14-08-2012, 02:54 PM
High Voltage Engineering
1High Voltage.pdf (Size: 4.48 MB / Downloads: 175)
Introduction
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.
Voltage stresses
Normal operating voltage does not severely stress the power system’s insulation
and only in special circumstances, for example under pollution conditions,
may operating voltages cause problems to external insulation. Nevertheless,
the operating voltage determines the dimensions of the insulation which forms
part of the generation, transmission and distribution equipment. The voltage
stresses on power systems arise from various overvoltages. These may be of
external or internal origin. External overvoltages are associated with lightning
discharges and are not dependent on the voltage of the system. As a result,
the importance of stresses produced by lightning decreases as the operating
voltage increases. Internal overvoltages are generated by changes in the operating
conditions of the system such as switching operations, a fault on the
system or fluctuations in the load or generations.
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.
Testing with power frequency voltages
To assess the ability of the apparatus’s insulation withstand under the system’s
power frequency voltage the apparatus is subjected to the 1-minute test under
50 Hz or 60 Hz depending upon the country. The test voltage is set at a level
higher than the expected working voltage in order to be able to simulate
the stresses likely to be encountered over the years of service. For indoor
installations the equipment tests are carried out under dry conditions only. For
outdoor equipment tests may be required under conditions of standard rain as
prescribed in the appropriate standards.
Testing with lightning impulse voltages
Lightning strokes terminating on transmission lines will induce steep rising
voltages in the line and set up travelling waves along the line and may
damage the system’s insulation. The magnitude of these overvoltages may
reach several thousand kilovolts, depending upon the insulation. Exhaustive
measurements and long experience have shown that lightning overvoltages are
characterized by short front duration,
D.C. voltages
In the past d.c. voltages have been chiefly used for purely scientific research
work. Industrial applications were mainly limited to testing cables with relatively
large capacitance, which take a very large current when tested with a.c.
voltages, and in testing insulations in which internal discharges may lead to
degradation of the insulation under testing conditions. In recent years, with
the rapidly growing interest in HVDC transmission, an increasing number of
industrial laboratories are being equipped with sources for producing d.c. high
voltages. Because of the diversity in the application of d.c. high voltages,
ranging from basic physics experiments to industrial applications, the requirements
on the output voltage will vary accordingly. Detailed description of the
various main types of HVDC generators is given in Chapter 2.
1High Voltage.pdf (Size: 4.48 MB / Downloads: 175)
Introduction
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.
Voltage stresses
Normal operating voltage does not severely stress the power system’s insulation
and only in special circumstances, for example under pollution conditions,
may operating voltages cause problems to external insulation. Nevertheless,
the operating voltage determines the dimensions of the insulation which forms
part of the generation, transmission and distribution equipment. The voltage
stresses on power systems arise from various overvoltages. These may be of
external or internal origin. External overvoltages are associated with lightning
discharges and are not dependent on the voltage of the system. As a result,
the importance of stresses produced by lightning decreases as the operating
voltage increases. Internal overvoltages are generated by changes in the operating
conditions of the system such as switching operations, a fault on the
system or fluctuations in the load or generations.
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.
Testing with power frequency voltages
To assess the ability of the apparatus’s insulation withstand under the system’s
power frequency voltage the apparatus is subjected to the 1-minute test under
50 Hz or 60 Hz depending upon the country. The test voltage is set at a level
higher than the expected working voltage in order to be able to simulate
the stresses likely to be encountered over the years of service. For indoor
installations the equipment tests are carried out under dry conditions only. For
outdoor equipment tests may be required under conditions of standard rain as
prescribed in the appropriate standards.
Testing with lightning impulse voltages
Lightning strokes terminating on transmission lines will induce steep rising
voltages in the line and set up travelling waves along the line and may
damage the system’s insulation. The magnitude of these overvoltages may
reach several thousand kilovolts, depending upon the insulation. Exhaustive
measurements and long experience have shown that lightning overvoltages are
characterized by short front duration,
D.C. voltages
In the past d.c. voltages have been chiefly used for purely scientific research
work. Industrial applications were mainly limited to testing cables with relatively
large capacitance, which take a very large current when tested with a.c.
voltages, and in testing insulations in which internal discharges may lead to
degradation of the insulation under testing conditions. In recent years, with
the rapidly growing interest in HVDC transmission, an increasing number of
industrial laboratories are being equipped with sources for producing d.c. high
voltages. Because of the diversity in the application of d.c. high voltages,
ranging from basic physics experiments to industrial applications, the requirements
on the output voltage will vary accordingly. Detailed description of the
various main types of HVDC generators is given in Chapter 2.