How to detect cable faults?
来源：本站 点击：46 时间：2021-03-16
With the development of the economy and the acceleration of modernization construction, China's demand for electricity is increasing. Power cables used to connect various electrical equipment, transmit and distribute electrical energy have already been widely used.
Currently, the majority of power supply failures are caused by faults in power cables. Therefore, how to accurately and quickly determine the location of the fault point and determine the type of fault has become a crucial technology in the use and operation of power cables.
Types of cable faults:
According to the nature of the fault, it can be divided into high resistance grounding or short circuit fault, low resistance grounding or short circuit fault, wire breakage fault, wire breakage and grounding fault, and flashover fault.
Reasons for cable faults:
1. Mechanical damage
Mechanical damage causes a significant proportion of cable failures in cable accidents. Some mechanical damages are very minor and did not cause a malfunction at the time, but it takes several months or even years for the damaged area to gradually develop into a malfunction.
Damage during installation: Accidentally damaging the cable during installation, pulling the cable due to excessive traction, or damaging the cable due to excessive bending;
Direct external damage: Construction on or near the cable laying route causes direct external damage to the cable; The vibration or impact load passed by vehicles can cause the lead (or aluminum) package of underground cables to break;
2. Insulation is damp
Malfunction caused by insulation moisture. The main reasons for cable moisture are: water ingress due to unsealed or poorly installed joint or terminal box structures; Poor cable manufacturing, with small holes or cracks in the metal sheath; The metal sheath is punctured or corroded by external objects;
3. Insulation aging and deterioration
The internal air gap of the cable insulation medium becomes free under the action of an electric field, resulting in a decrease in insulation performance. When the insulating medium ionizes, chemical compounds such as ozone and nitric acid are generated in the air gap, which corrode the insulation; The moisture in the insulation causes hydrolysis of the insulation fibers, resulting in a decrease in insulation performance;
Overheating causes insulation aging and deterioration: The internal air gap of the cable generates electrical ionization, causing local overheating and carbonization of the insulation.
Diagnosis of Cable Fault Nature
The nature of diagnosing cable faults refers to determining whether the fault resistance is high or low; Is it a flashover or a closed fault; Is it grounding, short circuit, disconnection, or a mixture of them; Is it a single-phase, two-phase, or three-phase fault.
1. Bridge method
Connect the fault phase and non fault phase of the tested cable terminal, and connect the two arms of the bridge to the fault phase and non fault phase respectively. Adjust the resistance to achieve balance in the bridge, and calculate the distance of the fault point through the formula.
2. Low voltage pulse reflection method
Inject low voltage pulses into the faulty phase of the power cable during testing. When the pulse propagates along the cable to the impedance mismatch point, that is, the fault point, the pulse is reflected back to the test point and recorded by the instrument. Based on the round-trip time difference between the transmitted pulse and the reflected pulse and the wave velocity of the pulse propagating in the cable, the distance between the fault point and the test point can be calculated.
3. Pulse current method
The pulse current method is to puncture the cable fault point with high voltage, collect and record the current traveling wave signal generated by the fault click through using instruments, and calculate the fault distance by analyzing and judging the time it takes for the current traveling wave signal to travel back and forth between the measuring end and the fault point. The pulse current method uses a linear current coupler to collect the current traveling wave signal in the cable.
Common cable fault location methods
1. Acoustic fixed-point method
The acoustic fixed-point method is the main method for locating cable faults, mainly used to measure high resistance and flashover faults. During measurement, high-voltage equipment is used to cause the fault to click through and discharge. The mechanical vibration generated during the fault gap discharge can be transmitted to the ground, and a "pop, pop" sound can be heard. This phenomenon can be used to accurately locate cable faults, but the disadvantage is that it is greatly affected by external interference.
2. Acoustic magnetic method
When a high voltage signal is applied to the cable to discharge the fault point, a circulating current is induced in the circuit formed by the outer skin of the cable and the ground. This circulating current generates a pulsed magnetic field around the cable. By listening to the sound signal and receiving the pulsed magnetic field signal, it can be determined that the sound is generated by the discharge of the fault point, which is nearby.
3. Audio induction method
The audio induction method is generally used to detect low resistance faults with a fault resistance of less than 10 Ω. During detection, a 1 kHz audio signal generator is used to apply audio current to the tested cable and emit electromagnetic waves; Then use a probe on the ground to receive electromagnetic field signals along the tested cable path, and send them to an amplifier for amplification.
When detecting power cable faults, one should remain calm and carefully analyze the type and nature of the fault, correctly apply search methods and instruments, and accumulate more search experience.
