Stops machines unplanned caused by faults in the electrical insulation of the power electric system equipment is a big problem, because account for high losses to plan and undertake performance indicators due long stop period generating unit necessary for the generation function restoration. In order to solve this problem, many researchers and scholars in the field of large rotating machines found that these failures were often related to the increased level of partial discharges (PD) of electrical insulation of the machine. Occurrences of PDs in insulating systems of high voltage equipment are symptoms of weakness in their dielectric withstand, whose evolution can cause serious consequences for the equipment. The PDs can be detected by measuring quantities associated with current pulses, radio frequency signals (electromagnetic waves), acoustic signals, among others. Among these methods the detection of electromagnetic waves radiated by PDs are proving to be a reliable technique to follow the default process of evolution. In comparison with other techniques such as the detection method based on capacitive couplers (conventional method), the use of electromagnetic broadband sensors is not invasive to isolation. In this way, this paper aims to show the evaluation of three different electromagnetic sensors of Ultra High Frequency (UHF) designed to be installed in hydrogenerators. The sensors used in the experiments are: microstrip directional coupler, loop antenna and log-periodic antenna. The sensors were evaluated by high-voltage tests to measure partial discharges which were evaluated all kids of discharges found in hydrogenerators, like internal discharges, delaminations, slot discharge, bar-bar and surface discharges, and through the accelarated aging test. The results obtained in the laboratory demonstrated that the directional coupler showed a high sensibility to all types os simulated discharges which results were validated by log-periodic antenna, and the loop antenna showed a low sensibility. The results also demonstrated that is possible to identify discharges through the UHF characteristics. The evolution of the PDs levels were evaluated through the directional coupler during the accelarated aging test, it was possible to observe the appearance of two types of discharges, delaminations and corona, the results were similar to those measured by the capacitive coupler.
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