Partial discharge (PD) online monitoring of transformers by ultra-high-frequency (UHF) approaches has been paid increasingly attention recently. High-qualified UHF antennas are focused on as one key technique. This paper presents two types of compact UHF antennas in small sizes, a Hilbert fractal antenna and a small loop antenna. PD experiments of three typically artificial insulation defects are executed and the both antennas are used for PD measurement. The spectrums of power via frequency of detected PD signals are analyzed and compared. The experimental results show that the Hilbert fractal antenna and small loop antenna are qualified for PD online UHF monitoring.

Winding status is an important part of transformer fault diagnosis. In this paper, the diagnosis test platform includes a reduced-scale transformer with the connection group model of Ynyn0, and its voltage ratio is 10 kV/0.4 kV. The voltage response of single-phase windings of three-winding transformers with varying degrees of short-circuit faults is measured under a damped oscillation wave with an amplitude of 4 kV and an equivalent frequency of 20 kHz. The transfer function of high-voltage phase and its corresponding low-voltage phase in the normal and fault condition is calculated. Three characteristic criterion of the transfer function are analysed in order to diagnose short-circuit faults in single-phase windings of three-winding transformers.

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