Converter transformers play an important role in the high-voltage DC power transmission system. The insulation of these transformers is stressed by both AC and DC voltages. Particle contamination is one of the major problems in insulation structures, as they generate partial discharges. A methodical experimental study was carried out to understand the partial discharge formation because of single macroscopic conducting particle movement in transformer oil at different temperatures under DC voltages using ultra-high-frequency (UHF) technique. It is observed that the single particle moves in the vertical electrode gap under the DC voltage, and generates UHF signal in the range of 1-3GHz. At high electric fields, the levitated particle sometime floats in the oil medium and as and when the applied voltage is removed, the particle slowly drops down. When it comes in contact with ground electrode, it generates UHF signals with the frequency content lying in the range of 1-3GHz. Oil breakdown tests were carried out (without any particle in the oil electrode gap) under DC voltages and fast Fourier transform (FFT) analysis of the signal generated by the UHF sensor because of breakdown indicates that the frequency content of the UHF sensor signal lies up to 1GHz. © 2009 The Institution of Engineering and Technology.