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Summary This paper investigates the influence of metallic sheaths on alternating current (AC) losses and ampacity of power cables. The AC losses are expressed as AC/direct current (DC) resistance in power cables, which is the main factor in loss computations. With regard to the importance of harmonic contents of the currents, a comprehensive study is performed on AC/DC resistance of medium‐voltage cables in the presence of harmonics. Different harmonic frequencies include triple and nontriple orders are considered. A derating factor is proposed on the basis of cable constructions. Two types of sheathed and unsheathed medium‐voltage power cables are considered. Also for metallic sheathed cables, 2 types of sheath grounding are investigated. It is shown that single‐point bonding and both‐ends bonding of sheaths have different effects on cable ampacity in harmonic frequencies. Moreover, different conductor cross sections and distances between cables are considered, and their effects on AC/DC resistance and ampacity derating are assessed. It is shown that these factors have important effects on AC/DC resistance and thus power cable losses. Furthermore, all cases are investigated in both flat and trefoil formations that are the most popular formations in power cables. To achieve precise results, we performed this study on the basis of finite element simulation, which is an exact way to cable losses computations in each configuration.

This paper presents a novel technique for transformer differential protection to prevent incorrect operation due to inrush current. The proposed method in this paper is based on time-frequency transform known as the Wavelet transform. The discrete Wavelet transform is used for analysis the differential current signals in time and frequency domains. The investigation on the energy distribution of the signal on the discrete Wavelet transform components shows the difference distribution between inrush and internal fault current signals of power transformer. The correlation factor which is a statistical parameter is used here to express the pattern of the energy distribution for different current signals. The proposed algorithm is based on the correlation factors to distinguish between internal fault and inrush currents in the transformer differential protection. The proposed algorithm is tested and simulated for several cases by simulating inrush and internal fault currents. The simulation of inrush and internal fault currents are performed using electromagnetic transient program PSCAD/EMTDC software. Simulation results show that the proposed scheme accurately identifies inrush and fault currents at the distance of the power transformer protection in a time period less than quarter of power frequency cycle. In addition the proposed method has high sensitivity and reliability. The method has low computation work and not requires determining the threshold for each new power system.