-The life of a power transformer is dependent on the life of the cellulose paper, which influenced by the hot spot temperature. Thus, the determination of the cellulose paper's life requires identifying the hot spot temperature of the transformer. Currently, however, the power transformer uses a heat run test is used in the factory test to measure top liquid temperature rise and average winding temperature rise, which is specified in its specification. The hot spot temperature is calculated by the winding resistance detected during the heat run test. This paper measures the hot spot temperature in the single-phase, 154kV, 15/20MVA power transformer by the optical fiber sensors and compares the value with the hot spot temperature calculated by the conventional heat run test in the factory test. To measure the hot spot temperature, ten optical fiber sensors were installed on both the high and low voltage winding; and the temperature distribution during the heat run test, three thermocouples were installed. The hot spot temperature shown in the heat run test was 92.6℃ on the low voltage winding. However, the hot spot temperature as measured by the optical fiber sensor appeared between turn 2 and turn 3 on the upper side of the low voltage winding, recording 105.9℃. The hot spot temperature of the low voltage winding as measured by the optical fiber sensor was 13.3℃ higher than the hot spot temperature calculated by the heat run test. Therefore, the hot spot factor (H) in IEC 60076-2 appeared to be 2.0.

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-VSC-HVDC system is vulnerable to a DC fault because the fault current can be injected from AC system to DC system during the fault. Therefore, DC circuit breaker is required to isolate faults in VSC-HVDC system. The inverse current injection method of circuit breaker has been considered as DC circuit breaker. However, the topology has drawback that the breaking time is longer than hybrid circuit breaker using semiconductor devices. In order to solve this problem, this paper proposes an improved topology of circuit breaker based on inverse current injection method . In addition, the proposed topology will be compared with the existing topology. And we will verify its effects by using the simulation results.

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