JPH0675090B2 - Cable insulation defect detection method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention shows extremely high resistance under a low DC measurement voltage, such as a cable having an insulation failure due to the generation of a water tree. The present invention relates to a cable insulation defect detecting method and device used when it is impossible to detect or the orientation error is large.

(B) Conventional technology In detecting an insulation defect of a cable showing high resistance, an inverted bridge is generally used because of its high sensitivity. However, when the resistance value exceeds 10 MΩ, the sensitivity becomes insufficient in the inverted bridge, which is the null method, and the potential difference comparison method, which is the displacement method, becomes effective.

FIG. 2 is a circuit configuration diagram showing the potential difference comparison method, in which 1
Is a shield of a poorly insulated cable (exemplified by a three-core cable), which contains three-phase high-voltage conductors 2R, 2S, and 2T. Of these conductors, 2R and 2S are sound conductors, and 2T is a poorly insulated conductor having a poor insulation point 3 at a position X% of the entire cable length (100% is the total length) from the measurement side end. The non-measurement side end S ₁₀₀ of the sound conductor 2S and the non-measurement side end of the shield 1 are used to form a loop with the shield 1 as the forward line and the sound conductor as the return line.
1 ₁₀₀ and are connected to each other, the measurement end side between the measuring side end S ₀ of the series-connected DC low voltage power supply 5 and the switch 6 and a healthy conductor 2S a measurement end 1 ₀ shielding 1 It is connected. The sound conductor 2R is used to guide the potential of the non-measurement side end 1 ₁₀₀ to the measurement side end, and the non-measurement side end R _{100 of the} sound conductor 2R is used.
Is connected to the non-measuring end 1 ₁₀₀ of the shield 1, the measurement end R ₀ is connected to the terminal L ₁₀₀ is one of the switch end of the measuring end changeover switch 7. Terminal L ₀ is the other switch end of the changeover switch 7 is connected to the measuring end 1 ₀ shielding 1, which led to the 1 ₀ potential. Between the fixed terminal of the changeover switch 7 and the terminal E, a minute voltage detecting means 8 and a stray current canceling device 9 are connected. In FIG. 2, since the stray current canceling device 9 is shown as a series type (voltage canceling system), the external resistance seen from the minute voltage detecting means 8 becomes extremely high, and the time response of the means 8 tends to deteriorate. A shunt resistor 10 is connected in parallel to prevent this. When the parallel type (current canceling method) is used as the stray current canceling device 9, the stray current canceling device 9 is provided in parallel with the minute current detecting means 8. In this case, the shunt resistance 10 is not necessary. The terminal E is generally connected to the measurement side end T ₀ of the poorly insulated conductor 2T without being grounded.

The reason why the terminal E is floated is that there is a finite but sometimes very low value of the shield insulation resistance 4 in the shield 1 and the ground to the extent that it can be said that the ground point of the entire measurement circuit is automatic. This is because it is determined by the position of the shield insulation resistance 4. Although the terminal E can be grounded if the value of the shielding insulation resistance 4 is infinity or is higher than the value of the insulation failure point 3 by two digits or more, this condition is not normally established, and if the terminal E is forcibly grounded, it is not preferable stray current. Flows into the shield 1 and causes an error.

The insulation failure point measuring method of the circuit of FIG. 2 is performed as follows. The stray current canceling device is operated while the switch 6 is open, and the instruction of the minute voltage detecting means 8 is set to zero. At this time, the measuring end changeover switch 7 may be at the changeover position L ₀ or the position L ₁₀₀ . Next, the switch 6 is turned on, and the DC low voltage power source 5
To flow a current through the loop circuit composed of the shield 1 and the sound conductor 2S. The reading | E ₀ | of the minute voltage detecting means 9 is obtained at the switching position L ₀ in the state where a stable instruction can be obtained, and the reading | E ₁₀₀ | when switching to the switching position L ₁₀₀ is obtained.
From this measurement result, the position where the insulation failure point 3 exists, that is, X% of the total length from the measurement side end, and (100-
X)% is Can be oriented as

(C) Problems to be Solved by the Present Invention The above-described conventional method and apparatus for detecting cable insulation defects have the following problems.

The measurement target is a poorly insulated cable caused by the generation of a water tree, and in many cases it is still in operation without electrical breakdown. Therefore, since the operation must be restarted after the operation of locating the insulation defect point is completed, it is absolutely not allowed to apply a high voltage to lower the resistance value (so-called firing operation) in order to facilitate the orientation. However, the insulation failure resistance value due to the water tree is extremely voltage-dependent, unlike the metal resistance and the carbon film resistance. That is, when the measurement voltage is increased, the resistance value decreases, and when the measurement voltage is decreased, the resistance value increases. For example, even a cable that has a value of about 10 MΩ measured at 1000 V meggers will increase to several hundred MΩ when measured at a voltage of about 10 V.

FIG. 3 shows an example of the change in insulation failure resistance value with respect to the DC measurement voltage. In this example, 14 MΩ was measured at 1000 V megger. Generally, the voltage value of the DC voltage power supply that can be handled in the orientation work is 50V or less. In Fig. 3, at 100V, it is already 75M.
At Ω and 10V, it has risen to over 400MΩ. Here, even if the measured power supply voltage is 6 V or less and the maximum measurement sensitivity is 100 MΩ, the actual measurement is impossible. As a measurement power source
Even if 100V is used (actually, the loop current becomes enormous in proportion to the voltage and becomes out of practicality), the following second problem occurs. The voltage measured by the minute voltage detecting means 8 is the voltage drop of the shield 1, that is, the length X and 100-X.
However, the reading of the minute voltage detecting means is not proportional to the length except when X is 50%, resulting in an error. In the example of FIG. 3, assuming that X is 20%, 20
Resistance value at V voltage is 245 MΩ, resistance value at 80 V is 87
Since it is MΩ, It will be greatly deviated from 20% of the true value in the direction of decreasing.

As described above, in addition to the problem that the insulation resistance value becomes too high at the practical DC measurement voltage and the orientation cannot be performed, there is a problem that the orientation error becomes large even if the orientation can be performed.

The object of the present invention is to show an extremely high resistance value under a low DC measurement voltage, which is accurate even when it is used when it is impossible to detect the defect point due to insufficient sensitivity and when the orientation error is large. It is an object of the present invention to provide a method and apparatus for detecting a cable insulation defect point that can perform various measurements.

(D) Means for Solving the Problem In the cable insulation defect point detection method of the present invention, the secondary output voltage between the insulation defective conductor and the shield remains in the low voltage range, and the primary winding Winding resistance is 10000 MΩ
By applying the output of the AC low-voltage transformer having the above value through a capacitor whose electrostatic capacitance is one digit or more larger than the electrostatic capacitance between the insulation faulty conductor and the shield, insulation faulty The insulation failure point is located by the minute voltage detecting means having the filter circuit while the resistance of the point is kept low.

Further, the cable insulation defect detecting device of the present invention is an AC low voltage transformer for applying a secondary side output voltage between the insulation defective conductor and the shield, and is inserted in series on the output side at that time. A secondary voltage of the AC low-voltage transformer, comprising: a capacitor; and a minute voltage detecting means having a filter circuit for measuring a potential difference between the measurement-side end and the non-measurement-side end of the shield and a poorly insulated conductor. Side output voltage stays in the low voltage range,
In addition, the insulation resistance of the primary winding has a value of 10,000 MΩ or more, and the capacitance of the capacitor has a value larger than the capacitance between the insulation faulty conductor and the shield by one digit or more. Therefore, the filter circuit has an attenuation characteristic in which the function of the minute voltage detecting means is not hindered by the application of the secondary side output voltage of the AC low voltage transformer.

(E) Embodiment FIG. 1 is a block diagram of a measuring circuit according to an embodiment of the present invention. In FIG. 1, the secondary side output of the AC transformer 13 is connected via the capacitor 14 between the switching terminal L ₀ and the terminal E of the measuring end changeover switch 7 shown in FIG. The primary side of the AC transformer 13 is connected to a normal commercial frequency low voltage power supply. Between the fixed contact of the measuring end changeover switch 7 and the terminal E, the minute voltage detecting means 8 is provided via a choke coil 11.
A stray current canceling device 9 and a shunt resistor 10 are connected, and a capacitor 12 is connected in parallel to the minute voltage detecting means 8. Other configurations are similar to those in the case of FIG. 2, and therefore, the same reference numerals as those in FIG.

Here, the reason why the AC voltage is applied by superimposing it on the DC voltage as the measurement power source is based on the following knowledge of the inventor.

The insulation resistance of a cable with poor insulation caused by a water tree, when measured at a constant DC voltage under a variable AC voltage, decreases significantly as the superimposed AC voltage rises. FIG. 4 shows an example of the change of the insulation resistance value with respect to the superimposed AC voltage. In this example, the insulation resistance is measured when the superimposed AC voltage is changed from 0V to 500V with the DC measurement voltage being 50V. 14000 when measured with a DC voltage of 50 V without AC voltage applied
The insulation resistance value, which was MΩ, is reduced to 400 MΩ under the condition of 500 V AC superposition.

Under constant AC voltage superposition, even if the DC voltage changes in the range of 0 to 100V, there is no great difference in the insulation resistance value. FIG. 5 shows an example of the insulation resistance value with respect to changes in the DC measurement voltage. According to FIG. 5, when the DC voltage is 60 V or less, the insulation resistance value does not increase even if the DC measurement voltage is decreased, and the insulation resistance value may be almost constant.

In the present invention for measuring by superimposing an AC voltage for the above-mentioned reason, it is a low DC measurement voltage that is practically easy to handle, and it is impossible to measure due to insufficient sensitivity because it originally shows an extremely high insulation resistance value. Even if it is possible, it is possible to perform the measurement without the problem of giving a large error to the orientation value due to the difference in the distribution value of the DC side voltage in the cable shielding length direction.

Said transformer end of the secondary side output of the 13 switching terminal L _0, that is, connected to the measuring end 1 ₀ shielding 1, the measurement end of the sound conductor 2R
It may be connected to R ₀ (that is, equivalent to the non-measurement side end 1 _{100 of} the shield 1) or to the measurement side end S ₀ of the sound conductor 2S. The point is that an appropriate AC voltage may be applied between the shield 1 and the poorly insulated conductor 2T. However, connecting to the fixed terminal of the measuring end changeover switch 7 is not preferable for stable measurement because the application of the AC voltage is interrupted during the changeover.

The capacitor 14 is for preventing the minute voltage detecting means 8 from being bypassed by the secondary winding resistance of the transformer 13 having a low DC resistance and losing its sensitivity. This capacitance must have a value that is at least an order of magnitude larger than the capacitance between the shield 1 and the poorly insulated conductor 2T so that the AC output voltage does not drop significantly. Capacitor 14 is terminal E
Side or the switching terminal L ₀ side may be connected.

If there is no filter circuit consisting of the choke coil 11 and the capacitor 12, most of the secondary side voltage of the transformer 13 is applied to the minute voltage detecting means 8, and the DC voltage measured by the minute voltage detecting means 8 is measured. Its function is lost due to the penetration of an alternating voltage that is too large compared to the voltage. Therefore, it is necessary to design the filter circuit so that only the minute AC voltage reduced to the extent that it does not affect the performance of the minute voltage detection means 8 is superposed and applied.

The value of the secondary output voltage of the AC transformer 13 is determined as follows. Referring to the example of FIG.
Under V superposition, it is measured as only 50 MΩ at 50 V DC. However, a high AC voltage of 1900V cannot be used to ensure the safety of the measurer. The maximum output voltage of the secondary side is 440V, which is the maximum voltage used in the factory premises, and the measurer can get the same potential as the measuring device by riding on the insulating sheet that mounts the entire device. It is important to prevent accidents. The required capacity of the AC transformer 13 is determined by the capacitance value of the cable to be measured. If the secondary side capacity is
When set to 25VA, the current capacity is 25/440 = 0.0568
(A) and 60Hz. It can charge a capacitance of 0.343μF under 440V. That is, a normal high voltage cable is about 1 km
A small and lightweight transformer with a length of about 25VA is sufficient, which is convenient for practical use. Insulation resistance performance between the primary and secondary sides of the AC transformer 13 is also important. Normally, the primary side low voltage system is grounded at one point, so the secondary side winding should include the primary side winding (including iron core and other metal parts, shield plate) in order to avoid creating an undesired stray current path in the measurement circuit. ), It is necessary to have a high insulation resistance value of at least 10,000 MΩ or more at the maximum operating temperature of the transformer.

Next, a method for detecting an insulation failure point using the measurement circuit shown in FIG. 1 will be described. First, the AC transformer 13 is operated with the switch 6 open. Then, most of the AC output current flows through the capacitance between the shield 1 and the poorly insulated conductor 2T. Although there is a capacitance between the sound conductor 2R or 2S and the shield 1, since they are all at the same potential, no current flows and the AC transformer 13 does not become a load. By the application of this AC voltage, the value of the insulation failure point is shifted to and held at a measurable value.
The position X% of the insulation failure point can be determined by exactly the same method as the conventional method described in FIG. In addition, the intrusion of the AC voltage that causes the minute voltage detection means 8 to malfunction is caused by the filter element 11,
It is prevented by the combination of 12, and therefore the minute voltage detecting means 8 reacts only to the minute DC component. Further, when the voltage proportional to X% and (100-X)% of the shielding length is measured by the minute voltage detecting means 8, the value of the insulation resistance 3 that enters in series is constant even if the voltage changes, so the orientation result There is no error in.

(F) Effect According to the present invention, it is possible to easily realize the insulation failure point detection of the water tree generation cable. Moreover, conventionally, there was a drawback that the orientation error was large when the DC voltage was forcibly increased to enable the measurement, but this invention eliminates the problem, and accurate measurement is possible. For this reason, it is possible to partially replace the cable only in the defective insulation part, and it is possible to search the cause of the defective insulation from the relationship between the defective insulation part and the installation environment, and to obtain suggestions for the method of laying a new cable, etc. Various advantages can be practically obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of a measuring circuit showing an embodiment of the present invention, FIG. 2 is a block diagram of a conventional measuring circuit, and FIG. Fig. 4 is a diagram illustrating the relationship between the superimposed AC voltage applied to the insulation-defective cable and the change in the insulation resistance, and Fig. 5 is a DC measurement voltage and insulation resistance under the application of a constant superimposed AC voltage. It is a figure which illustrates the relationship of the change of. 1 ... Shielding, 2R, 2S ... Sound conductor (high voltage conductor), 2T ... Insulation poor conductor (high voltage conductor), 3 ... Poor insulation point, 5 ... DC low voltage power supply, 6 ... Switch, 7 ... Measuring end changeover switch, 8 ... Micro voltage detecting means, 9 ... Stray current canceling device, 11 ... Choke coil, 13 ... AC transformer.

Claims (2)

[Claims] 1. A loop is constructed with a shield of a poorly insulated cable as a forward line and a sound conductor as a return line, and a DC low-voltage power supply is connected to the open end of the loop to energize the loop. In the method of detecting the insulation failure point by the ratio of the potential difference between the measurement side end and the insulation failure conductor, between the insulation failure conductor and the shield, the secondary output voltage remains in the low voltage range, and the primary A capacitor with an output of an AC low-voltage transformer that has a winding resistance of 10,000 MΩ or more for the side winding and whose capacitance is one digit or more larger than the capacitance between the poorly-insulated conductor and the shield. A method for detecting a cable insulation defect point characterized by locating the insulation defect point by a minute voltage detection means having a filter circuit in a state where the resistance of the insulation defect point is maintained at a low level by applying the voltage via the above. 2. A loop is constructed with a shield of a poorly insulated cable as a forward line and a sound conductor as a return line, and a DC low-voltage power source is connected to the open end of the loop to energize the loop, and the measurement side end and the non-side of the shield are connected. A device used in a method for detecting an insulation failure point by a ratio of a potential difference between a measurement side end and an insulation failure conductor, wherein a secondary side output voltage is applied between the insulation failure conductor and the shield. An AC low-voltage transformer, a capacitor inserted in series at the output side at that time, and a filter circuit for measuring the potential difference between the measuring side end and the non-measuring side end of the shield and the poorly insulated conductor were added. And a minute voltage detection means, the secondary output voltage of the AC low voltage transformer remains in the low voltage range, and the insulation resistance to the primary winding is 10000 MΩ
The capacitor has a value above, the capacitance of the capacitor has a value larger than the capacitance between the insulation faulty conductor and the shield by one digit or more, and the filter circuit has the AC low voltage. A cable insulation defect detecting device having a damping characteristic in which the function of the minute voltage detecting means is not hindered by the application of the secondary side output voltage of the transformer.