A polymer barrier inserted in the interelectrode gap can significantly improve the insulation performance of air‐insulated high‐voltage electrical equipment. With the development of power equipment towards integration and miniaturisation, multiple barriers can be used to further increase the insulation level. This paper investigated the withstand characteristics of the air‐insulated rod‐rod gap with multiple barriers by changing the barrier numbers, the barrier position and the barrier insertion direction. Four control modes were set up based on the assumed breakdown paths under alternating voltages. It was found that the control modes with barriers inserted from the opposite direction had a more significant improvement effect than that from the same direction. When the horizontal barrier position was closer to the electrode and the vertical barrier position was farther to the electrode axis, the breakdown voltage was higher. The vertical barrier position was a key factor affecting the probability of breakdown path type, and the breakdown voltages along different types of breakdown paths were almost the same when the barriers were at the same position. The increase of the breakdown voltage was mainly resulted from the change of electric field distribution in the insulated gap and the extension of the breakdown path length.

In this article, the effect of residual charges on the lightning impulse voltage testing in the air insulation gap and the hybrid gas–solid insulation gap within the range of 30–50 cm gap distances was studied using the modified up-and-down test (UDT) method. The impulse voltage testing was performed by changing the gap distance, the resting time, and the initial voltage. Experimental results show that the residual charges cause continuous breakdown and withstand during the testing when the gap distance is 30 cm, which cannot be considered by the standard UDT. When the residual charges are removed after the gap breakdown, the continuous breakdown and withstand phenomenon is disappeared, and the 50% impulse breakdown voltage ${U}_{50}$ is decreased under different gap distances, resting times, and initial voltages. The effect of the residual charges is strengthened with the decrease in the gap distance. When the breakdown path length is close to 30 cm in the air insulation gap and the hybrid gas–solid insulation gap, the effect of residual charges is significant. The modified UDT method is recommended when residual charges are present during impulse testing, which can be indicated by the number of tests.

When submarine cable line fails or other lines need load transfer, it often suffers from emergency ampacity that exceeds the steady-state ampacity. The layout environment of the submarine cable is always complex and changeable, and the overload capacity of the submarine cable in different layout environments is also different. Therefore, it is necessary to analyze the overload capacity of the submarine cable. In this paper, a coupled multi-physical field model by finite element method is established for AC 500 kV XLPE extra high voltage submarine cable in landing section, which is the ampacity bottleneck section of the whole line. The overload capacity of submarine cable in two typical layout environments which are direct buried and within pipeline is analyzed. The results show that the overload capacity of submarine cable in the direct buried environment is much higher than that in the pipeline environment. The allowable emergency time in the direct buried environment is 2–3 times that of the pipeline environment under the same condition. In the two typical layout environments, when the emergency current are 2500 A and 3500 A, the ratio of the emergency time allowed to run in the direct buried environment to that in the pipeline environment is about 5 times under the same initial capacity. The proposed model can provide a reference for dynamic capacity control of the extra high voltage submarine cable.

With the large-scale development of deep-sea offshore wind power, the insulation safety of high-voltage submarine cable outer sheaths faces severe challenges, yet the propagation characteristics and coupling mechanisms of cable-core overvoltages to the metal sheath remain unclear. To address this, a physical experimental platform was established for lightning impulse testing based on a 66 kV submarine cable, and the transient potential response characteristics of the metal sheath under various operating conditions were systematically investigated. The results indicate that under fully suspended conditions, the sheath voltage is primarily determined by the voltage division caused by the radial coaxial distributed capacitance. When a 30.6 kV lightning impulse voltage is applied, the overall potential of the sheath approaches that of the cable core; under conditions where the sheath is grounded at the end, the capacitively coupled charge is rapidly discharged, and the transient potential is then dominated by inductive coupling, with its variation strictly corresponding to the rate of change in the cable-core’s transient current; under conditions where the sheath is fully suspended and the cable core is grounded, the potential of the metal sheath exhibits a dynamic superposition of the radial capacitive common-mode voltage and the axial inductive differential-mode potential difference. These findings demonstrate that metallic-sheath grounding conditions significantly influence the relative contributions of capacitive and inductive coupling to transient sheath overvoltages under lightning impulse excitation. The experimental results provide a physical basis for further investigation of sheath grounding arrangements and overvoltage protection strategies.