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Why insulated cables break?

Why insulated cables break?
Why do insulated cables break?

Article Background Information:

In May 2020, our engineers found that 72 cables connected to a high-power, high-voltage inverter device had different degrees of broken insulation when overhauling the machine. Meanwhile, self-inspection and self-checking were conducted at three other gas compressor stations, and the same problems were found.

The cable is between the phase-shifted isolation transformer and the power unit cabinet and the power cabinet and the motor feeder cabinet in the high-power, high-voltage inverter device, and the cable model is JEH-W-10kV-1×120mm2. Three pressurized gas stations were operated in 2017 and 2020, respectively. This part of the cable was put into operation along with the inverter device, and it has been used for a total of 1.32~1.46 million hours. The lines are damaged at the outer sheath insulation layer, the cable bridge cross-arms, and the cable glands on the secondary side of the isolation transformer.

What are our inspection methods?

1. Inspection of the cable between the isolation transformer and the power cabinet

Inspection of the cable between the isolation transformer and the power cabinet at the top of the inverter cabinet revealed apparent cracks, and the outer sheath of the cable insulation in contact with the grounding lead showed noticeable burn marks.

2. Inspection of cables in contact with the bridge

Inspection of the wires in the cable bridge at the top of the cabinet revealed apparent cracks in the outer sheath insulation of the wires at the location where the wires are in contact with the bridge.

3. Isolation transformer cable glen head cable inspection

Inspection of the cable glands at the location of the secondary side outlet of the isolation transformer reveals evident traces of discharge in the outer sheath insulation layer of the cable at this location.

Cause analysis

Accident cable for high-power high-voltage frequency conversion device within the phase-shifting isolation transformer to the power unit cabinet and power cabinet to the motor feeder cabinet between the line, cable model JEH-W-10kV-1×120mm2, it was found that this cable for the rubber-coated flexible wires does not have a metal shielding outer sheath. At the same time, through the cable quality, cable selection and other normal aging cracking, cable induced current, corona discharge, extreme sheath discharge, close laying, and additional internal force accumulation, Glen head, cable fixture fixing and winter construction, and additional external force accumulation analysis of cracking aspects of a comprehensive analysis.

1. Is the insulated cable aged and cracked?

From the perspective of normal cable aging and poor cable quality inspection and analysis, after investigation, as of the accident, several pressure station cables run for up to 7 years, far from reaching the average service life of the line, according to the third-party laboratory on the aging and failure analysis of the line, can be excluded from the aging caused by cracking factors. Third-party laboratory from the conductor material, single wire diameter, insulation thickness, insulation minimum thickness, the average thickness of the sheath, and the minimum thickness of the test results are in line with the norms and can be excluded from the cable core quality factors.

2. Analyze whether the conductor meets the heat conditions.

Frequency conversion device parameters: Frequency conversion devices with the load are compressors, a single variable device with a total power of 25MVA, isolation transformer ratio 10/1.76kV, unit rated capacity of 2083kVA, unit input voltage of 1.76kV, unit input current of 683A, frequency converter rated output voltage of 10kV, inverter rated output current of 1250A, the form of unit series of four series. Phase shifting transformers to each inverter unit cable use 3 JEH-W-10kV 1 × 120mm2 parallel cable.

Cable capacity and voltage calibration: cable rated voltage of 10kV is much larger than the system operating voltage of 1.76kV. The line meets the requirements: cable capacity according to the ambient temperature, laying mode, routing correction: single-core cable actual ambient temperature of 40 ℃, take the temperature correction factor of 1, the single-core cable zigzag arrangement (take the correction factor of 0.97), and more circuits share a bridge (600 wide) The cable is arranged in 2 layers (with a correction factor of 0.65), and the comprehensive correction factor is 0.63.

Frequency conversion device unit rated current of 683A, the correction factor to be selected by the laying of permissible load (40 ℃ environments in the air) should not be less than 1084A; after checking the JEH-W-10kV 1 × 120mm2 single-core cables in the environment at 40 ℃, air laying, zigzag arrangement of the permissible load of 365A, 3 and with the allowable load of 1095A, to meet the requirements.

3. What is the thermal stability of the cable?

After calculating the RMS value of the short-circuit cycle component of the secondary side of the isolation transformer 38.1kA, the minimum cable cross-section to meet the thermal stability should be 128mm2, 3 120mm2 cables in parallel is much larger than the minimum cable cross-section to meet the requirements of thermal equilibrium.
According to the cable standards and design manuals, three 1 × 120mm2 cables in parallel can meet the basic requirements, excluding the cable selection factors.

4. Is the insulation cracked due to the build-up of internal forces?

Cable surcharge and induced current factors

AC three-phase single-core unshielded cable through the cable bridge with magnetic flux, resulting in alternating induced current, the measured leakage current of the cable bridge can be up to 7.3A, alternating induced current generates rotating magnetic field, three-phase single-core unshielded cable through this alternating magnetic field, will produce additional current in the contact point of the cable and cable bridge to increase the temperature of the local cable wire core. At the same time, the cable bridge in the cable around the formation of the closed electromagnetic circuit produces eddy current and hysteresis loss, improving the local cable core temperature. In the superimposed effect, the cable and cable bridge contact point cable core temperature reaches more than 90 ℃. Insulation and outer sheath thermal conductivity are relatively low; the insulation layer is heated to produce a significant expansion, and the external sheath expansion is relatively tiny, accumulating internal stress. To a certain extent, the cable appeared in small cracks, paraffin wax precipitation, and continued to develop into visible cracks.

Corona discharge factors

Rubber flexible cable is concentrated at the bottom so that the cable outer sheath insulation layer is subjected to a stronger magnetic field; the cable outer sheath insulation layer and the cable bridge contact surface produce corona discharge, resulting in localized high temperature, resulting in the accumulation of internal stresses, resulting in cracks in the cable, the distribution schematic shown in Figure 1.

Corona discharge factors

Figure 1: Schematic diagram of field strength distribution of cables with shielded and unshielded layers
Figure 1: Schematic diagram of field strength distribution of cables with shielded and unshielded layers

Cable outer sheath discharge factors

Epoxy resin composite bridge (with metal keel) on non-shielded cables will induce a high potential difference between the cable sheath discharge, resulting in localized high temperatures, the accumulation of internal stress, and cracks in the line.

Cable parallel close laying factors

According to “Power Engineering Cable Design Standards,” the configuration of the cable arrangement on the same layer of support should be in line with the following provisions: In addition to the AC system with a single-core power cable for the same circuit can be taken to the Pinzigang (trefoil) configuration, for the important of the same course of more than one power cables, it is not suitable for stacking. Article 5.1.5 stipulates that the phase sequence configuration of single-core power cable for AC system and its inter-phase distance shall comply with the following provisions: the average induced voltage of the metal jacket of the cable shall be satisfied without exceeding the permissible value; single-core power cable without zigzag configuration, two or more circuits configured in the same pathway shall be counted as mutual influence.
Single-core power cables should be laid in a zigzag shape and reasonably arranged to increase the cable spacing. Isolation transformer to power cabinet cables are applied in parallel closely, the formation of proximity effect and skin effect, so that the charge is concentrated in the conductor cross-section of the local, reducing the permissible conductor current-carrying capacity, and leading to heat accumulation, causing local high temperature, resulting in the build-up of internal stresses, resulting in cracks in cables.

5. Analysis of the causes of cracking caused by the accumulation of external forces

The granular head at the cable joint force factors

After the scene of the actual inspection, it was found that the transformer to the power cabinet, the power cabinet to isolate the output cabinet cable parallel close laying, and direct contact with the metal base plate, and the design requirements of the zigzag laying do not match. At the same time, it also found that the cable at the glen head was to withstand external forces, and part of the cable’s outer sheath insulation layer was damaged. In summary, if the cable path selection requirements’ cable standards do not match, the line should avoid suffering mechanical external forces, overheating, corrosion, and other hazards. The actual site is inconsistent with the requirements of the cable acceptance standards for the distance between the cable pivots.

Cable fixing and bending radius factors

The inspection also found that the secondary side of the transformer cable is not fixed with cable clamps but directly sag into the bridge, so that the head of the cable sag all the weight of the individual cable bending radius does not meet the requirements of the soft cable turning radius of 6 times the bending radius of the sheath to lead to the tensile stress increases, the external force accumulation triggered by cracking.

Winter construction factors

In winter cable construction, the too large temperature differences in the process, too significant temperature differences to the cable acceptance standards, the line allows the laying of the minimum temperature, the average temperature in the 24h before laying, and the temperature of the radiation site should not be lower than the provisions; when more melancholy than the provisions of the measures should be taken. After reviewing the completion data, we did not find the relevant records during the winter construction period can be excluded from the winter construction factors.

Handling of the accident and summary of experience

According to the above analysis, the cable itself, without a metal shielding layer and eddy current in the cable bridge, is the main reason for the accident. Therefore, it is necessary to replace all the cables in the batch and replace the line with cross-linked polyvinyl chloride (XLPE) with a metal shielding layer, type YJV-8.7/15kV (with shielding), rated voltage 8.7/15kV. Replace the fiberglass-lined cable bridges with Aluminum alloy cable trays. At the same time, the following six aspects should be noted in the design and construction of this type of project:

(1) The single-core cable of a cross-linked system should have a metal shielding layer, and the cable bridge for laying the single-core cable of an AC system should be made of non-hysteresis material, such as aluminum alloy. It should not be made of hot-dip galvanized steel or glass fiber reinforced plastic cable bridge with steel lining.

(2) for cross-linked system single-core cable metal sheath at least at one end of the direct grounding, in any non-direct grounding end of the average induced potential maximum value to meet the specification requirements. Therefore, in selecting single-core cable by the specification in Appendix F, the calculation of the AC system single-core cable metal layer of the ordinary induced potential equation ultimately determines the metal jacket’s grounding choice.

(3) 35kV and the following single-core cable fixed with the selection of components in line with the following provisions: in addition to AC single-core power cables, can be used by the corrosion-resistant flat steel fixture, nylon cable ties or plastic-plated metal ties; corrosive solid environments should be used nylon cable ties or plastic-plated metal ties; AC single-core power cables are suitable for rigid fixing of aluminum alloy, etc., does not constitute a magnetically closed loop of the clamp, and other fixing methods can be used Nylon cable ties or ropes; wire shall not be used to tie the cable directly. AC single-core power cable selected parts of the mechanical strength of the short-circuit electric power conditions should be calculated. Therefore, the cross-linked system of single-core cable fixing fixtures must use aluminum alloy and other materials tied with nylon material to avoid installation errors.

(4) 35kV and the following single-core cable laying needs to meet the bending radius requirements sing fixed fixtures per the cable standards and acceptance criteria to do an excellent job setting the line to prevent the cable head from external forces.

(5) Replacement of the cable part of the region, such as the inability to take the zigzag laying, it is recommended to use insulation boards to ensure the cable spacing.

(6) According to the cable construction and acceptance standards, the average temperature of the cable within 24 hours before laying, as well as the temperature of the laying site, should not be lower than the following requirements:

Rubber-insulated polyethylene sheathed cables (e.g., JEH-W) are allowed to be laid at a minimum temperature of -15℃;

The lowest temperature permitted for laying PVC insulated PVC sheathed cables -10°C”.


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