IMPROVED CURRENT-SENSING ARRANGEMENT AND METHOD FOR A CIRCUIT-INTERRUPTER CONTROL CIRCUIT

Abstract: An improved current-sensing arrangement (10) and method are provided for supplying a signal (eg. at 14) that suitably represents line current (e.g. in 12) over a wide current range. The current-sensing arrangement utilizes the signals from an iron core transformer (32) and an air core trans­ former (42) along with transition and scaling arrangements (e.g. 56,58,70,62,64 and 68) to provide the sensed current signal. The range of operation includes currents for which the iron core transformer exhibits saturation effects and currents for which the air core transformer does not contribute to the combined output signal. Additionally, the range of operation includes currents for which both the transformers contribute to the combined output signal. The output of the iron core transformer dominates the combined output signal for low currents, the output of the air core transformer dominates at high currents, and the transition from the domination of the iron core transformer to the air core transformer is gradual and occurs over a mid-range of current with contribution from both transformers. Accordingly, the combined output signal suitably repre­ sents the sensed current over a wide current range due to the contribution of the iron core transformer at low currents where no saturation effects are present and the contribution of the air core transformer at high currents with no saturation characteristic; a smooth transition being provided over a wide current range where both transformers contribute to the output and dominant contribution being provided by the transformer that is most accurate for the particular current.

Description: The present invention relates generally to the field of circuit interrupters and control circuits therefor, and more particularly, to an improved current-sensing arrangement and method for providing a signal that suitably represents line current over a wide current range. Arrangements for sensing the current in an AC power line and providing a signal representing the current are shown in U.S. Patent Nos. 4, 434, 415, 4, 567, 540 and 4, 571, 658 and in EP-A-0129623. For example, FIG. 1 of U.S. Patent No. 4, 434, 415 illustrates a current transformer 46 positioned coaxially about a conductive member through which line current is passed. As further disclosed in the aforementioned patent documents, the current transformer is utilized to provide an output signal for use by control circuitry to provide appropriate trip signals in accordance with predetermined time-current characteristics. Where the control circuit is required to operate over a wide current range, e.g. 400-40,000 amperes rms, the use of an iron core transformer for this wide current range is not practical since the output signal no longer represents the current as the transformer begins to saturate. Some control arrangements utilize two transformers which function independently of each other. For example, U.S. Patent No. 4, 297, 741 utilizes an iron core transformer for providing current signals to long-­ time and short-time trip circuits while an air core transformer is utilized to develop signals representing the rate of change of the current, di/dt, as an input to an instantaneous trip circuit. A digital electronic current transducer is disclosed in an article entitled "Digital EHV Current Transducer" by L.B. Berkebile, 1980, IEEE, 80 SM 647-8. A line potential encoder unit is also disclosed and two types of current-drive power supplies are discussed: a saturable current transformer; and a controlled current-switching current transformer. DE-B-1 241 526 discloses an arrangement having an air core transformer and an iron core transformer, the output signal being obtained solely from-the former at high currents and solely from the latter at low currents. US-A-4,532,571 discloses a similar arrangement, in which the output signal at high currents is, instead, obtained from a current detector using a Hall element. According to a first aspect of the present invention there is provided a method for providing a signal that suitably represents the current in an AC line over a wide range of currents, the method comprising the steps of: rectifying the output from a first transformer that senses the line current; and rectifying the output from a second transformer that senses the line current; and filtering and scaling each of the resultant rectified outputs, such that the output from the first transformer is dominant in the signal in a low current region of the range, and such that the output derived from the second transformer is dominant in the signal in a high current region of the range, the method being characterised in that, in an intermediate current region between the low and high current regions, the signal is obtained by combining at each point in time the filtered and scaled outputs with both of the filtered and scaled outputs contributing significantly to the signal throughout the intermediate current region. According to a second aspect of the invention, there is provided a current-sensing arrangement for providing a sensed-current signal that suitably represents the sensed current in an AC line over a wide range of currents, the range having a lower current region, an intermediate current region and a higher current region, the current-sensing arrangement comprising: first sensing means responsive to the current in an AC line for providing a first output signal which increases with sensed current and which suitably represents the current in the AC line over the low current region of the range, wherein, at higher intermediate region currents, said first output signal increases with increasing current at a lower rate than in the lower current region, and wherein, in the higher current region of the range, said first output signal is essentially unchanging for increasing current; second sensing means responsive to the current in the line for providing a second output signal which increases with sensed current and which suitably represents the current in the AC line in said higher current region of the range, wherein, over said intermediate region, said second output signal reasonably accurately represents the current, and wherein, over said lower current region of the range, said second output signal is not usable as an accurate representation of the current; scaling and outputting means responsive to said first and second output signals for appropriately scaling each of said first and second output signals and for outputting a third output signal that suitably represents the sensed signal over said lower, intermediate and higher current regions, the arrangement being characterized in that both said first and second output signals contribute significantly to said third output signal throughout said intermediate current region, said scaling and outputting means comprising means for comparing said first and second output signals after scaling. Preferably, the first transformer or first sensing means is an iron core transformer and the second transformer or second sensing means is an air core transformer. Thus the range of operation includes currents for which the iron core transformer exhibits saturation effects and currents for which the air core transformer does not contribute to the combined output signal. Additionally, the range of operation includes currents for which both the transformers contribute to the combined output signal. The output of the iron core transformer dominates the combined output signal for low currents, the output of the air core transformer dominates at high currents, and the transition from the domination of the iron core transformer to the air core transformer is gradual and occurs over a mid-range of current with contribution from both transformers. Accordingly, the combined output signal suitably represents the sensed current over a wide current range due to the contribution of the iron core transformer at low currents where no saturation effects are present and the contribution of the air core transformer at high currents with no saturation characteristic; a smooth transition being provided over a wide current range where both transformers contribute to the output and dominant contribution being provided by the transformer that is most accurate for the particular current. Reference will now be made, by way of example, to the accompanying drawings, in which:­ FIG. 1 is a block diagram and schematic representation of the improved current-sensing arrangement of the present invention for use by a control circuit to control operation of a circuit interrupter; FIG. 2 is a graphic representation of a time-­ current characteristic curve that is illustrative of the overcurrent conditions for which the control circuit operates the circuit interrupter; FIG. 3 is a graphic representation of illustrative time-current characteristic curves that are useful in describing the operation of the arrangement of FIG. 1; and FIGS. 4 and 5 are graphic representations of output waveforms at illustrative points in the arrangement of FIG. 1. Referring now to FIG. 1, the current-sensing arrangement 10 of the present invention and to practice the method of the present invention is illustrated in a preferred usage to sense the AC current in the line 12 and to provide a signal at 14 representing the sensed current to a trip control circuit 20. The trip control circuit 20 operates in response to the sensed current signal 14 and in accordance with a predetermined time-­ current characteristic to provide a trip signal at output 22. The trip signal 22 is provided to a gate of a power supply 30 to control an interrupter operator 40 to operate a circuit interrupter 50. An illustrative example of a time-current characteristic describing the overcurrent conditions which are interrupted by the circuit interrupter 50 in response to the trip signal at 22 is depicted in FIG. 2. The illustrated inverse-time characteristic provides for circuit interruption in the presence ofovercurrents such that the time delay is inversely related to the magnitude of the overcurrent. The lowest current at which interruption will be provided is called the pick-up current. In a particular example, the pick-up current is 800 amperes. If the interrupting range of operation of the trip control circuit 20 is, for example, 800 to 40,000 amperes, the trip control circuit must be provided with a suitable representation of the sensed current for at least the range of interruption operation; for example, from 400-40,000 amperes. If the sensed current signal at 14 does not suitably represent the current over this operating range, the time-current characteristic will be distorted from the desired characteristic. For example, if the signal at 14 does not increase with increasing current after a predetermined current, for example 5000 amperes, then the trip control circuit 20 will operate along the curve portion 100 instead of the desired portion 102. If an iron core transformer is utilized to provide the signal at 14, saturation effects will cause the signal at 14 to approach a fixed value that no longer represents the sensed current for currents above a certain level; e.g. 5­ 10kA. On the other hand, if an air core transformer is utilized, the output at low currents, for example in the range of 400 to 2000 amperes, is et such a low level that such a signal is difficult to utilize and does not provide a suitable representation of the current. Further, if the iron core transformer were utilized for a low current range and the air core transformer for a high current range, i.e. in the respective ranges of accuracy, a smooth transition between the two transformers is difficult and neither of the two transformers is particularly accurate in the mid-range of currents; e.g. 4-10kA. In accordance with the features of the present invention, the current-sensing arrangement 10 includes an iron core transformer 32 which provides outputs on lines 34, 36 to a bridge rectifier 38. Additionally, the current-sensing arrangement 10 includes an air core transformer 42 which provides outputs on lines 44, 46 to a bridge rectifier 48. The iron core trans­ former 32 and the air core transformer 42 are arranged to sense the current in the line 12. For example, the transformers 32 and 42 are coaxially mounted about a conductor in a cavity referred to by reference numeral 52 in FIGS. 1 and 2 of U.S. Patent No. 4, 434, 415. In that case, the transformers 32 and 42 occupy the position of the current transformer 46 as shown in FIG. 1 of that patent. The output 52 of the bridge 38 and the output 54 of the bridge 48 are scaled and utilized to provide a combined output at 14 with a suitable, gradual transition to supply a suitable representation of the sensed current in the line 12 over a wide current range, e.g. 400 to 40,000 amperes, to the trip control circuit 20. The scaling of each of the outputs 52,54 and the transition regions are arranged such that the output derived from the iron core transformer 32 dominates the output 14 for low currents and the output derived from the air core transformer 42 dominates at high currents. Further, the transition from the domination by the output 52 to the output 54 is gradual so as to occur over a wide mid-range of current over which both outputs 52 and 54 contribute to the signal at 14. To this end, the output 52 of the bridge 38 is connected through a resistor 56 to a signal point 63 which is connected to the signal output 14. The parallel combination of a resistor 58 and a capacitor 59 is connected between the signal point 63 and the circuit common 60. The resistors 56 and 58 and the capacitor 59 provide filtering and the resistors 56 and 58 provide scaling of the signal at 52 derived from the iron core transformer 32. The output 54 derived from the air core transformer 42 is connected through a resistor 62 to one side 61 of the parallel combination of a resistor 64 and a capacitor 66. The other end of the parallel combination of the resistor 64 and the capacitor 66 is connected to the circuit common 60. The junction of the resistors 62,64 is connected to the cathode of a diode 68, the anode of which is connected to the signal output 14. The resistors 62,64 provide scaling of the signal at 54 derived from the air core transformer 42 and along with capacitor 66 provide filtering. With the provision of the diode 68, the output at 61 derived from the air core transformer 42 contributes to the signal at 14 only when the signal at 61 is greater than the signal at 63 that is derived from the source at point 52 as will be explained in detail hereinafter. Since the voltage at the output 44,46 of the air core transformer 42 is a voltage representing the instantaneous value of di/dt, the rate of change of the sensed line-current, a capacitor 72 across the output 44,46 decreases the rate effect and filters out the high frequency components. A varistor 74 is connected across the output 44, 46 in specific embodiments where it is desirable to limit the peak output voltages to the trip control circuit 20. Referring now to FIG. 3, the plot ACT for the air core transformer, the plot ICT for the iron core transformer, and the combined ACT&ICT plot represent the respective time-current characteristic curves as derived from the air core transformer 42 alone, the iron core transformer 32 alone, and the combination of both of the individual waveforms at 61 and 63 respectively. The ICT plot illustrates saturation effects starting at 4-5kA. At higher currents, e.g. 10-40kA, the signal contributed by the iron core transformer 32 does not represent the current and approaches a fixed value. Additionally, it can also be seen that the ACT plot in the same range of higher current increases with current. Accordingly, at higher currents, the signal at 14 becomes dominated by the output derived from the air core transformer 42. The combined signal at 14 is represented in FIG. 3 by the plot ICT&ACT. Referring additionally to FIGS. 4 and 5, the plots of amplitude of the current versus time represent the waveforms at the respective points in FIG. 1. Specifically, the waveforms of FIG. 4 correspond to a mid-range current at which the output at 52 derived from the iron core transformer 32 exhibits saturation effects. The output at 52, which is before the filtering and scaling, illustrates the saturation effects. For this mid-range current, the outputs at 54 and 61 that are derived from the air core transformer 42 are significant and contribute to the composite waveform at 14. It should be noted that, as depicted in FIG. 1, the outputs as referenced are negative voltages with respect to the circuit common 60. The waveforms of FIG. 5 correspond to a higher current, e.g. 20-30kA, at which the output 61 derived from the air core transformer 42 is dominant as can be seen by the near coincidence between the waveform at 61 and the combined output at 14 at all points along the waveforms. For compairison purposes, note that in FIG. 4 the output 52 that is derived from the iron core transformer 32 is predominant over the output 54 derived from the air core transformer 42. This is illustrated over the range 110 where the signal at 14 is determined by the output at 52 as can be seen by the output 54 being less than the output 14. Accordingly, it should be realized that the combined signal at 14 includes contribution by signals derived from both transformers at different points of the combined waveform at 14 at a given current as dependent on the amplitude of each signal at a particular time; i.e. at each point along the plots of FIGS. 4 and 5. Further, the combined waveform at 14 also includes contribution from both transformers at various points. In accordance with the present invention, in addition to utilizing the outputs of both the air core transformer 42 and the iron core transformer 32 and performing scaling and filtering of the outputs to provide the appropriate combined signal, it should be realized that, as discussed hereinbefore, the change from the dominance by the iron core transformer to the air core transformer is not an abrupt transition as would be performed by switching from one output to another above a particular current. Further, it should also be noted that the combined signal at 14 is not derived by a simple addition or superpositioning process. Instead, the combined signal at 14 includes contribution from both transformers over a wide mid-range of current at different times in the waveform at 14 for any given current in the range. To this end, the diode 68 at times performs a switching function between the output 61 and the output supplied from 52 through the resistor 56. In circuit terminology, when the output at 61 is more negative than the output at 63, the diode 68 is rendered conductive and the signal at 14(offset by a diode drop) follows the output at 61 via current flow through the diode 68. Otherwise, the signal at 14 is provided by the output at 52 via the scaling and filtering circuitry. A second diode 70 can be utilized between 63 and 14 but desirable efficiencies and circuit performancd are achieved without the diode 70. Additionally, the output at 44, 46 from the air core transformer 42 is 90 degrees out of phase with respect to the output 34, 36 from the iron core transformer 32. The outputs being out of phase along with the significant amount of ripple at the outputs 61 and 63 are other factors in the gradual transition from the dominance of the iron core transformer to the air core transformer as the current increases. In a preferred arrangement, the output of the bridge 38 also provides operating current to the power supply 30 over line 39 and through line 52 via a series resistor 41. The power supply 30 provides operating power to the trip control circuit 20 at output 31. Upon the generation of a trip signal 22, the interrupter operator 40 is supplied with operating energy over the lines 27, 29 from the current in the line 39 and from stored energy in the power supply 30. Additionally, the circuit interrupter 50 and the interrupter operator 40 in a specific arrangement are of the type shown in U.S. Patent No. 4, 467, 307. It should be realized that in other specific implementations, the two outputs 61 and 63 can be appropriately sampled and/or combined by means of a micro­ processor or the like. Additionally, the present invention is also useful in obtaining desirable time-current characteristics by means of various scaling and combining methods of the signals at 52 and 54 or by scaling of the combined signal at 14.

Claims: 1. A method for providing a signal that suitably represents the current in an AC line (12) over a wide range of currents, the method comprising the steps of: rectifying the output from a first transformer (34) that senses the line current; and rectifying the output from a second transformer (42) that senses the line current; and filtering and scaling each of the resultant rectified outputs, such that the output from the first transformer is dominant in the signal in a low current region of the range, and such that the output derived from the second transformer is dominant in the signal in a high current region of the range, the method being characterized in that, in an intermediate current region between the low and high current regions, the signal is obtained by combining at each point in time the filtered and scaled outputs with both of the filtered and scaled outputs contributing significantly to the signal throughout the intermediate current region. 2. The method of Claim 1, wherein the first transformer is an iron-core transformer and the second transformer is an air-core transformer. 3. The method of Claim 1 or 2, wherein said filtered and scaled outputs are compared to each other. 4. The method of Claim 3 wherein the result of the comparison between the filtered and scaled outputs is used to determine the appropriate combination of the filtered and scaled outputs. 5. The method of Claim 3, wherein, depending upon the result of the comparison, one of the filtered and scaled outputs is supplied das the signal. 6. The method of any preceding claim wherein each of the rectified outputs is scaled in a predetermined manner to provide a desired time-current characteristic when the combined signal is used as the input to a trip signal generator. 7. The method of any preceding claim, wherein the combined signal is scaled in a predetermined manner to achieve a desired amplitude versus current relationship. 8. The method of any preceding claim, wherein he filtered and scaled output signals from he first and second transformers are out of phase with each other. 9. A current-sensing arrangement for providing a sensed-current signal that suitably represents the sensed current in an AC line (12) over a wide range of currents, the range having a lower current region, an intermediate current region and a higher current region, the current-sensing arrangement comprising: first sensing means (32, 38) responsive to the current in an AC line for providing a first output signal which increases with sensed current and which suitably represents the current in the AC line over the low current region of the range, wherein, at higher intermediate region currents, said first output signal increases with increasing current at a lower rate than in the lower current region, and wherein, in the higher current region of the range, said first output signal is essentially unchanging for,increasing current; second sensing means (42, 48) responsive to the current in the line for providing a second output signal which increases with sensed current and which suitably represents the current in the AC line in said higher current region of the range, wherein, over said intermediate region, said second output signal reasonably accurately represents the current, and wherein, over said lower current region of the range, said second output signal is not usable as an accurate representation of he current; and scaling and outputting means (56, 58, 62, 64, 68) responsive to said first and second output signals for appropriately scaling each of said first and second output signals and for outputting a third output signal that suitably represents the sensed signal over said lower, intermediate and higher current regions, the arrangement being characterized in that both said first and second output signals contribute significantly to said third output signal throughout said intermediate current region, said scaling and outputting means comprising means (68) for comparing said first and second output signals after scaling. 10. The arrangement of Claim 9, wherein said firs sensing means is an iron-core transformer (32) and said second sensing means is an air-core transformer (42). 11. The arrangement of Claim 9 or 10, wherein said comparing means comprises means (68) for utilizing the highest of said scaled output signals at each point in time to provide said third output signal. 12. The arrangement of Claim 9, 10 or 11, wherein the filtered and scaled output signals from the first and second transformers are out of phase with each other.

Inventors: RUTA, JOSEPH W.
Applicants: S & C ELECTRIC COMPANY
Application year: 1986
Main classification: H02H3
IPC codes: H02H3