Effects of Electric Arc on The Dielectric Properties of Liquid Dielectrics

In high voltage equipment, insulation is an important part. There are solid, liquid, gaseous insulations and combination between them used in various equipments in a power system. Liquid dielectric is widely used in transformer, circuit breaker, capacitor and oil-filled cable. During the operation of the equipment, electric arc may take place in the liquid insulation. The arc contains high energy in the form of thermal energy and photonic energy radiated by the arc. Electric arc appearance in a liquid insulation may degrade the performance of the insulation and may lead to the failure of the insulation. This paper reports the results of investigation on the influence of electric arc on the dielectric properties of liquid insulation. Several transformer oils were used as samples. The samples were subjected to an electric arc generated by applying AC voltage to steel needle electrodes in a needle-plane electrode system. The distance between needle and plane electrode was varied from 1 to 3 mm corresponding to applied voltage varied from 12 to 24 kV in order to obtain several sizes of electric arc. After applying electric arc to the samples, dielectric measurements were done. Dielectric properties measured were breakdown voltage (V_bd), dissipation factor (tan delta) and relative permittivity (epsiv_r). The influences of electric arc size and the duration of arc application were investigated. Chemical change of the oil samples was investigated using Fourier Transform Infra Red (FTIR). The results showed that electric arc reduced significantly the breakdown voltage and increased the losses factor. The application of arc slightly reduced the relative permittivity of the samples. FTIR analysis indicated that C=O bonding was generated during arcing in the oil samples