M.R. Raghuveer

Numerical prediction of HVDC wall bushing flashover caused by uneven wetting

In this paper, a numerical model is developed to determine the critical flashover voltage of an unevenly wetted HVDC wall bushing, The electric field of a 600 kV HVDC wall bushing is calculated by using the Finite Element Method, and the E-field distribution, particularly that along the critical line, is used to determine the critical flashover voltage (CFO). The simulation results show that predicted critical flashover voltages agree well with experimental data in literature. Furthermore, various aspects of uneven wetting flashover phenomena are verified. The influence of the dry zone length, rain conductivity, air pressure as well as altitude on the critical flashover voltage are also investigated using the developed model. The results presented in this paper suggest that the proposed E-field based model is an effective-tool to evaluate the performance of an HVDC wall bushing.

Detection of temporary faults during impulse tests using wavelets

The impulse test on power transformers simulates the conditions that exists in service when a transformer is subjected to an incoming high voltage surge due to lightning or other disturbances on the associated transmission line. There have been several studies on the application of wavelets to identify faults during impulse testing. Most of these studies have utilized simulated waveforms in which the fault is introduced by a suitable mathematical model. In this paper the authors have used a new technique to simulate such faults. Experimentally obtained waveforms are used to investigate the suitability of wavelets to detect faults during impulse testing and this paper presents preliminary results of this research work.

Optimal wavelet selection to identify faults during impulse tests

The purpose of conducting lightning impulse tests on a transformer is to check the dielectric strength of the transformer insulation against overvoltages of atmospheric origin. Several signal analysis methods have been proposed to overcome the limitations associated with conventional fault detection techniques. In one such group of methods test waveforms are analyzed to localize the disturbance in both time and frequency. There have been several studies on the application of wavelets to identify faults during impulse testing. Since there are a number of wavelets available, the question arises as to how to choose the most appropriate wavelet for a particular application. In this paper, a method based on statistical analysis is used to select the optimum wavelet to detect faults during impulse testing and preliminary results of the research work are presented.

Technique to examine the influence of voltage harmonics on leakage current based MOSA diagnostic indicator

The degradation of Metal Oxide Surge Arresters (MOSA) in service is diagnosed by using the third harmonic content of the resistive portion of the arrester leakage current as an indicator. The third harmonic content consists of two components, i.e., a component due to the nonlinear volt-ampere characteristics of the MOSA and an error component due to the presence of voltage harmonics. In order to assess the discerning ability of an on-site diagnostic method to the error component its indicator has to be compared with that of a benchmark method. This paper discusses such a method.

Analytical solution and scaled model of a unipolar HVDC transmission line

Existing analytical solutions for the ionized field of a unipolar HVDC transmission line are reviewed. The analytical relations indicate that the corona current is directly proportional to the square of the corona onset field strength if the ratio of the applied voltage to the corona onset value and that of the conductor height to its radius remain fixed. This is confirmed by numerical results. It is pointed out that this observation makes it possible to derive the basis of a scaled model to evaluate the corona current and hence the resulting power loss of a full-scale unipolar DC line.

On the corona voltage-current characteristic of unipolar HVDC transmission lines

Simplified analytical analysis of ionized fields associated with HVDC transmission lines is possible if it is based on Deutschs assumption. The resulting voltage-current (V-I) characteristic may be conveniently used to calculate the power losses of a HVDC transmission line. However, such an approach has been criticized because Deutschs assumption is considered invalid for the line-plane geometry. This paper verifies the validity of the V-I characteristic thus obtained by comparing it with numerical results which are generated using a new method developed by the authors, in which Deutschs assumption is not employed.

A new method for solving ionized fields associated with HVDC transmission lines

In this paper, a new method for solving the ionized fields associated with HVDC transmission lines is presented. The boundary value problem is transformed into an optimization problem by using the finite element method. Numerical tests on a coaxial cylindrical geometry show that the new method is very efficient. This method has also been applied to a practical line-plane geometry.

The influence of electrode material upon prebreakdown current in viscous dielectric oil

Silicone fluids are now being used as an insulator. Watson,2 Hakim,3 and Hosticka4 have carried out some investigations on this liquid. There is still, however, a great deal that is unclear about the breakdown mechanism under high electric field. It is of importance to know the behavior of the current stress characteristic in the region below breakdown, since the breakdown itself in the liquid dielectric is the ultimate point of the conduction stage.1 It is, therefore, worthwhile characterizing more fully their dielectric performance in this region of electric field. Watson2 has studied the influence of electrode separation upon current conduction in silicone oil and has shown that a linear relationship between the slope of the log voltage vs. log current [d(ℓnV)/d(ℓnI)] ani gap separation exists. He has also shown this phenomenon to be consistent with a model involving Fowler-Nordheim emission which initiates a convective charge transport mechanism with the fluid flowing in a vortex cell enclosing a steady irrotational funnel flow through its core at the cathode. In his interpretation, negative charge will be trapped in the moving core of liquid dielectric, and the space charge is contained laterally by the tendency of the vortex ring to resist stretching by the outward component of the electrostatic Maxwell stress. He reports that the Maxwell stress from this space charge will increase and enlarge the vortex by raising the applied voltage. This suggests that the phenomenon may be influenced by the viscosity of the liquid. The effect of electrode materials was also considered from the viewpoint of their capability to inject charge under stress.

Some aspects or a digital method for the measurement of loss factor of high voltage insulation

Periodic monitoring of the insulation condition of power system apparatus is necessary in order to estimate its deterioration and maintain the reliability of electric power supply. In this paper a digital technique for the on-line evaluation of loss factor is presented. The Discrete Fourier Transform is applied to the acquired voltage and current signals and the loss factor computed by consideration of the phase shift between the fundamental quantities of voltage and current. The effects of sampling rate, the number of measurements and system frequency fluctuation on the estimated value of loss factor are discussed.

Analysis of the ionized field associated with a bundled DC line including effect of wind

In this paper, the ground level ionized field quantities of a bundled line are investigated by using an upwind node-centered finite volume method (FVM) based algorithm recently introduced by the authors. The geometries considered are typical of practical HVDC transmission lines and the effect of wind is included. The validity of the equivalent single conductor approach is verified.