HIGH VOLTAGE ENGINEERING (HVE)

CHAPTER 3. GENERATION AND MEASUREMENT OF HIGH VOLTAGES

Generation of high voltages Three main kinds of high voltages are used in modern industry – alternating, direct and pulsed ones. Alternating voltages As electric power transmission with high alternating current (a.c.) voltages predominates in our transmission and distribution systems, the most common form of testing high voltage (h.v.) apparatus is related to high a.c. voltages. It is obvious then that most research work in electrical insulation systems has to be carried out with this type of voltage.

It may well be understood that the design of the h. v It may well be understood that the design of the h.v. winding become difficult if voltages of more than some 100 kV must be produced within one coil. Better constructions are available by specialized techniques, mainly by “cascadin transformers”. For voltages higher than about 300 kV, the cascading of transformers is a big advantage, as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier

Direct voltages In high voltage technology direct voltages are mainly used for pure scientific research work and for testing equipment related to high voltage direct current (HVDC) transmission systems The rectification of alternating currents is the most efficient means of obtaining HVDC supplies.

Impulse voltages Impulse voltages have applications in such areas as plasma, discharge, particle beam sources, nuclear and thermonuclear researches, overvoltage pulses modeling, high voltage testing of high voltage equipment and others. Basic circuits for single–stage impulse generators are shown in Figure:

In order to overcome these difficulties, in 1923 German researcher and engineer Marx suggested an arrangement where a number of condensers are charged in parallel through high ohmic resistances and then discharged in series through spark gaps.

Measurement of high voltages Measurement of high voltages – d.c., a.c. or impulse voltages – involves unusual problems that ma not be familiar to specialists in the common electrical measurement techniques. These problems increase with the magnitude of the voltage, but are still easy to solve for voltages of some 10 kV only, and become difficult if hundreds of kilovolts or even megavolts have to be measured.

The difficulties are mainly related to the large structures necessary to control the electrical fields, to avoid flashover, corona and sometimes to control the heat dissipation within the circuits. This subchapter is devoted to the measurement of voltages applied for the testing of h.v. equipment or in research. Voltage–measuring methods used within the electric power transmission systems, e.g. instrument transformers, conventional or non–conventional ones, are not discussed. Such methods are summarized in specialize courses and given in references.

Electrostatic voltmeter is used to measure both – d.c. and a.c. voltages. Main part of the device is pair measuring electrodes. Electrical field forces on metal electrode with some mechanical force. This force is proportional to voltage square. Electrode is connected with device scale. Scale shows the value which is corresponding electrical field force, which, in turn, corresponds to measuring voltage. The electrostatic measuring device can be used for absolute voltage measurements,

Technology of high voltage capacitor The main requirements for technology of H.V. capacitors are: the capacitance C shall be independent of magnitude of voltage level and shall not change with time of application (no ageing effects); the temperature coefficient shall be small or very small; dependent on the kind and temperature range of application, and shall at least be known; the effective inductivity of C shall be as small as possible, if used for high–frequency applications, i.e. voltage dividers for impulse voltages.

Main condition of measurement procedure is following: Voltage dividers Any voltage divider consists of two main parts: high voltage arm and low voltage arm. Main condition of measurement procedure is following: signal form on low voltage arm must exactly repeat signal form in measurement circuit.

Voltage dividers are three main types, depending on which kind of circuit element is used for dividing purpose: resistive, capacitive resistive-capacitive (mixed type). Signal level on low voltage arm is appropriate to match it with accurate measurement device which allows to analyze the signal parameters.