JP2014006229A - Insulation defect part searching method for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a searching method for identifying a position of an insulation defect part of a power cable in a temporally and economically efficient manner.SOLUTION: A DC voltage 1 is applied between the ground and a conductor part 2 at one end of a power cable to which an insulation defect has occurred; a current 4 is caused to be leaked to the ground from the insulation defect part 3; and a current value on the cable is measured by a clamp type ammeter 5 while maintaining the situation. Since a current value of a part on the cable farther than a leakage point when seen from a voltage application side is not measured, a boundary between a part where the current flows and a part where the current does not flow is determined to identify the insulation defect part 3.

Description

  It relates to searching for poor insulation in power transmission cables.

  Although there are equipment for investigating defective insulation in overseas products, etc., the work efficiency is very poor and the reliability is vague. That is, when a high-voltage pulse is transmitted to the cable conductor, the receiver searches for a pulse leaking to the ground from the location of poor insulation. In this method, the total length of the cable must be searched sequentially. If the cable is long, 10-20km, all of which must be confirmed by the receiver. In this case, it takes an enormous amount of time to identify the insulation failure location, and this pulse signal method cannot be applied to a cable used for an overhead wire.

  There are other methods using high frequency. However, this method may cause erroneous measurement because current is also induced in a portion that is not energized by electromagnetic induction with other cables.

  As described above, there are many methods for investigating the insulation failure portion of the cable, but each has advantages and disadvantages. As a result of trial and error, a method using a DC voltage has been developed.

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There is no widespread method for reliably identifying an insulation failure location in any wiring method with the current technology for identifying an insulation failure location. Insulation surveys are mainly done with an insulation resistance meter, so measure the insulation value with an insulation resistance meter only between the part where the cable conductor is exposed (such as the connection to the switchboard) and the part where the next conductor is exposed. Since there is no method, it is unavoidable to identify an insulation failure location only at long intervals. At present, there is no other way but to replace the cable in units where the cable sheath is connected. For this reason, a cable in which an insulation failure has occurred cannot be repaired unless all healthy parts are also replaced. Power outage time is long and repair costs are expensive.
These problems can be solved if there is a method that can identify an insulation failure location even in a location where the cable is covered.

  A DC voltage is applied between the conductor of the cable whose insulation has been lowered and the ground, and current is leaked from the place where the insulation is lowered to the ground. In this state, the current value at each location on the cable is measured with a clamp-type ammeter. Since the current value is zero on the cable far from the location where the current leaks to the ground when viewed from the DC voltage application side, the insulation failure location can be found by specifying this boundary point.

According to the present invention, when an insulation failure location occurs in the power cable, the insulation failure location can be specified within a very short length, so that the length of the cable to be repaired and replaced can be shortened. As a result, the replacement time can be shortened and the power failure time can be shortened. Also, repair and replacement costs can be greatly reduced.
In addition to this, according to the present invention, since the insulation investigation can be performed while the insulation value can be transmitted, it is possible to perform a power failure in a time zone in which the power is not used, and to remove the defective insulation portion of the cable. According to the law, a leakage current of about 1/20 of the rated current is allowed. Only a pressure resistance test is shown at high voltage, but as a general guideline, it is possible to transmit power to an insulation value of several MΩ or more. According to the present invention, the DC voltage to be applied is increased within the allowable voltage of the cable, so that it is possible to investigate and specify even in the range of a considerably high insulation failure value. For example, when an insulation failure portion of 10 MΩ is generated, a current of about 2000 V ÷ 10 MΩ = 0.2 mA can be flowed by a DC voltage of 2000 V, so that it can be investigated. However, the clamp type ammeter to be used must have a high accuracy capable of measuring up to 0.1 mA. In any case, it can be used as preventive maintenance because the insulation failure value can be investigated in a considerably high state. If the insulation failure is left until the circuit is interrupted by the ground fault relay, it will be a power outage during operation and the loss will be immeasurable. Therefore, if the insulation starts to drop, it is possible to avoid a power failure by removing the defective insulation portion from the insulation value that can be transmitted.

  It is the figure which showed distribution of direct-current when a direct-current voltage is applied to a cable.   It is sectional drawing of a three-phase three-core cable. In particular, it is called a shielded cable, and each conductor has a metal shield. Furthermore, the metal shield is given to the three cores together. This three-core metal shield is grounded, and by quickly energizing a ground fault current due to a decrease in insulation due to deterioration due to water trees inside the cable insulator, the ground fault relay is operated and the circuit is shut off. We detect insulation deterioration early and try not to cause a short circuit accident.   The current distribution when a DC voltage is applied to a three-phase three-core cable with reduced insulation is shown.

The present invention can be implemented with a DC voltage generator and a clamp type DC ammeter capable of measuring up to DC 0.1 mA. The DC voltage generator can be a DC withstand voltage tester. A clamp type DC ammeter capable of measuring 0.1 mA is also available as an off-the-shelf product. However, since these off-the-shelf products are not used for the purpose of the present invention, excessive performance and conversely insufficient performance are mixed for the purpose of the present invention.
It is necessary to further lower the lower limit of the voltage as the performance that is insufficient in the DC withstand voltage tester, and the excessive performance does not require the function necessary for the DC withstand voltage test.
In addition, the clamp-type ammeter is only for indoor use, and requires durability that can be used in harsh environments where power cables are laid, and also requires a jig for fixing the clamp to the cable. Since the clamp diameter is 30 mm at the maximum, a clamp with a larger diameter is required.

FIG. 1 schematically illustrates the principle of the present invention. Reference numeral 1 denotes a DC voltage generator, which can be replaced by a normal DC withstand voltage tester. (However, since the DC withstand voltage tester has advanced functions necessary for the withstand voltage test, it is better to manufacture equipment excluding these at low cost.)
Reference numeral 2 is a conductor of the cable. When a DC voltage is applied thereto, a DC current indicated by a broken line 4 circulates from the location of defective insulation 3 through the ground. This direct current does not flow to the right from the leakage point code 3. Therefore, the current value is sequentially measured on the cable as in the clamp type ammeter code 5. It is possible to investigate efficiently by dividing and measuring 1/2, 1/4, 1/8,... Of the entire length. Finally, the boundary point between where there is current and where there is no current can be identified.

FIG. 2 is a cross-sectional view of a three-core power cable. As indicated by reference numeral 11, a deteriorated portion due to intrusion of water or the like from the outside occurs, and is enlarged due to electromagnetic influence. If it is connected between the conductor and the ground, a leakage current is generated. In the present invention, the location can be specified while the deterioration is small.

FIG. 3 is a connection diagram when investigating an insulation failure location according to the present invention in a three-core cable, and a voltage is applied in a three-core package. Power cables have a metal shield called a sheath, which is grounded and important to remove from the grounding pole before investigation.

  The owners of business electric works and private electric works usually change the distribution line over the entire length when the insulation of the distribution line is lowered, but sometimes it extends over several kilometers. This is very inefficient in terms of time and economics. According to the present invention, it is possible to identify an insulation failure point in a considerably short range, so that it is very efficient both in terms of construction period and economically. Good news for the world.

1 DC voltage generator (DC voltage tester can be used instead)
2 Cable conductor 3 Insulation drop 4 DC current 5 Clamp section of clamp-type ammeter that measures DC current 6 Metal shield of cable, generally called sheath
7 Cable sheath 8 Conductor metal shield to reduce capacitance between conductors.
9 Conductor through which the cable current flows 10 Cable insulator 11 Insulation deterioration part generated in the insulator Causes of foreign matter mixed in during manufacture, water entering from the outside, scratches due to stress received during construction, etc. due to electric and magnetic fields due to current Degradation is accelerated by electromagnetic factors.
12 Grounding on metal shield Usually called shield grounding.
13 Cable sheath 14 Cable conductor 15 Location where one end of conductor is short-circuited 16 Metal shield sheath (not grounded)
17 DC pressure tester

Claims (1)

The insulation failure location of the power cable where the insulation failure occurred can be identified with very high accuracy. In this method, a DC voltage is applied from one end to a conductor of a cable where a defective insulation point has occurred, current is leaked from the defective insulation point to the ground, and the current distribution on the cable is investigated using a clamp-type ammeter. Since there is no current in the portion far from the current leakage point when viewed from the voltage application point, the insulation failure location can be identified by finding the boundary between where the current flows and where it does not flow. This principle is claimed.

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