JP4681391B2 - Degradation diagnosis method for low-voltage cables - Google Patents

Description

  The present invention relates to a method for diagnosing deterioration of a low voltage cable for 600 V or less used for supplying power to various facilities in a factory.

  As a low-voltage cable as described above, the one having a structure in which the conductor is covered with a rubber insulation coating and a vinyl insulation coating is used, but the insulation coating deteriorates during use for a long period of time, and cracks occur. Insulation breakdown may cause a ground fault. For this reason, it is necessary to take measures to prevent the occurrence of accidents such as periodically diagnosing the deterioration state and replacing the low-voltage cable at the time of deterioration.

  As a diagnosis method for a low-voltage cable, a method is generally implemented in which the insulation resistance is measured and the deterioration is determined based on the absolute value of the insulation resistance measured in accordance with the electrical equipment technical standards. However, the low-voltage cable often has a high insulation resistance value that is higher than the reference value even if the insulation coating is cracked, and there is a problem that deterioration cannot be accurately detected. In addition, a method of performing online diagnosis using a direct current component included in a circuit charging current during operation has also been implemented, but even this method cannot detect the occurrence of cracks in a state where the insulation resistance is high.

  Further, Patent Document 1 proposes a method of measuring the surface electrical resistance of the insulator or sheath on the surface of the electric wire / low voltage cable and diagnosing the degree of deterioration of the electric wire / low voltage cable from the change in the surface electric resistance. . However, in this method, it is necessary to provide electrodes at an appropriate interval in the length direction of the low-voltage cable, and it is difficult to diagnose a long low-voltage cable over the entire length.

As a method for diagnosing the deterioration state of the high voltage cable, a leakage current test is known which is performed by applying a DC high voltage to the high voltage cable to be diagnosed. However, this method requires cable disconnection and takes a long time of about 20 minutes. Therefore, although this test is necessary for high-voltage cables with a high management level, it is practical to apply it to the diagnosis of low-voltage cables with a low management level. is not.
JP-A-8-233896

  The present invention has been made in order to solve the above-described conventional problems and to provide a low-voltage cable deterioration diagnosis method that can easily and accurately determine a deterioration state such as crack occurrence of a low-voltage cable within a short time. Is.

The degradation diagnosis method for a low-voltage cable according to the present invention made to solve the above-described problem is when a change in insulation resistance is measured immediately after application of a DC voltage to a low-voltage cable and the insulation coating is degraded. Discharge pulsation rate defined by the equation of discharge pulsation rate = (maximum value of insulation resistance−minimum value of insulation resistance) / maximum value of insulation resistance for each pulsation of a plurality of pulsations occurring in Among them, it is judged that the deterioration is caused when the maximum discharge pulsation rate exceeds 1%. Note that the DC voltage to be applied is preferably 500 V or more, and the change in insulation resistance is preferably measured within 20 to 60 seconds after the voltage application is started.

  According to the degradation diagnosis method for a low-voltage cable of the present invention, the degradation is detected not by the absolute value of the insulation resistance as in the prior art but by the inherent discharge pulsation rate generated in the degraded low-voltage cable. There is an advantage that deterioration due to the occurrence of cracks can be reliably detected. In the deterioration diagnosis, as in the conventional insulation resistance measurement, it is only necessary to connect a diagnostic device to a low voltage cable to be diagnosed and apply a voltage, and there is an advantage that diagnosis can be performed easily in a short time.

Next, the best mode for carrying out the present invention will be specifically described with reference to the drawings.
FIG. 1 shows a connection between a low-voltage cable 1 and a diagnostic device 2 that are diagnostic targets when the low-voltage cable degradation diagnostic method of the present invention is carried out, and a conducting wire 3 of the low-voltage cable 1 is connected to the diagnostic device 2. . The diagnosis device 2 basically measures the insulation resistance between the conductors 3 of the low-voltage cable 1 and diagnoses the deterioration of the low-voltage cable, and one pole of the diagnosis device 2 is connected to the ground.

  The diagnostic device 2 generates a constant voltage and applies a voltage between the conductor 3 and the ground, a current detector 5 that detects a current flowing through the conductor 3, and a diagnosis when the low voltage cable 1 is broken down. An overcurrent protection device 6 that protects the device 2; an arithmetic device 7 that calculates an insulation resistance value from the output voltage value of the DC power supply device 4 and the current flowing through the conductor 3 detected by the current detector 5; It comprises a storage device 8 for storing the insulation resistance value calculated by the device 7 in time series, an analysis device 9 for analyzing data stored in the storage device 8, and a determination device 10.

The analysis device 9 has a function of calculating the discharge pulsation rate. In the present invention, as the discharge pulsation rate,
The value defined by the equation of discharge pulsation rate = (maximum value of insulation resistance−minimum value of insulation resistance) / maximum value of insulation resistance is used. The determination device 10 has a function of comparing the discharge pulsation rate obtained by the analysis device with a reference value and outputting a determination signal for determining deterioration when the discharge pulsation rate exceeds the reference value. An alarm display or the like can be performed by the output determination signal. In the figure, reference numeral 11 denotes an insulation coating for the low-voltage cable.

  When the low-voltage cable 1 and the diagnostic device 2 are arranged as described above, and the DC power supply device 4 is operated to apply a DC voltage to the conductor 3, the charging current and the insulation coating for charging the capacitance between the conductor 3 and the ground 11 absorption current flows, and the leakage current of the insulating coating 11 flows. This current is detected by the current detector 5 and is calculated as an insulation resistance including a charging current by the arithmetic device 7. The calculated insulation resistance is stored in the storage device 8 in time series. .

  FIG. 2 is a graph showing changes in insulation resistance immediately after the start of voltage application. When the insulation resistance measurement is performed by applying a DC voltage to the conducting wire 3 of the sound low-voltage cable 1, the current flowing therethrough decreases with time after the voltage application starts and eventually becomes a constant value. This current is almost equal to the displacement current that flows immediately after the voltage is applied to charge the capacitance including the insulating coating 11, the polarization current that flows slowly while absorbing the insulating coating 11, regardless of the passage of time. When the insulation resistance is obtained from this current, the insulation resistance gradually increases as shown in A and changes so as to approach a constant value determined by the leakage current. It has been known.

  On the other hand, when the insulation coating 11 is deteriorated, the insulation resistance changes so as to pulsate such that the insulation resistance changes from increasing to decreasing as time passes and then increases again as shown by B. To do. This is assumed to be an influence of a micro discharge or the like inside the crack of the insulating coating 11. A plurality of pulsations of the insulation resistance value due to this discharge pulsation phenomenon may appear, and FIG. 2 shows a state where two pulsations appear. The analysis device 9 calculates the above-described discharge pulsation rate for each pulsation. The determination device 10 compares the discharge pulsation rate given from the analysis device with a reference value, and outputs a determination signal if the discharge pulsation rate exceeds 1% which is the reference value. Thereby, it can be known that the low-voltage cable 1 is deteriorated.

  In the present invention, the DC voltage applied to the low voltage cable 1 is preferably 500 V or higher. This is because discharge pulsation may not occur if the voltage is less than 500V. The analysis of the change in insulation resistance is preferably performed from 20 seconds to 60 seconds after the polarization current after voltage application converges. This is because the discharge pulsation can be detected most clearly in this time zone. The great advantage of the present invention is that the diagnosis can be completed in such a short time.

  Deterioration diagnosis and dielectric breakdown voltage measurement were performed by the method of the present invention for low voltage cables for overhead cranes with various deterioration states. The results are as shown in Table 1, and the method of the present invention for judging deterioration when the discharge pulsation rate exceeds 1% accurately corresponds to the actual deterioration state of the low-voltage cable visually confirmed. Confirmed to do. It can also be seen that the absolute value of the insulation resistance does not necessarily match the degree of cracking.

It is a figure which shows arrangement | positioning of a low voltage | pressure cable and a diagnostic apparatus. It is a graph which shows the example of the change of the insulation resistance value with respect to time.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Low voltage cable 2 Diagnostic apparatus 3 Conductor 4 DC power supply apparatus 5 Current detector 6 Overcurrent protection apparatus 7 Arithmetic apparatus 8 Storage apparatus 9 Analyzing apparatus 10 Judgment apparatus 11 Insulation coating

Claims (3)

Discharge pulsation rate defined by the following equation for each pulsation of multiple pulsations that occur when the insulation resistance is deteriorated by applying a DC voltage to the low-voltage cable and measuring the change in insulation resistance. A degradation diagnosis method for a low-voltage cable, characterized in that the degradation is determined when the maximum discharge pulsation rate exceeds 1%.
Discharge pulsation rate = (maximum value of insulation resistance-minimum value of insulation resistance) / maximum value of insulation resistance   2. The method for diagnosing deterioration of a low-voltage cable according to claim 1, wherein the DC voltage to be applied is 500 V or more.   The method for diagnosing deterioration of a low-voltage cable according to claim 1, wherein the change in insulation resistance is measured within 20 to 60 seconds after the start of voltage application.

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