Download Measurement of high voltages

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. 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.
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.