The problems of HVAC/DC gas insulated systems (GIS) dielectric strength coordination and support spacers design criteria are discussed in this paper. The most important challenge is to protect the spacers against flashover and consequently exclude them from consideration when coordinating an insulation of GIS. Both experimental and numerical studies demonstrate that electric field formation mechanisms and insulation behavior at SF/sub 6//epoxy interface are substantially different for AC and DC voltage excitations. Nevertheless, in actual power systems, HVDC apparatus are frequently subjected to impulse and time-varying voltages, implying capacitive field grading, and vice versa, HVAC apparatus can actually be subjected to harmful pseudo-DC voltages occurring in certain operating modes. Therefore, the above-mentioned problem is considered in order to find generalized criteria which will provide a reliable spacer performance in both HVAC/DC gas insulated systems. To illustrate the specificity of SF/sub 6//epoxy insulation behavior at HVDC stress, static V-T characteristics for post-type spacer models were studied experimentally. Dielectric strength coordination requirements and constraints arising for AC/DC GIS conditions were discussed. Some design and technological alternatives directed to improve the DC spacer reliability were analyzed. Based on these results, generalized design criteria for AC/DC conical and post type spacers were formulated. For conventional range of GIS operating stress and insulation parameters, it has been shown that proposed design rules can be realizable with no additional enlargement of apparatus dimensions. Such an approach provides unification and interchangeability of support spacers in HVAC/DC SF/sub 6/ apparatus, which could be technologically worthwhile and economically beneficial for manufacturers and power utilities.

Dielectric strength coordination and generalized spacer design rules for HVAC/DC SF/sub 6/ gas insulated systems

This paper reports on a new cross-equipment study on the charging phenomena of solid insulators, evaluating the influence of charging on high-voltage equipment such as gas insulated switchgear (GIS), oil-immersed transformers, gas-insulated transformers and high-voltage vacuum-insulated equipment. New parameters of normalized charge density and surface resistance are introduced to consistently explain all the characteristics of the surface charge density on solid insulators and the decay time constant of the accumulated charges between the equipment. The results show that the surface charge density and the decay constant decrease as the surface resistance of the insulators is lowered. Combinations of insulating media of solid insulators and fluids (gas or liquids) are discussed for GIS and oil-immersed transformers in terms of the relation between the relative permittivity and the volume resistivity. The decay time constant of the accumulated charges is generally dominated by the insulating media of a fluid like oil which has lower volume resistivity than solid insulators in cases like transformers. However, the time constant is determined by the resistivity of the solid insulator rather than the fluid (gas) for GIS. Furthermore, it is suggested that low resistivity coatings on solid insulators are effective in suppressing the charge accumulation on the insulators. These new cross-equipment studies on the charging phenomena are an effective approach to reliable insulation design especially when downsizing high-voltage equipment

Cross-equipment study on charging phenomena of solid insulators in high voltage equipment

Hardware for use on HV systems is designed to be corona free. Recent hardware design activity has been centered around attempts to increase the corona onset voltage by optimizing the electric field distribution on its surface. Corona onset, however, is not only a function of the surface electric field, but also its rate of decay away from the surface, and the temperature and pressure of the gas in which it is immersed. While corona onset is a better criterion, formulas for predicting it have been validated only for simple geometries. The first goal of this work, then, is to validate corona onset conditions for more practical electrode geometries. The second goal is to determine whether the use of corona onset rather than surface electric field as an optimization criterion can result in hardware with a measurably higher corona onset voltage. To test this idea, two electrodes were designed, one using electric field optimization and the second using corona onset optimization. The corona onset voltage of each electrode was then measured and the results compared with predicted values. It was found that the measured results compared favorably with the predicted values and that the use of corona onset optimization can result in a modest but measurable increase in corona onset voltage.

Corona onset as a design optimization criterion for high voltage hardware

This paper reports on the electric field behavior in HVDC SF/sub 6/ gas insulated systems. In the field model definitions, the SF/sub 6//epoxy interfaces are assumed as thin slightly conducting layers with non-zero thickness. The boundary conditions involving surface-reduced parameters of these layers are adopted. This allows avoidance of problems of calculation regarding the proximity of interfaces between subdomains. The boundary-value problem describing capacitive-resistive time-varying electric field is formulated. On this basis, the transient HVDC electric field model and time domain-calculating algorithm are constructed. The model accounts simultaneously for both the surface layer properties and bulk conduction parameters of the bounding dielectrics, while surface conductivity may be specified as a non-linear or an arbitrary function. The method employs direct integration of a continuity equation in the time domain with the calculation of an electrostatic field approximation at each integration step. The basic features and applicability of the method are discussed. The model has been applied for analysis of HVDC electric field formation mechanism in SF/sub 6//epoxy insulation. It is demonstrated that charge accumulation phenomena cannot be described by a single time-stable mechanism. Some directions regarding further development of the field model and calculating algorithm are suggested.

Electric field modeling and field formation mechanism in HVDC SF/sub 6/ gas insulated systems

Capacitive-resistive field computations are carried out around post-type HV insulators of varying shapes. The boundary element method (BEM) has been employed for electric field computations. Different insulator shapes have been obtained by varying several parameters, which define the shape of the HV insulator contour. For each insulator shape, the maximum stress occurring on the insulator surface has been determined with no surface pollution, uniform surface pollution and also partial surface pollution. For partial pollution, several cases have been studied, in which different sections of the insulator surface are polluted. Furthermore, the effect of electrode radius on the maximum stress on insulator surface has been investigated. The results obtained are presented in this paper in detail.

http://schakravorti.info/pdf-papers/sc_tdei_2000_april_bem_insu.pdf

Boundary element studies on insulator shape and electric field around HV insulators with or without pollution

Capacitive-resistive field distributions including surface and volume resistivities have been computed in and around HV porcelain and capacitor bushings. The boundary element method has been applied for field computation in two axi-symmetric bushing configurations including four dielectrics. Effects of uniform and non-uniform distributions of surface resistivity and volume resistivities of different dielectric media are studied in detail. Special emphasis has been given to the stress distributions along the various dielectric-dielectric boundaries. Further, the values and locations of maximum stresses in different dielectrics are determined and reported in this paper. The plots of equipotential lines for a few typical cases of field distributions are also presented here for both bushings.

Capacitive-resistive field calculation on HV bushings using the boundary-element method

To avoid a large number of iterations, optimization of electrode shapes has been done by artificial neural networks (NN). Two practical examples have been considered, an axisymmetric single-phase GIS bus termination and an axisymmetric transformer shield ring. The shape of the electrodes has been taken as quarter-ellipse or half-ellipse because an ellipse has more flexibility than a circle. For NN, the so-called resilient propagation algorithm, learning faster than the standard back-propagation algorithm, has been employed. The training sets as well as the test sets of NN have been prepared by charge simulation method.

Optimization of HV electrode systems by neural networks using a new learning method

The high reliability and compactness achieved by compressed gas insulation in HVAC systems, has led to the development of HVDC Gas Insulated Substations (GIS). As in case of HVAC GIS, the basic insulation components of HVDC GIS are SF6 gas and solid spacers. In this respect, it has been recognized that the breakdown strength of GIS is mainly influenced by spacers. For HVAC GIS, the problems related to spacers can be eliminated to a great extent by a careful electric stress control design1-3. However, for HVDC GIS stress control designs based on dielectric interfaces with no trapped charges are not valid, since charges get accumulated on spacer surfaces which distort the designed Laplacian field distribution. In this context, it is also kept in mind that SF6 insulation is very sensitive to local enhancement of electric field. According to this surface charge accumulation and its effects in HVDC GIS have been studied in various aspects by several researchers4-11. In order to improve the dielectric performance of epoxy spacers by properly shaping the gas-dielectric interfaces, studies on electric field optimization along the profile of the gas-dielectric interface in gas insulated systems have been reported in literature12,13. For HVDC GIS, investigations on the effects of spacer shapes on flashover characteristics have also been carried out14. Since, in contrary to HVAC GIS, the electric field distribution at steady-state in HVDC GIS is mainly controlled by the conductivity k, of the dielectric media, studies on the influence of volume and surface resistivities on the electric field distribution around GIS spacers have been done15.

H. Steinbigler

Papers

Boundary element studies on insulator shape and electric field around HV insulators with or without pollution

Capacitive-resistive field calculation on HV bushings using the boundary-element method

Optimization of HV electrode systems by neural networks using a new learning method

Enhanced Field Calculation for HVDC GIS

capacitive-resistive Field Calculation around a HV Insulator Using Boundary Element Method