High-voltage switch-mode assisted linear amplifier based voltage source for dielectric low frequency-domain spectroscopy measurements

This study presents a switch-mode assisted linear amplifier (SMALA) designed for use as a frequency-domain spectroscopy (FDS) excitation voltage source for the diagnostic testing of oil/paper insulation systems in large power transformers. The developed voltage source has a low output voltage noise level and low harmonic content and is capable of driving highly capacitive loads with a maximum output voltage of 800 Vp-p in the frequency range of DC to 100 Hz and current limited to 20 mA. Disturbances in the output voltage associated with switching of the main switching amplifier are minimised through the control of the switching time, taking the slew rate of the correction amplifier into account. The switching time of the main switching amplifier can be effectively controlled using a combination of a simple RC filter on the output of the main switching amplifier and controlling the rise-time and fall-time of the MOSFET gate drive voltage. Using the developed SMALA unit, it was possible to perform FDS measurement on an oil/paper-insulated transformer at an elevated excitation voltage. This resulted in a non-linear response current, manifesting as odd harmonics in the measured current.