Cable fault can be summarized as grounding, short circuit, broken line three categories, its fault types mainly have the following aspects:

(1) Flashover fault. The cable is in a good insulation state at low voltage, and there will be no fault. However, as long as the voltage value rises to a certain range, or a certain voltage continues to rise after a period of time, it will instantly break down the insulator, causing a flashover fault.

(2) a core line break or multi-phase break. In the continuous test of cable conductors, the insulation resistance of each conductor of the cable is consistent with the relevant regulations, but in the inspection, it is found that one phase or multiple phases cannot be continuous, then it means that one phase core wire is broken or multiple phase broken.

(3) The three-core cable is grounded with one or two cores. The discontinuity of one or two conductors of a three-core cable is tested with an insulation gauge, and then the insulation resistance of one or two conductors to the ground is telemetered. If there is an insulation resistance between the core and the core that is much lower than the normal value, this insulation resistance value higher than 1000 ohms is called a high resistance ground fault; Otherwise, it is a low resistance ground fault. Both of these faults are called disconnected and grounded faults.

(4) three-phase core short circuit. The size of the ground resistance is the basis for judging the short-circuit fault of the three-phase core of the cable. There are two types of short-circuit faults: low-resistance short-circuit fault and high-resistance short-circuit fault. When the three-phase core wire is short circuit, the ground resistance below 1000 ohms is a low resistance short circuit fault, and the opposite is a high resistance short circuit fault.

Ii. Reason analysis

The direct cause of cable failure is that the insulation is reduced and broken down, which can be summarized as follows:

(1) External damage. External force damage is a common cause of cable faults. When the cable is damaged by external force, there will be a large area of power failure. For example, in the process of underground pipeline construction, the cable is pulled off because the traction force of construction machinery is too large; The insulation layer and shielding layer of the cable are damaged due to excessive bending of the cable; Excessive cutting and deep knife marks during cable cutting and stripping. These direct external factors will cause certain damage to the cable.

(2) Insulation damp. The cable manufacturing process is not fine, which will lead to the rupture of the protective layer of the cable; The cable terminal joint is not sealed enough; The cable protective cover is punctured or corroded by objects during cable use. These are the main causes of cable insulation moisture. At this time, the insulation resistance decreases, the current increases, and the power failure problem is caused.

(3) Chemical corrosion. Long-term current action will cause cable insulation to generate a lot of heat. If the cable insulation work is in a poor chemical environment for a long time, it will change its physical properties, make the cable insulation aging or even lose effect, and power failures will result.

(4) Long-term overload operation. Power cables are in a high-current operating environment for a long time, if there are impurities or aging in the line insulation, coupled with the impact of external causes such as lightning on overvoltage, overload operation generates a lot of heat, which is prone to power cable failure.

(5) Cable and cable accessories quality. Electric cable and related accessories are two important cable materials, whose quality problems have a direct impact on the safe operation of power cables. Cable and its accessories, the production of cable three heads is easy to appear quality problems, such as wire and cable will be damp because of transportation, storage sealing; The insulation tube is rough, the thickness is uneven, and there are bubbles in the tube. Cannot accurately strip the three ends of the prefabricated cable; The designer did not manufacture the cable joint as required. In addition, improper material selection and poor water resistance in the design of cable products will also cause cable quality problems.

In order to judge the fault of 35kv power cable, it is necessary to do a simple test according to the fault situation and judge the nature of the fault.

There are mainly the following ways to judge cable faults:

1. Basic methods

(1) Bridge method.

The application of the bridge method has a long history, but in today's new technology continues to emerge, the bridge method still has its advantages. This method is more convenient to detect single-phase grounding, interphase short circuit and other problems of power cable, and the error is small. Traditionally, the fault is found by calculating the distance measurement point between the data obtained from the bridge wall balance adjustment and the total length of the cable. However, the deficiency of the bridge method is to accurately know some raw information such as the length of the cable, and the phase of the cable should have good insulation. However, the cable fault in reality is basically high resistance and flashover fault, and the measured time by this method is relatively long.

(2) Low voltage pulse reflection method.

In the power cable fault detection, the so-called low-voltage pulse reflection method is to emit high-frequency low-voltage pulses into the cable, and the pulse will reflect electromagnetic waves when it encounters the fault point or mismatch point in the propagation, and the measuring instrument will receive the reflected pulse.

(3) DC flashover method and high voltage flashover method.

The DC flashover method is used to query the fault point in the flashover fault. The DC voltage is applied to the power cable fault point, and it will break down immediately, then the fault point will appear flashover, and the distance between the measuring point and the fault point is obtained by measuring the wave. This method can be used if the flashover fault is immediately broken down at high voltage. The DC flashover method is simple, easy to understand, and has a high precision reading. If the resistance of the cable fault point is not high, this method is not suitable. Because this will let the DC leakage of larger current, resulting in the voltage of the cable line becomes smaller. At this time, the high-voltage flashover method (flash method) should be used. This method can be used to determine whether the fault point has a strike discharge, but it does not indicate that the gap discharge is generated, and the fault point is broken down.

2, accurately determine the point measurement method

The above method of measuring fault points is applicable to a large range of fault points, and is not suitable for construction treatment. Cable routing and deep search can be used to find the exact location of the fault point using accurate search methods. The method used in this case is the acoustic measurement method and the harmonic magnetic synchronization method.

(1) Acoustic measurement method. Use high-sensitivity acoustoelectric converter to amplify the sound generated when the fault point is released, so that it is converted into sound signal and current signal, and then use headphones and instruments to determine the fault point on the cable line. However, the disadvantage of this method is that the rapid measurement results have a large randomness, and the error is large. If the cable is buried too deep in the ground, it is difficult to measure, and the advantage is that the requirements for equipment are not high.

(2) acoustomagnetic synchronization method. It is well known that electromagnetic field signals travel close to the speed of light, but sound travels relatively slowly. In this way, there is a large difference between the speed of electromagnetic signal and the speed of sound, and the receiving instrument will treat the two signals as being sent from the fault point at the same time when receiving the acoustic and magnetic signals, so the detection position is close to the fault point, and the reception time difference of the signal will become smaller, and vice versa.

Third, strengthen the daily management and maintenance of the power system

1, carefully manage the power cable operating environment and its own quality

The power supply company (or the entrusted external construction enterprise) should first investigate the surrounding environment when laying the cable pipeline. If there are corrosion factors or other factors that are easy to cause failure in the environment, they should be avoided as far as possible. In addition, it is necessary to investigate the geological pollution in the environment in detail, and it is necessary to make corresponding anti-pollution preparatory measures in different geological environments. For example, it is necessary to carefully choose the construction of channels in chemical plants and groundwater pollution areas. The type of cable should also be selected with reference to the operating environment of the power grid, attach importance to the quality of the cable, and cannot let the cable be damaged and corroded by the environment. The main core cross section of the cable should bear the operating load of the line, and the cable should not be overloaded or run at overvoltage. Power supply companies should vigorously promote the knowledge of power grid protection, set cable signs around the operation of power cables to prevent cable damage, such as setting warning signs in prominent locations, warning not to touch, climb and change pressure; Do not damage the cable; Strictly crack down on the behavior of stealing, selling and destroying cable facilities, and create a good cable safe operation environment.

2, pay attention to power cable construction operation management

The power supply company should clarify the responsibility of power construction and operation through clear management measures for cable construction and operation. According to the "Electricity Law" and other relevant national laws and regulations on the protection of electric power facilities, to promote the normal construction, operation and management of cable. Provide technical training for construction personnel to ensure the construction and operation quality of power cables. The safe operation of cable lines is closely related to the quality and normal operation of power engineering. Therefore, strictly training the technical ability of cable construction and operation personnel, and strengthening the assessment is not only responsible for the power enterprises themselves, but also responsible for the normal construction and operation of power cables. The cable laying and installation in the cable construction need to design a reasonable line, and the corresponding laying method is adopted according to the terrain environment. For example, the distance between the power user is relatively far, the overhead or waterproof cable can be used. In the areas where the user is relatively concentrated, the cable tunnel and cable well can be used to protect the cable and reduce the cable damage. For the newly operated power is a construction project, it should be strictly constructed and accepted according to the technical requirements of the state. Power enterprises should select the type and quantity of cables according to the situation when purchasing cables, which must meet the compliance standards of the power supply line and cannot be overloaded. The artificial failure and mechanical wear in the construction should be reduced to a small range, and the installation path of the cable should also be reasonably arranged and considered. As far as possible, the use of brackets, pipelines, cable trenches to lay cables, cable trenches, cable racks and other auxiliary facilities should be scientifically designed. In particular, we must pay attention to the production quality of the cable middle head and the terminal head, choose more new silicone rubber prefabricated joints, and construct and accept the cable project according to the national technical requirements.

3, strengthen the supervision and daily maintenance of wire and cable

The power supply company shall formulate relevant supervision and inspection system according to the provisions of the national ministries and commissions, closely monitor the load current of the line, and cannot allow the overload to break down the edge, and the cable that is overloaded for a long time will cause cable failure. Establish a special maintenance department, establish a regular inspection system for power distribution equipment, and let employees with professional skills training and certain operation management experience regularly inspect power cables and other equipment. If they find that there is a fault in the line, they should report it in time and propose a maintenance plan. Pay attention to the special inspection of cable lines with safety hazards, and make inspection records according to the provisions of the power supply company, and truthfully fill in the operation of the line. When inspecting the line, be sure to pay attention to the operation around the cable line, such as whether there is construction around the line, and whether the normal operation of the line is damaged.

Qingdao Tianxing Cable Co., Ltd. focuses on the production of customized wire and cable manufacturers, main: fire wire and cable, power cable, control cable and other products, exported to more than 40 countries and regions around the world, the annual production capacity of 1 billion yuan. 6000 square meters of storage, standing hundreds of millions of inventory, order demand for rapid distribution and delivery, large prices more favorable. Hotline: 16678668098