Method and Apparatus for Monitoring Faults by Means of a Polygonal Trigger Region

U.S. patent application Ser. No. 953,637, filed Oct. 23, 1978, and U.S. patent application Ser. No. 953,788 filed Oct. 12, 1978, relate to similar subject matter by the same applicant herein.

The present invention relates to a new and improved method of, and apparatus for, monitoring faults at electrical lines, wherein a trigger or trip signal is formed for each line phase composed of line current and line voltage as a function of the distance between the measuring location and the fault location. Furthermore, the present invention relates to apparatus for the performance of such method employing measuring value-transducers or transformers for line current and line voltage for each phase to be monitored of the network system as well as a fault distance-measuring device for the distance-selective generation of a trigger or trip signal.

One proposal for distance-selective fault monitoring of the aforementioned character is known, for instance, from Brown Boveri Mitteilungen, 1966, Volume 53, pages 784-790, wherein electronic means detect relative phase angles between different voltage signals, partially derived by means of line simulation impedances, and such voltage signals are subjected to threshold monitoring. The trigger or trip curve--e.g. in the imaginary input impedance plane--in this case is circular. On the other hand, a relay trigger or trip curve which can be freely configured, according to the momentary protection requirements, cannot be readily realized, especially where there is used only one measuring system for each monitored phase.

Hence, it is a primary object of the present invention to provide a new and improved method of, and apparatus for, monitoring faults at electrical lines which is not associated with the aforementioned drawbacks and limitations of the prior art proposals.

Another and more specific object of the present invention aims at providing a new and improved distance-selective fault monitoring, whose trigger or trip curve in an imaginary impedance or voltage plane can be defined with comparatively simple means as a polygon having a randomly selectable arrangement of the sides of the polygon or vertices.

Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the method aspects of the present invention are generally manifested by the features that there are formed a multiplicity of fault detection signals or phasors, and the phase angle between timewise successive fault detection signals is subjected to threshold monitoring. According to a first aspect of the invention none of the detection signals has a magnitude equal to the line voltage signal i.e. none of the detection signals is equal to the inverted line voltage. As a further aspect one of the detection signals can comprise an external signal. A still further feature contemplates that one of the detection signals is a difference signal formed by the line voltage signal and an external signal.

Not only is the invention concerned with the aforementioned method aspects, but also relates to apparatus for the performance thereof. According to the invention there are provided a multiplicity of reference impedances connected with a line current-measuring value transducer or transformer. The voltage drop at a respective reference impedance together with the output of a line voltage-measuring transducer is connected with a difference former, and the output of such difference former is connected with a phase angle-monitoring device.

The contemplated reference impedances, which determine at least certain of the vertices of the trigger or trip curve, can be freely selected, thereby affording the strived for adaptation of the trigger or trip range. Also possibly desired, curved sections or portions of the trip region or curve can be approximated by means of a corresponding number of reference impedances. The impedance-reference voltages employed for the formation of the difference signals can be obtained quite simply by applying to the impedances a current signal corresponding to the line current, and equally there can be formed the difference signal required for the phase monitoring while employing a line approximation signal. The phase angle monitoring then delivers a simple criterion for the location of the fault within or without the boundary limits of the trigger or trip region in accordance with the position of the line voltage vector within or without, as the case may be, such trigger or trip region, because with a position within the trigger or trip region none of the relative phase angles reaches the boundary value of 180.degree., whereas externally of such regions always one of such angles is greater than 180.degree.. A threshold value comparison thus delivers an unambiguous, distance-selective trigger or trip signal.

The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 is a vector diagram of the impedance-reference voltages in the imaginary voltage plane U.sub.X -U.sub.R for a quadrilateral trigger or trip region having a vertex or corner point at the coordinate-null point;

FIG. 1a is a vector diagram of the difference signals with line voltage signal within the trigger or trip range according to FIG. 1;

FIG. 1b is a vector diagram according to FIG. 1a, however for a line voltage signal externally of the trigger or trip range;

FIG. 2 is a vector diagram showing the relationship during reversal of the current direction in the presence of a fault;

FIG. 3 is a vector diagram incorporating a replacement-difference signal derived from non-monitored phases;

FIG. 4 is a vector diagram according to FIG. 3, however for reversal of the current direction in the presence of a fault or short-circuit;

FIG. 5 is a vector diagram with an additional reference voltage derived from non-monitored phases for the formation of a related difference signal by means of the line-voltage signal;

FIG. 6 is a circuit diagram of fault distance-measuring equipment functioning according to the previously enumerated diagrams;

FIG. 7 is a circuit diagram of a phase angle-monitoring device with threshold monitoring for connection with the measuring apparatus shown in FIG. 6; and

FIG. 8 respectively illustrates signal diagrams as a function of time in FIG. 8(a), FIG. 8(b) and FIG. 8(c) for explaining the function of the circuitry of FIGS. 6 and 7.

Turning now to the drawings, there will be initially explained on the basis of the vector diagrams of FIGS. 1 to 5 the function of the fault monitoring. In FIG. 1 there is shown a quadrilateral trigger or trip region having the vertices or corner points 1, 2, 3 and 4. The vertices or corner points 2, 3 and 4 are defined by the impedance-reference voltages U.sub.2, U.sub.3 and U.sub.4. According to FIG. 1a there is then formed with the aid of the line voltage signal U.sub.K, obtained by the measurement as will be explained more fully later, the difference signals Ud.sub.2 =U.sub.2 -U.sub.K, Ud.sub.3 =U.sub.3 -U.sub.K and Ud.sub.4 =U.sub.4 -U.sub.K and additionally a reference signal Ud.sub.1 =-U.sub.K, which is incorporated into the monitoring of the relative phase angle between all of the vectors starting from the tip of the vector U.sub.K and thus determines the coordinate-null point in the form of the vertex or corner point 1. For the here assumed position of the vector tip of the vector U.sub.K within the quadrilateral trigger or trip range 1-2-3-4, FIG. 1a clearly shows that none of the relative phase angles between the detection signals or phasors i.e. between the successive difference signals Ud.sub.2, Ud.sub.3, Ud.sub.4 or between the additional reference signal Ud.sub.1 and the neighboring difference signals Ud.sub.2 and Ud.sub.4 has reached the threshold value 180.degree.. This is only first then the case with a position of the vector tip of the vector U.sub.K at the boundary of the trigger or trip region 1-2-3-4. On the other hand, with a position of the vector tip of the vector U.sub.K externally of the trigger or trip region, as shown in FIG. 1b, the relative phase angle between Ud.sub.3 and Ud.sub.4 is greater than 180.degree..

In the case of mutually coupled or intermeshed networks, for instance in the case of parallel lines between two busses or bus bars, it is possible, under circumstances, that in the case of a fault with reversal of the current direction i.e. reversal of the direction of flow of the energy or power at a measuring location, there will be simulated a fault location within the boundaries ascribed to the protective device of such measuring location. This is eliminated with the contemplated fault monitoring, as will be explained more fully hereinafter, by formation of the reference voltages determining the vertices as the line current-voltage drop at the reference impedances in a manner apparent from the showing of FIG. 2, because such reference voltages, during reversal of the current direction, shift their phase by 180.degree., and thus, define the now inversely arranged trigger or trip region 1-2-3-4, within which the vector U.sub.K which is not affected by the reversal of the current direction does not lie. For comparison purposes the trigger region of the original current direction has been shown in broken lines in FIG. 2.

In the case of a fault occurring close-up to the measuring location, the line voltage then collapses almost to null. This corresponds to an equally small value of the vector U.sub.K and renders uncertain triggering or tripping at the region of the coordinate-null point. To counteract this, there can be advantageously resorted to the embodiment of FIG. 3, where a line voltage signal U.sub.g, derived from another phase of a multi-phase line system or network or from a number of such other phases by suitable signal mixing or superimposing, is incorporated as a replacement-difference signal or replacement-reference signal--in comparison to FIG. 1a and FIG. 1b e.g. as the replacement for the signal Ud.sub.1 --directly into the phase angle monitoring. The trigger region 1-2-3-4 thus contains a threshold course which is adequately spaced from the coordinate-null point, because the other phases i.e., the phases differing from the momentarily monitored phase, only are affected in the case of multi-phase faults, and therefore, in many cases deliver a sufficiently high measuring voltage. As concerns multi-phase faults, especially three-phase faults, there can be utilized so-called "remember or sustaining circuits", which approximately continues the course of the line voltage prior to the occurrence of the fault for a certain time, and therefore delivers measuring voltages adequate for the formation of the signal U.sub.g. Also such fault monitoring is insensitive to faulty tripping or triggering operations due to reversal of the current direction, as illustrated in FIG. 4, owing to the transition of the trigger or trip region 1-2-3-4 to the trigger region 1-2'-3'-4' by virtue of the phase reversal of the corresponding impedance-reference voltages.

Another possibility for moving the trigger or trip boundary further away from the coordinate-null point contemplates deriving from the line voltage of at least one phase of the line system or network, and which phase differs from the monitored phase, and in a manner as will be explained more fully hereinafter, a voltage signal U.sub.g and to form therefrom together with the line voltage signal U.sub.K of the monitored phase a difference signal Ud.sub.1 and to incorporate such into the phase angle monitoring. The thus resultant configuration of the trigger or trip region has been shown in FIG. 5. Here again there is afforded the possibility of continuing the line voltage signal of the non-monitored phases for the formation of the signal U.sub.g in consideration of multi-phase faults.

The phase angle monitoring occurs advantageously with a threshold monitoring of at least approximately 180.degree.. In this way there is obtained an effective boundary of the trigger or trip region, defined by the lines extending between the vertices or corner points 1, 2, 3, 4. Basically, phase angle monitoring also can be carried out with threshold monitoring at threshold values deviating from 180.degree., especially at larger threshold values. In this way there are formed curved boundary line sections of the trigger region between the vertices, something useful for certain purposes.

Now the circuitry shown in FIGS. 6 and 7 for the performance of fault monitoring according to the aforementioned method aspects, will be seen to comprise a line voltage-measuring value transducer or transformer UMR which is connected with the monitored phase R of a three-phase line system or network R, S, T. The line voltage-measuring value transducer UMR comprises a voltage converter 50. There is also provided a line current-measuring value transducer or transformer IM, likewise connected to the phase R, and comprises a current converter, generally indicated by reference character 60, having three secondary windings 65, each of which applies to a related reference impedance Z.sub.2, Z.sub.3 and Z.sub.4, respectively, serving as the load a line current signal. Thus, there appear at these impedances Z.sub.2, Z.sub.3 and Z.sub.4 the previously discussed impedance-reference voltages U.sub.2, U.sub.3 and U.sub.4, respectively, whereas there directly appears at the secondary winding 55 of the line voltage-measuring value transducer UMR the line voltage signal U.sub.K. Differential amplifiers D.sub.2, D.sub.3 and D.sub.4 are connected in circuit with the line voltage-measuring value transducer UMR and also with the impedances Z.sub.2, Z.sub.3 and Z.sub.4, respectively, delivering at their outputs A.sub.2, A.sub.3 and A.sub.4 the aforementioned difference signals Ud.sub.2, Ud.sub.3 and Ud.sub.4, respectively. By means of an inverter IV.sub.1 and a reversing switch S.sub.1, which when located in a switched position opposite to that shown in FIG. 6, the line voltage-measuring value transducer UMR delivers at its output A.sub.1 the signal Ud.sub.1 =-U.sub.K i.e. the inverted line voltage signal. Thus there are initially prepared the signals required and which are of interest for the method explained in FIGS. 1, 1a and 1b.

For the modified method aspects of FIG. 3 and FIG. 5 the reversing switch S.sub.1 is shifted into the illustrated switch position. The line voltage-measuring value transducers UMS and UMT, which are connected to the non-monitored phases S and T, respectively, of FIG. 6, deliver to the potentiometers PS and PT adjustable voltage signal components U.sub.S and U.sub.T. These voltage signal components can be superimposed in a summing amplifier SV to form the voltage signal U.sub.g =k.sub.1 .multidot.U.sub.S +k.sub.2 .multidot.U.sub.T with the adjustable coefficients k.sub.1 and k.sub.2 which can be set at the potentiometers PS and PT. By means of a further differential amplifier D.sub.1, which with a further reversing switch S.sub.2 in the illustrated position, to thus deliver the signal U.sub.K to its subtraction or negative input, there is formed a difference signal Ud.sub.1 =U.sub.g -U.sub.K which appears at the output A.sub.1 for introduction into the phase angle monitoring, as explained with reference to FIG. 5. Upon reversing the position of the reversing switch S.sub.2 there is dispensed with the subtraction of U.sub.K, so that U.sub.g can be directly incorporated into the phase angle monitoring, as previously discussed in conjunction with FIG. 3. By means of a broken line indicated inverter IV.sub.2, shown as a modification, it is furthermore possible to form a voltage signal from the signal U.sub.g shifted by a phase shifter Ph through 90.degree. and corresponding to the coupled or linked voltage between the phases S and T. There can be provided resonant circuits SWS and SWT which use the converter-secondary windings as resonant inductances and, as illustrated, appear in the synchronized control connection with the relevant phase voltages (by means of the transformer or converter-primary windings), for sustaining the voltage signals in the case of multi-phase faults.

The phase angle monitoring circuit PU illustrated in FIG. 7 encompasses trigger circuits TR1, TR2, TR3 and TR4 connected with the amplifier outputs A1, A2, A3 and A4, respectively, to form square-wave pulses corresponding to the positive half-waves of the signals Ud.sub.1, Ud.sub.2, Ud.sub.3 and Ud.sub.4. All of the square-wave pulses are superimposed by means of an OR-gate GO having four inputs 70, and the output 75 of which only then carries the logic signal "O" when none of the square-wave pulses is present, i.e. when there occurs a gap in the superimposing of the total train of square-wave pulses. In this case a flip-flop FF.sub.1, which is reset to its starting state by means of an input RS, is switched such that a subsequently connected AND-gate GU.sub.2 is blocked, whereas when there does not occur any gap in the sequence of square-wave pulses such remains in a preparatory state.

At an input AR there is delivered an excitation signal which is produced in the usual fashion during fault detection, and which, as a function of the position of the vector tip of the vector U.sub.K within or without the trigger region, would be switched through to the trigger or trip output AS or be blocked. This selection is accomplished by means of an AND-gate GU.sub.1 which is prepared by the input AR by means of a timing element ZT having a slight time-delay. The simultaneously energized or driven monoflop MF having a reset time corresponding to a period of the network voltage, i.e., a period of the repetition frequency of the square-wave pulse train, is connected by means of a further flip-flop FS.sub.2 for initially blocking the AND-gate GU.sub.1 and switches such in accordance with the reset switching of the monoflop MF only then through to the trip output AS if up to the expiration of the reset or return switching time of the monoflop MF there has not occurred any gap in the superimposed train of square-wave pulses. Moreover, the dynamic input 80 of the flip-flop FF.sub.1, which is connected with the OR-gate GO, responds to the negative starting edge of such gap, in contrast to the shown inputs 90 and 95 of the monoflop MF and flip-flop FF.sub.2.

Now in FIG. 8(a) there is shown superimposing of the square-wave pulse sequence according to the positive half-waves of the detection signals Ud.sub.1 to Ud.sub.4 in the case of a phase angle of 180.degree. between the signals Ud.sub.4 and Ud.sub.3. This is the boundary case for the position of the vector tip of U.sub.K at the boundary line or extremity of the trigger or trip region. The graph of the showing of FIG. 8(b), on the other hand, illustrates superimposing of the square-wave pulse sequence for the case where the position of the signal U.sub.K is within the trigger or trip region, for instance according to the showing of FIG. 1a. Here there does not occur any gap in the superimposed pulse train, because none of the relative phase angles of neighboring signals or square-wave pulses Ud.sub.1 to Ud.sub.4 has reached 180.degree.. On the other hand, with superimposing of the signals of the square-wave pulse train according to the graph as shown in FIG. 8(c), corresponding to a position of the vector tip U.sub.K outside of the trigger or trip region, there occurs a gap, the negative starting edge of which blocks an excitation signal in the manner explained above in conjunction with the circuitry of FIG. 7, whereas such, in the case of the pulse graphs of FIGS. 8(a) and 8(b), after expiration of a time interval corresponding to a network voltage period, is switched-through for performing a triggering or tripping operation. In this way there is insured for the effectiveness of the phase angle monitoring with the circuitry PU shown in FIG. 7, and equally for the entire distance-selective fault monitoring. Basically, there also can be employed other types of known phase angle monitoring or phase angle detection.

Furthermore, it has been found that each phase of the line system is monitored by a measuring system which is uniform with regard to the temporal function course. In contrast to heretofore known equipment having an effective combination of a number of measuring systems which as a function of time operate in parallel and the fault selection first can be accomplished upon the presence of evaluation or result signals delivered by all of the measuring systems, the monitoring system of the present invention has the notable advantage of greater operational security. In the case of multiple systems with possibly differently rapidly operating partial measuring systems it can happen that in the interval between preparing the different partial results there occurs a change in the operating state of the line system, so that the finally present partial results, which are to be combined with one another, are predicated upon different conditions and under circumstances no longer are compatible with one another.

While there are shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims. Accordingly,