The influence of frequency of AC component on space charge behaviours in polyethylene under combined AC and DC electric fields
Combined ac and dc voltages may occur in high voltage power transmission system, which bring challenges to the dielectric materials. Generally speaking, space charge is well recognised under high voltage dc conditions but has received little attention under high voltage ac conditions. Under ac electric fields, the formation space charge in the material depends strongly on the frequency. The knowledge of space charge behaviour in dielectric materials under combined voltages is desirable in terms of both experimental and simulation. In this paper, a numerical model based on the bipolar charge injection/transport model is used to analyse characteristics of space charge in polyethylene under combined electric fields at room temperature. The DC component ratio and the overall root mean square (RMS) are kept the same, while the frequency of the AC component is altered from 0.05 Hz to 50 Hz. It has been found that the amount of the accumulated charges reduces with increasing frequency due to the more frequent polarity reversal. Moreover, the time to reach the steady states of charge amount increases with frequency as well.
A new numerical model applied to bipolar charge transport, trapping and recombination under low and high dc voltages
In this paper, we present a new numerical formulation that we have applied to the model for bipolar charge transport, trapping and recombination in polymeric insulators, such as low density polyethylene. The numerical model is based on the high precision Runge–Kutta method for determining mobile and trapped charge local density within the sample, during the application of a dc voltage. In addition, we have applied the finite element method considered as the most general one, which can be applied to any sample shape and boundary conditions. It is applied to resolve Poisson's equation, thus providing the potential and its gradient within the sample and at the dielectric–electrode interfaces. In a first step, the new formulation is validated for the charge dynamic under low dc applied voltage. In a second step, model outputs are analyzed under high dc applied voltage. The appearance of charge packets is revealed for the first time in modelling works although they have long been reported in experimental studies on polyethylene materials. The conditions for the appearance of such packets are discussed.
Pulsed electro-acoustic method for measurement of space charge distribution in power cables under both DC and AC electric fields
A modification of an earlier technique, to measure the distribution of space charge in the insulation of a coaxial cable, under both DC and AC electric fields, is described. The mathematical analysis is developed on the basis of acoustic waves propagating in media with cylindrical geometry. In order to measure the charge distribution in AC electric fields at any phase angle, a phase shift technique has been employed. The method of calibration of charge density is given, and the charge distribution in the insulation (2 mm in thickness) of XLPE cable has been measured under DC voltages (+or-40 kV, at room temperature) and AC voltage (17 kV peak, at 24 degrees C and 105 degrees C).