HVDC gas insulated apparatus: electric field specificity andinsulation design concept

This paper demonstrates the field model and time domain calculating methods and some of their important applications, helping advance an understanding of the complicated charge accumulation phenomena and its effect on the dielectric strength of a HVDC SF₆ insulation system     

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

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.     

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

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.