Chapter 8. High Voltage Insulation Design

Chapter 8. High Voltage Insulation Design - 8 HIGH VOLTAGE...

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285 8. HIGH VOLTAGE INSULATION DESIGN Summary Transformer insulation must not only be designed to withstand the normal operating voltages but to survive the effects of lightning strikes and other possible disturbances such as switching operations which may occur on the electrical system. In order to assess the adequacy of the insulation, it is necessary to have some understanding of the breakdown process, especially in liquids and solids which are normally used in combination in large power transformers. Although our understanding in this area is very incomplete, some trends or correlations have been deduced from test data. Using these and much accumulated experience, design rules have been formulated whose main justification is that they work in practice at least in most cases. It is also necessary to have some means of calculating the voltages and electric stresses which occur in a transformer under normal and especially abnormal conditions such as lighting strikes. From such calculations, voltages, voltage differences, and electrical stresses can be obtained and compared with the breakdown limits. Although a standardized waveform has been developed to represent a typical lightning strike which reaches a transformer, simple step function approximations are often used in approximate analytical calculations. The test of design adequacy comes when the unit is built and is subjected to a variety of electrical tests to simulate the abnormal conditions. The ultimate test comes from the unit’s survival in service for long periods of time. 8.1 INTRODUCTION A transformer’s insulation system must be designed to withstand not only the a.c. operating voltages, with some allowance for an ~15% overvoltage, but also the much higher voltages produced by lightning strikes or switching operations. These latter voltages can be limited by protective devices such as lightning or surge arresters but these devices are usually set to protect at levels well above the normal a.c. operating voltage. Fortunately the transformer’s insulation can withstand higher voltages for the shorter periods of time characteristic of lightning or switching disturbances. Thus, insulation designed to be adequate at the © 2002 by CRC Press
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HIGH VOLTAGE INSULATION DESIGN 286 operating voltage can also be sufficient for the short duration higher voltages which may be encountered. Insulation design is generally an iterative process. A particular winding type is chosen such as disk, helix, or layer, for each of the transformer’s windings. They must have the right number of turns to produce the desired voltage and must satisfy thermal, mechanical, and impedance requirements. The voltage distribution is then calculated throughout the windings, using a suitable electrical model together with the appropriate input such as a lightning impulse excitation. Voltage differences and/or electric fields are then calculated to determine if they are high enough to cause breakdown, according to some breakdown
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Chapter 8. High Voltage Insulation Design - 8 HIGH VOLTAGE...

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