(a) Causes of Cable Faults
1, external damage. The main reason for external damage to the cable is that mechanical construction such as an excavator directly damages the cable, causing a short circuit trip or failure to insult the insulation and leaving an accident. In actual operation, it is shown that the failure of the external force type cable accounts for more than half of the total cable faults.
2, the construction quality of the cable. The quality problems that arise in the cable construction process are mainly divided into two aspects: external environmental factors and production technology levels. The external environmental factors mainly include the shallow burying of cables, which leads to the protection of exposed cables; the excessively small bending radius; the excessive accumulation of debris in the cable trenches; and hidden dangers caused by scratches during cable laying. The production technology level mainly includes the installation of cable head accessories that do not meet the technical requirements; the cable head heat-shrinking materials are unevenly baked or over-baked, causing the thermal insulation of the cable material to be not tight or hot melt excessive, thereby reducing the degree of insulation itself; or cold-shrink manufacturing At that time, it was not produced in accordance with the instructions of the technical workbook and did not meet the prescribed production process.
3, cable operation problems. The user's overloaded power will cause the cable insulation to dry, embrittlement, reduce the cable insulation strength, surface temperature is too high, it will cause cable failure, in serious cases may cause a fire.
4, the quality of the cable itself.
5, cable aging.
(B) cable fault type
The main types of cable faults are mainly divided into low-resistance faults, high-resistance faults, three-phase short-circuit faults, disconnection faults, and flashover faults. The 500V-2500V rocking table is usually determined before the fault is found.
Second, the cable fault detection method
1. Bridge method. At the test end of the cable line, the good phase and fault phase conductors are respectively connected to the test instrument as two bridge arms of the bridge, and the two-phase conductors at the other end are bridged to form a loop. Adjusting the bridge, when the bridge is balanced, the product of the corresponding bridge arm resistance is equal, and the resistance value of the cable conductor that is the two bridge arms of the bridge is proportional to the length, so the ratio of the cable conductor resistance can be converted to the ratio of the cable length. According to the value of the adjustable resistance and standard resistance on the bridge, the initial distance of the cable fault point can be detected. It is mainly used for single-phase, two-phase, three-phase and phase-to-phase short-circuit (grounding) faults with resistance values ​​below 100kΩ. Generally not suitable for testing high resistance and flashover faults. Because the bridge method mainly calculates the distance of cable fault based on the field voltage meter and resistance ratio, the accuracy is not high and it is not used within the port area.
2, pulse method. The pulse method is a method of applying pulse wave technology to cable fault location. Which is divided into low-voltage pulse reflection method, DC high-voltage flash test method, the impact of high-voltage flash test method three.
The working principle of low-voltage pulse method is to inject a low-voltage pulse wave at the test end. The pulse wave propagates along the cable to the fault point to generate reflections and send it back to the test instrument. The time interval Δt between the transmitted wave pulse wave and the reflected pulse wave is recorded together. The known pulse Waves propagate velocity V in the cable to calculate the fault point distance.
The working principle of the direct flash method is that a DC voltage is applied to the faulty phase of the cable line at the test end. When the voltage rises to a certain value, a flashover discharge occurs at the fault point, and the pulse wave and its reflected wave generated by the flashover discharge are recorded together. The time interval Δt is used to calculate the fault point distance.
In the actual work process, we found that cable faults are generally high-resistance faults and low-resistance faults. The low-voltage pulse method and the flashover method in the pulse method are highly accurate in solving low-resistance and high-resistance cable faults, and are not affected by the artificial factors, and thus become the main application methods for cable fault detection.
Third, DFA cable fault instrument application
1, pulse reflection. The low voltage pulse emitted by the pulse reflectometer travels along the cable. When the pulse signal reaches the position where the impedance of the cable changes, it will change reflection of this impedance. By observing these reflections on the display, the distance to the reflection point can be determined. The cable pulse reflectometer consists of a pulse generator and a cathode oscilloscope. This oscilloscope usually requires special circuitry to determine the distance and change the pulse width for different distance ranges. After the pulse is generated, it is applied to a cable with a uniform distributed capacitance, and pulse reflection occurs when the impedance changes. The rising reflected signal represents a high impedance change; the falling reflected signal represents a low impedance change. When the impedance at the reflection is higher than the characteristic impedance of the cable, the signal rises. When the impedance at the reflection is lower than the characteristic impedance of the cable, the signal drops.
2, arc reflection. Because the low pulse signal emitted by the pulse reflectometer does not reflect at the high resistance fault point, it directly reaches the end of the cable to form an open-circuit reflection. Therefore, under the pressure situation, only the track waveform of an “integrity†cable can be measured. Therefore, for a high-impedance fault, an arc-reflection method is used to perform a shock discharge to the fault point through a high-voltage impactor to cause an arc at the fault point to form an instantaneous short-circuit state (less than 50 ohms). At this time, the pulse reflectometer is connected to the faulty cable through the coupler, and when the arc is generated, the device triggers the trigger pulse signal to form a short-circuit reflection at the arc point (short-circuit transient point), and the fault waveform is displayed in the pulse with a falling signal. Reflector on. The short-circuit reflection waveform measured under the arc reflection method and the open-circuit reflection waveform measured under the low-voltage pulse method will be displayed on the pulse reflectometer at the same time. The two trace waveforms will be clearly separated at the fault point. The separation point is The point of failure, the distance of the fault point is also automatically displayed on the pulse reflectometer.
Fourth, the problems encountered and solutions
After the DFA cable fault meter is used, the efficiency of finding a cable fault is much greater than before. However, with the increase in the use of time, but also found some problems. DFA cable fault meter in the search for high-voltage cable fault, the accuracy is very high, can reach about 90%, but in the search for low-voltage cables are still missing. Low-voltage cables have a large difference in insulation performance and shielding performance. Therefore, faults are often interfered when using the faulty instrument to find faults, resulting in inconspicuous points of measurement failure and large distance errors.
In the process of cable fault detection, it is impossible to blindly use technical equipment, but also to combine the experience of practical work in peacetime. In the long-term detection of cable faults, we learned that most of the points of cable faults are concentrated on the two sides of the road passing through the buried pipe, the middle position of the cable, and the external force of the construction damage. This requires finding out the route of the cable, the position of the middle of the cable, and the position of the passing pipe before searching for cable faults. At the same time, it also requires strict management in the technical level of cable head production, daily power supply operation management, and protection of the cable route at the construction site to avoid unnecessary cable faults.
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