HV Bushings Research Articles

Temperature Distribution in the Insulation System of Condenser-Type HV Bushing—Its Effect on Dielectric Response in the Frequency Domain

HV bushings are an important part of the equipment of large power transformers, responsible for their many serious (including catastrophic) failures. Their proper exploitation needs to apply correct and reliable diagnostics, e.g., the use of dielectric response methods, that take into account their specific construction and working conditions. In this article, based on laboratory tests carried out on a real bushing, it has been shown that the significant temperature distribution within its core significantly affects the shape of the dielectric response of its insulation; therefore, the approach to its modeling should be changed. Hence, a new method for interpreting the results, using the so-called the 2XY model, is proposed. Subsequently, based on the measurements made on the insulators in operation, a new modeling method was verified. In conclusion, it can be stated that the 2XY model significantly improves the reliability of the dielectric response analysis, which should be confirmed in the future by tests on withdrawn and revised insulators.

Condition Assessment of HV Bushings with Solid Insulation based on the SVM and the FDS Methods

This paper presents the dielectric response of the insulation of bushings as an effect of the simulated long-term aging process. The experiment was conducted under a condition of a high temperature difference between the current circuit and the cover. The dielectric response was measured with the FDS (Frequency Dielectric Spectroscopy) and the SVM (Step Voltage Measurement) methods. The research has shown the correlation between the aging time and some parameters obtained with the FDS and SVM analysis.

Test Results and Conclusions From a 1 MVA Superconducting Transformer Featuring 2G HTS Roebel Cable

We present results from commissioning testing of a 1 MVA three-phase transformer demonstration project. The three-phase transformer utilizes a 15 strand Roebel cable for the low voltage windings and 4 mm wide conductor for the high voltage windings. Both windings use ReBCO conductor and the iron core is external to the cryogenic region. The target operating temperature is 70 K. Key design features and operational performance measures of the advanced sub-cooled liquid nitrogen circulation system are described. The overall thermal budget is presented along with ac loss measurements for the windings and electrical test results of the complete transformer. There remain some unresolved issues with the stability of the cryogenic cooling system when operated sub-cooled, and with the high voltage withstand performance of the HV bushings. These issues are described and discussed. The utility of HTS Roebel cable as a convenient means of producing a compact, high current winding with low ac loss characteristics has been successfully demonstrated.

Fluid influence on dynamic characteristics of transformer-bushing system using fluid structure interaction (FSI) approach

Currently used procedures for seismic qualifications of HV bushings are based on the analytical equations, and very often are verified by shake table tests. As many specialists claim this approach does not fully reflect the real behavior of the bushing coupled with the transformer tank during seismic loads. In the paper a new methodology for seismic analysis of the transformer/bushing system has been presented using complex numerical simulations. In the proposed approach Fluid Structure Interaction is involved. The methodology has been experimentally verified and can provide useful information for proper design of transformer tank and bushings with high resistance to seismic loads.

A study of electrical stress grading of composite bushings by means of a resistive silicone rubber coating

Local discharges in the vicinity of the mounting flange of composite bushings due to the electric field enhancement can cause insulating material degradation and finally lead to failure. In this paper, a proposed solution to this problem by employing a resistive silicone rubber coating on composite HV bushings was investigated. Finite Element Method (FEM) analysis of the electric field distribution along the surface of bushings by FEMLAB ® simulation and potential measurements by an electrostatic probe (ESP) technique confirmed that the stress grading effect of a 100 MΩ resistive coating results in a linear potential distribution and the tangential field is significantly reduced in the region of the mounting flange.

Numerical performance analysis of semiconductor coatings for corona suppression

Corona is initiated in high field regions. To suppress corona or to increase the corona inception voltage in HV insulators or bushings, suitable semiconductor coatings, able to relieve high electric fields, are commonly used. The choice of appropriate materials to be used for the most effective corona suppression is typically made on the basis of expensive and time consuming trial and error tests or on the basis of approximate theoretical analysis. In this paper a guideline, based on the adoption of a suitable numerical method, is provided in order to lead to the optimal choice of the electrical characteristics of semiconductor materials for the most effective corona suppression.

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

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.

Seismic qualification of transformer high voltage bushings

The paper is born as a contribution to the IEC 36A Working Group 2 "Seismic qualification of bushings" in order to evaluate the coefficients K (amplification factor between the ground and the transformer bushing flange) and R (response factor of the bushing itself) as defined in the IEC 61463. These coefficients allow the definition of the seismic severity at the flange level to be used for static calculations and vibrational tests on high voltage bushings. For the evaluation of these coefficients, both experimental on-site dynamic tests and numerical finite element analyses have been performed for a 160 MVA, 230/135 kV power transformer. The natural frequencies and the seismic response of the whole transformer (tank, oil conservator and HV bushings), subjected to different earthquakes, have been calculated. A comparison between the calculated coefficients (K and R) and those established by the IEC 61463 is illustrated.