10kV cable Intermediate joint fault test case

Ⅱ.Test process

Step 1: Determine the nature of the fault

Use a 2500V megohmmeter to test the insulation resistance of the three phases of cables A, B, and C, and determine the nature of the fault as follows:

Step 2: Fault pre-location

1.From the first step, it can be seen that a high-resistance grounding fault occurs in phase C of the cable, and the grounding resistance is low. According to the test process, the full length of the three-phase cable is first tested using the low-voltage pulse method of the wave reflectometer to verify whether the cable is broken. The full length of phase C is shown in Figure 1, and the measured total length is 2471m;

Figure 1 Full-length waveform of low-voltage pulse of phase C

2.Use the low-voltage pulse method to test the full length of the AB phase cable and compare it with the full length of the C phase. As shown in Figure 2 below, the full length is consistent, but there is a difference at the 877m position. From the waveform, it can be seen that this should be an intermediate joint. Because the insulation of the C phase is low, there is a weak "low resistance" reflection in the low-voltage pulse waveform. It is suspected that this is the fault location;

Figure 2 Comparison of the full length of low-voltage pulse waveform

3.Next, we use the pulse current method to test and verify again. After adding voltage to phase C, we perform waveform testing again. The waveform shown in Figure 3 below is obtained. The fault distance is 887m, which is basically consistent with the distance measured by the low-voltage pulse. It is basically confirmed that the fault point is at the middle joint of about 880m;

Figure 3 phase C pulse current waveform

Step 3: Cable path search

The cable comes out of the ring main unit and is laid along the road. There are cable wells at certain locations along the way. The path information is clear and no need to search.

Figure 4 Path diagram

Step 4: Accurately locate the fault

1.After adding voltage to phase C, go to the 877m position for positioning. Because the cable is a user cable, the path from the substation to the user is basically clear. The cable is laid along the roadside pipes, and there are observation wells at certain intervals. The path information after reaching the user is unknown. After estimating the 877m position, find the nearby cable well and open it for confirmation. As shown in Figure 5 below, all nearby cable wells are basically filled with rainwater, and the fault point cannot be confirmed.

Figure 5 Cable well near the fault point

2. Because there is a cable well every 50m near the fault point, the fault point is measured to be an intermediate joint. The intermediate joint well must be found to locate the fault. The user's internal path is unclear, about 200m away, and there is a deviation in the distance estimation. At this time, a more accurate cable well must be selected to start pumping. If no joint is found, other nearby cable wells will be replaced to continue pumping. If the wrong choice is made, the workload of pumping will be large.

3. At this time, among the three or four cable wells nearby, it was found that there was silicone grease in the joint installation accessories floating in the water in a cable well about 600 meters away from the user, as shown in Figure 6 below. Although the cable well was also filled with rainwater, it was suspected that there should be a cable joint here. It was about 600 meters from here to the user, plus the cable in the user was about 200 meters, which just matched the measured fault distance of 877m. It was decided to pump water here;

3. Because the drainage pipes of the nearby cable wells were interconnected and the pipe openings were not effectively blocked, the rainwater in the wells was interconnected, and the pumping workload was large. Several pumps and generators were replaced, and it took nearly 20 hours to pump out the rainwater in the cable well until the cables could be observed. After the pumping stopped, the rainwater was still flowing back. At this time, obvious cable joints were observed, and there were obvious discharge marks on the joints, as shown in Figure 7 below. The fault was found.

Figure 7  Faulty joint

III. Test summary

1. The C-phase low-voltage pulse waveform has a suspected "low-resistance" reflection waveform, because the faulty joint is immersed in water, and water enters the fault point, resulting in a low resistance value, but the inside of the joint is still closed and not completely grounded, so the low-resistance waveform reflection amplitude is small. When analyzing this waveform alone, it is not possible to make a direct judgment. It can be judged as the fault distance by comparing it with the intact one;

2. Cable faults that are damp and flooded are generally not easy to measure. If the resistance value of the fault point is high, the general waveform test is difficult. The low-voltage pulse waveform has no bifurcation point, and the pulse current waveform is mostly irregular. If the fault point is in the water when locating the point, it also affects the propagation of sound. The use of the device is greatly affected;

3. Path information is very important for fault finding. The reservation, coiling, and turning of the cable have a great influence on the estimation of the road distance and should be paid attention to.

IV. Cause analysis of failure

The cable has been in operation for less than 5 years. It is suspected that there are process problems in the production of cable joints. In addition, the joints are usually immersed in water, and the internal defects are increasingly magnified until they break out into failures.

V. Cable operation and maintenance suggestions

Improve the process of accessory installation and manufacturing, strengthen the inspection of cable joints and terminals, and conduct targeted partial discharge measurements in addition to conventional voltage resistance tests to determine whether there are obvious hidden dangers in the joints; in addition, daily cable operation and maintenance work should also be carried out in a timely manner, and standardized cable management should be carried out in cable channels and cable wells.