• Energy Research

With advancements in cable manufacturing processes, the physical parameters of certain cable conductors fall outside of the scope specified by the IEC60287-1-1 standard, and the alternating current (AC) resistance calculated using the IEC standard may lead to reliability issues in the thermal evaluation of cable lines. Therefore, conducting an AC resistance test on cable conductors becomes critical for the thermal evaluation of cable lines. The source of error in the existing AC resistance test was analyzed first. It was found that the characteristics of the source used in the test lead to an error between the test value and the actual value of AC resistance. Moreover, an optimized AC resistance testing method based on active power was proposed to decrease the error. The accuracy of the method was also demonstrated. Finally, AC resistance tests were conducted on cable conductors with different cross-sectional areas, segmental methods, and oxidation methods by using the proposed method. The test results are also thoroughly discussed.

A converter transformer is important primary equipment in a DC transmission project. The voltage on the valve side winding is complex when the equipment is running, including DC, AC, and AC–DC combined voltage. The insulation structure of the valve side winding of a converter transformer is an oil-paper insulation structure, which may have a variety of defects in the manufacturing stage and daily use, resulting in partial discharge. Therefore, it is the key to studying the partial discharge characteristics and mechanism of oil-paper insulation under AC–DC combined voltage. In this paper, we build a two-dimensional air gap model of oil-paper-insulated pressboard considering the actual particles and actual reaction based on the fluid model. The characteristics and evolution mechanism of partial discharge (PD) in pressboard under different AC/DC combined voltages are studied by numerical simulation. The results show that when the DC component increases, the polarity effect of partial discharge is more obvious, while the potential and discharge intensity in the air gap decrease. Further analysis revealed that the DC component in the combined voltage accumulated a large number of surface charges on the surface of the air gap, and the space charge distribution was more uniform and dispersed, which generated an electric field with opposite polarity to the DC component in the air gap and, then, inhibited the development of local discharge in the paperboard. The results of the simulation are consistent with the previous experimental phenomena, and the mechanism analysis of the simulation results also verifies the previous analysis on the mechanism of experimental phenomena. This will lay a theoretical foundation for the further study of partial discharge phenomenon of oil-paper insulation structures in practical operation in the future.

The ac resistance of power cable conductors is an important parameter for accurately estimating the ampacity of power cable and optimally designing power cable. Although international standard IEC-60287 has recommended a simplified formula for calculating the ac resistance, it is not capable of taking into account the effect of stranded conductor parameters, such as the direction and the length of lay, on the ac resistance because the stranded conductor is simplified as a solid conductor. In this paper, the method which has been used for analyzing the ac resistance of aluminum-conductor steel-reinforced overhead power lines is applied to single-core power cable conductors. Basically, the magnetic fluxes around each layer of the stranded wires are divided into circular magnetic fluxes and longitudinal magnetic fluxes, and thus each layer is equivalent to a circuit with resistor and inductor in series. Then, the ac resistance of single-core power cable conductors is calculated from the paralleled equivalent circuits representing all layers. A case study of 800 mm 2 single-core power cable is given to investigate the effect of the parameters of the stranded wires on the ac resistance. Results show that the ac resistance for bidirectional stranding of copper wires is a few percentage points higher than that for unidirectional stranding. Such difference will be significant in the optimization of cable design as even a slight decrease in ac resistance can save considerable copper in large-scale industrial application.