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8
Insulation Design
Insulation design is one of the most important aspects of the transformer design. It is the heart of transformer design, particularly in high voltage transformers. Sound design practices, use of appropriate insulating materials, controlled manufacturing processes and good house-keeping ensure quality and reliability of transformers. Comprehensive verification of insulation design is essential for enhancing reliability as well as for material cost optimization.
With the steady increase in transmission system voltages, the voltage ratings of power transformers have also increased making insulation content a significant portion of the transformer cost. Also, insulation space influences the cost of active parts like core and copper, as well as the quantity of oil in the transformer, and hence has a great significance in the transformer design. Moreover, it is also environmentally important that we optimize the transformer insulation which is primarily made out of wood products. In addition, with the associated increase in MVA ratings, the weight and size of large transformers approach or exceed transport limits. These reasons together with the ever-increasing competition in the global market are responsible for continuous efforts to reduce insulation content in transformers. In other words, margin between withstand levels and operating stress levels is reducing. This requires greater efforts from researchers and designers for accurate calculation of stress levels at various critical electrode configurations inside the transformer under different test voltage levels and different test connections. Advanced computational tools (e.g., FEM) are being used for accurate calculation of stress levels. These stress levels are compared with withstand levels which are established based on experimental/published data.
For the best dielectric performance, reduction in maximum electric stress in insulation is usually not enough; the following factors affecting the withstand
327 Copyright © 2004 by Marcel Dekker, Inc.
328 Chapter 8
characteristics should be given due consideration, viz. waveform of applied voltage and corresponding response, volt-time characteristics of insulation, shape and surface condition of electrodes, partial discharge inception characteristics of insulation, types of insulating mediums, amount of stressed volume, etc. Minimization of non-uniform dielectric fields, avoiding creepage stress, improvement in oil processing and impregnation, elimination of voids, elimination of local high stresses due to winding connections/crossovers/ transpositions, are some of the important steps in the insulation design of transformers. Strict control of manufacturing processes is also important. Manufacturing variations of insulating components should be monitored and controlled. Proper acceptance norms and criteria have to be established by the manufacturers for the insulation processing carried out before high voltage tests. The transformer insulation system can be categorized into major insulation and minor insulation. The major insulation consists of insulation between windings, between windings and limb/yoke, and between high voltage leads and ground. The minor insulation consists of basically internal insulation within the windings, viz. inter-turn and inter-disk insulation. The chapter gives in details the methodology of design of the major and minor insulations in transformers. Various methods for field computations are described. The factors affecting the insulation strength are discussed. In transformers with oil-solid composite insulation system, two kinds of failures usually occur. The first kind involves a complete failure between two electrodes (which can be jump/bulk-oil breakdown, creepage breakdown along oil-solid interface or combination of both). The second one is a local oil failure (partial discharge), which may not immediately lead to failure between two electrodes. Sustained partial discharges lead to deterioration of the insulation system eventually leading to a failure. The chapter discusses these failures and countermeasures to avoid them. It also covers various kinds of test levels and method of conversion of these to an equivalent Design Insulation Level (DIL) which can be used to design major and minor insulation systems. Statistical methods for optimization and reliability enhancement are also
introduced.
8.1 Calculation of Stresses for Simple Configurations
For uniform fields in a single dielectric material between bare electrodes, the electric stress (field strength) is given by the voltage difference between the electrodes divided by the distance between them,
(8.1)
The above equation is applicable to, for example, a parallel plate capacitor with one dielectric.
Copyright © 2004 by Marcel Dekker, Inc.
Insulation Design 329
Figure 8.1 Multi-dielectric configuration
For non-uniform fields (e.g., cylindrical conductor—plane configuration), the stress (Enu) is more at the conductor surface; the increase in stress value as compared to that under the uniform field condition is characterized by a non-uniformity factor (η),
(8.2)
The non-uniformity factor is mainly a function of electrode configuration.
For a multi-dielectric case between two parallel plates shown in figure 8.1, the stress in any dielectric for a potential difference of V between the plates is
(8.3)
where εi is relative permittivity of ith dielectric. This expression for the configuration of parallel plates can be derived by using the fact that the stress is inversely proportional to permittivity. The stress value is constant within any dielectric.
For two concentric cylindrical electrodes of radii r1 and r2, with a single dielectric between them as shown in figure 8.2, the stress in the dielectric is not constant and varies with radius. The stress at any radius r(r1
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