GB2080549A - Methods and apparata for measuring capacitances and locating breaks in cables and conductors - Google Patents

Abstract

In a method of measuring capacitance or capacitance difference, or of locating a break in a cable or conductor by such measurement, the two balancing resistors of a Murray bridge circuit are replaced by a changeover switch S1, S2 with a variable switching time ratio. The bridge is balanced by altering the switching time ratio, and the switching time ratio is measured and displayed when the bridge is balanced. The polarity of a constant charging current which charges the capacitances to be measured is periodically reversed by switches 53, 54. Minor modifications of the circuit permit measurement and display of capacitance differences as well as of capacitances, after the insertion of standard capacitances. Unnecessarily large measuring voltages are avoided by such a method. <IMAGE>

Description

SPECIFICATION Improvements in or relating to methods of and apparatuses for measuring capacitances and for locating breaks in cables and conductors The invention relates to a method of and apparatus for measuring capacitances, capacitance differences and for locating breaks in cables and conductors, by balancing of a Murray bridge circuit.

The method is intended mainly for locating breaks in cables and conductors and for measuring capacitance ratios. Minor modifications permit the measurement and display of capacitance differences and, after the insertion of standard capacitances, of capacitance values.

Murray's measuring bridge circuit for locating earth faults on cables and conductors is generally known. (pp. 22-24 "Fehlerortungen" by Dr.-lng. E.

Widl, published by Dr. Alfred Hüthig-Verlag, Heidelberg, 1962). In this system, a fault-free cable core and the end of the faulty cable core connected thereto and situated downstream of the earth fault forms one branch of a bridge while the section of the faulty core situated upstream of the earth fault forms the second bridge. The two other branches of the bridge are represented by a potentiometer system.

The measuring voltage source is connected between the tapping of the potentiometer and one earth connection. A null indicator, connected between the two accessible ends of the aforementioned cable cores, is provided for balancing the bridge by adjustment of the potentiometer. This known Murray bridge circuit can also be driven by a.c. and, given a choice of suitable null indicator, permits breaks in cables and conductors to be located and capacitance ratios to be measured.

This known bridge circuit suffers from a number of disadvantages, which render measurement more difficult and introduce complexities in the precise determination of the fault location. The bridge cicuit must be manually balanced, and the distance of the fault location must be calculated in accordance with the potentiometer setting. The limit of the system is defined directly by the quality of the potentiometer in use. Costs of a sufficiently accurate potentiometer are very high and the accuracy diminishes in the course of the service life. Furthermore, the faulty core and the fault-free core must be connected to the measuring system terminals associated with them.

Also, measurement is disturbed or rendered completely impossible if spurious voltages appear.

Prior art also discloses a method for measuring resistances, resistance differences and for the locating of faults on cables and conductors by the balancing of a Murray bridge circuit (German Auslegeschrift 29 49 467). In this known method, the two balancing resistors of the bridge circuit are replaced by a changeover switch with a variable switching time ratio At/(T-At). The bridge is balanced by varying the switching time ratio At/(T-At) and the switching time ratio AV(T-At) is measured and displayed when the bridge is balanced (in this expression, T refers to the duration of the measuring cycle and At is the time during which a connection is established which is associated with a specific switching position of the change-over switch).

This method is suitable only for measuring resistances, resistance differences and for locating faults in cables and conductors which are caused by an earth short-circuit of a cable core. The use of such a method for measuring capacitances, capacitance differences and for locating breaks in cables and conductors suffers from the disadvantage that constantly increasing d.c. voltages appear while the measurements are carried out on the capacitors or conductor dividers, and these voltages also appear on the switching contacts and represent a risk.

According to one aspect of the invention, there is provided a method of measuring capacitance or capacitance difference, comprising providing a Murray bridge circuit in which the two balancing resistors are replaced by a changeover switch with a variable switching time ratio, balancing the bridge by varying the switching time ratio, measuring and displaying the switching time ratio when the bridge is balanced, and reversing the polarity of a constant charging current Im which charges the capacitances which are to be measured after each plurality of measuring cycles T.

According to another aspect of the invention there is provided an apparatus for measuring capacitance or capacitance difference, comprising a pair of test terminals connected across a null indicator and via change-over switch means to a power supply, means for repeatedly changing over the switch means with an adjustable mark-space ratio, and means for periodically reversing the polarity of the power supply.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of a preferred apparatus; Figure 2 is a block circuit diagram of a preferred apparatus; Figure 3 shows graphs of voltage with respect to time at some important points in the apparatus of Figure 2; Figure 4 shows a graph of current and voltage with respect to time at measuring points Xa and Xb to a time scale smaller than that of Figure 3; and Figure 5 is a block circuit diagram of an apparatus constituting another embodiment of the invention.

Figure 1 shows a basic circuit diagram of a measuring system for illustrating a preferred method. Afaulty cable is assumed to have length L.

A fault-free core a is supplemented by a capacitance whose value is defined by the core length L and the capacitance loading c' of the core. At the distant end of the cable the said core a is connected to the faulty core b. The faulty interrupted core b is represented by the partial capacitances thereof upstream and downstream of the break. The distance from the measuring point to the fault is assumed to be Lx. The distance from the fault to the distant cable end is designated by Ly. The bridge circuit is balanced by means of a nul indicator N.

Instead of potentiometer, as used in the known Murray bridge for interconnecting the cables cores and forming two additional branches of the bridge, a changeover switch S1,S2 with a variable switching time ratio or mark/space ratio is used. The junction of the two switch contact S1 and S2 is connected via a current limiting resistor R3 and a changeover switch S3, S4 to two measuring voltage sources Um of opposite polarity, whose second poles are connected to earth.Thus, under specific conditions, for example a sufficiently high switching frequency fm or a correspondingly high inertia of the null indicator N, the bridge can be balanced to zero by changing the switching time ratio of the changeover switch S1 and S2, even if the bridge circuit were driven buy a constant d.c. current. However, during measurement, this would result in constantly rising voltages across the partial capacitors and these voltages would also appear at the switch contacts S1 and S2.

To avoid this effect, the polarity of the current Im which feeds the bridge is reversed after every few, for example wight, switching cycles of the changeover switch S1 and S2. This is achieved by means of the changeover switch S3 and S4 which alternately connects the two measuring voltage sources Um of opposite polarity to the measuring bridge.

In practice, the mechanical switches S1 and S2 indicated in Figure 1 as well as the switch contacts S3 and S4 for the two measuring voltage sources Um are replaced by controllable semiconductor elements, as indicated in Figure 2 and described subsequently. However, the effect on the measuring accuracy of any differences of forward resistances of the semiconductor elements used as switches can be reduced to almost any desired extent by the insertion of a sufficiently large current limiting element, represented in this case by a resistor R3, into the connection between the changeover switch esS1,S2andS3,S4.

Figure 2 is a block circuit diagram of a circuit for performing a preferred method. The element to be measured, already described in detail in Figure 1, is represented in simplified form and without reference numerals in the top part of the drawing. The null indicator N is provided with an amplifier so thatthe sensitivity ca be increased as desired to improve the measuring accuracy. An instrument rectifier (not shown) may be connected between the amplifier and the indicator if, as is common practice, a moving coil instrument is used as the null indicator N. The switches S1 adn S2 are constituted by controllable semiconductor elements T1 and T2. In Figure 2, these are shown as field effect transistors. However, other suitable transistors types or modern digital semiconductor elements can also be used.The measuring voltage source Um is again connected via a current limiting resistor R3 to the changeover switch, but can be omitted if a constant current source is used as measuring voltage source. The regular reversal of the measuring voltage source polarity is obtained by means of a frequency divider which is driven either by a timing pulse generatorTG or, as in example shown, by pulses delivered by an up/down counter Z to a sawtooth generator SG. The output of a comparator K supplies pulses whose mark-space ratio may be varied by a potentiometer P, which pulses are further processed and supplied as control voltages to the senniconductor elements T1 and T2.In particular, the inverting input of the comparator K-is connected to reeiSfde a d.c. voltage which can be adjusted by means of the potentiometer P and which is compared with the sawtooth voltage provided by the sawtooth generator SG.

Varying the d.c. voltage enables the mark-space or switching time ratio At/(T-At) to be varied within wide limits and the bridge can thus be balanced to zero. The starting signal for each sawtooth of the sawtooth generator SG is obtained from one output.' of the up/down counter Z. This in turn is driven by the clock pulse generatorTG whose clock frequency is a multiple, for example 20 000 times, of the desired measuring frequency fm. A pulse former derives a display instruction from the control voltage for the semi-conductor element T2 for transfer to the up/down counter Z, so that the output A thereof displays the number of pulses delivered by the clock TG during the closing time At of the element T2.It can be shown that this display is directly proportionalto the distance of the fault from the beginning of the cable. The maximum4ndication of the up/down counter Z can be set to any desired value by means of a coding switch at input E. If the known total length L of the cable and the measurement is entered in any desired units at the input E, it can be shown that the length Lx of the cab-is as far as the fault location appears at the out A when the bridge is balanced.

Figure 3 shows a number of voltage graphs with respect to time in schematic form for various points of the circuit of Figure 1 when the bridge is balanced.

The top part of the illustration shows a pulse of duration T, i.e. the duration of the measuring cycle.

Its pulse frequency fm is therefore 1 :T. According to Figure 3, the time is defined by the number of clock pulses shown therebelow and preselected by the input T. The third illustrated pulse series in which the duration of the individual pulses amounts to T:2 is derived from the two pulse series illustrated above.

The first-mentioned pulse series is used to change the counter Z from up to down counting. The resultant counter position is shown below. Starting pulses for the sawtooth generator SG are derived from the first pulse series. The sawtooth generators SG supplies the sawtooth wave to the non-invertin'q input of the comparator IC The d.c. voltage, also shown and obtained from the potentiometer P, is applied to the inverting input of the comparator. The pulses, which appear at the output of the comparator K when the two voltages under comparison are identical, are processed and serve as control vol tages for the semiconductor sw-rtch elements T1 and T2. The changes of the two control voltages with respect to time are shown beneath the sawtooth.

The display instruction, causing the output A to indicate the counter position reached at that moment of time, is again derived from the control voltage for the switch element T2.

Figure 4 shows the change with respect to time of the charging current Im supplied by the measuring voltage source Um, the change Cf contrnIvoltage for the switching transistors T1 and w below the latter the change of voltage at the measuring points b, a and of the voltage Uab at the input of the amplifier for the null indicator N when the bridge is not balanced, when the switching times of the switching transistors T1 and T2 happen to be identical, to a time scale which is smaller than that of Figure 3. In this example, the polarity of the charging current Im is reversed after every four measuring cycles T, in order to ensure that the illustration remains simple and neatly grouped.The aforementioned number is defined directly by the frequency divider ratio n of the frequency divider in Figure 2 between the up/down counter Z and the measuring voltage source Um. The switching voltage illustrated in the second row of this Figure for the switching transistors T1 and T2 indicates a condition in which the switching times of both transistors are of approximately equal magnitude, so that the switching time ratio At/(T-AT) amounts to approximately 1. Each charging current current surge results in a defined voltage rise Ub at the measuring connection b of Figure 2 to which the first part member of the broken core b and its small partial capacitance is connected.

A smaller voltage rise Ua, offset with respect to time, is obtained at the measuring connection a to which the fault-free core a and the remainder of the broken core dwith its larger partial capacitance is connected. The voltage difference Uab = Ub - Ua, shown below, is connected to the input of the amplifier for the null indicator N. By varying the switching time ratio of the changeover switch T1, T2 it is possible for the voltage rises at the measuring points a and b to be adjusted to identical values, i.e.

forthe measuring brdigeto be balanced. This illustration also shows the regular polarity reversal of the charging current Im. By these means it is possible for the voltages across the partial capacitances and across the switching transistors T1 and T2 to be confined to readily controllable limits.

Without such a step the voltages could rise to dangerously high values during prolonged measurement.

The circuit arrangement of Figure 2 still has some of the disadvantages of the known bridge circuit. In this arrangement, zero balancing must still be performed manually and the display of the output A is unequivocal only if the faulty core is connected to the measuring system terminal associated therewith. Furthermore, the measuring frequency fm depends on the value to which the input E is set.

Figure 5 shows in the form of a block circuit diagram an arrangement which is extended with respect to the circuit arrangement of Figure 2.

Identical components have the same reference symbols as those in Figure 2. The inverting input of the comparator K in this case can be connected by means of a switch S5 to the previously described potentiometer P. In the illustrated position of the switch S5, the inverting input is however connected to the output of an integrator I, the input of which receives the voltage applied to the null indicator N. It can be seen that the output voltage of the integrator assumes a constant value only if the null voltage is equal to zero, i.e. the bridge circuit is balanced. Such balancing therefore takes place automatically. This represents a substantial simplification of the measuring procedure.Isolation means may be introduced for safety reasons in the circuit of Figure 5 between the output of the rectifier and the input of the integrator, for example taking the form of a known opto-couplers. The null indicator N can also be well protected in this manner against spurious voltages. Furthermore, a logic circuit L is shown which compares the control voltage for the switching element T2 with signal of the counter Z, which signals indicate the counting mode of the counter Z as "forward" or "reverse". This indicates the connected cable core in which the measured fault location is situated. This method offers the advantage that when connecting the.cable cores it is no longer necessary to observe the designation of the connections but an unequivocal measured result is nevertheless obtained.In another embodiment, the clock frequency of, which drives the counter Z, is generated by a phase locked loop circuit pLL and is adjusted so that the measuring frequency fm, which starts the sawtooth generator SG, is made equal to the fixed frequency fl of the clock TO This arrangement offers the advantage that the switching time T of the changeover switch Ti, T2 can be kept constant and independent of the input E.

The described method and the illustrated circuit of Figure 5 offer the following possibilities: 1. Measuring the distance of the fault location in percent or tenths of a percent of the cable length or of the core capacitance if full decades such as 100, 1000 are set as input E.

2. Measurement of the fault location distance in length units if the length of the cable in metres, kilometres or some other units are entered at the input E.

3. Random connection of defective and healthy cores, since the display indicates which of the cores has the defect.

4. Manual null balancing of the bridge, if this is considered desirable.

5. Automatic null balancing of the system if the switch S5 is set in the indicator position.

In the measuring procedure described so far, the starting pulse for the sawtooth generator FG is provided by the counter Z as soon as this is in the zero position. By appropriate setting of the switch S6, it is also possible to trigger the starting pulse if the internal counter position of the counter Z corresponds to the predefined value of the input E.

This converts the circuit for measuring the fault location into a circuit for measuring a capacitance difference which in turn permits the following additional measurements: 6. Relative capacitance difference between the two cores in percent or tenths of a percent or other fractions of the total capacitance if the corresponding decade values are set at the input E.

7. Absolute capacitance difference between the two cores if the total capacitance is set at the input E.

8. Fault location measurement as indicated under 1 and 2 above as measured from the cable beginning but indicated from the cable end.

These measuring facilities can also be obtained with manual as well as automatic null balancing of the bridge system. Such methods and circuits can also be used for comparing an unknown capacitance with a standard capacitance. Absolute values of unknown capacitances can therefore also be defined. A direct display of absolute capacitance values can be obtained when using automatic balancing by setting the clock frequency fo, which drives the counter Z, to specific preselected values. If the counter over-run is then monitored and a changeov erswitching of the corresponding standard capacitances or timing frequency derived therefrom, it will provide automatic range selection for capacitance measurement.

New claims or amendments to claims filed on 23rd September 1 981 Superseded claims 1 New or amended claims: 1. A method of measuring capacitance or capacitance difference, comprising providing a Murray bridge circuit in which the two balancing resistors are replaced by a changeover switch with a variable switching time ratio At/(T-At), balancing the bridge by varying the switching time ratio, measuring and displaying the switching time ratio when the bridge is balanced, and reversing the polarity of a constant charging current which ch charges the capacitances which are to be measured after each plurality of measuring cycles, where T is the duration of each measuring cycle and At is the time during which the changeover switch is in one of its switching positions during each cycle.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   the bridge system. Such methods and circuits can also be used for comparing an unknown capacitance with a standard capacitance. Absolute values of unknown capacitances can therefore also be defined. A direct display of absolute capacitance values can be obtained when using automatic balancing by setting the clock frequency fo, which drives the counter Z, to specific preselected values. If the counter over-run is then monitored and a changeov erswitching of the corresponding standard capacitances or timing frequency derived therefrom, it will provide automatic range selection for capacitance measurement.

   New claims or amendments to claims filed on 23rd September 1 981 Superseded claims 1 New or amended claims:

   1. A method of measuring capacitance or capacitance difference, comprising providing a Murray bridge circuit in which the two balancing resistors are replaced by a changeover switch with a variable switching time ratio At/(T-At), balancing the bridge by varying the switching time ratio, measuring and displaying the switching time ratio when the bridge is balanced, and reversing the polarity of a constant charging current which ch charges the capacitances which are to be measured after each plurality of measuring cycles, where T is the duration of each measuring cycle and At is the time during which the changeover switch is in one of its switching positions during each cycle.