The principles and methods for precise location of power cable fault points (I) 1. Acoustic detection method: The acoustic detection method is the main method for locating cable faults, mostly used for testing high-resistance, flashover faults, and some low-resistance faults. The equipment used is the same as that for the impulse flashover method, using an acoustic-electrical converter to convert very small vibration waves into electrical signals for amplification and processing, and listening with headphones. 2. Acoustic-magnetic synchronization method: In actual testing, interference from environmental noise increases the difficulty of accurately identifying with the acoustic detection method. When the fault point discharges, in addition to producing discharge sounds, it also generates high-frequency electromagnetic waves that propagate to the ground. By simultaneously receiving sound waves and electromagnetic waves, it can be determined whether the current sound wave is caused by the fault point discharge. This is the acoustic-magnetic synchronization method. It is an improvement over the acoustic testing method, enhancing anti-interference capability. The location environment inevitably has various continuous noises and pulse impact noise interference. Currently, simple acoustic detection locators have been phased out, replaced by acoustic-magnetic synchronization locators. Such instruments artificially eliminate on-site noise interference by observing whether the radiated electromagnetic waves received at the site when the cable is broken down by high-voltage impact and the vibration sound waves from the fault point are synchronized. The precise fault point location is determined by the maximum point of the fault point vibration sound. Although this method's positioning accuracy is not high, it generally meets requirements. Most domestic manufacturers' locators belong to this type. A few manufacturers also display the time difference between electromagnetic waves and seismic waves on a liquid crystal screen to precisely determine the fault point location, which is undoubtedly a significant improvement. Our company has developed and produced the DDY-3000 digital display synchronous cable fault locator, which possesses all the advantages of cable path finding, acoustic-magnetic synchronization method, and displaying acoustic-magnetic time difference method. It also converts the acoustic-magnetic time difference into the actual distance between the locating probe and the cable fault point, directly displayed on the liquid crystal screen. The screen simultaneously displays fault distance, electromagnetic signal strength, and acoustic signal strength, and also has storage and recording functions. Directly above the fault point, the seismic wave sound is the loudest (at this time, the change in seismic wave sound level is no longer important), and the reading is the smallest, and this reading is the burial depth of the fault point from the ground surface. Directly above the fault point, whether the probe moves left or right, or forward or backward, the reading will increase, even though the seismic wave sound change is not obvious. That is, this function also achieves precise judgment of the cable path on-site. Therefore, the DDY-3000 digital display synchronous cable fault locator is currently the most fully functional, strongest anti-interference, and most accurate cable fault precise locator among similar domestic products. The DDY-3000 cable fault locator adopts our company's original new theory for cable location. It is a new type of cable fault precise location testing instrument developed by our company based on the latest research results, with high anti-interference, high sensitivity. This instrument combines advanced analog low-noise design with high-performance filtering circuits, greatly improving its anti-interference performance. With original design and exquisite assembly craftsmanship, this instrument has the highest domestic standard. It can precisely locate various cable faults, especially having unique testing effects for cross-linked cables and cable sealing faults. It is the latest upgraded product among cable testing instruments. 3. Electromagnetic method positioning method:
The electromagnetic method testing can determine the cable path and the approximate location of some cable fault points (open circuit faults). For cable faults, especially open circuit faults, it is very convenient and accurate to judge by observing changes in electromagnetic waves. Based on practical work experience, the precise location of the fault is about 1 meter before and after the point where the signal suddenly disappears (related to burial depth, etc.). If there are several short-circuit grounding points, there will be several jump points. Generally, the signal change at the short-circuit grounding point farthest from the transmitter is the most obvious. When solving, the farthest or most obvious signal change point should be addressed first. After solving, retest. You will find that the signal changes at the remaining fault points are much more obvious than in the original test. Repeat the above process to solve each fault separately. Testing conditions: ① The distance from the entire cable to the ground has no significant elevation difference (horizontal laying), and the ground at the testing site is relatively flat; ② The insulation resistance at the fault point is less than 2000 ohms. Theoretical analysis shows that the fault point is the boundary point of signal strength. Within about 1 meter before and after the fault point, the electromagnetic signal will suddenly increase or attenuate significantly from a stable state. This method can also be used for some phase-to-phase short circuits. At this time, attention should be paid to keeping the receiver direction consistent with the cable direction to find the signal jump point.
If there are multiple faults, along the cable path, multiple "polarity change points" can be measured, and then the "zero" potential points of ground potential changes can be found separately. The step voltage method can have a good effect on the rapid and accurate location of ground insulation faults. It is especially useful for dead grounds of directly buried cables, because the discharge energy at a single-phase metallic grounding fault point is proportional to the square of the discharge current and the grounding resistance. Therefore, when the fault point breaks down and discharges through the gap, the sound is light, making precise location difficult or even impossible. When the fault point is under a road surface, using the A-frame can be somewhat difficult. In such cases, the A-frame can be used in the soil on both sides of the road. Since the underground return current flows out from the fault point in a spoke pattern, when the A-frame current is perpendicular to the spoke lines, there is a minimum reading. First, place the A-frame on the side to be measured, note a zero-value segment reading and remember it. Then rotate the A-frame until the minimum zero-value segment reading appears. Using the bottom edge of the A-frame as the baseline, the intersection of the two bisectors is the fault point.
With the continuous expansion of the power system, the short-circuit current of cables also increases, causing more severe impact on cable fault points, and the insulation resistance at fault points tends to decrease continuously. Metallic grounding faults (insulation resistance generally below 10 ohms; in practice, grounding resistance of tens of ohms is also called metallic grounding) occur from time to time, most of which are single-phase metallic grounding. When high-voltage impact is applied to metallic grounding faults, the discharge sound through the gap is very light, making testing difficult. Generation and characteristics of metallic grounding faults: With the continuous expansion of the system, the short-circuit current of cables also increases, causing the insulation resistance at cable fault points to tend to decrease continuously. Metallic grounding faults occur from time to time, most of which are single-phase metallic grounding. The discharge energy at the fault point is proportional to the square of the discharge current and the grounding resistance. Since the grounding resistance is very small, the discharge sound through the gap at the fault point is light or even absent, making precise location impossible. 1. Due to long-term impact discharge at the fault point, heat accumulates, causing local overheating near the fault point. This can be judged by hand touch, but high-voltage impact must be stopped and the cable fully discharged before doing so.
2. Many metallic grounding faults are caused by external force damage, so the site should be carefully observed for any suspicious conditions. Testing without measuring grounding resistance can waste a lot of time when encountering metallic grounding, affecting efficiency. Therefore, the nature of the fault should first be accurately determined. For faults with grounding resistance around tens of ohms, they should be tested as metallic grounding faults. Use the low-voltage pulse method for rough measurement. Based on the rough distance, find the corresponding position on the drawings, and accurately determine the actual cable route on site. Within 20 meters on both sides of the rough distance, the direct buried cable fault tester can quickly find the fault point.
