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High Voltage Engineering
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Introduction
1.1 Generation and transmission of electric energy
The potential benefits of electrical energy supplied to a number ofconsumersfrom a common generating system were recognized shortly after the developmentof the ‘dynamo’, commonly known as the generator.The first public power station was put into service in 1882 in London(Holborn). Soon a number of other public supplies for electricity followedin other developed countries. The early systems produced direct ccurrent atlow-voltage, but their service was limited to highly localized areas and wereused mainly for electric lighting. The limitations of d.c. transmission at lowvoltagebecame readily apparent. By 1890 the art in the development of an a.c.generator and transformer had been perfected to the point when a.c. supplywas becoming common, displacing the earlier d.c. system. The first majora.c. power station was commissioned in 1890 at Deptford, supplying powerto central London over a distance of 28 miles at 10 000 V. From the earliest
‘electricity’ days it was realized that to make full use of economic generationthe transmission network must be tailored to production with increasedinterconnection for pooling of generation in an integrated system. In addition,the potential development of hydroelectric power and the need to carry thatpower over long distances to the centres of consumption were recognized.Power transfer for large systems, whether in the context of interconnectionof large systems or bulk transfers, led engineers invariably to think in termsof high system voltages. Figure 1.1 lists some of the major a.c. transmission
systems in chronological order of their installations, with tentative projectionsto the end of this century.
Generation of high voltages
A fundamental knowledge about generators and circuits which are in use forthe generation of high voltages belongs to the background of work on h.v.technology.Generally commercially available h.v. generators are applied in routinetesting laboratories; they are used for testing equipment such as transformers,bushings, cables, capacitors, switchgear, etc. The tests should confirm the efficiency
and reliability of the products and therefore the h.v. testing equipment
is required to study the insulation behaviour under all conditions which theapparatus is likely to encounter. The amplitudes and types of the test voltages,which are always higher than the normal or rated voltages of the apparatusunder test, are in general prescribed by national or international standards orrecommendations, and therefore there is not much freedom in the selection ofthe h.v. testing equipment. Quite often, however, routine testing laboratoriesare also used for the development of new products. Then even higher voltages
might be necessary to determine the factor of safety over the prospectiveworking conditions and to ensure that the working margin is neither too highnor too low. Most of the h.v. generator circuits can be changed to increasethe output voltage levels, if the original circuit was properly designed. Therefore,even the selection of routine testing equipment should always considera future extension of the testing capabilities.The work carried out in research laboratories varies considerably from oneestablishment to another, and the type of equipment needed varies accordingly.As there are always some interactions between the h.v. generating circuits usedand the test results, the layout of these circuits has to be done very carefully.The classes of tests may differ from the routine tests, andthereforespecially
designed circuits are often necessary for such laboratories. The knowledgeabout some fundamental circuits treated in this chapter will also support thedevelopment of new test circuits.
Voltage dividing systems and impulse voltage
measurements
The measurement of impulse voltages even of short duration presents nodifficulties, if the amplitudes are low or are in the kilovolt range only. Thetremendous developments during the last three decades related to the techniqueof common CROs, digital scopes or transient recorders provide instrumentswith very high bandwidth and the possibility to capture nearly every kind ofshort-duration single phenomena. Although the usual input voltage range ofthese instruments is low, h.v. probes or attenuators for voltages up to ome10 kV are commercially available.
The problems arise with much higher voltages and it is well known that
impulse voltages with magnitudes up to some megavolts are used for testingand research. The voltage dividers necessary to accommodate these voltagesare specialized apparatus, and there are only a few manufacturers throughoutthe world who are ready to produce such dividers with adequate accuracy. Selfprovidedconstructions are often adequate if the problems are known. But alsohe application of such voltage dividers needs a fundamental understanding ofthe interactions present in voltage dividing systems. Hence an attempt is made
to introduce the reader to the theory as well as to some hints on constructionaldetails on this quite difficult field of h.v. measuring techniques.We will start with a generalized voltage generation and dividing system andbriefly discuss the layout (section 3.6.1). Depending upon the voltage shape tobe measured, the requirements related to the whole measuring system must bewell defined (section 3.6.2). A generalized analytical treatment of the transfercharacteristics of this system involves the complex interactions between thedifferent parts of the circuit (section 3.6.3). The theory of the ‘isolated’ voltage
dividers as the most essential part of the circuit demonstrates the different typesof these devices and their possible applications (section 3.6.4). For fast transient
voltages the interactions between the dividers and their adherent circuitsare briefly discussed and methods for the evaluation of the transfer propertiesare presented (section 3.6.5). Some advice on a proper design of the 1.v. armof the voltage dividers is given (section 3.6.6). As the transient digital recorderhas recently become the most powerful tool for the evaluation of impulse voltages,an up-to-date introduction in this kind of instrument is provided in a separatesection (see 3.7), which is partly still related to voltage dividing systems.