FR2485205A1 - METHOD FOR MEASURING CAPACITY AND FOR LOCATING CUT IN ELECTRIC CABLES OR PIPES - Google Patents

Abstract

A.PROCEDE FOR MEASURING CAPACITIES AND LOCATING BREAKS IN ELECTRICAL CABLES OR PIPING. B. PROCESS CHARACTERIZED IN THAT THE TWO BALANCING RESISTORS IN THE BRIDGE MOUNTING ARE REPLACED BY A SWITCH S1 S2 WITH A VARIABLE SWITCH-TIME RATIO D (T-DT). C. PROCEDURE APPLICABLE TO MEASUREMENTS ON CABLES IN SERVICE.

Description

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  The invention relates to a method for measuring capacitances, differences in capacitance, and for locating breaks in cables and lines, by balancing an arrangement

in Murray Bridge.

  The method is mainly used to locate cables and lines, as well as to measure capacitance ratios. By slightly modifying the assembly according to the method, it is also possible to measure and indicate differences in capacities, and after having introduced normal capacities, values

of capacity.

  It is generally known to use for local

  In the case of ground faulting of cables and lines, the so-called Murray bridge measuring arrangement (see "Fehlerortungen" by Dr. E. Widle, Dr. Alfred HUthig-Verlagp Heidelberg, 1962, pp. 22-24). . The intact strand of the cable and the end of the damaged strand which is located after the ground fault, and which is connected with it, form a branch of the bridge, while the part of the damaged strand which is before this defect forms the second branch of the bridge. The other two branches of the bridge are represented by a potentiometer, or a potentiometer-resistance device. The measurement voltage source is mounted between the potentiometer tap connection and a ground connection. A zero-setting device that is placed

  between the two accessible ends of the strands of the cable

  is used to balance the bridge by adjusting the potentiometer.

  This known Murrayp next bridge arrangement can also be supplied with alternating current, and then makes it possible, by suitably choosing the zero-setting device, to locate cable and line breaks or to measure transmission ratios.

capacity.

  This known bridge arrangement, however, has a series of drawbacks which make the measurement more difficult and

  make the precise determination of the cutoff point expensive.

  The bridge mounting must be balanced by hand, the distance from the cutoff point must be calculated from the potentiometer setting. The error limit of the device is determined directly by the quality of the potentiometer used. The cost of a sufficiently accurate potentiometer is very high and accuracy decreases over time. In addition, the faulty wire and the strand in good condition must be connected to the terminals of the device.

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  of measurement attributed to them. In addition, if parasitic voltages occur, the measurement is disturbed or

is made absolutely impossible.

  A method is also known for measuring resistances, differences in resistances, and for locating faults on cables and lines by balancing.

  a Murray bridge fitting (German document AS 29 39 467).

  In this known method, the two balancing resistors,

  provided in the bridge arrangement, are replaced by a

  (Si, S2) with a switching time ratio

  & yl (T - At) variable, the balancing of the bridge is effected

  relying on this ratio of the switching time At / (T - At), and the ratio of the switching time & t / (T - ht) is measured and indicated when the bridge is balanced. The sign T here designates the duration of the measuring cycle, and & t is the time during which the connection is established at this switching position in a

  determined switching position of the switch.

  This method is suitable only for measuring resistances, differences in resistances and for locating cable and line faults that have been caused by earthing of one of the strands of the target. Its use for measuring capabilities, differences in capabilities and for

  locate cable and line breaks has the disadvantage

  that during the measurement, it occurs on the condensa-

  conductors or parts of the conductors, always increasing DC voltages, which even appear on the switching contacts and which represent a danger to the accuracy

of the measure.

  The object of the invention is to improve the process described above.

  in such a way that uncontrolled measurement voltages

  can no longer occur and that, as a result,

  what errors they could produce.

  For this purpose, the invention proposes that both resist

  The compensation currents provided for in the bridge arrangement are replaced by a switch in which the switching-to-time ratio is variable, the balancing of the bridge being effected by modifying this commutation-time ratio, which is measured and indicated, when the bridge is balanced, and after a few periods of inverse measurement, the polarity of a charge current which is supplied to the capacities which must be determined,

  or a constant current supplying the bridge circuit.

  Further improvements of the embodiment of the invention reside in that: - as a switch, adjustable semiconductor components are used,

  - the influence of possible differences is diminished

  switch resistors of the switch components on the accuracy of the measurements, by introducing an element of

  limitation of the intensity on the conductor which connects the

  controller and the voltage source of measurement,

  - the commutation-time ratio has t / (T -Et) of the

  mutator is measured and displayed by a digital counter in advance and recoil controlled by a rate indicator, - the counter is programmed by the input of a

  measuring range of the cable length for the absolute emission

read the measurement value,

  the At / (T-At) ratio of the switch for the equi-

  libration of the bridge is adjusted by a comparator by comparison of a sawtooth voltage with an adjustable reference voltage, - the ratio A t / (T - At) of the switch is set,

  for the automatic balancing of the bridge, by means of a comparison

  by comparison of a sawtooth voltage with the

  voltage of the zero branch of the integrated bridge.

  The invention will be better understood with regard to the

  description hereafter and the attached drawings, representing

  embodiments of the invention, drawings in which - Figure 1 is a schematic diagram of the principle

of the process of the invention.

  FIG. 2 is a block diagram of an assembly

  intended for the practice of the invention.

  FIG. 3 represents the shape of the voltage in

  different important points of the assembly.-

  FIG. 4 represents the curve of the intensity and the voltage, in time, at the measurement points Xa and -Xb,

  at a reduced time scale compared to Figure 3.

  FIG. 5 is a block diagram of an assembly

  comprising alternative embodiments.

  Figure 1 shows the schematic diagram of a

  measuring instrument, with important details for the exhibition

  process. The faulty cable has a length 1.

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  The strand in good condition is supplemented by a capacity whose value comes from the length 1 of the strand and the capacity

  linear c 'of the strand. This strand is connected at the far end

  Target's ghost, with a rough edge b. The representation of strand b, cut, defective is completed by its partial capabilities before and after the cut. The distance from the measuring point to the cutoff is lx. The distance of the cut

  to the far end of the cable is designated ly.

  The bridge is balanced using the instrument at zero N. A potentiometer connecting the strands of the cable and forming two other branches of the measuring bridge according Murray known is

  replaced by the switches S1 and S2, the switching ratio

  time being variable. The junction of the two contacts Si and S2

  switches is connected via the resistor

  R3 intensity limiting, and switches S3 and S4, with the two voltage sources measuring 9, p8les

  opposite, whose second heads are grounded.

  It can be appreciated without a more precise explanation that under given conditions, for example at a sufficiently high frequency fm, or with an inertia of an appropriate importance of the instrument at zero N, it is possible to obtain an equilibrium. zero of the bridge circuit by changing the commutation-time ratio of the switch S1 and S2 or also by providing the bridge circuit with a constant DC current,

  It would occur here, however, during measurement, on

  partial densities, ever increasing voltages on the contacts of the switch S1 and S2. To avoid this phenomenon, the polarity of the current Im feeding the bridge is reversed, after, for example, eight switching cycles of the switch S1 and S2. This is done in this example by means of the switch S3 and S4, which connects, alternately, to the measuring bridge, the two

  Measuring voltage sources Um of opposite polarities.

  In practice, replace the mechanical switches S1 and S2 shown in Figure 1, as soon as the

  switch contacts S3 and S4, with both voltage sources

  measuring voltage Um, by means of semiconductor

  as shown in Figure 2, and as it will be

explained later.

  We can however realize here already that we

  may diminish to any desired extent in the conduct

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  between switches S1 and S2, as well as between S3 and S4 switches, the influence of the differences

  possible resistance of passage of the semi-

  conductors, used as switches, on the accuracy of the measurements, by interposing a sufficient intensity limiting element

  important, illustrated here in the form of a resistance

this R3.

  Figure 2 shows an illustration in the form of a block diagram of an assembly for application of the method. The object to be measured, already described in detail in relation to FIG. 1, is indicated in a simple way in the part

  top of the figure and without reference numerals. The instrument

  zero is equipped with an amplifier intended to increase

  its sensitivity and to improve the accuracy of

  measures. If one uses, as is common, as

  instrument at zero, a rotating coil instrument connected to

  tera, between the amplifier and the instrument, a measuring rectifier. The switches S1 and S2 have been replaced by

  adjustable semiconductor components T1 and T2, which are indi-

  2 in the form of field effect transistors. But other suitable types of transistors or modern digital semiconductor components can also be used. The measurement voltage source Um is again connected to the switch via an intensity limiting resistor R3 which can be suppressed if a measurement voltage source is used. a

constant current source.

  The regular polarity inversion of the measuring voltage source is ensured by a frequency divider, which is controlled either by the cadence indicator TG or, as in the example, by start pulses emitted by the generator. Z forward and backward on the sawtooth generator

  SG. The output of comparator K provides pulses which can

  can be modified by the conditions prevailing at its entry, which are sent, after appropriate preparation, in the form of control voltage to the semiconductor components

T1 and T2.

  The negative input of the comparator K is connected to a DC voltage, adjustable by the potentiometer P, which is compared with a sawtooth voltage coming from the generator

  of saw teeth SG. We realize that by changing the

  equilibrium, the commutation-time ratio can be varied to a large extent and thus ensure zero bridge equilibrium. The start signal for each sawtooth from the sawtooth generator SG comes from an output of a counter advance and recoil Z. The latter is in turn controlled by a rate indicator, whose frequency of cadence is a multiple, for example equal to 20,000 times, of the

  desired measurement frequency fm. From the tension of com-

  Mande T2 semiconductor component, it is given, by a pulse trainer, a display order to the counter in advance and recoil Z, so that its output A indicates the number of pulses provided by the indicator of TG rate during the closing time & t component T2. It can be shown that this indication is directly proportional to the distance between the cut-off point and the beginning of the cable. The maximum counting state of the counter can be set at any advance value with an encoder switch of the input E. If the total length L of the cable to the input E is entered at the input E measure, in any units, it can be shown that,

  if the bridge is balanced, it appears at exit A, the length

  lx of the measured cable to the location of the cut.

  In Figure 3 is shown schematically a series of temporary voltage curves, as they emerge at different points of the assembly of Figure 2 when the bridge is balanced. In the upper part of the figure is represented an impulse whose T weighting equals the duration of the measurement cycle. Its pulse frequency fm is therefore 1: T. Its duration appears, according to FIG. 3, from the number of pulses of the cadence indicator, represented below which is preselected by the input E. From these two series

  pulses, we deduce the third series of pulses repre-

  in which the duration of the different pulses amounts to T: 2. This series of pulses is used to

  invert the counter Z of the rising and falling digits.

  The counting state thus obtained is shown below. From the first series of pulses, the starting pulses of the saw-tooth generator SG are derived. This

  sawtooth generator SG provides the sawtooth illus-

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  the negative input of the comparator K arrives the DC voltage also illustrated, which is taken from the potentiometer P. The pulses, which occur at the output of the comparator K if the two compared voltages are equal, are reformatted prepared, and serve as control voltage for the elements

  semiconductor switching circuits Tl and T20 The chronological traces

  Both control voltages are shown below the saw teeth. From the control voltage of the switching member T2, again is derived the display order which allows the output A to display the metering status reached a

this moment.

  In FIG. 4, a chronological scale of the charging current Im supplied by the measurement voltage source Um, the control voltage curve of the transistors T1 and T2 is represented on a chronologically reduced scale with respect to FIG. switching point, and below, the voltage curve at the measurement points b, a, and the voltage Uab at the input of the instrument amplifier at zero N when the bridge is not balanced, for which for example the switching times of the transistors T1 and T2 are just equal. To maintain the simple and clearer illustration, it is assumed that in this example, the polarity of the charging current lm is reversed every four measurement periods T. This number directly emerges from the frequency division ratio n of the frequency divider of FIG. 2, between the counter Z and the measurement voltage source Um. The switching voltage illustrated in the second row of this figure for the switching transistors T1 and T2 shows a state in which the switching times

  switching of the two transistors are approximately equal, that is

  that is, the commutation-time ratio At / (T-4t) is approximately

ron equal to 1.

  At the measuring terminal B of FIG. 2 where the first portion of the cut strand b is applied, with its low partial capacity, it appears, with each load current shock, a determined voltage rise Ub. At the measuring terminal a, on which the intact strand a and the remainder of the cut strand b are applied with their partial capacity more impotent, it appears, with an offset in time, a small elevation

  of voltage Ua, the differential voltage Uab = Ub-Ua, illus-

  24852e5 tren below applies to the input of the amplifier of

  the instrument to zero N. By modifying the switching ratio

  time of the switch T1, T2, it is possible to bring to the same values the voltage increases at the measurement points a and b, the measurement bridge being thus balanced. In this illustration, the advantageous mode of action of the inversion of

  the polarity of the charging current Im. We thus arrive at

  within limits that one can well control the tensions

  to the partial capacitors and the T1 and T2 transistors of

  mutation. Without this precaution, if the measurement continues, the voltages could increase to high and dangerous levels.

  It can be seen that the following

  FIG. 2 still shows some defects in known bridge mounts. In this arrangement, the zero balance must still be manually operated, the display of the output A shows sufficient sharpness only if the damaged strand is attached to the terminal of the measuring device assigned to it. It also appears that the measurement frequency fm depends on the value adjusted to the input E. In FIG. 5 is shown in block diagram form a variant of the arrangement illustrated in FIG. 2. The negative input of FIG. Comparator K can here be connected by a switch S5 to the potentiometer P that has already been described. In the illustrated position of this switch S5,

  the negative input is, however, connected to the output of an integra-

  I, at the input of which is applied the voltage which appears on the instrument at zero N. It can be seen that the

  The output voltage of integrator I does not take a con-

  only if the zero voltage is equal to zero, that is to say

  if the bridge is balanced. This balancing is thus automatically

  and it results in an essential simplification of

the measurement operation.

  For safety reasons, in a circuit according to FIG. 5, between the output of the rectifier and the input of the integrator, a potential barrier is interposed, for example by means of one of the known opto-couplers. This way, you can also effectively protect the instrument from scratch against

  stray voltages. The figure also shows a logical assembly

  L, which compares the control voltage of the switching element

  T2 with the signals of the counter Zu which indicate the type of count "before" or "back" 'of this counter. It is indicated in this way that of the connected strands of the cable in which is the defective location which is the object of the measurement, This possibility has the advantage that, when connecting the strands of the cable, we no longer need to ensure the identification of the terminals and we are however in

  presence of a very clear measurement result.

  Another embodiment is that the cadence frequency fo controlling the counter Z is produced in a phase-locked PLL arrangement and is adjusted to such an extent.

  way that the measuring frequency fm which sets the gen-

  The sawtooth frequency SG is equal to the final frequency of the cadence indicator TG. This provision has the advantage that

  the switching time T of the switch TI, T2 can be now

  constant holding regardless of the entry E.-

  The described process and the assembly illustrated by the

  FIG. 5 for the execution of the method then offer the possibi-

  - Measuring the distance of the defective point in percent or per thousand of the length of the cable or the capacity of the strand, if we adjust as input E whole decades such

than 100, 1000.

  Measure the distance of the defective point in units of long

  if we introduce, at the entrance E, the length of the cable

  in meters, kilometers or any other unit.

  Easy connection of defective wires and intact, because

  it is indicated which of the strands has the defect.

  - Zero balancing by hand of the bridge fitting if it is

longed for.

  - Automatic zero balancing of the assembly, if the switch

  S5 is in the illustrated position.

  In the measurement process described so far, the impulse

  - starting voltage of the sawtooth generator SG is given

  counter Z, as soon as the latter is in the position

  zero. By setting up the switch SG, however, the start pulse can be triggered when the counting state of the counter Z corresponds to the prescribed value of the input E. By this means the measuring assembly of a point

  defective becomes a measuring device of difference in capacitance

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  cited which additionally allows the following measures: - Relative differences in capacity of the two strands in percent, per thousand, or other fractions of the total capacity, if

  the corresponding decimal values are adjusted to the input E.

  - Absolute difference in capacity between the two strands, if one

  adjusts to input E the total capacity.

  - Measurement of the defective location, as indicated under 1 and 2 measured from the beginning of the cable but displayed

  from the end of the cable.

  These measurement possibilities can be obtained with zero balancing of the bridge assembly, by hand, also

  although by automatic balancing. It will be easy to see

  the process and assembly can also be used

  to compare an unknown ability with a normal ability.

  It is therefore also possible to determine absolute values of unknown capacities. You can get a direct display of

  absolute capacity values using automatic balancing

  by adjusting the cadence frequency for which controls the counter Z to preselected determined values. If we take care here to exceed the capacity of the meter and if we derive from it a reversal of the corresponding normal capacities

  or the frequency of cadence, we arrive at a self-selection

  the field of capacity measurement.

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Claims (6)

R E V E N D I C A T I 0 N S   1) Process for the measurement of capacities or diffe-   capacitances, and for locating wire breaks and balancing lines of a Murray bridge arrangement, characterized in that the two balancing resistors in the bridge arrangement are replaced by a switch (S1, S2). with a variable switching-time ratio A t / (T-At),   the balancing of the bridge taking place by modifying the   tation-time, which is then measured and displayed, and after a number of measurement cycles <T) the polarity of a constant charging current (Im), which loads the capacitances to be determined, is reversed (T here being the duration of the measurement cycle, and tt the time during which is established, when the switch is in a determined switching position there link to this switching position) Process according to claim 1, characterized   in that, as a switch (Sie S2),   semiconductors (T1, T2) adjustable.   Process according to one of claims 1 and   2, characterized in that the influence of the different   these possible switching resistances of the components (T1, T2) of the switch on the accuracy of the measurements, by introducing an element (R3) of limitation of the intensity on the conductor which connects the switch (SI, S2) and the-source (A) of tension measurement.   4) Process according to any one of the claims   1 to 3, characterized in that the commutation-time ratio & t / (T-AEt) of the switch (S1, S2) is measured and displayed by a digital (Z) counter in advance and in reverse controlled by a cadence indicator (TG).    Method according to Claim 4, characterized in that the counter (Z) is programmed by the input (E) of a   measuring range or cable length (1) for the transmission   absolute value (A) of the measured value.   60) Process according to any one of the claims   1 to 5, characterized in that the ratio 4t / (T-At) of the switch (Sl, 52) for balancing the bridge is adjusted by a comparator (K) by comparison of a voltage in teeth of   saw with adjustable reference voltage.   70) Process according to claim 6, characterized 12 2485205   in that the ratio At / (T- t) of the switch (S1, S2) is set, for the automatic balancing of the bridge, by means of a comparator (K), by comparison of a sawtooth voltage   with the voltage of the zero branch of the integrated bridge.   8) Method according to claim 5, characterized in that the switching time (T) of the switch (Si, S2)   is kept constant, and the cadence frequency (fO),   the counter (Z) is changed according to the input   (E) measuring fields or cable lengths.   9) A method according to claim 8, characterized in that the cadence frequency (fO) is produced in a mounting (PLL) said "phaselocked-loop" at the first input of which is sent a frequency (fi) of the indicator constant rate, and at the second input, the measurement frequency (fm = 1: T).    Process according to any one of claims   1 to 9, characterized in that a logic circuit (L) is mounted between the output of the comparator (K) and the auxiliary terminals of the counter (Z) to measure the faulty location.   logic assembly whose outputs indicate the faulty wire.