WO1980002748A1

WO1980002748A1 - Method and apparatus for measuring and displaying the distance to an occured short circuit or similar line fault on an alternating current powerline

Info

Publication number: WO1980002748A1
Application number: PCT/NO1980/000019
Authority: WO
Inventors: H Hoel
Original assignee: Aktuell As; H Hoel
Priority date: 1979-05-31
Filing date: 1980-05-30
Publication date: 1980-12-11
Also published as: NO144612C; NO144612B; SE426104B; SE8100748L; NO791815L; FI801723A; EP0029439A1

Abstract

This method and apparatus is primarily intended for use in combination with a distance relay adapted to actuate a line switch or breaker (B) when sensing a line fault on the line, the short time interval available within the tripping time of the line breaker before disconnecting the line being utilized for performing measurements to locate the occured fault. These measurements are performed by means of an impedance relay (XR, R) disposed to receive two measuring signals (E, dI/dt) derived from sensed line voltage and current, respectively, and to respond to these signals when their mutual proportion corresponds to a line impedance which is less than a limiting impedance of the relay. During the time interval available for measurements this limiting impedance is reset by time controlled switching means (K) to a different value for each half cycle or other suitable given time period subsequent to the detected short circuit. Each such different reset limiting impedance value corresponds to a predetermined portion of the line length, and the sequence of limiting impedances is set in accordance with a predetermined switching programme (Fig. 1), which may be optimised to obtain maximum information as to the line fault location during the time span available before the power line is disconnected. During said switching programme a detector unit (A1-A4, B1-B4, a, b, c, d) registers consequently whether the respective set limiting impedances allow or do not allow the impedance relay (XR, R) to respond to the received measuring signals. Based on the information thus obtained by the detector, the location of the line fault is displayed on a distance indicator.

Description

"Method and Apparatus for Measuring and Displaying the Distance to an Occured Short Circuit or similar Line Fault on an Alternating Current Power Line".

The present invention is related to method and apparatus for measuring and displaying the distance to an occured line fault on an AC power .line. The object of the invention is primarily to achieve a fast distance measurement which is automatically initiated by the short circuit itself and may be performed with reasonable accuracy in the course of a few cycles of the AC line current after the occurence of the short circuit.

The invention is based on principles which are known from socalled distance relays which in practice are used for monitoring power lines and fast disconnection of such lines when a short circuit or similar line fault occurs. The power line monitoring performed by a distance relay consists in continuous sensing of the line impedance and disconnection of the line when the sensed impedance assumes a value within a predetermined impedance range, which may be set by appropriate adjustment of the relay itself. This is performed by selecting a suitable value for at least one limiting impedance of the relay, which then is tripped when the line impedance is less than said limiting impedance.

Any fast acting operative principle of a distance relay function may theoretically be used as a basis of the present invention, but relay functions satisfying the following three conditions are considered more suitable than others:

1) The tripping of the relay should be related to a fixed interval of the half cycles of the line current or voltage.

2) The limiting impedance of the relay should be easily resetable.

3) The resetable limiting impedance of the relay should exhibit a reactance characteristic.

The distance relay principle described in Norwegian Patent Specification No.- 126.104 and the corresponding British Patent Specification No. thus is well suited for use in connection with the present invention.

Thus, generally the present invention concerns a method for measuring and displaying the distance to an occurred short circuit or similar line fault on an alternating current power line by means of two measuring signals which are deduced from voltage and current, respectively, on the power line at a fixed time-interval within the half cycles of the alternating current, the measuring signals being supplied to an impedance relay which i operatively adapted to respond to the supplied measuring signals when their mutual proportion corresponds to a line impedance which is less than a characteristic limiting impedance of the relay.

On this background of general prior art the fundamental characteristic features of the method according to the invention are that the impedance relay in accordance with a predetermined programme and by means of time controlled switching means, for each half cycle or other given time interval subsequent to a detected short circuit is reset to a different limiting impedance corresponding to a predetermined portion of the total impedance of the line, and thus of the length of the line, and a detector unit is operative to register consecutively whether the respective set limiting impedance values allow or alternatively dot not allow the impedance relay to respond to the supplied measuring signals, and the information obtained as to the location of the short circuit by means of such registrations as to which limiting impedance values permit or alternatively do not permit the relay to respond, is utilized to display on a distance indicator in which specified range ofthe total length ofthe line the short circuit is located.

In order to determine the line section in which the short circuit is located with the highest possible accuracy, it is found favourable according to the invention to apply a predetermined programme including the following steps:

a) for the first half cycle or given time interval the limiting impedance is set to a first value corresponding to a first predetermined portion of the total length of the power line, and this limiting impedance value is maintained during the subsequent half cycles or given intervals in the case detector unit registers that said value does not allow the impedance relay to respond to the supplied measuring signals, but is deleted in the alternative case, b) for a second half cycle or given time interval a second limiting impedance value corresponding to a second predetermined portion of the total length of the power line, is set separately or in addition to the possibly maintained impedance value from the first half cycle or interval, this set impedance value being maintained during the subsequent half cycles or intervals in the case the detector unit registers that the total impedance present does not allow the impedance relay to respond to the supplied measuring . signals, but is deleted in the alternative case. c) for each subsequent half cycle or given time interval a further limiting impedance value having a predetermined relation to the previously set value for the immediately preceding half cycle or interval, is set separately or in addition to possibly mainted impedance values from preceding half cycles or intervals, said impedance being maintained during the subsequent half cycles or intervals in the case the detector unit registers that the total impedance present does not allow the impedance relay to respond to the supplied measuring signals, but is deleted in the alternative case.

In this programme, for the different half periods or given intervals mutually differing further limiting impedance values may be set in proportion as the set limiting value for the preceding half cycles or given interval allows or does not allow the relay to respond. Generally, however, the limiting impedance for the first half cycle or given time interval is set to a first value corresponding to about half the total length of the power line, and for each subsequent half cycle or interval is set to a further value corresponding to about one half of the valie set for the immediately preceding half cycle or interval, separately or in addition to possibly maintained values from earlier half cycles or intervals.

The invention also comprises an apparatus for measuring and displaying the distance to an occurred short circuit on an alternating current power line, the apparatur comprising means for deducing two measuring signals from voltage and current, respectively, on the power line at a fixed time interval within the half cycles of the alternating current, and an impedance relay disposed to receive the measuring signals and adapted to respond to these signals when their mutual proportion correspond to a line impedance which is less than a characteristic limiting impedance of the relay.

On this basis the general characteristic features of the apparatus according to the invention are that it further comprises time controlled switching means disposed and

O adapted in accordance with a predetermined programme initiated by the short circuit, for each half cycle or given time interval subsquent to the detected short circuit, to reset the impedance relay to a different limiting impedance corresponding to a predetermined portion of the total impedance of the line and thus also of its length; a detector unit arranged and designed to register consecutively whether the respective set limiting impedances allow or do not allow the impedance relay to respond to the supplied measuring signals and a distance indicator designed to display in which specified range of the total length of the line the detected short circuit is located, based on the information obtained as to the location of the short circuit by means of the detector unit, registering said limiting impedance values which allow or alternatively do not allow the relay to respond.

The apparatus according to the invention is preferably combined with a distance relay adapted to sensing the impedance of the power line and to actuate a line switch having a certain tripping time, to disconnect the line when the line impedance is within a predetermined impedance range which corresponds to short circuits or similar line faults, the apparatus having said means for deducing measuring signals and possibly the impedance relay in common with the distance relay.

In such a combination of distance measuring apparatus and distance relay the time controlled swithcing means should advantageously be designed to perform the settings of the limiting impedance of the impedance relay in accordance with the above programme a) through c) throughout the half cycles availbale within the tripping time of the line switch, preferably be having the further impedance value to be set for each half cycle.

In this way optimum utilization of the half cycles available between a detected short circuit and the disconnection of the line is achieved, for the purpose of effectively collecting information as to the location of the short circuit and suitably displaying such information by means of distance indicator of the apparatus. Also, a conventional distance relay needs only a small amount of auxilliariesto be able to measure automatically the distance to a short circuit and to display this distance with reasonable accuracy approximately simultaneously with the notification of the short circuit and the disconnection of the power line.

The invention will now be further illustrated by means of examples of embodiments and with referance to the accompanying drawings, in which;

Figure 1 shows a diagram illustrating the preferred swithcing programme for an apparatus according to the invention in combination with a distance relay,

Figure 2 shows such combination of distance relay and distance measuring apparatus according to the invention, and

Figure 3 shows a more detailed circuit diagram for a preferred embodiment of the distance measuring apparatus.

In order to illustrate how the distance to a short circuit on a power line is registered, four examples of short circuits on one and the same line will now be described with reference to Figure 1. In the rectilinear coordinate system shown in Figure 1 the distance along the line is indicated on the vertical axis in percents ofthe total line length, while the time from the short circuit instant is marked out in half cycles of the AC line current along-the horizontal axis of the diagram. The different set limiting impedance values of the impedance relay are indicated as horizontal line sections above the respective corresponding half cycles and designated with the perceήtual portion of the line length which corresponds to each such limiting impedance value. The distance measuring apparatus is in this case assumed to operate in combination with a distance relay.

Short Circuit Example 1

The location of the short circuit is assumed to corres¬ pond to 90% of the line length. Promptly upon the response of the distance relay on the short circuit, the distance measuring process in accordance with the previously indicated optimum programme is initiated. The limiting impedance of the impedance relay is first set to such a value that the relay is tripped within the first half cycle occuring after the detected short circuit, if the fault is located within the nearer 50% of the total line length. Consequently the impedance relay will not respond during this half cycle, and 50% line length will be recorded, e.g. by lighting a lamp and/or maintaining a relay contact closed.

As the impedance relay of the distance measuring ' apparatus did not respond to the supplied measuring signals during the first half cycle, the limiting impedance setting is increased with .25% for the second half cycle, and the response of the relay to the measuring signals is then tested with the increased limiting impedance now corresponding to 75% of the total line length. However, neither was the relay tripped during this half cycle, and the additional impedance of 25% is recorded in the same manner as the previous recording of 50% of the total line length during the first half cycle, e.g. by lighting a further lamp marked with "25".

OMPI For the third half cycle the limiting impedance setting was increased in accordance with the optimum programme with a value corresponding to a line portion of 12% (approximately 1/8 of the total line length), and this value is also recorded by lighting a lamp, as the impedance relay neither in this case did respond to the supplied measuring signals.

For the forth half cycle the value of the limiting impedance of the relay is increased with further 6% (approximately 1/16 of the line length), and the relay will then have a greater distance range than 90% of the total line length. The relay will consquently react on the supplied measuring signal with this setting of the limiting impedance, and the added impedance value of 6% is not recorded.

At the end of the fourth half cycle after the response of the distance relay to the detected short circuit, the tripping action of the relay effects the disconnection of the line and thus prevents a continuation of the measuring programme. In the course of the four half cycles which are available for the distance measurement, two limiting impedance values have however been detected, which respectively allows and does not allow the impedance relay to react and exhibit a mutual difference corresponding to 6% of the total line length. This represents the best tolerance range obtainable with a measuring programme of the present type within a time period of four half cycles of the AC line current. The sum total of the percentual indications of the lamps shining when the whole measuring process is completed, thus represents the lower boundary of the measuring tolerance range comprising the short circuit distance to be determined. With an addition of 3%, corresponding to half this tolerance range and indicated by a further lamp which always is lit upon a distance measurement, the following total distance value is obtained: 50% + 25% + 12% + 3% = 90%.

Short Circuit Example 2

The short circuit is in this case assumed to be located at a distance corresponding to 54% of the total line length. The measurement gives for the first half cycle the same result as in Example 1. Neither in this case the impedance relay reacts during the first half cycle, and the value 50% is registered by lighting a corresponding lamp on the distance indicator.

For the second half cycle the limiting impedance is supplemented by an additional value corresponding to 25% of the total line length, so that the impedance relay assumes a measuring range corresponding to 75% of the length of the power line. With this impedance value the relay responds to the supplied measuring signals and the additional value of 25% is thus not recorded, and the addition of 25% to the limiting impedance of the relay during this half cycle is not maintained.

For the third half cycle the previously- set value correspond to 50% of the total line length is increased with 12% to assume a total value of 62%, and even in this case the impedance relay reacts. Consequently, this addition of 12% is not recorded and the. set.impedance increase of 12% is not maintained.

For the fourth half cycle the prior impedance setting of 50% is increased by only 6%, but even this small increase does not prevent the relay from reacting, and thus neither the set value of 6% during this half cycle is maintained.

As in Example 1, the measuring programme is terminated after the fourth half cycle, and thus the displayed distance on the distance indicator is in this case: Set value 50% + fixed value 3% = 53%.

Short Circuit Example 3

The short circuit is in this case assumed to occur at a distance corresponding to 35% of the total line length.

For the first half cycle subsequent to the occurred circuit the impedance relay is as usual set to a response range corresponding to 50% of the total line length. Thus, the relay will in this case react during the first half cycle. Consequently, the line length portion of 50% will not be recorded and the corresponding set limiting impedance value will not be maintained for the subsequent half cycles.

For the second half cycle the limiting impedance is set in accordance with the optimum measuring programme to a value corresponding to 25% of the total line length. The∑Jiort circuit is then beyond the response range of the relay, and the set value of 25% will be registered by lighting the corresponding lamp on the distance indicator.

For the third half cycle the maintained 25% is increased with 12% to assume a total value corresponding to 37% of the length of the power line. The short circuit is now located barely within the reaction range of the relay and thus the impedance relay will react. The addition of 12% is consequently neither recorded nor maintained during the subsequent half cycles.

For the fourth half cycle the maintained limiting relay impedance of 25% is increased with 6% to a value corresponding to a line length portion of 31%. The location of the short circuit is now outside the range og the relay, and thus the impedance addition of 6% is

^Q duly registered. Consequently, the indication on the distance indicator is:

'25% + 6% + 3% = 34%.

Short Circuit Example 4

The short circuit is now assumed to be located at a distance corresponding to 13% of the total length of the power line.

With the set distance range of 50% for the first half cycle, the fault is within the reaction range of the relay, and consequently the set value of 50% is not recorded and is deleted for the subsequent half cycles.

For the second half cycle a limiting impedance value corresponding to 25% of the power line length is set according to the programme. The fault is still located within the reaction range of the relay, and thus this last set value of 25% is neither recorded nor maintained during the subsequent part of the measuring programme.

For thethird half cycle the setting of the impedance relay corresponds to 12% of the total line length. The fault is now located outside the response range of the relay, so that the impedance relay does not respond to the supplied measuring signals and the set value of 12% is duly recorded.

For the fourth half cycle the maintained limiting impedance value of 12% is increased with further 6% to a total value of 18%, which means that the impedance relay now will react and the last set value of 6% is not recorded.

Thus, at the termination of the measuring programme the distance indicator displays:

f ^'OMPI 12% + 3% = 15%

The measuring programme which is used in the above examples allows maximum information as to the short circuit location to be obtained during the four half cycles which are at the disposal of the distance measuring before the power* line is disconnected by the distance relay.

In the following table the obtained measuring results within the various percentual ranges of the line length are indicated, and the excited display units of the distance indicator in the various cases are specified.

Range of short Measured circuit location value (% of total

O cont. 81 - 87 84

87 - 93 90

93 - 100 96

According to this table the maximum theoretical deviation of the measured value from the actual short circuit distance is - 3,5% of the total line length. Possible additional system errors may be of the same order of magnitude.

Other measuring programmes may of course be used for performing the method according to the invention and the number of measurements may thus be varied as desired and in accordance with the prevailing operative conditions.

Figure 2 shows a block diagram of a distance relay connected with a distance measuring apparatus M according to the invention. The shown distance relay is of the type described in Norwegian Patent Specification No. 126.104 and the corresponding British Patent Specification No. and the operative function of which is based on a comparison of the line voltage with the derivative of the line current at the zero crossing points. It is also shown that the distance relay is provided with a switching arrangement adapted to supply its impedance relay XR with appropriate measuring signals representing the line voltage E and the zero crossing time derivative dl/dt of the line current, respectively, both with short circuits between different line phases and between the line phases and earth. Further, the distance relay is furnished with a tripping device U to be actuated by the impedance relay XR when said relay detects a short circuit on the power line, said device activating inturn the line switch B to

OMPI_ ,, WIPO ^ disconnect the power line. Distance relays of the present type are described in detail in the patent specifications indicated above, and thus will not be further discussed here.

The two measuring signals which represent the line voltage E and the zero crossing time derivative dl/dt of the line current, respectively, are supplied to the distance measuring apparatus M through a phase selector F. The tripping signal from the impedance relay XR of the distance relay arrangement is also transferred through the phase selector F to the distance measuring apparatus M in order to activate the same upon a detected short circuit on the line. The phase selector F is operative to sequentially connecting the power line phases to the distance measuring apparatus. In the block, diagram of Fig. 2 the distance measuring apparatus is shown in combination with a distance indicator having five display units, e.g. in the form of signal lamps, which during a distance measuring process in accordance with the previously described optimum measuring programme are lit in a combination which indicates the distance to the detected short circuit. Further details of the distance measuring and displaying apparatus are shown in Fig. 3.

In Fig. 3 it is illustrated that the distance measuring apparatus according to the invention comprises time controlled switching means K in the form of an automatic start and timing selector. This switching means is shown as a block at the upper left hand corner of Fig. 3, in a generally known embodiment which is commercially available for switching applications of the present type. The switching means is supplied with the tripping signal from the distance relay in the form of a positive voltage, which by means of a self-locking relay is fed to a common lead on the output side of

"£,\_. r. said means. Furthermore, on this output side there are four other output leads, which each receives said positive voltage during an associated AC half cycle subsequent to the tripping action, as indicated in the

Figure.

The measuring signals from the distance relay are^' supplied to the impedance relay of the distance measuring apparatus whown at the lower left hand corner of Fig. 3. As the supplied signals represent the line current E and the zero crossing time derivative dl/dt of the line current, this relay is suitably designed as a reactance relay R adapted to sense the reactance of the power line at each zero crossing of the alternating current. This reactance relay reacts on the supplied measuring signals if- their mutual proportion corresponds to a reactance which is smaller than the set limiting reactance of the relay. This limiting reactance may be set by means of the series connected resistors shown on the right hand side of the reactance relay in Fig. 3. These resistors are connected in series with the supplied measuring signal which represents the line voltage, and the response range of the relay is proportional to the combined resistance value of the resistors included in said series connection. All available resistors (800 KΛ ) connected in series with the line voltage signal give a response range of 100%. As shown in the Figure each resistor may be short-circuited by means of shunted relay contact b, which is controlled by the automatic timing selector K. A reactance relay R of the present type is known per se and further described in the above Norwegian Patent Specification No. 126.104 and the corresponding British Patent No. This relay is so designed that it has a very fast return after tripping, and thus reliably will go back to its initial state in the course of the half cycle at which

O I » the relay possibly responds to the supplied measuring signals, as- the signal representing the time derivative of the line current in fact is a very short pulse (about 1 ms).

The automatic switching means shown between the two blocks K and R in Fig. 3 utilizes reed relays as switching components in order to achieve fast tripping and possible return.

The switching means comprises four identical circuits, each provided with two reed-relays A and B, and each individually associated with one of the outputs 1-4 of the timing selector K and one of the series resistors of the reactance relay. Each circuit is active only during its associated half cycle, but all circuits function identically and thus this function will only be described in detail with reference to the first circuit. When the positive voltage is fed to output 1 of the timing selector, the relay Al operates and is maintained in operated condition throughout the first half cycle. Thereby the 400 K _Λ series resistor is short-circuited by means of the relay contact b and thus the reactance relay is set to a limiting reactance corresponding to 50% of the total length of the power line. If the short circuit distance now is less than 50% of the total length of the lpower line, the reactance relay R will react on the supplied measuring signals and close a current loop through the relay Bl, which responds by locking both itself and also the relay Al through operating the relay contact c.

The lamp designated "50%" in the Figure, which was lit together with the other display lamps when the measur¬ ing apparatus was activated by feeding the positive voltage to the common lead at the top of the diagram, is now extinguished. If the reactance relay R had not reacted on the supplied measuring signals with the said set limiting reactance, the relay Bl had not been operated and the lit state of the lamp 50% had been maintained, whereas the relay Al had been de-energised in order to reopen the contacts a and b, and to reinsert-the resistor of 400 K _ - in the series connection.

For the next half cycle the relay A2 is operated and the series resistor of 200 K-H. is short-circuited. In proportion as the reactance relay reacts or does not react with the set limiting reactance during the second half cycle, the relay B2 is operated or remains unactuated. As for the first half cycle, this results alternatively in the extinguishing of the lamp 25% and the maintenance of the short-circuiting of the resistor, or in_^ the continuation of the lit state of the lamp and cancelling the short-circuiting of 200 K_T resistor at the end of the second half, cycle. In the same manner the measuring apparatus continues to operate during the following half cycles in proportion as the reactance relay is actuated or not actuated by the supplied measuring signals in the course of each of these half cycles.

In the Figures 2 and 3 it is shown and assumed that the measuring apparatus M is furnished with an integrated impedance relay in the form of the reactance relay R, but the impedance relay XR of the distance relay arrangement may also be used for carrying out the measuring programme of the apparatus according to the invention, subsequent to the transfer of the tripping signal from this relay XR to the tripping device U for actuating the line switch B.

Claims

1. Method for measuring and displaying the distance to an occurred short circuit or similar line fault on an alternating current power line by means of two signals which are deduced from voltage and current respectively, on the power line at a fixed time-interval within the half cycles of the alternating current, the measuring signals being supplied to an impedance relay which is operatively adapted to respond to the supplied measuring signals when their mutual proportion corresponds to a line impedance which is less than a characteristic limiting impedance of the relay, c h a r a c t e r i z e d in that the impedance relay in accordance with a predetermined programme and by means of time controlled switching means for each half cycle or other given time interval subsequent to a detected short circuit is reset to a different limiting impedance corresponding to a predetermined portion of the total impedance of the line, and thus of the length of the line, and a detector unit is operative to register consequently whether the respective set limiting impedance values allow or alternatively do not allow the impedance relay to respond to the supplied measuring signals, and the information obtained as to the location of the short circuit by means of such registrations as to which limiting impedance values permit or alternatively do not permit the really to respond, is utilized to display on a distance indicator in which specified range of the total length of the line the short circuit is located.

2. Method as claimed in claim 1, c h a r a c t e r i z e d in that the limiting impedance of the impedance relay is set and reset by said time controlled switching means in accordance with the following predetermined programme:

O P a) for the first half cycle or given time interval the limiting impedance is. set to a first value corresponding to a first predetermined portion of the total length of the power line, and this limiting impedance value is maintained during the subsequent half cycles or given intervals in the case the detector unit registers that said value does not allow the impedance relay to respond to the supplied measuring signals, but is deleted in the alternative case,

b) for a second half cycle or given time interval a second limiting impedance value corresponding to a second predetermined portion of the total length of the power line, is set separately or in addition to the possibly maintained impedance value from the first half cycle or interval, this set impedance value being maintained during the subsequent half cycles or intervals in the case the detector unit registers that the total impedance present does not allow the impedance relay to respond to the supplied measuring signals, but is deleted in the alternative case,

c) for each subsequent half cycle or given time interval a further limiting impedance value having a predetermined relation to the previously set value for the immediately preceding half cycle or interval, isset separately or in addition to possibly maintained impedance values from preceding half cycles or intervals, said impedance being maintained during the subsequent half cycles or intervals in the case the detector unit registers that the total impedance present does not allow the impedance relay to respond to the supplied measuring signals, but is deleted in the alternative case.

3. Method as claimed in claim 2, c h a r a c t e r i z e d in that for at least one half

OMPI cycle or given time interval mutually differing further limiting impedance values are set in proportion as the set limiting impedance value for the preceding half cycle or given interval allows or does not allow the relay to respond.

4. Method as claimed in claim 2, c h a r a c t e r i z e d in that the limiting impedance for the first half cycle or given time interval is set to a first value corresponding to about half the total length of the power line, and for each subsequent half cycle or interval is set to a further value corresponding to about one half of the value set for the immediately preceding half cycle or interval, separatel -or in addition to possibly maintained values from earlier half cycles or intervals.

5. Method as claimed in claims 2 - 4, c h a r a c t e r i z e d in that the set limiting impedance values which are maintained beyond the respective half cycles or given intervals for which they are set, are displayed on the distance indicator by indicating the corresponding portionsof the power line length by means of individually associated indicator units whereas those limiting impedance values which are deleted after being set, are not indicated, and said indicated line length portions are summed to indicate the distance to the lower boundary of the line section in which the short circuit is located.

6. Method as claimed in claim 5, c h a r a c t e r i z e d in that a fixed value corresponding to half the line section in which the short circuit is located, is included in the sum of the indicated line length portions for indicating the distance to the center of said section.

O

Λ WI

7. Apparatus for measuring and displaying the distance to an occurred short circuit or similar line fault on an alternating current power line, the apparatus (M) comprising means for deducing two measuring signals (E, dl/dt) from voltage and current, respectively, on the power line at a fixed time interval within the half cycles of the alternating current, and an impedance relay (R) disposed to receive the measuring signals and adapted to respond to these signals when their mutual proportion corresponds to a line impedance which is less than a characteristic limiting impedance of the relay, c h a r a c t e r i z e d in that the apparatus further comprises time controlled switching means ( ) disposed and adapted, in accordance with a predetermined programme (Fig. 1) initiated by the short circuit, for each half cycle or given time interval subsequent to the detected short circuit, to reset the impedance relay^* (R) to a different limiting impedance corresponding to a predetermined portion of the total impedance of -the line and thus also of its length, a detector unit (A1-A4,. B1-B4, a,b,c,d) arranged and designed to register consecutively whether the respective set limiting impedances allow or do not allow the impedance relay (R) to respond to the supplied measuring signals (E, dl/dt) and a distance indicator designed to display in which specified range of the total length of the line the detected short circuit is located, based on the information obtained as to the location of the short circuit by means of the detector unit, registering said limiting impedance values which allow or alternatively do not allow the relay to respond.

8. Apparatus as claimed in claim 7 and combined with a distance relay adapted to sensing the impedance of the power line and to actuate a line switch (B) having a certain tripping time, to disconnect the line when the line impedance is within a predetermined impedance range which corresponds to short circuits or 'similar line faults, the apparatus (M) having said means for deducing measuring signals and possibly the impedance relay XR) in common with the distance relay, c h a r a c t e r i z e d in that the time controlled switching means (K) is designed to perform the settings and resettings of the limiting impedance of the impedance relay (XR) ^'in accordance with the predetermined programme indicated in claims 2 through 4 during the half cycles which are available within the tripping time of the line switch.

9. Apparatus as claimed in claim 7 or 8, c h a r a c t e r i z e d in that the impedance relay comprises a number of resistors connected in series with the supplied measuring signal deduced from the line voltage, the switching means (K) being disposed and adapted for resetting the limiting impedance of the relay (R,XR) by means of successively short-circuiting preselected resistors each half cycle or given time interval subsequent to the detection of a short-circuit or similar line fault, in accordance with said predetermined programme.

10. Apparatus as claimed in claim 9, c h a r a c t e r i z e d in that each series connected resistor is combined with an individually associated display unit of the distance indicator and that the time controlled switching means (K) is disposed and adapted to leave each display unit in actuated displaying state after completed switching programme only in those cases when the short-circuiting of the associated resistor does not result in a response of the impedance relay (R,XR) on the supplied measuring signals.

O

11. Apparatus as claimed in claim 10, c h a r a c t e r i z e d in that the said display units are lamps and the switching means (K) is disposed and adapted, upon detection of a short-circuit, to light all lamps, and sebsequently during carrying out of the switching programme to extinguish each lamp only in those cases when the short-circuiting of the associated resistor results in a response of the impedance relay on the supplied measuring signals.

12. Apparatus as claimed in claim 7 - 11, c h a r a c t e r i z e d in that the impedance relay is a reactance relay (R) adapted to sense the reactance of the power line.