GB2046039A - Method and apparatus for detecting the direction of a fault on an electrical transmission line - Google Patents

Abstract

The detection of the direction of a fault on an electrical transmission line in the case of faults close to the measuring station is problematical because of the low measuring voltage, particularly for bipolar short-circuits, with which the measuring voltage UK appears as the difference between the phase voltages U'S, U'T which have a small difference in phase angle. Measuring errors and deviations in the measuring channels can then lead to extensive errors in the decision regarding direction. The invention solves this problem by introducing an additional phase angle alpha into the phase position of at least one of the phase voltages U''T used for the formation of the fault measuring voltage UK. <IMAGE>

Description

SPECIFICATION Method and apparatus for detecting the direction of a fault on an electrical transmission line The invention relates to a method of detecting the direction, relative to a reference location, of a fault on an electrical transmission line, and to an apparatus for carrying out such a method.

A new method of detecting the direction of a fault on an electrical transmission line is known, for example, from the AEG-Telefunken Zeitschrift, Vol. 78, 1 978, No. 5, pages 177-182.

According to this, the detection of the direction of the fault is based essentially on a limiting-value monitoring of the phase angle between the fault measuring voltage and fault measuring current: on the hypothesis of a direct use of the fault measuring current as a reference vector for the phase angle measurement of the voltage, asymmetrical limiting valuers are necessary for leading and lagging measurement voltage.

Therefore an additional phase rotation of the measuring current may be carried out for use as a reference vector.

In the case of the detection of the direction of a fault for bipolar short-circuits, the corresponding interlinked voltage must be used as fault measuring voltage. Above all in the case of short range faults, this measuring voltage has very small amplitudes and in some circumstances does not ~ permit satisfactory evaluation. In addition, in the case of a bipolar short-circuit, the affected phase voltages in the short-range fault region have approximately the same phase, so that the difference vector suffers from great uncertainty not only in its magnitude but also in its direction, particularly in view of the inevitable phase-angle errors in the two phase voltages (deviation of the measuring transformer and the like). Thus the decision regarding direction is uncertain and unusable even in the close-range fault region.

In accordance with this invention, there is provided a method of detecting the direction of a fault on a polyphase electrical transmission line, wherein a fault measuring voltage associated with the faulty phase is formed from at least one phase voltage and is used together with a fault measuring current associated with the faulty phase to produce a trip-signal indicating the direction of the fault with respect to a reference location on the line, and further wherein at least one faulty phase voltage used to form the fault measuring voltage is rotated into a modified phase position which is offset by an additional angle in the direction of the faultless normal phase position of this faulty phase voltage.

Also in accordance with this invention, there is provided apparatus for carrying out a method as claimed in claim 1, comprising means for deriving signals representing said at least one phase voltage and said fault measuring current, means for rotating said at least one phase voltage signal into said modified phase position offset in the direction of the faultless normal phase position of said phase voltage, means for forming a fault measuring voltage from said fault measuring current and the modified phase voltage signal, and means responsive to the fault measuring current to produce a trip signal indicating the direction of the fault. Useful results are produced, even in the close-range fault region for multi/polar shortcircuits.The additional phase-angle displacement introduced accordingly in at least one of the phase voltages used to form the fault measuring voltage leads to the fact that an adequate amplitude of this measuring voltage is always available even with phase voltages which are close together in phase, and so a satisfactory determination of the phase angle between this measuring voltage and the measuring current is possible.With reference to the change-over of the measuring voltage from one phase position into the opposite phase, there is a reversal of the phase position, but this is not noticeable in the case of short-range faults in front of and behind the measuring or reference station, because the phase-angle monitoring of the measuring voltage is related to the measuring current, which in turn reverses its sign during the change of the error position between "forwards" and "backwards" direction, while that of the measuring voltage remains unaltered.

Embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: FIGURE 1 is a vector diagram of measured current 1K and measured voltages UKg1 and UKg2 in the complex plane for a conventional detection of the direction of a fault relative to a reference location, on an electrical transmission line; FIGURE 2 is a vector diagram of a three-phase system with the associated phase and measured voltages for bipolar short-circuit with additional phase-angle displacement of one measuring voltage in accordance with the invention; FIGURES 3a and 3b are vector diagrams of the type of Figure 2, showing details only to illustrate various possibilities for the additional phase-angle displacement of phase voltages; and FIGURE 4 is a basic circuit diagram of an apparatus for the detection of the direction of a fault in accordance with the invention.

In Figure 1, the rotation of the fault measuring current 1K is illustrated to a modified phase position, shown by the vector IIKE for the purpose of detecting the direction of fault. Two limiting angles yg, disposed substantially symmetrically at both sides of 1,K' then result for the fault measuring voltage, the exceeding of which corresponds to a change-over between "forwards" and "backwards" direction of the fault with respect to the measuring or reference station.

The displacement indicated from 1K to 1,K has the advantage of making the monitoring of the limiting angle symmetrical, but fundamentally it is also possible to work with an unaltered current vector IK An inversion of the tripping region, which is often desired for reasons of simplification in circuitry, can be achieved by inversion of voltages and currents in accordance with the boundary lines g1 and g2 indicated as broken lines.

In Figure 2, the formation of a fault measuring voltage UK for a bipolar fault between phases Us and UT is indicated as the difference between the measured phase voltages U's and U'T reduced in their amplitude and now close to one another in their phase position. The conditions indicated do not correspond to a short-range fault, in which the vectors U's and U'T approximate much more closely to one another and lead to a very small amplitude of UK. In accordance with the invention, therefore, the measured phase voltage U'T for example is transferred to a modified position U"T by rotation through an additional angle , towards its fault-free position.

In Figure 3a the conditions are represented with a greater approximation to reality for a bipolar short-range fault. The additional rotation with the angle a from U'T to U"T leads, with adequate dimensioning, with certainty out of the region of uncertainty ST indicated by hatching to the measurement of this phase voltage. Of course, the second phase voltage U's also suffers from such a region of uncertainty, which, with the position of the two vectors indicated, can lead to an incorrect decision regarding direction.As a result of the additional angular displacement, it can be seen that in any case an adequate amplitude results for the monitoring of the phase angle with regard to the measuring current and a clear direction of the fault measuring voltage UIK.

As can be seen from Figure 3a the additional rotation of the phase voltage is associated with a certain rotation of the measuring voltage UIK also, but it is a question of an error of a higher order which scarcely matters. It is advisable, however, particularly with a monitoring of the phase angle of the measuring voltage with respect to the non rotated measuring current, to effect the additional rotation at the phase voltage which is lagging in the phase sequence, because associated with this is a rotation of the measuring voltage in the sense of greater symmetry, but in any case not of an increased asymmetry, but in any case not of an increased asymmetry, in the limiting phase angle at both sides of 1K (see Figure 1).

As indicated in Figure 3b, both fault measuring voltages U's and U'T of a bipolar short-circuit may be provided with additional rotations into modified positions U"5 and U"T respectively. It is true that a somewhat greater expenditure is necessary for this, but a rotation of the measuring voltage UIIK is thus largely avoided.

The direction of the additional rotation must be selected in each case so that the faulty phase voltage is displaced out of its faulty phase position in the direction of its faultless normal phase position within the polyphase system. This can be seen from Figure 2 and Figures 3a and 3b.

In the circuit of Figure 4, a phase selection circuit PA is started by an exciting means AR when it responds to a detected fault, and phase selection signals s,, S5, ST, s, delivered by the phase selection circuit are used in a manner not illustrated for means forming measuring quantities of a distance protection or the like (see the lines leading upwards) but also, in the present case, for a control circuit MZ.This is a conventional control circuit having binary input signals from the phase selection circuit PA and analogue inputs for the phase connections R, S, T, 0, analogue inputs for the measuring currents IR 15, ITW IRSF ISTT ITR to be connected through according to the different cases of fault, and corresponding analogue outputs A, to A3. This control circuit has a logic function which, for the sake of simplicity, is shown in Figure 4 by a logic table with the input and output quantities already given.For the connecting through of the phase connections and measuring currents, this control circuit comprises, in conventional manner which will not be expiained, the necessary analogue switches which are controlled according to the logic functions shown in the logic table.

Connected to the outputs A, and A2 are two channels 1 and 2 for the measuring voltages, which lead to a differential amplifier VD.

Connected into one of the channels, and possibly into both channels, as indicated in chain line for channel 1, is a phase rotation means PD for the introduction of the additional angle a. The output of the differential amplifier VD and the output A3 associated with the relevant measuring current are taken through respective rectangular signal shapers RF to a comparator as shown and render possible, by means of a following timer ZG and a simple blocking AND-gate UG, the detection of an exceeding of the limiting phase angle vg determined by the time interval of timer ZG. In the event of such an exceeding, a flip-flop FF is set and a fault direction signal aR is produced which is associated with a predetermined direction of fault.

A reset input R for the flip-flop FF is provided for the resetting of the overall direction detection circuit FRD. Equivalent channels may be provided for detecting the opposite direction of fault.

As the logic table of MZ shows in its first three horizontal lines, the circuit is also suitable for detecting the direction of fault in the case of single-phase faults. In this case, on the voltage channels, one phase connection is connected through to zero (A2) while in the current measuring channel, one phase current is connected through.

In order to avoid a phase rotation in the case of a single-pole fault, channel 1 preferably does not include a phase rotating means, which corresponds to a mode of operation according to Figure 3a. A greater influencing of the decision regarding the direction of the single-pole fault is avoided as a result.

Claims (8)

1. A method of detecting the direction of a fault on a polyphase electrical transmission line, wherein a fault measuring voltage associated with the faulty phase is formed from at least one phase voltage and is used together with a fault measuring current associated with the faulty phase to produce a trip-signal indicating the direction of the fault with respect to a reference location on the line, and further wherein at least one faulty phase voltage used to form the fault measuring voltage is rotated into a modified phase position which is offset by an additional angle in the direction of the faultless normal phase position of this faulty phase voltage.

2. A method as claimed in claim 1, for a bipolar short-circuit, wherein, in order to form a fault measuring voltage as a difference between the two faulty phase voltages, phase voltages are turned through an additional angle in respective directions from their faulty to their faultless normal phase positions.

3. A method as claimed in claim 1, wherein, when the fault measuring voltage is formed from two faulty phase voltages, the phase voltage which is later in the phase sequence is rotated through said additional angle.

4. A method as claimed in any one of claims 1 to 3, wherein the additional angle is at least equal to the total error angle of voltage measuring means used to obtain the phase voltages.

5. A method as claimed in claim 1 and substantially as herein described.

6. Apparatus for carrying out a method as claimed in claim 1, comprising means for deriving signals representing said at least one phase voltage and said fault measuring current, means for rotating said at least one phase voltage signal into said modified phase position offset in the direction of the faultless normal phase position of said phase voltage, means for forming a fault measuring voltage from said fault measuring current and the modified phase voltage signal, and means responsive to the fault measuring current to produce a trip signal indicating the direction of the fault.

7. Apparatus as claimed in claim 6, comprising a phase selection circuit, a control circuit and a fault-direction detecting circuit, the control circuit comprising phase selection control inputs from the phase selection circuit, inputs for the phases to be monitored, measuring-current inputs from the polyphase system to be monitored, and control outputs for two affected phases (in the case of a bipolar fault), or one affected phase and neutral (in the case of a single pole fault), and for the associated fault measuring current, depending on the particular phase selection control inputs activated, and the fault direction detecting circuit comprising two control channels associated with respective faulty phase connections and connected to the inputs of a difference former, and a phase-rotating means for said additional phase angle is disposed in at least one of these control channels.

8. Apparatus substantially as herein described with reference to Figure 4 of the accompanying drawings.