introduction on high voltage engineering

UNIT I OVER VOLTAGES IN ELECTRICAL POWER
SYSTEMS
Causes of over voltages and its effects on power system –
Lightning, switching surges and temporary over voltages,
Corona and its effects – Bewley lattice diagram- Protection
against over voltages.

Course Objective
•To impart knowledge on the various types of
over voltages in power system and protection
methods

Course Outcomes
• Understand various types of over voltages in power
system and protection methods

Introduction
HIGH VOLTAGE ENGINEERING
High-voltage engineering is the science of planning, operating,
,measuring and testing high-voltage electrical devices and
generating, designing the insulation coordination in order to ensure
the reliable operation of the power network.
• Insulation co-ordination is the technique used to ensure that the
electrical strengths of the various items of plant making up the
transmission and distribution system and their associated protective
devices are correlated to match the system characteristics and
expected range of voltages.

Advantages of Transmitting Electricity At High
Voltage
Reduction in Power Loss
• Power from traditional sources like coal-fired plants is usually produced far
away from large cities. These power plants are built in areas that are rich in
the resources used to create power, but it means the power needs to be
transported long distances before it can be used.
• Wires of every size have some amount of resistance simply due to the
copper and aluminium they are made from. Over great distances, this
resistance adds up and the amount of power lost as heat can be expensive
for the energy provider. Transmitting power at high voltages is the simplest
way of reducing these losses. Increasing the voltage of the electricity
means the current can be decreased and the same amount of power can
be transmitted with lower losses due to the resistance of wiring and other
conductors.

• Increased Power Transmission Efficiency
Transmitting power at high voltage and low current makes the entire
system more efficient. Not only does it reduce infrastructure costs,
but more power can be transmitted over larger distances. When
designing high voltage power systems, engineers account for the
distance travelled and the expected losses due to resistance.
In a system where more power is lost between the power plant and
the substation, the power plant would need to supply more power to
keep up with demand. By reducing the amount of power lost in
transmission, power plants can produce less of the electricity that
substations need to pass onto paying consumers.

• Asset Power Solutions are High Voltage Electricians
High voltage power systems are an electrical staple that makes our
modern world possible. Asset Power Solutions are licensed and
trained for work on networks up to 132kV. We can provide safe
installations and repairs for your business’ high voltage power needs,
and our expert technicians can ensure your job is completed on time
and on budget. Contact the team at Asset Power Solutions today for
an appointment or to find out more about our high voltage electrical
services.

Lower Infrastructure Costs
• The size of the cabling needed to transmit power is proportional to the
amount of current that will pass through it. So, when transmitted at higher
currents, larger wire sizes are used. High voltage cabling can be a relatively
small diameter than to the low currents passing through.
• Over large distances, sometimes hundreds or thousands of kilometres, the
cost of power cabling quickly stacks up. When added to the cost of
substations and transformers, the size of the cabling makes a large
difference to infrastructure costs. Even transmission towers would need to
be engineered to handle larger wire sizes. With high voltage cabling already
weighing in at several tonnes per kilometre, supporting thicker gauges of
wire would require far more substantial transmission towers. Using high
voltages and thinner gauges of wire is cheaper for power producers and
infrastructure suppliers, as well as for end consumers wanting affordable
electricity.

APPLICATIONS OF HIGH VOLTAGE
• Cathode ray tubes
• Particle accelerators
• Xerography
• Spray painting
• Electrostatic precipitators
• Ignition in internal combustion engines
• Gas discharge lamps
• Ozone generators
• Nuclear research

Electrostatic precipitators
• Used in thermal power plants and industries
• Flue gas and exhaust gas are to be made free from dust particles and
other solid waste particles before they are let in to the atmosphere.
Principle:
For dust collection it uses the electrostatic force(an attractive or
repulsive force caused by the electric charge particles ,also called
columb force).
-Consists of discharge wire and collecting plates.

• A High voltage is applied to the discharge wires to form an electric field
between the wires and the collective plates.
• Also ionizes the gas around the discharge wires to supply ions
• When the gas that contains an aersol (dust ,mist)flows between the
collecting plates and the discharge wires, the aerosol particles in the gas
are charged by ions.
• The attractive force caused by the electric field causes the charged
particles to be collected on the collective plates and the gas is purified.
• Particles collected on the collective plates are removed by methods such :
-Dislodging by rapping the collecting plates
-Scraping off with a brush
-Washing off with water and removing from a hopper.

Why we are going for high voltage
• Demand of electrical energy increases day by day
• Size of the power system increases
• Need very high voltages to avoid the building cost ,operating cost of
smaller power stations.
• Transmission of voltages should be high to reduce power loss and reduce
the cost of transmission
• For measuring high generating voltages, we should have high voltage
measuring devices and high current measuring devices.
• Electrical power apparatus like insulators ,cables, power transformers
,circuit breakers, surge arresters should be designed to withstand such high
voltages and high currents.

Electric power is the product of two quantities: current
and voltage. These two quantities can vary with
respect to time (AC power) or can be kept at constant
levels (DC power).
Most refrigerators, air conditioners, pumps and
industrial machinery use AC power whereas most
computers and digital equipment use DC power (the
digital devices we plug into the mains typically have
an internal or external power adapter to convert from
AC to DC power).

• AC power has the advantage of being easy to
transform between voltages and is able to be
generated. DC power remains the only practical
choice in digital systems and can be more economical
to transmit over long distances at very high voltages
(see HVDC)

• The ability to easily transform the voltage of AC power is important
for two reasons: Firstly, power can be transmitted over long distances
with less loss at higher voltages. So in power systems where
generation is distant from the load, it is desirable to step-up
(increase) the voltage of power at the generation point and then step-
down (decrease) the voltage near the load.
• Secondly, it is often more economical to install turbines that produce
higher voltages than would be used by most appliances, so the ability
to easily transform voltages means this mismatch between voltages
can be easily managed Solid state devices, which are products of the
semiconductor revolution, make it possible to transform DC power to
different voltages, build brushless DC machines and convert between
AC and DC power.

Structure of Power Systems:
Structure of Power Systems –
• Generating stations, transmission lines and the
distribution systems are the main components of an
electric power system.
• Generating stations and a distribution system are
connected through transmission lines, which also
connect one power system (grid, area) to another. A
distribution system connects all the loads in a
particular area to the transmission lines.

• For economical and technological reasons, individual power
systems are organized in the form of electrically connected
areas or regional grids (also called power pools).
• Each area or regional grid operates technically and
economically independently, but these are eventually
interconnected to form a national grid (which may even form
an international grid) so that each area is contractually tied
to other areas in respect to certain generation and
scheduling features. India is now heading for a national grid.

• The siting of hydro stations is determined by the natural water power
sources. The choice of site for coal fired thermal stations is more
flexible. The following two alternatives are possible.
• Power stations may be built close to coal mines (called pit head
stations) and electric energy is evacuated over transmission lines to
the load centres.
• Power stations may be built close to the load centres and coal is
transported to them from the mines by rail road.

• In practice, however, power station siting will depend upon many
factors technical, economical and environmental. As it is considerably
cheaper to transport bulk electric energy over extra high voltage
(EHV) transmission lines than to transport equivalent quantities of
coal over rail road, the recent trends in India (as well as abroad) is to
build super (large) thermal power stations near coal mines.
• Bulk power can be transmitted to fairly long distances over
transmission lines of 400 kV and above. However, the country’s coal
resources are located mainly in the eastern belt and some coal fired
stations will continue to be sited in distant western and southern
regions.

• As nuclear stations are not constrained by the problems of fuel
transport and air pollution, a greater flexibility exists in their siting, so
that these stations are located close to load centers while avoiding
high density pollution areas to reduce the risks, however remote, of
radioactivity leakage.
• In India, as of now, about 75% of electric power used is generated in
thermal plants (including nuclear). 23% from mostly hydro stations
and 2%. come from renewables and others. Coal is the fuel for most
of the steam plants, the rest depends upon oil/natural gas and
nuclear fuels.

• Electric power is generated at a voltage of 11 to 25 kV which then is
stepped up to the transmission levels in the range of 66 to 400 kV (or
higher). As the transmission capability of a line is proportional to the
square of its voltage, research is continuously being carried out to
raise transmission voltages. Some of the countries are already
employing 765 kV. The voltages are expected to rise to 800 kV in the
near future. In India, several 400 kV lines are already in operation.
One 800 kV line has just been built.

For very long distances (over 600 km), it is economical to transmit bulk
power by DC transmission. It also obviates some of the technical
problems associated with very long distance AC transmission.
The DC voltages used are 400 kV and above, and the line is connected
to the AC systems at the two ends through a transformer and
converting/inverting equipment (silicon controlled rectifiers are
employed for this purpose). Several DC transmission lines have been
constructed in Europe and the USA.
In India two HVDC transmission line (bipolar) have already been
commissioned and several others are being planned. Three back to
back HVDC systems are in operation.

• The first stepdown of voltage from transmission level is at the bulk
power substation, where the reduction is to a range of 33 to 132 kV,
depending on the transmission line voltage. Some industries may
require power at these voltage levels. This step-down is from the
transmission and grid level to sub-transmission level.
• The next stepdown in voltage is at the distribution substation.
Normally, two distribution voltage levels are employed:
• The primary or feeder voltage (11 kV)
• The secondary or consumer voltage (415 V three phase/230 V single
phase).
• The distribution system, fed from the distribution transformer
stations, supplies power to the domestic or industrial and commercial
consumers.

Need for EHV Transmission
• To provide adequate grid system capacity ,electricity transmission
lines need to operate at 400KV or765 KV.The existing transmission
capacity is inadequate .
• So the developers are contracting to connect to EHV transmission
system.
• EHV transmission provide more reliable and less constrained
electricity network capacity.

• Increasing in size of generating units:
Volume α 1/VL
2
Cost α volume
As the voltage increases ,volume of conductor decreases and cost of
the line decreases and the size of generating unit increases
Transmission of large amount of power over long distance is
economically feasible by EHV Transmisson.

• Increase in transmission efficiency;
As the voltage increases ,current flows through the line decrease and I2
R loss reduces. so transmission efficiency increases.
Pithead steam plants and remote hydro plants:
Cost of transportation depends on cost of coal in thermal plants. To
avoid this stream of thermal plants are suited near coal mines is
called as pithead steam plants.
Hydro plants are mostly situated at remote areas. In remote
places, water availability is more ,land and labour cost is cheap. EHV
systems are needed to transmit large amounts of power over long
distances from pithead and remote hydro plants to load centres.

• Number of Circuits and land requirement:
As voltage increases number of circuits and land requirement for
transmission decreases.
Low Costs:
As voltage increases the line installation cost/MW/KM decreases.
The total cost including the cost of losses/MW/KM decreases with
increase in voltage.

Surge impedance loading:
Surge impedance loading is proportional to the square of the
operating voltage to surge impedance loading increases as voltage
increases.
• PSIL= VRL
2/ZC
• Where zc =Surge impedance= √(L/C)

Advantages of EHAC Transmission Voltage
Advantages of High Transmission Voltage
1. Reduces the Volume of Conductor Material
Consider the electric power being transmitting through the three-phase three-wire
transmission system.
Let,
P = Power transmitted (in Watts)
V = Line voltage (in Volts)
cosϕ= Load power factor
R = Resistance per conductor (in ohms)
ρ= Resistivity of conductor material
l = length of transmission line (in meters)
a = cross sectional area of conductor
Therefore, the load current is given by,

• And the resistance per conductor is
• Thus, the total power loss in the transmission line is

• As there are three conductors, the total volume of conductor material
required is given by,
From equation (1), it is clear that for the given values of P, ρ, l and W,
the volume of conductor material required is inversely proportional to
the square of transmission voltage and load power factor.
Therefore, if the power is transmitted at high voltage, then lesser is
the conductor material required.

2. Decreases Percentage Line Drop
The voltage drop in the transmission line is given by,
Let J is the current density of the conductor, then
From equation (2), it is clear that the percentage line drop is inversely
proportional to the transmission voltage. Therefore, the percentage line
drop decreases when the transmission voltage increases.

3. Increases Transmission Efficiency
• The input power to the transmission line is given by,

• Since the transmission efficiency is defined as,
• By using Binomial theorem, we get,
• Since ρ, l and J are constants, therefore the transmission efficiency
increases when the transmission voltage increases.

Disadvantages of EHV Transmission System
1) Corona loss and radio interference
The corona loss is greatly influenced by choice of transmission voltage.
If weather conditions are not proper then this loss further increases.
There is also interference between transmission line and
communication line which causes disturbance.
2) Line supports
In order to protect the transmission line during storms and cyclones
and to make it wind resistant, extra amount of metal is required in
the tower which may increase the cost.

3) Erection difficulties
There are lot of problems that arise during the erection of EHV lines. It
requires high standard of workmanship. The supporting structures are
to be
efficiently transported.
4) Insulation needs
With increase in transmission voltage, insulation required for line
conductors also increases which increases its cost.
5) The cost of transformers, switchgear equipments and protective
equipments increases with increase in transmission line voltage.
6) The EHV lines generates electrostatic effects which are harmful to
human beings and animals.

Benefits of High-Voltage Direct Current Transmission
Systems
• Both electrical science and the practical applications of electricity began with direct
currents and its first practical application was DC Telegraphy powered by
electrochemical batteries. Electrical lighting also began with dc power using dynamos.
The first electrical central station was built by Thomas Alva Edison at pearl street in
newyork and it began in the year 1882 with operating dc voltage at 110 volts.
• Nowadays, modern civilization mainly depends on the consumption of electrical
energy for domestic, industrial, commercial, and agricultural applications. For this
purpose, an efficient transmission system is very necessary since the generating
stations are located in remote areas. An efficient transmission system has to meet the
following requirements.
• Bulk power transmission over long distances,
• Low transmission losses.
• Less voltage fluctuations.
• Possibility of power transfer through submarine cables.
• System of interconnection.

•Up to the 1980s, ultra high voltage ac (UHV-AC)
transmission lines above 765 kV were used for
bulk power transmission, and due to the
development of accurate control in thyristor, the
HVDC (high voltage direct current) transmission
lines are using which are having a distinct
superiority over UHV-AC transmission lines.

What is HVDC Transmission System?
• The High Voltage Direct Current (HVDC) transmission
system uses direct current for the transmission of
power over long distances. The HVDC transmission
system provides efficient and economic transmission
of power even to very long distances that meet the
requirements of growing load demands. Due to its
simple constructional feature and less complexity,
research and development authority discovered its
usage in modern power transmission.

• By comparing ac and dc transmission, it is clear that for
transmission of power over long distances ac is not much
suitable, and for generation and utilization of power, dc is
not favourable compared to ac.
• Thus, for HVDC transmission it requires terminal equipment
for converting ac into dc at the sending end, and terminal
equipment is again required at the receiving end to invert
this dc supply into ac.

Principle of HVDC Transmission System :
• The HVDC transmission system mainly consists of
converter stations where conversions from ac to dc
(rectifier station) are performed at sending end and at
the receiving end the dc power is inverted into ac
power using an inverter station. Hence, the converter
stations are the major component of the HVDC
transmission system.

• Also, by changing the role of the rectifier to inverter and inverter to
rectifier the power transfer can be reversed which can be achieved by
suitable converter control. The below shows the schematic diagram
of the HVDC transmission system.

Smoothing reactor
• serially Connected reactors inserted in DC Systems.
• Reduce harmonic currents and transient over currents and
/or current ripples in DC Systems.
• Necessary to smooth the DC wave shape,reduce losses and
improve system performance.

• The ac substations at both ends of the HVDC line consist of
ac switchgear, bus bars, current transformers, voltage
transformers, etc. The converter transformers are connected
between converter values and ac bus valves which transfers
power from ac to dc or vice-versa.
• Smoothing reactors are necessary for converter operation,
and for smoothing the dc current by reducing ripples
obtained on the dc line. The electrode line connects the
midpoint of converters with a distant earth electrode.

Types of HVDC Transmission Systems :
• The HVDC transmission systems are mainly classified into the
following types on the basis of arrangement of the pole (line) and
earth return. They are,Monopolar HVDC System - An HVDC system
having only one pole and earth return.
• Bipolar HVDC System - An HVDC system with two poles of opposite
polarity.
• Homopolar HVDC System - It has two poles of the same polarity and
earth return.
• Back to Back HVDC Coupling System - It has no dc transmission line.
The rectification and inversion are taken place at the same substation
by a back-to-back converter.
• Multi-Terminal HVDC Systems - It has three or more terminal
substations.

• Advantages of HVDC Transmission System :
• Nowadays HVDC systems are preferred over HVAC systems because of the following
advantages, The HVDC transmission requires narrow towers, whereas ac systems require
lattice shape towers, this makes the construction simple and reduces cost.
• The ground can be used as the return conductor.
• No charging current, since dc operates at unity power factor.
• Due to less corona and radio interference, it results in an economic choice of the
conductor.
• Since there is no skin effect in dc transmission the power losses are reduced
considerably.
• Large or bulk power can be transmitted over long distances.
• Synchronous operation is not required.
• Low short-circuit current on dc line.
• Tie-line power can be easily controlled.
• Power transmission can be also possible between unsynchronised ac distribution systems
(interconnection of ac systems of different frequencies).
• Cables can be worked at a high voltage gradient, which makes them more suitable for
undersea cables.
• Power flow through the HVDC line can be quickly controlled.

Applications of HVDC Transmission System :
• Since HVDC transmission systems have various technical and
economic superiority features as compared to the EHV-AC
transmission systems. Hence, in these modern days, HVDC
transmission systems are mainly using in the following applications.
Long-distance bulk power HVDC transmission by overhead lines.
• Underground or underwater cables.
• Interconnection of ac systems operating at different frequencies.
• Back-to-back HVDC coupling stations.
• MTDC asynchronous interconnection between 3 or more ac networks.
• Control and stabilization of power flow in ac interconnection of large
interconnected systems.

•Multi-Terminal Direct Current (MTDC) can
achieve the function of Power transmission
among multi power sources or multi receiving
ends. And compared to conventional
transmission systems, it is more economical and
flexible as well as many other significant
advantages.

introduction on high voltage engineering

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