Ⅰ.Preparation before testing

Ⅱ.Test process

Step 1: Determine the nature of the fault

Insulation resistance measurements of R, S, and T phases were conducted using a megohmmeter, and the fault characteristics were determined as follows: A main insulation fault was identified between phases S and T (ST).

The test was performed from the outdoor termination at the substation. (Note: Since RR, SS, and TT phases are shorted together inside the GIS, the remote end of the test cable is effectively located at the same outdoor terminal position at the substation.)

Step 2: Fault pre-location

01. First, the healthy R-phase cable was tested along its full length as a reference. As shown in Figure 1, the single cable length is 2,743 meters. The two distinct sinusoidal reflection waveforms in the middle indicate the positions of the insulated joints, while the weaker sinusoidal reflection near the end indicates the location of the straight-through joint.

02. The low-voltage pulse method is used to test the full length of the S-phase cable compared to the full length of the R-phase, as shown in Figure 2 below. The red waveform represents the S-phase fault waveform, while the black waveform represents the full length of the R-phase. It can be observed that the R-phase experiences a disconnection at the "red marker" position, approximately 417.9 meters from the GIS terminal, and the disconnection point coincides exactly with the position of the straight-through joint. The fault is suspected to be located at the straight-through joint.

Step 3: Cable Path Search

The cable path information is clear and does not require searching.

Step 4: Fault Location Precision

S phase:

01. After applying pressure to the S-phase, we went to the straight-through joint chamber to observe. The appearance of the joint had been inspected previously and showed no issues. However, when approaching the joint in the chamber, a faint discharge sound could be heard, leading to the suspicion that an internal insulation fault occurred at the joint.

02. It was decided to dissect the joint. As shown in Figure 3 below, the main end of the cable joint has been scorched, and the metal shielding is damaged. However, the cable itself is not disconnected, indicating that the fault affecting power transmission is not located here.

03. Upon reanalyzing the waveform shown in Figure 2 above, the test gain was increased, and the local cursor position was zoomed in, as shown in Figure 4 below. It was found that the disconnection waveform position does not exactly coincide with the straight-through joint position, but is actually about 15 meters apart.

04. After applying pressure to the S-phase cable, the fault location was pinpointed 15 meters before the joint. The equipment detected a distinct discharge sound, and the minimum acoustic time difference at the fault point was 5.8 ms. The fault location is shown in Figure 5 below.

05. Since excavation cannot be immediately carried out at this location and on-site verification is not possible, confirmation will be made after subsequent excavation. The S-phase fault point has been successfully located.

T phase:

01. The low-voltage pulse test waveform for the T-phase is also a full-length waveform, indicating that the T-phase has not experienced a disconnection, but rather a high-resistance grounding fault due to insulation breakdown. The pulse current method, used in conjunction with the high-voltage unit, is required for distance measurement. The fault distance was measured to be approximately 5430 meters, which exceeds the single-circuit cable length (TT-phase short circuit at the GIS), suggesting that the fault point is on the T-phase of the other circuit.

02. The testing end was switched, and the fault waveform for the T-phase of the other circuit was measured under pressure. The fault waveform is shown in Figure 6 below. One cycle of this waveform corresponds to a fault distance of 50 meters.

03.After removing the 30-meter reserved coil near the near-end station, the fault point was found near the cable terminal. After applying pressure, noticeable ground vibrations were felt at a certain location. Excavation was carried out randomly, and the T-phase fault point was successfully located, as shown in Figure 7 below.

04. After the cable at the fault point was sawed through, insulation tests were conducted on both cable sections, and both passed. The T-phase fault was successfully located.

III. Test summary

01. The insulation joint metal sheath is split and disconnected. The waveform at the joint is usually more distinct. In the case of a straight-through joint where the metal shielding is fully connected, the waveform reflection is weaker and harder to detect. At this point, the distance between each joint can be compared for judgment. Generally, the three segments of cables in a cross-interconnected large section are of equal length.

02. Fault testing should carefully analyze multiple certifications. Until the fault is identified, all special situations are possible.

03. This straight-through joint is the endpoint of a large section of cross-interconnected cables, and the metal sheath must be directly grounded. The metal sheath at the GIS end connected to it must also be properly grounded for protection. Otherwise, the sheath in that section will continue to heat up during cable current operation or during a short circuit to ground. This is because the single-core voltage generates induced voltage on its metal sheath, and the existence of the loop leads to circulating current, which in turn causes the cable to heat up. The reason for the failure of the metal shielding in both straight-through joints is due to this issue.