High Voltage Engineering

Generation and transmission of electric energy
The potential benefits of electrical energy supplied to a number of consumers
from a common generating system were recognized shortly after the development
of 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 followed
in other developed countries. The early systems produced direct ccurrent at
low-voltage, but their service was limited to highly localized areas and were
used mainly for electric lighting. The limitations of d.c. transmission at lowvoltage
became 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. supply
was becoming common, displacing the earlier d.c. system. The first major
a.c. power station was commissioned in 1890 at Deptford, supplying power
to 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 generation
the transmission network must be tailored to production with increased
interconnection for pooling of generation in an integrated system. In addition,
the potential development of hydroelectric power and the need to carry that
power over long distances to the centres of consumption were recognized.
Power transfer for large systems, whether in the context of interconnection
of large systems or bulk transfers, led engineers invariably to think in terms
of high system voltages. Figure 1.1 lists some of the major a.c. transmission
systems in chronological order of their installations, with tentative projections
to the end of this century.


Testing voltages
Power systems equipment must withstand not only the rated voltage (Vm),
which corresponds to the highest voltage of a particular system, but also
overvoltages. Accordingly, it is necessary to test h.v. equipment during its
development stage and prior to commissioning. The magnitude and type of
test voltage varies with the rated voltage of a particular apparatus. The standard
methods of measurement of high-voltage and the basic techniques for
application to all types of apparatus for alternating voltages, direct voltages,
switching impulse voltages and lightning impulse voltages are laid down in
the relevant national and international standards.


Direct voltages
In h.v. technology direct voltages are mainly used for pure scientific research
work and for testing equipment related to HVDC transmission systems. There
is still a main application in tests on HVAC power cables of long length, as
the large capacitance of those cables would take too large a current if tested
with a.c. voltages


Testing transformers
The power frequency single-phase transformer is the most common form of
HVAC testing apparatus. Designed for operation at the same frequency as the
normal working frequency of the test objects (i.e., 60 or 50 Hz), they may also
be used for higher frequencies with rated voltage, or for lower frequencies, if
the voltages are reduced in accordance to the frequency, to avoid saturation
of the core.


Generating voltmeters and field sensors
Similar to electrostatic voltmeters the generating voltmeter, also known as
the rotary voltmeter or field mill, provides a lossless measurement of d.c.
and, depending upon the construction, a.c. voltages by simple but mainly
mechanical means. The physical principle refers to a field strength measurement,
and preliminary construction was described by Wilson,
36 who used the
principle for the detection of atmospheric fields which are of small magnitude.


The measurement of peak voltages
Disruptive discharge phenomena within electrical insulation systems or highquality
insulation materials are in general caused by the instantaneous
maximum field gradients stressing the materials. Alternating voltages or
impulse voltages may produce these high gradients, and even for d.c. voltages
with ripple, the maximum amplitude of the instantaneous voltage may initiate
the breakdown. The standards for the measurement and application of test
voltages therefore limit the ripple factors for d.c. testing voltages, as the peak
value of d.c. voltages is usually not measured, and claim for a measurement
of the peak values of a.c. and impulse voltages whenever this is adequate.


Voltage dividing systems and impulse voltage measurements
The measurement of impulse voltages even of short duration presents no
difficulties, if the amplitudes are low or are in the kilovolt range only. The
tremendous developments during the last three decades related to the technique
of common CROs, digital scopes or transient recorders provide instruments
with very high bandwidth and the possibility to capture nearly every kind of
short-duration single phenomena. Although the usual input voltage range of
these instruments is low, h.v. probes or attenuators for voltages up to some
10 kV are commercially available.


Electrostatic fields and fieldstress control
In response to an increasing demand for electrical energy, operating transmission
level voltages have increased considerably over the last decades.
Designers are therefore forced to reduce the size and weight of electrical
equipment in order to remain competitive. This, in turn, is possible only
through a thorough understanding of the properties of insulating materials
and knowledge of electric fields and methods of controlling electric stress.


Electrical breakdown in gases
Before proceeding to discuss breakdown in gases a brief review of the fundamental
principles of kinetic theory of gases, which are pertinent to the study of
gaseous ionization and breakdown, will be presented. The review will include
the classical gas laws, followed by the ionization and decay processes which
lead to conduction of current through a gas and ultimately to a complete
breakdown or spark formation.


Breakdown in solids
Solid insulation forms an integral part of high voltage structures. The solid
materials provide the mechanical support for conducting parts and at the same
time insulate the conductors from one another. Frequently practical insulation
structures consist of combinations of solids with liquid and/or gaseous media.
Therefore, the knowledge of failure mechanisms of solid dielectrics under
electric stress is of great importance.


Non-destructive insulation test techniques
This chapter is dedicated to test techniques, which provide information about
the quality of insulation systems which form part of an equipment or apparatus.
The tests as described here take advantage of well-known or desirable electrical
properties of either a specific dielectric material or an insulation system as
formed by a combination of different (fluid and/or solid) materials. Although
also mechanical or chemical tests are often applied to assess the insulation
quality, such tests are not taken into account.


Overvoltages, testing procedures and insulation coordination
Power systems are always subjected to overvoltages that have their origin in
atmospheric discharges in which case they are called external or lightning
overvoltages, or they are generated internally by connecting or disconnecting
the system, or due to the systems fault initiation or extinction. The latter
type are called internal overvoltages. This class may be further subdivided
into (i) temporary overvoltages, if they are oscillatory of power frequency or
harmonics, and (ii) switching overvoltages, if they are heavily damped and
of short duration. Temporary overvoltages occur almost without exception
under no load or very light load conditions. Because of their common origin
the temporary and switching overvoltages occur together and their combined
effect has to be taken into account in the design of h.v. systems insulation.

:references
High Voltage Engineering
Fundamentals
Second edition
E. Kuffel
Dean Emeritus,
University of Manitoba,
Winnipeg, Canada
W.S. Zaengl
Professor Emeritus,
Electrical Engineering Dept.,
Swiss Federal Institute of Technology,
Zurich, Switzerland
J. Kuffel
Manager of High Voltage and Current Laboratories,
Ontario Hydro Technologies,
Toronto, Canada