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Because of excellent electrical properties, epoxy resin is widely used in packaging and casting power equipment. Moisture and temperature in the environment are inclined to seriously affect the insulation tolerance of epoxy resin. This work focuses on the aging characteristics of epoxy resin in hygrothermal environment. The effect of hygrothermal aging on the properties of epoxy resin was deeply studied. Scanning electron microscopy images show that there are micro-crack, micro-slit and deterioration of resin in the hygrothermal aging process. The moisture adsorption process undergoes three equilibrium stages and it does not follow the Fick's second law. In the hygrothermal environment, the content of small free molecules produced by the oxidation and hydrolysis of the material and the absorbed water molecules dynamically changes, which leads to an increase-decrease-increase variation in e'. After the aged sample is dried, its e' descends, but because the internal structure is destroyed, its original dielectric properties cannot completely restored. It is found that after chemical moisture adsorption emerging, the equipment is damaged severely.

We propose a class of Alamouti-like Orthogonal Space-Time Block Codes (STBC) for two additional information bits without source such as power, bandwidth and time slot. To maintain the full rank property of the coding gain matrix, we propose a set of STBCs for 4 transmit antennas transmission to transmit one additional information bit achieving rate-9/8. Another orthogonal STBC code with rate-1 is proposed in this paper within the set. It shows by computer simulation results that by employing the set of STBCs, it achieves better bit error rate (BER) performance and throughput than that of [1] with a valid BER improvement at high SNR region above 20dB.

The mechanism of thermogenesis and thermolysis of transformer was analyzed in this paper, the position of the hot-spot temperature was also analyzed. By calculating winding average surface temperature rising, a hot-spot temperature calculating model of dry -type transformer was established, and the calculating model of insulation life loss of dry-type transformer was established based on the hot-spot temperature calculating model. In the end, according to a practical example, the winding hot-spot temperature was calculated, the ageing rate of transformer winding under rated load was calculated by the calculating model, and the insulation life loss of the transformer was evaluated. The result shows that the hot-spot temperature of LV coil is higher than HV coil; the winding has a smaller aging rate and insulation life loss when less than rated load.