xm represents digital data sampled at a point of time, X(m-1) digital data sampled previous to Xm, K a gain correction value peculiar to each input channel, and A a phase correction value peculiar to the input channel. The phase-variable block 23 fixes, at the time of performing calculation using the equation (8), the gain correction value K at 1 (i.e., assumes that there are no gain errors) and varies the phase correction value A within a range of 0 - 1 in accordance with a signal from the phase-error detecting section 25, thereby determining such phase correction values A for the respective input channels as make the phases of the channels identical to one another. The phase-detecting section 24 is responsive to digital data supplied from the phase-variable block 23 for detecting the phase ϑ of the digital data Xmo of each input channel, using the zero-cross method expressed by the following equation (9): ϑ = {N + X(n-1)/(X(n-1) - X)} × 2π × Ts/T where, as is shown in Fig. 3, X(n-1) represents a value sampled immediately before the digital data crosses zero, Xn a value sampled immediately after it crosses zero, N the number of occasions of sampling from a reference sampling point to a sampling point n-1, Ts a sampling cycle, and T the cycle of the input AC waveform. The phase-error detecting section 25 compares the phases of the input channels with one another, sets as a reference phase the phase of an input channel which is most retarded in phase in all the input channels, and causes the phase-variable block 23 to vary the phase correction values A of the input channels whose phases are advanced relative to the reference phase. Thus, the phase correction values A, which can cause the phases of all the input channels to be identical to one another, are determined for the input channels, respectively. The phase-variable block 23 stores the determined phase correction values A in the storage section 21, and outputs digital data Xmo calculated using the determined correction values A, to the effective value-calculating part 26. The effective value-calculating section 26 calculates the root means square values X E of the digital data Xmo thus calculated, using the equation (6). The gain correction value-calculating section 27 reads the effective values X E from the calculating section 26, thereby calculating the gain correction values K of the input channels using the equation (5), and storing the values K in the storage section 21. Thus, the storage section 21 stores the phase correction values A supplied from the phase-variable block 23, and the gain correction values K supplied from the gain correction value-calculating section 27. The correction section 22 reads the phase correction values A and gain correction values K from the storage section 21, and performs calculation using the equation (8). Then, the operation of the apparatus of the invention will be explained more specifically. Assume that the analog input circuit 1 has first and second input channels, and that these input channels ideally generate outputs with a gain of 1/10 and without phase errors, with respect to inputs having a reference waveform (50 Hz). In reality, however, in the circuit 1, the first input channel has a gain error of + 5% and a phase error of + 0.5°, while the second input channel has a gain error of - 7% and a phase error of + 1.0°. An analog amount Vi of electricity input to the analog input circuit 1 is: Vi = 100 sin (100πt) An ideal output Vo for each of the first and second channels is: Vo = 10 sin (100πt) Actually, however, each channel has the above-described errors, so that the outputs Vo1 and Vo2 of the first and second channels are: Vo1 = 10.5 sin (100πt + 0.5π/180) Vo2 = 9.3 sin (100πt + π/180) Here, assume that the analog amount of electricity is sampled with a frequency of 4800 Hz. At this time, the sampling time series t0, t1, t2, ..., tK are: t0 = 0 × Ts, t1 = 1 × Ts, t2 = 2 × Ts, ..., tK = K × Ts where Ts = 1/4800. Assume that the sampling data series of the first and second channels with respect to the sampling time series are: Vo10, Vo11, Vo12, ..., Vo1K Vo20, Vo21, Vo22, ..., Vo2K At this time, the following are satisfied: Vo195 < 0, Vo196 > 0 Vo295 < 0, Vo296 > 0 This means that the data value of each of the first and second channels crosses zero between the 95th sampling point from the reference sampling point and the 96th sampling point from the same. Accordingly, the phases ϑ1 and ϑ2 of the data of the first and second channels are: ϑ1 = {95+Vo195/(Vo195-Vo196)} × 2π × Ts/T = 6.2744547 ϑ2 = {95+Vo295/(Vo295-Vo296)} × 2π × Ts/T = 6.2657277 where T = 1/50. Since the phase ϑ1 of the first channel is larger than the phase ϑ2 of the second channel, the phase-error detecting section 25 supplies the phase-variable block 23 with an instruction for changing the phase correction value A of the second channel so as to cause the phases ϑ1 and ϑ2 to coincide with each other, thereby obtaining a phase correction value A2 which enables the both phases to coincide with each other. Specifically, the phase correction value A2 is obtained as follows, assuming that the phase correction value A1 of the first channel is zero: Since Vo2K* = Vo2K + A2 × Vo2 (K-1) , the following equation is satisfied by substituting 0.1539267 for A2: ϑ2* = {95+Vo295*/(Vo295*-Vo296*)} × 2π × Ts/T = 6.2744547 = ϑ1 (= ϑ*1) Accordingly, the phase correction value A2 is determined to be 0.1539267. Further, the amplitude values (i.e., root means square values) V1* and V2* are: V1* = {(Vo1 (K-24)*² + Vo1K*²)/2} 1/2 = 7.4246212 V2* = {(Vo2 (K-24)*² + Vo2K*²)/2} 1/2 = 7.5864511 Since the true value V T of V is 10/2 1/2 , the following are satisfied: K1 = V T /V1* = 0.9523809 K2 = V T /V2* = 0.9320652 Vo1K* and Vo2K* are calculated using the obtained K1, K2, A1, and A2. Vo1K* = K1 × (Vo1K + A1 × Vo1(K-1)) Vo2K* = K2 × (Vo2K + A2 × Vo2(K-1)) Thus, Vo1K* and Vo2K* coincide with each other, and the phase error between the first and second channels is corrected. Further, the effective value obtained using Vo1K* or Vo2K* is 10/(1/2) 1/2 , which means that the gain error is corrected. In the embodiment, the timing error of sampling between the channels is also corrected, in addition to the gain error of each channel and the phase error between the channels. As is described above, the third embodiment can provide the gain correction value K of each input channel and the phase correction value A between the input channels, thereby simultaneously correcting the gain errors and phase error. As a result, the protective relay calculating section can perform reliable calculation. Moreover, in the third embodiment, the correction section 22 can perform correction processing using the following equation (10) in place of the equation (8): where Xm, Xm-1, ..., and X(m-N) represent digital data sampled at different sampling time points, K0, K1, ..., and K N correction values peculiar to each input channel. That is, the degree of phase correction is determined from the relative rate of each of the correction values K0, K1, ..., and K N , while the degree of gain correction is determined from the absolute value of each of the correction values K0, K1, ..., and K N . In all the embodiments, the correction section and correction value-calculating section can be realized by hardware or software.

Claims: A digital protective relay apparatus for outputting a trip instruction to a breaker provided in a power system for protecting the same when a fault occurs therein, comprising: an analog input section (1) for sampling an analog electrical signal of the power system and A/D converter means (6) for receiving the analog electrical signal outputted from the analog input section (1) to convert the same to digital data; characterised by: correction value-calculating means (11; 20) for receiving the digital data outputted from the A/D converter means (6) when a predetermined calibration signal is applied to the analog input section (1) to calculate a correction value for correcting an error in the digital data caused by the analog input section (1) and the A/D converter means (6); storage means (12; 21) for storing the correction value obtained by the correction value-calculating means (11; 20); correction means (13; 22) for correcting the digital data outputted from the A/D converter means (6) by using the correction value stored in the storage means (12; 21); and protective calculating means (14) for receiving the corrected digital data, and performing predetermined calculation using the corrected digital data, and outputting the trip instruction to the breaker when a calculation result indicates that a fault occurs in the power system. The digital protective relay apparatus according to claim 1, characterized in that the correction value calculating-means (11) obtains the correction value for correcting a gain error of the analog input section (1) and A/D converter means (6), when a predetermined calibration signal having a DC waveform is applied to the analog input section (1), by using the following equation: K = X DC /Xm where K represents the correction value for correcting the gain error, X DC represents digital data that would be outputted from the A/D converter means (6) in response to said predetermined calibration signal having an DC waveform being applied to the analog input section (1) if there were no gain errors in the analog input section (1) and converter means (6), and Xm represents the actual digital data that are outputted from the A/D converter means (6) in response to said predetermined calibration signal having a DC waveform being applied to the analog input section (1). The digital protective relay apparatus according to claim 2, characterized in that the correction means (13) corrects the digital data Xm outputted from the A/D converter means (6) by using the following equation: Xmo = Xm × K where Xmo represents the corrected digital data. The digital protective relay apparatus according to claim 1, characterized in that the correction value calculating-means obtains the correction value for correcting gain errors of the analog input section (1) and A/D converter means (6), when a predetermined calibration signal having an AC waveform is applied to the analog input section (1), by using the following equation: K = X ACE /X E where K represents the correction value for correcting the gain errors, X ACE represents the root means square value of digital data that would be outputted from the A/D converter means in response to said predetermined calibration signal having an AC waveform being applied to the analog input section (1) if there were no gain errors in the analog input section (1) and converter means (6), and X E represents the root means square value of the actual digital data that are outputted from the A/D converter means (6) in response to said predetermined calibration signal having an AC waveform being applied to the analog input section (1). The digital protective relay apparatus according to claim 4, characterized in that the correction means corrects the digital data Xm outputted from the A/D converter means (6) by using the following equation: Xmo = Xm × K where Xmo represents the corrected digital data. The digital protective relay apparatus according to claim 1, characterized in that the analog input section (1) has a plurality of input channels for respectively processing a plurality of analog electrical signals simultaneously supplied from the power system; the correction value-calculating means includes phase-correcting value detection means (23, 24, 25) for obtaining phase-correcting values for correcting phase errors between the input channels when a predetermined calibration signal having an AC waveform is applied to all input channels of the analog input section (1), and gain correction value-detecting means (26, 27) for obtaining gain correction values for correcting the gain errors of the analog input section (1) and A/D converter means (6) when said predetermined calibration signal having an AC waveform is applied to all input channels of the analog input section (1). The digital protective relay apparatus according to claim 6, characterized in that the phase correction value-detecting means has phase detection means (24) for detecting the phases of the signals corresponding to said predetermined calibration signal passing each of the input channels, and phase correction value-determining means (25, 23) for detecting phase correction values of the input channels such that said phases of the input channels which are detected by the phase detection means, can be made to coincide with one another. The digital protective relay apparatus according to claim 7, characterized in that the phase detection means (24) detects the phase value of each of the input channels by using a zero-crossing method. The digital protective relay apparatus according to claim 6, characterized in that the gain correction value detecting-means (26, 27) obtains the gain correction value of each of the input channels for correcting the gain error thereof by using the following equation: K = X ACE /X E where K represents the gain correction value for correcting the gain error of the input channel, X ACE represents the root means square value of the digital data that would be outputted from the A/D converter means (6) in response to said predetermined calibration signal having an AC waveform being applied to the analog input section (1) if there were no gain errors in the analog input section (1) and converter means (6), and X E represents the root means square value of the digital data Xmo obtained by correcting, using each phase correction value, the actual digital data Xm that are outputted from the A/D converter means (6) in response to said predetermined calibration signal having an AC waveform being applied to the analog input section (1). The digital protective relay apparatus according to claim 6, characterized in that the correction means (22) corrects the digital data outputted from the A/D converter means (6) by using the following equation: Xmo = K × (Xm + A × X(m-1)) where Xmo represents the corrected digital data, K represents the gain correction value, Xm represents digital data sampled at a certain time point, A represents the phase correction value, and X(m-1) represents digital data sampled previous to Xm by one sampling cycle. The digital protective relay apparatus according to claim 6, characterized in that the correction means corrects the digital data outputted from the A/D converter means (6) by using the following equation: where Xmo represents the corrected digital data, Xm, Xm-1, ..., and X(m-N) represent digital data sampled at different sampling time points, K0, K1, ..., and K N represent correction values peculiar to each input channel. The digital protective relay apparatus according to claim 1, characterized by further comprising operation mode-changing means (16) for controlling the operation of the correction value calculating-means (11 ; 20), thereby changing the mode of the apparatus from a calibration mode to an operation mode or vice versa.

Inventors: NISHITANI YASUCHIKA C O INT PR, NISHITANI, YASUCHIKA C/O INT. PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H02H3
IPC codes: H02H3, G01R35, G01R19