Power cables have safe and reliable power supply and are suitable for special environments in coal mines and have been widely used in mines. Commonly used high-voltage power cables mainly include paper-insulated wrap-around type, cross-linked polyethylene insulated and extruded type, and rubber-insulated extruded type. The operating environment of the underground cable is relatively harsh, and the production load is uneven and other comprehensive external factors, which makes the cable easy to malfunction, affecting the safety and reliability of the power supply. Correctly analyze the causes of cable faults, understand the cabling environment, determine the nature of cable faults, select appropriate detection methods, and quickly and accurately determine the fault point, which can improve power supply reliability, reduce fault repair costs and power outage losses.
1 cable failure analysis
1.1 Cause of failure
(1) Mechanical damage Mechanical failure caused by cable damage accounts for a large proportion of cable accidents. The cable is damaged during installation, the mechanical traction is too large, the cable is pulled and the cable is excessively bent to damage the cable; the damage caused by external damage and natural phenomena, such as vehicle crushing, rock fall and bruise, environmental corrosion, etc. The body is faulty.
(2) Insulation and moisture is mainly caused by poor sealing process caused by poor connection process of intermediate joints and terminal joints, resulting in moisture intrusion and damage to insulation performance.
(3) Insulation aging metamorphic cable The internal air gap of the insulating medium is free under the action of electric field to reduce the insulation; overheating will also cause the insulation layer to deteriorate and cause insulation degradation.
(4) Overvoltage Atmospheric overvoltage and operation overvoltage, fault transient overvoltage cause cable insulation breakdown to form a fault.
(5) Bad design and manufacturing process The moisture-proof and electric field distribution of the intermediate joints and terminal joints are not perfect, the materials are improperly selected, the manufacturing process is bad, and the production is not required according to the operation rules.
(6) Material defect The defect of the insulation material of the cable itself; during the process of wrapping the insulation layer, defects such as wrinkles, cracks, breaks and overlapping gaps appear on the insulation layer; the defects of the cable joint accessories are not in conformity with the regulations or are not sealed during assembly. Etc.; poor maintenance and management of insulation materials, causing moisture, dirt and aging of the cable insulation.
1.2 Fault nature Cable faults can be divided into series and parallel faults in form. A series fault means that one or more conductors of the cable are disconnected; a parallel fault is a guide to the insulation between the outer insulation layer or the conductor and cannot withstand normal operating voltage. There are many kinds of combinations of actual faults on the site. According to operational experience statistics, most of the high-voltage cable faults are caused by a single insulation drop. The cable fault equivalent circuit is shown in Figure 1.
According to the fault resistance Rf and the breakdown gap G, the cable fault properties are classified into open circuit, low resistance, high resistance and flashover faults. The open circuit fault Rf≈∞, the breakdown gap G is broken down by a direct current or high voltage pulse. Low-resistance fault Rf is generally less than 100Ω, and can be broken down by high-voltage pulse; high-resistance fault insulation resistance Rf is generally greater than 400Ω, and high-voltage pulse can be used for breakdown. The flashover fault insulation resistance Rf≈∞ can be broken down by DC high voltage or high voltage pulse. The faults that occur in preventive tests are mostly flashover faults. There is also a closed fault on the site, which occurs mostly in cable joints and cable sheaths with no obvious damage marks. Insulation breakdown at a certain test voltage. When insulation is restored, the breakdown phenomenon disappears, but it cannot be maintained normally. Operating voltage.
2 cable fault detection
2.1 Fault Detection Steps Cable fault detection generally involves three steps: judgment, ranging, and fixed point.
(1) Cable fault quality judgment should firstly understand cable laying, fault and repair, fault location and elimination, cable specifications, insulation mode, joint form, insulation type, precise position of joint, surrounding environment, operation and calibration The situation includes test voltage, time, leakage current and insulation resistance value, historical fault record, etc. These conditions are important to determine the type and severity of the failure. The site can initially determine the nature of the fault based on various signal indications, trip ranges, etc. that occur when the fault occurs. Use Megohmmeter to measure cable insulation resistance value, short-circuit discharge spark size to judge insulation condition, conduct multi-meter conduction test to determine whether the fault resistance is high resistance or low resistance; flashover or closed fault; grounding, short circuit, disconnection , or a combined fault; is a single-phase, two-phase or three-phase fault. Statistics of coal mine underground cable faults, high resistance and flashover faults account for 95% of the total, generally mostly phase-to-phase or relatively high-resistance or low-resistance faults, while leakage high resistance is the most common, and most faults are concentrated Expressed on a variety of cable heads.
(2) Cable fault location According to the nature of the cable fault and the cable laying condition, the traveling wave method is commonly used in the field for fault location, that is, the test instrument is used at one end of the cable to determine the fault distance. Low-resistance, short-circuit, and open-circuit faults are measured by the low-voltage pulse reflection method by observing the time difference between the reflected pulse and the transmitted pulse at the fault point. By identifying the polarity of the reflected pulse, the nature of the fault is determined and the distance to the fault is calculated. The open circuit fault reflection pulse has the same polarity as the emission pulse, and the reflected pulse of the short circuit fault is opposite to the polarity of the transmitted pulse. High-resistance and flashover faults are applied by pulse voltage method and pulse current method. The test instrument is used to break the cable fault under the action of DC high voltage or pulse high voltage signal. The instrument tests the instantaneous pulse signal generated by the breakdown of the fault, and observes the discharge voltage pulse. Time ranging between the observation point and the fault point. The difference between the pulse current method and the pulse voltage method is that the pulse current method measures the current pulse signal generated when the cable is broken through the linear current coupler; the pulse voltage method measures the voltage pulse signal through the capacitor, the resistor and the inductor divider, and the instrument Electrically coupled to the high voltage circuit. The faulty cable is tested by the common voltage pulse method in the underground. The wiring principle is shown in Figure 2. This test method is suitable for leaky high resistance faults and flashover high resistance faults. The large-capacity capacitor that has been charged in the figure is used as a high-power DC power supply. The voltage is applied to the faulty cable through a ball gap breakdown short circuit to cause a flashover of the fault point to form a short-circuit. After the ball gap is broken, the pulse waveform is recorded by the flash meter. Perform complex mathematical processing to calculate the fault point distance. According to the cable type and the nature of the fault, adjust the ball gap spacing so that the highest impact voltage of the cable is 3~5 times of the cable withstand voltage, so that the fault point is fully discharged.
(3) Common methods for fault fixed-point cable fault fixed point are shock discharge sound measurement method, audio induction method, and acoustic magnetic synchronous detection. There are many bare suspensions in underground coal mines. The use of impact discharge sound measurement legal points on site is relatively straightforward, simple and convenient. That is to say, the distance of the fault point is preliminarily calculated by the flash meter, the approximate position of the fault point is determined, and the fault point is determined by the principle that the flashover discharge of the fault point is instantaneously synchronized with the ball gap breakdown discharge. The fault clicks through and discharges, generates strong mechanical vibration, and then hears the sound of “啪†and “啪â€, and this phenomenon can be used to locate the fault very accurately under the well.
2.2 Fault Detection Precautions
(1) The pulse voltage method uses a resistor and a capacitor divider to perform voltage sampling, and is electrically connected to the high voltage circuit, and performs wiring and disconnection operations according to the operating procedures. The discharge rod must be fully discharged before the device is used.
(2) The storage capacitor is short-circuited to the high-frequency traveling wave signal. Pulse capacitors should be used, or 6 kV power capacitors can be used, and the capacity is 1~4μF.
(3) Wiring in strict accordance with the requirements, the high-voltage generator grounding wire and the capacitor outlet wire are connected together to connect the cable sheath, and the connection between the capacitor and the cable should be shortened as much as possible. The high-voltage equipment, capacitor casing and cable core must be connected to the grounding grid. .
(4) Adjust the ball gap so that the voltage applied to the cable through the ball gap exceeds the critical breakdown voltage of the fault point, and the fault point is broken through the arc short circuit, which is beneficial to improve the seismic wave intensity generated by the fault point discharge, and is convenient for finding the fault point.
(5) The gap interval of the ball gap is 2~6 s. The discharge is too fast, which is easy to damage the control equipment. It is too slow to distinguish the external interference. (6) In the case of impact discharge, if the grounding is poor, there may be a discharge phenomenon between the cable sheath and the grounding part, which may be misjudged. Especially in the metal part of the bare part of the cable, the true fault point should be carefully and carefully identified. It sounds, and there will be vibrations accompanied by local temperature rise.
(7) The output lead should be in reliable contact with the terminal and cable core wire, otherwise an arc will be generated during the impact discharge, which will affect the measurement effect.
3 Conclusion The application of computer technology has promoted the development of cable fault detection technology. Xi'an Sifang Electromechanical Company SDC series intelligent cable fault finder and fixed-point instrument have high test accuracy and easy operation, and have been widely used in mine sites. However, due to the special nature of underground cable fault detection, field testers still need to accumulate experience, promote and apply advanced cable fault detection technology to improve cable fault detection.
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