Abstract
This paper reports the experimental investigation of the corona discharge activity of punga oil under the influence of a local magnetic field and different voltage profiles (AC and DC voltages) using fluorescent fiber sensor as well as UHF sensor technique. The corona inception voltage (CIV) of punga oil is higher under negative DC voltage followed by positive DC and AC voltage, with a marginal reduction in its CIV magnitude observed on the impact of external magnetic field. The dominant frequency of the UHF signal obtained under AC voltage shifted towards lower frequency (0.6 GHz) with the influence of magnetic flux density to about 85 mT. The rise time, pulse width and energy content of the fluorescent signal formed due to corona activity under AC voltage is found to vary under high magnetic fields confirming the inception results. The fluorescent signals formed due to corona discharge under both AC and DC voltages have its dominant frequency at 1 MHz with no shift observed in the presence of magnetic field. Also, there is no variation in the phase resolved partial discharge (PRPD) pattern observed due to corona discharge (with and without magnetic field) signal using both UHF sensor and the fluorescent sensor. The fluorescent fiber-based technique provides a better accuracy on detecting the corona discharges in punga oil at an early stage compared to conventional UHF sensor. The breakdown voltage of punga oil under different voltage profiles with and without the effect of magnetic field follows normal distribution. The dielectric dissipation factor and electrostatic charging tendency (ECT) of punga oil is observed to be higher than the limit set for insulating fluids towards power transformer operation.
Highlights
The insulation design of oil-filled power transformers is vital for their longevity and reliable operation in a power system network
The application of different fibers tested towards partial discharges (PD) detection in transformers [29] and in the present work, the red and white fiber was chosen since its excitation and emission characteristics are similar to that of punga oil
One of the objectives of the present study is to identify a suitable fluorescent fiber (Table I), which need to be chosen for its better performance towards corona detection
Summary
The insulation design of oil-filled power transformers is vital for their longevity and reliable operation in a power system network. Mineral oils have been traditionally used as an insulant and coolant in the power transformers. The higher thermal class of ester fluids [2] compared to mineral oil makes them suitable to be placed closer to the buildings, resulting in lower power losses. Alternative insulants such as vegetable oils (soybean, sunflower, punga) are being tested for transformer applications [3]. In comparison to the conventional mineral oil, these ester-based fluids derived from various plant seeds have shown better fire class properties with increased biodegradability [4,5]. Punga oil (non-edible) have been used nowadays for transformer applications [6]. Punga oil has been considered to understand its performance in partial discharge behaviour and dielectric properties. Punga oil derived from Milletia pinnata tree has a variety of fatty acids (oleic, palmitic and stearic) that determines the physicochemical and dielectric properties of the insulating oil [7]
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