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PhD
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Course Code: MDSP-805
Course Name: Understanding Power Industry
CENTRE FOR CONTINUING EDUCATION (CCE)
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Contents
Part-1 Power Generation
Unit 1 Power Scenario in India .................................................................................. 3
Unit 2 Power Demand ............................................................................................... 13
Part-2 Power Transmission
Unit 3 Overview of Power Transmission Structure ................................................ 25
Unit 4 HVDC .............................................................................................................. 81
Part-3 Power Distribution
Unit 5 Distribution Systems ..................................................................................... 99
Unit 6 Metering, Billing and Revenue Collection ................................................. 113
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The electricity sector in India is predominantly controlled
by the Government of Indias public sector undertakings
(PSUs). Major PSUs involved in the generation of electricity
include National Thermal Power Corporation (NTPC),
National Hydroelectric Power Corporation (NHPC) and
Nuclear Power Corporation of India (NPCI). Besides PSUs,
several state-level corporations, such as Maharashtra State
Electricity Board (MSEB), are also involved in the generationand intra-state distribution of electricity. The PowerGrid
Corporation of India is responsible for the inter-state
transmission of electricity and the development of national
grid.
The Ministry of Power is the apex body responsible for the
development of electrical energy in India. This ministry
started functioning independently from 2 July, 1992; earlier,
it was known as the Ministry of Energy. The Union Minister
of Power at present is Sushilkumar Shinde of the CongressParty who took charge of the ministry on the 28th of May,
2009.
India is worlds 6th largest energy consumer, accounting for
3.4% of global energy consumption. Due to Indias economic
rise, the demand for energy has grown at an average of 3.6%
per annum over the past 30 years. In March 2009, the installed
power generation capacity of India stood at 147,000 MW while
the per capita power consumption stood at 612 kWH. The
Objectives
After reading this unit you will be able to:
Know present sector-wise generation of power
Learn the Power Sector Strategies
Understand Restructuring of Power Sector & steps
Unit 1
Power Scenario in India
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countrys annual power production increased from about 190
billion kWH in 1986 to more than 680 billion kWH in 2006.
The Indian government has set an ambitious target to add
approximately 78,000 MW of installed generation capacity
by 2012. The total demand for electricity in India is expected
to cross 950,000 MW by 2030.
About 75% of the electricity consumed in India is generated
by thermal power plants, 21% by hydroelectric power plants
and 4% by nuclear power plants. More than 50% of Indias
commercial energy demand is met through the countrys vast
coal reserves. The country has also invested heavily in recent
years on renewable sources of energy such as wind energy.As of 2008, Indias installed wind power generation capacity
stood at 9,655 MW. Additionally, India has committed
massive amount of funds for the construction of various
nuclear reactors which would generate at least 30,000 MW.
In July 2009, India unveiled a $19 billion plan to produce
20,000 MW of solar power by 2020.
Electricity losses in India during transmission and
distribution are extremely high and vary between 30 to 45%.
In 2004-05, electricity demand outstripped supply by 7-11%.
Due to shortage of electricity, power cuts are common
throughout India and this has adversely effected the
countrys economic growth. Theft of electricity, common in
most parts of urban India, amounts to 1.5% of Indias GDP.
Despite an ambitious rural electrification program, some 400
million Indians lose electricity access during blackouts.
While 80 percent of Indian villages have at least an electricity
line, just 44 percent of rural households have access to
electricity. According to a sample of 97,882 households in
2002, electricity was the main source of lighting for 53% ofrural households compared to 36% in 1993. Multi Commodity
Exchange has sought permission to offer electricity future
markets.
Shortage level is 13.8% for peak demand and 10% for base
demand. The 16th Indian Electricity Power survey estimates
a capacity addition requirement of 80,000 MW by the end of
the Eleventh Five Year Plan.
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The major reasons for the inadequate, erratic and unreliable
power supply are
1. Inadequate power generation capacity
2. Lack of optimum utilization of the existing capacity.
3. Inadequate inter-regional transmission links
4. Inefficient use of electricity by the end consumers
5. Slow pace of rural electrification
All India installed capacity (in MW) of power stations located
on the main land and on islands expressed in terms ofterritorial regions/various energy sources as on 28.02.2010,
is as per the given table.
Future Power Generation
Plan
The Govt of India, Ministry of Power has taken a realistic
view on the demand requirement and set itself a target of
installing around 80000 MW capacity required to be added
by the end of the XIth Five Year Plan. The capacity shall be
further augmented by 125000GW in 12th5 year plan.
FuelLarge coal reserves of the country which are expected to last
for more than 150 years provide a ready and economical
resource and energy security. Hence coal has been identified
as the mainstay fuel for power generation. Special emphasis
has been laid to encourage setting up of large Mega Sized projects
at the coal pit head to avoid high costs associated with the
transportation of high ash bearing Indian coal and overstraining
the already overloaded Rail Networks of the country.
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Funds
It is estimated that building over 100,000 MW of additionalpower capacity and associated transmission & distribution
Infrastructure would require an investment of more than
USD 200 billion. The Govt of India is seriously looking at
long term solutions to attract investments in the sector and
have taken large scale reforms at the Federal level and the
Provincial levels for prompt and efficient revenue collection
from all the electricity consumers to provide the necessary
comfort to the investors.
Transmission facilityFurther, to solve the inter-grid transfer of power, plans have
been approved by Govt of India for construction of 37,500
MW interregional capability, through the formation of
National Power Grid. This would improve reliability, quality
and economics of power and provide some stability to the
generation units. The Govt. has also permitted private
investments in the transmission projects in the country.
Power Trading
It has also created a Power Trading Corporation which is
supposed to source, buy and transmit the surplus power from
one area / Region to another and act as a payment security
mechanism for inter-regional sale of power. This corporation
has started its activities and has been trading the power from
the currently surplus Eastern Grid to the other deficit areas.
The corporation is actively discussing with various Electricity
Boards and power Distribution Companies to identify their
demand pattern on the one hand, and discussing with the
potential mega Power projects for the supply of Power, onthe other hand. Power trading and power distribution has
also been opened up to private parties.
Mega Power Project
The Govt. of India has announced the Ultra Mega Power
Project policy for thermal power projects of more than 4000
MW which could be located at the coal pit head and would
supply power to more then one state. The policy permits the
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import of plant and equipment for the project, duty free, to
get the tariff at the international level. The power tariff for
such projects works out to be less then Rs 3/ Kwh, which is
comparable to the international level. Indias power sector
is growing at an annual growth rate of 5 - 8%.
Electricity Generation and Supply Act
The Electricity supply Industry is presently governed by
three enactments, namely,
The Indian Electricity Act, 1910
The Electricity Supply Act, 1948
The Electricity Regulatory Commission Act, 1998 and The
Electricity Act, 2003.
The Indian Electricity Act 1910 created the basic framework
for these electric supply industries in India which was then
in its infancy. The Act envisaged growth of electricity
generation through private licensees. Accordingly, it
provided for licensees who could supply electricity in a
specified area. The Electricity (Supply) Act 1948, mandated
the creation of state Electricity Boards with responsibilityof arranging the supply of electricity in the state. It was felt
that electrification which was limited to cities needed to be
extended rapidly and the states should step in to shoulder
this responsibility through the respective state Electricity
Boards (SEBs). Accordingly, the SEBs through the successive
Five Year Plans undertook the rapid growth and expansion
by utilizing the plan funds of Govt. of India.
With the policy of encouraging private sector participation
in generation, transmission and distribution and theobjective of distancing the regulatory commission, and the
need for harmonising and rationalising the provisions in the
Indian Electricity Act 1910, the Electricity (supply) Act 1948
and the Electricity Regulatory Commission Act 1998, a new
legislation has now been enacted: the Electricity Act, 2003.
The main provisions of Electricity Act 2003 are:
1. Generation shall be delicensed and captive generation
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would be permitted. Large capacity coal based plants
would be encouraged.
2. Transmission would be handled by a Central Govt
owned Transmission Utility. However private
participation through transmission licensees would be
encouraged.
3. There would be open access in transmission with
provision for the surcharge for taking care of cross
subsidy which would be gradually phased out.
4. Distribution licensees would be free to undertake
generation, and generating companies likewise wouldbe free to take up distribution.
5. Trading of power shall be identified as a separate
activity related to the interstate / inter grid transfer.
6. Where there is direct commercial relationship between
a consumer and a generating company or a power trading
company, the price of power would not be regulated and
only the transmission & wheeling charges (with the
surcharge) would be regulated.
This Act is now being implemented progressively by all the
state governments.
Power Sector - Proposed Strategies
1. Separation of Generation, Transmission & Distribution
Organization along with responsibility of the respective
functions.
2. Encourage Fast track coal (thermal) and gas based power
projects.
3. Increase generation by hydro and nuclear power plants
for mid range planning.
4. Formation of National Grid and Regional Grids The
Power Grid Corporation Ltd. (PGCL) was established
in 1989 at national Level for formation of a national
transmission network with responsibility of transmission.
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NTPC was relieved of the responsibility of transmission.
Hierarchical levels at national, regional, state and city
levels were identified for transmission and distribution
of electrical energy.
5. Load side management for better utilization of existing
capacity, and handling peak demand by adopting load
side management (peak shaving, load shaping, load
shifting distribution management.
6. Improved Power Factor, reduction of losses and better
voltage control and reducing peak MVA demand by
installing shunt capacitors in distribution system.
7. Reforms in Energy & Power Sector. The Energy sector
as a whole, including the power sector was opened to
private sector and joint sector. Investments from
multinational power companies were encouraged.
Competition has been introduced by bringing in private
sector, in an effort to boost efficiency and productivity.
8. To improve Plant Load Factors from 50-60% to 75-80%.
9. To use energy efficient plant and equipment.
Modernization projects for power generating plants and
power consuming plants have been sanctioned.
10. Develop Non-conventional energy technologies for
augmenting the power supply and conserving the
conventional raw energy forms while reducing pollution:
uplift of rural areas.
11. Encourage Human Resource Development (HRD) in
energy sector.
12. Encourage Research & Development in the energysector.
13. Accelerate gas based projects for quick increase in
installed capacity. Encourage use of naphtha for power
generation.
14. Develop modern coal gasification technologies.
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Reform efforts and privatisation
1. Since liberalisation in 1992, both the Central and Stategovernments have sought to increase the quantum of
private sector participation, specially in power sector.
2. Initially, the focus of the reform has been on encouraging
private participation in generation. Most of the projects,
however, could not achieve financial closure.
3. Private investors and lenders were wary of supporting
power projects that have to rely exclusively on
financially weak SEBs for evacuating their power.
Hence, the state governments gradually shifted the focus of
their efforts to areas such as SEBs, reform and regulation,
while the Central Government has been focusing on the areas
of regulation, transmission, privatization and power trading
companies.
Institutional Structure of Indian Power Sector is given in
Figure 1 & 2.
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The base load is the load below which the demand never falls
and is supplied for 100% of the time. The peak load occurs
for about 15 - 20% of the time. The intermediate load
represents the remaining region. The daily load curves of
one week can be superimposed, thereby generating the week
load curve.
Since, peaking load on plants are only for small fraction of
the total time, the fuel cost is not of major importance.
Minimum capital cost should be the criteria. The base load
plants being loaded heavily, operating costs of such plants
are important.
The variable load problem affects power plant design and
operation as well as the cost of generation. A careful study
of the load duration curve helps to decide capacity of the
base load plant and also of the peak load plant. The base
load plant should be run at high load factor. The peak load
plant should be of smaller capacity to reduce the cost of
generation. It could be a gas turbine unit, pumped hydro-
system, compressed air energy storage system or a diesel
engine, depending on the size and scope of availability. If
the whole of load is to be supplied by the same power plant,
then the prime movers and generators should act fairly
quickly and take up or shed load without variation of the
voltage or frequency of the system. It is the function of the
governor to control the supply of fuel to the prime mover
according to load. The capacity of the generators should be
so chosen as to suit and fit into the portions of the predicted
load curve. If the load conditions differ too much from this
capacity, the cost of energy increases.
When planning a power plant, the two basic parameters to
be decided are:
1. Total power output to be installed
2. Size of the generating units
The total installed capacity required can be determined from:
1. First demand estimated
2. Growth of demand anticipated
3. Reserve capacity required
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The size of generating units will depend on:
1. Variation of load (load curve) during 24 hours (summer,winter, week-days, holidays)
2. Total capacity start-up and shut-down periods of the
units
3. Maintenance programme planned
4. Plant efficiency vs. size of unit
5. Price and space demand per kW vs. size of unit
Effect of Variable load on Power Plant Design: Thecharacteristics and method of use of a power plants
equipment is largely influenced by the extent of variable load
on the plan. Supposing the load on the plant increases. This
will reduce the rotational speed of the turbo-generator. The
governor will come into action, operating a steam valve to
admit more steam and increase the turbine speed to bring it
up to its normal value. This increased amount of steam will
have to be supplied by the steam generator. The governing
response, however, will be somewhat slower.
The reason is explained below:
In most automatic combustion control systems, steam
pressure variation is the primary signal used. The steam
generator must operate with imbalance between heat
transfer and steam demand long enough to suffer a slight
but definite decrease in steam pressure. The automatic
combustion controller must then increase fuel, air and water
flow in the proper amount. This will affect the operation of
practically every component of auxiliary equipment in the
plant. Thus, there is a certain time lag element present evenin an combustion efficient design, but in general, they are
quick to cope with the variable load demand.
Variable load results in fluctuating steam demand. Due to
this, it becomes very difficult to secure good combustion since
efficient combustion requires the co-ordination of so many
various services. Efficient combustion is readily attained
under conditions where a steady head of steam is allowed to
be maintained. In diesel and hydro power plants, the total
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governing response is prompt, since control is needed only
for the prime mover.
The variable load requirements also modify the operating
characteristics built into equipment. Due to non-steady load
on the plant, the equipment cannot operate at the designed
load points. Hence for the equipment, a flat-topped load
efficiency curve is more desirable than a peaked one.
Regarding the plant units, if their number and sizes have
been selected to fit a known or a correctly predicted load
curve, then, it may be possible to operate them at or near
the point of maximum efficiency. However, to follow the
variable load curve very closely, the total plant capacity has
usually to be sub-divided into several power units of different
sizes. Sometimes, the total plant capacity would more nearly
coincide with the variable load curve, if more units of smaller
unit size are employed than a few units of bigger unit size.
Also, it will be possible to load the smaller units somewhere
near their most efficient operating pints. However, it must
be kept in mind that as the unit size decreases, the initial
cost per kW of capacity increases.
Again, duplicate units may not fit the load curve as closelyas units of unequal capacities. However, if identical units
are installed, there is a saving in the first cost, because of
the duplication of sizes, dimensions of pipes, foundations,
wires insulators, etc., and also because spare parts required
are less.
Effect of variable load on Power Plant Operation: In
addition to the effect of variable load on power design, the
variable load conditions impose operation problems also,
when the power plant is commissioned. Even though theavailability for service of the modern central power plants
is very high, usually more than 95%, the public utility plants
commonly remain on the "readiness-to-service". This capacity
is called "spinning reserve" and represents the equipment
standby at normal operating conditions of pressure, speed,
etc. Normally, the spinning reserve should be at least equal
to the largest unit actively carrying load. This will increase
the cost of electric generation per unit (kWh).
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In a steam power plant, the variable load on electric
generator ultimately gets reflected on the variable steam
demand on the steam generator and on various other
equipments. The operating characteristics of such
equipments are not linear with load, so their operation
becomes quite complicated.
As the load on electrical supply systems grow, a number of
power plants are interconnected to meet the load. The load
is divided among various power plants to achieve the utmost
economy in the whole system. When the system consists of
one base load plant and one or more peak load plants, the
load in excess of base load plant capacity is dispatched tothe best peak system, all of which are nearly equally efficient.
The best load distribution needs thorough study and full
knowledge of the system.
Co-ordination Base load and Peak load Power Plants: If the
load represented by figure is to be supplied from one power
plant only, then the installed capacity of the plant should be
equal to the peak load. Such a plant will be uneconomical
since the peak load occurs only for a short period in a year
and therefore the capacity equal to the difference of peak
load and base load will remain idle for the major part of theyear. Hence such a demand for power would not be met by a
single power station. There would be some stations supplying
the base load and others, possibly of different type, supplying
the peak load.
One method of meeting this varying load demand is to co-
ordinate the operation of hydro and steam stations. The
steam plant capacity and the available water power energy
are fitted into the load curve. Peak load demand can be
conveniently met by hydro-stations, the base load being
supplied by the steam power plants. A hydro-station can be
started up quickly at any time to meet a sudden emergency.
Also the load on a hydro-station can be reduced more quickly
than is possible with steam plant. There are two methods
for utilizing the hydro-electric power for supplying the peak
load:
1. By storing the natural run-off from a catchment area
during hours of light load and employing the water to
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operate the station at full capacity during periods of
peak demand.
2. Pumped storage system. In this the water is pumped
into a high level reservoir at off-peak periods, and is
utilized to drive the turbines and generators at the time
of peak demand.
Peak load can also be supplied by diesel engine power plant
and gas turbine power plants. Base load stations operate
almost continuously, i.e., at a load factor of about 80%. They
are shut down only for small periods for maintenance and
overhaul. The load factor of peak load plants is very low,
normally 5 to 15%, since they operate only for a small period
in a day, week, month or a year. As already discussed, the
cost of supplying the electric energy may be divided into two
parts:
a. Fixed cost, which mainly consists of the interest on the
capital cost and depreciation. It is independent of the
amount of electric energy actually supplied. It is
however, approximately proportional to the capacity of
plant installed, i.e., proportional to kW.
b. Running cost, which depends upon the actual energy
generated, i.e., it is proportional to the kWh.
Since electric power plants are very expensive to install, it
is desirable and even essential to generate as much energy
as possible in order to spread the fixed cost over the highest
possible number of units (kWh) supplied. Therefore the plants
should run at a high load factor, which will result in
minimising the cost per unit. If the plant is idle for most of
the period, it will generate only a small number of units and
hence the fixed charges will have to be spread over a smaller
number of units, resulting in high cost per unit supplied.
Therefore, the load factor has a very important effect on the
cost of the electric energy supplied from a power plant.
Hence, while base loads are cheaper to supply, peak load
units are expensive to produce. A base load power station
should have the highest possible efficiency. For peak load
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plants, since the units to be generated are small, efficiency
is not of much importance. Of course, the capital cost should
be minimum since it is to be distributed over the small
number of units supplied or generated. Since a peak load
plant may have to be started once or perhaps twice in a day
and possibly for some unexpected emergency condition, it
should be capable of quick starting and quick load pick up.
Significance of Various Factors
1. Load Factor: High load factor is a desirable quality.
Higher load factor means greater average load, resulting
in greater number of power units generated for a givenmaximum demand. Thus, the fixed cost, which is
proportional to the maximum demand, can be
distributed over a greater number of units (kWh)
supplied. This will lower the overall cost of the supply
of electric energy.
2. Diversity Factor: High diversity factor (which is always
greater than unity) is also a desirable quality. With a
given number of consumers, higher the value of diversity
factor, lower will be the maximum demand on the plant,
since
grouptotaltheofdemandMaximum
demandsmaximumindividualtheofSum=factorDiversity
The capacity of the plant will, therefore, be smaller,
resulting in fixed charges.
3. Plant Capacity Factor: Since the load and diversity
factors are not involved with 'reserve capacity' of the
power plant, a factor is needed which will measure thereserve, likewise the degree of utilization of the installed
equipment. For this, the factor "Plant factor, Capacity
factor or Plant Capacity factor" is defined as
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Thus, the annual plant capacity factor will be
The difference between load and capacity factors is an
indication of reserve capacity.
4. Plant use factor: This is a modification of Plant Capacity
factor in that only the actual number of hours that the
plant was in operation are used. Thus, Annual Plant Use
factor is
The Power Plant capacity study needs understanding of
following related terms:
1. Load Factor: It is defined as the ratio of the average
load to the peak load during a certain prescribed period
of time. The load factor of a power plant should be high
so that the total capacity of the plant is utilized for the
maximum period that will result in lower per unit cost
of the electricity being generated.
2. Utility Factor: It is the ratio of the units of electricity
generated per year to the capacity of the plant installed
in the station. It can also be defined as the ratio of
maximum demand of a plant to the rated capacity of the
plant. Supposing the rated capacity of a plant is 200 MW.
If the maximum load on the plant is 100 MW at load
factor of 80%, then the utility will be = (100x0.8)/200 x
100 = 40%
3. Plant Operating Factor: It is the ratio of the durationduring which the plant is in actual service, to the total
duration of the period of time considered.
4. Capacity Factor: It is the ratio of the average load on
a machine or equipment to the rating of the machine or
equipment, for a certain period of time considered.
5. Demand Factor: The actual maximum demand of a
consumer is always less than his connected load since
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all the appliances in his residence will not be in
operation at the same time or to their fullest extent.
This ratio of the maximum demand of a system to its
connected load is termed as demand factor.
6. Load Curve: It is a curve showing the variation of power
with time. It shows the value of a specific load for each
unit of the period covered. The unit of time considered
may be hours, days, weeks, months or years.
7. Firm Power: It is the power which should always be
available even under emergency conditions.
8. Prime Power: It is Power, be it mechanical, hydraulicor thermal, that is always available for conversion into
electric power.
9. Reserve: It is that reserve generating capacity which is
in operation but not in service.
10. Spinning reserve: It is that reserve generating capacity
which is connected to the bus and is ready to take the
load.
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Growth of Power Systems in India
India is fairly rich in natural resources like coal and lignite;
while some oil reserves have been discovered so far, intense
exploration is being undertaken in various regions of the
country. India has immense water power resources also; ofwhich only around 20% have so far been utilised, i.e., only
36800 MW has so far been commissioned up to 2010. As per a
recent report of the CEA, the total feasible potential of hydro
power is 148000. As regards nuclear power, India is deficient
in uranium, but has rich deposits of thorium which can be
utilised at a future date in fast breeder reactors. Since
independence, the country has made tremendous progress
in the development of electric energy and today it has the
largest system among the developing countries.
When India attained independence, the installed capacity
was as low as 1900 MW. In the early stages of the growth of
power system, the major portion of generation was through
thermal stations. But due to economical reasons, hydro
development received attention in areas like Kerala, Tamil
Nadu, Uttar Pradesh and Punjab.
Objectives
After studying this unit you should be able to:
Get an overview of power systems in India
Understand the problems Indian power sector is facing
Get a technical overview of Power Transmission
Unit 3
Overview of PowerTransmission Structure
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In the beginning of the First Five Year Plan (1951-56), the
total installed capacity was around 2300 MW (560 MW
hydro, 1004 MW thermal, 149 MW through oil stations and
587 MW through non-utilities). For transporting this power
to the load centers, transmission lines of up to 110 KV
voltage level were constructed. The emphasis during the
Second Plan (1956-61) was on the development of basic and
heavy industries and thus there was a need to step up
power generation. The total installed capacity which was
around 3420 MW at the end of the First Five Year Plan
became 5700 MW at the end of the Second Five Year Plan.
The introduction of 230 KV transmission voltage came upin Tamil Nadu and Punjab. During this Plan, totally about
1009 circuit kilometres were energized. In 1965- 66, the
total installed capacity was increased to 10,170 MW. During
the Third Five Year Plan (1961-66) transmission growth
took place very rapidly, with a nine-fold expansion in
voltage level below 66 KV. Emphasis was on rural
electrification. A significant development in this phase was
the emergence of an interstate grid system. The country
was divided into five regions, each with a regional
electricity board, to promote integrated operation of theconstituent power systems. Figure 1 shows these five
regions of the country with projected installed capacity in
MW for the year 1989-90. During the Fourth Five Year
Plan, India started generating nuclear power. At the
Tarapur Nuclear Plant 2 x 210 MW units were
commissioned in April-May 1969. This station uses two
boiling water reactors of American design.
By August 1972, the first unit of 220 MW of the Rajasthan
Atomic Power Project, Kota (Rajasthan), was added to thenuclear generating capability. The total generating capacity
at Kota is 430 MW with nuclear reactors of Canadian design
which use natural uranium as fuel and heavy water as a
moderator and coolant. The third nuclear power station of 2
x 235 MW has been commissioned at Kalpakkam (Tamil
Nadu). This is the first nuclear station to be completely
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Bengal), Korba (Madhya Pradesh), Ramagundam (Andhra
Pradesh) and Neyveli (Tamil Nadu), all in coal mining areas,
each with a capacity in the range of 2000 MW. Many more
super thermal plants would be built in future. Intensive work
must be conducted on boiler furnaces to burn coal with high
ash content. National Thermal Power Corporation (NTPC)
is in charge of these large scale generation projects.
Also the concept of UMPP (ultra Mega power Project) has
been implemented with capacity 4000MW and above working
on supercritical technology and unit size in excess of 660MW.
TATA power, Reliance are the major private players that
has entered into this field in a big way.
Hydro power will continue to remain cheaper than other
types for the next decade. As mentioned earlier, India has
so far developed only around 25% of its estimated total hydro
potential of 148000 MW. The utilization of this perennial
source of energy would involve massive investments in dams,
channels and generation-transmission system. The Central
Electricity Authority, the Planning Commission and the
Ministry of Energy are coordinating to work out a perspective
plan to develop all hydroelectric sources by the end of this
century, to be executed by the National Hydro Power
Corporation (NHPC).
Nuclear energy assumes special significance in energy
planning in India. Because of the limited coal reserves and
its poor quality, India has no choice but to keep going on
with its nuclear energy plans. According to the Atomic Energy
Commission, India's nuclear power generation will increase
to 20000 MW by year 2020 and 63000MW by 2032. Everything
seems to be set for a take off in nuclear power production
using the country's thorium reserves in breeder reactors. In
India, concerted efforts to develop solar energy and other
non-conventional sources of energy need to be emphasized,
so that the growing demand can be met and depleting fossil
fuel resources may be conserved. To meet the energy
requirement, it is expected that the coal production will have
to be increased considerably to meet the growing demand.
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A number of 400 kV lines are operating successfully as
mentioned already. This is the first step in working towards
a national grid. There is a need in future to go in for even
higher voltages (750/1000 kV).
There is a need for constructing HVDC (High Voltage DC)
links in the country since DC lines can carry considerably
more power at the same voltage and require fewer
conductors. A 400 kV Singrauli Vindhyachal line of 500
MW capacity is the first HVDC back-to-back scheme that
has been commissioned by NPTC (National Power
Transmission Corporation), followed by first point-to-point
bulk EHVDC transmission of 1500 MW at 500 kV over a
distance of 915 km from Rihand to Delhi. At the time of
writing, the whole energy scenario is so clouded with
uncertainty that it would be unwise to try any quantitative
predictions for the future. However, certain trends that
will decide the future developments of electric power
industry are clear.
Generally, unit size will go further up from 1000 MW. A
higher voltage (765/1200 kV) will come eventually at the
transmission level. There is a little chance for six-phase
transmission becoming popular though there are few such
lines in USA. As the population grows in India, we may see
a trend to go toward underground transmission in urban
areas.
Shortfall in the tenth Plan has been around 55%. There have
been serious power shortages and generation and availability
of power in turn have lagged too much from the industrial,
agricultural and domestic requirements. Because of power
shortages, many of the industries, particularly power-
intensive ones, have installed their own captive power plants.
Currently 12% of the electricity generated in India comes
from the captive power plants and this is bound to go up in
the future. Consortium of industrial consumers should be
encouraged to put up coal-based captive plants. Import
should be liberalized to support this activity.
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With the ever increasing complexity and growth of power
networks and their economic and integrated operation, it is
planned to establish central automatic load dispatch centers
with real time computer control.
Energy Conservation
Energy conservation is the cheapest new source of energy.
We should resort to various conservation measures such as
cogeneration (discussed earlier), and use energy-efficient
motors to avoid wasteful electricity uses. We can achieve
considerable electric power savings by reducing unnecessary
high lighting levels, oversized motors etc. Everyone shouldbe taught how consumption levels can be reduced without
any essential lowering of comfort. Rate restructuring can
have incentives in this regard. There is no consciousness on
energy accountability yet and no sense of urgency as in
developed countries.
Load Management
By various "load management" schemes, it is possible to shift
demand away from peak hours. A more direct method would
be the control of the load either through modified tariff
structures that encourage the individual customers to
readjust their own electric use schedules or direct electrical
control of appliances in the form of remote timer controlled
on/off switches with least inconvenience to the customer.
Systems for load management are varied. Ripple control has
been tried in Europe. Remote kWh meter reading by carrier
systems is being tried. Most of the potential for load control
lies in the domestic sector. In USA, power companies are
planning the introduction of system-wide load managementschemes.
Maintenance
Management and plant utilization factors of existing plants
must be improved. Maintenance must be on schedule rather
than an emergency. Maintenance manpower training should
be held on war footing. [PSE, Nagrath & Kothari]
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Structure of Power Systems
Generating stations, transmission lines and the distributionsystems 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 (which will be
discussed in detail in later chapters), 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.
Interconnection has the economic advantage of reducing
the reserve generation capacity in each area. Under
conditions of sudden increase in load or loss of generation in
one area, it is immediately possible to borrow power from
adjoining interconnected areas. Interconnection causes
larger currents to flow on transmission lines under faulty
condition with a consequent increase in capacity of circuit
breakers. Also, the synchronous machines of all
interconnected areas must operate stably and in a
synchronized manner. The disturbance caused by a short
circuit in one area must be rapidly disconnected by circuit
breaker openings before it can seriously affect adjoining
areas. It permits the construction of larger and more
economical generating units and the transmission of large
chunk of power from the generating plants to major load
centres. It provides capacity savings by seasonal exchange
of power between areas having opposing winter and summer
requirements. It permits capacity savings from time zones
of random diversity. It facilitates transmission of off-peak
power. It also gives the flexibility to meet unexpected
emergency loads.
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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.
1. 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.
2. 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 uponmany 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 trend in India 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 these stations are located close to load
centres, avoiding high density pollution areas to reduce the
risks, however remote, of radioactivity leakage.
In India, as of now, about 65% of electric power used is
generated in thermal plants (including nuclear). The
remaining 35% comes from hydro stations. 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
is then 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
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voltages are expected to rise to 1200 kV in the near future.
In India, several 400 kV lines are already in operation.
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 U.S.A. In India, the first
HVDC transmission line has recently been commissioned
and several others are being planned.
The first step down of voltage from transmission level is at
the bulk power substation, where the reduction is to the
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 gr idlevel to subtransmission level.
The next step-down in voltage is at the distribution
substation. Normally, two distribution voltage levels are
employed:
1. The primary or feeder voltage (11 kV)
2. The secondary or consumer voltage (440 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. Thus, the power
system operates at various voltage levels separated by
transformer. Figure 2 depicts schematically the structure of
a power system.
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Figure 2: Schematic diagram depicting power system
structure
Though the distribution system design, planning and
operation are subjects of great importance, we are compelled,
for reasons of space, to exclude them from the scope of this
book except for a short appendix (M) which gives elementary
description of a distribution system. [PSE, Nagrath &Kothari]
Technical Overview of Transmission Lines
Short transmission lines
For short lines of length 100 Km or less, the total 50 Hz shunt
admittance (jCl) is small enough to be negligible resulting
in the simple equivalent circuit as shown in figure 3.
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__________________ Figure 3: Simple equivalent circuit
This being a simple series circuit, the relationship between
sending-end receiving-end voltages and currents can be
immediately written as:
The phasor diagram for the short line is shown in Figure 2
for the lagging current case. From this figure we can write
The last term is of negligible order and so,
Expanding Binomially and retaining first order terms, we
get
or,
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Figure 5: Medium line, localized load end capacitance
Starting from fundamental circuit equations, it is fairly
straight forward to write the transmission line equations in
the ABCD constant form given below:
Nominal T Representation
If all the shunt capacitance is lumped at the middle of the
line, it leads to the nominal-T circuit shown in Figure 6.
Figure 6: Medium line nominal T representation
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For the nominal T circuit, the following circuit equations can
be written,
Substituting for Vc and Is in the last equation, we get
Rearranging the results , we get
Nominal- Representation
In this method the total line capacitance is divided into two
equal parts which are lumped at the sending and receiving-
ends resulting in the nominal- representation as shown in
Figure 7.
Figure 7: Medium line, II representation
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The equations for the above circuit are:
Finally we have,
Long Transmission Lines-Rigorous Solution
For lines over 250 km, the fact that the parameters of a line
are not lumped but distributed uniformly throughout its
length must be considered.
Figure 8: Schematic diagram for a long transmission line
Figure 8 shows one phase and the neutral return (of zero
impedance) of a transmission line. Let dx be an elemental
section of the line at a distance x from the receiving-end
having a series impedance zdx and a shunt admittance ydx.
The rise in voltage to neutral over the elemental section in
the direction of increasing x is dVx. We can write the
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following differential relationships across the elementalsection,
Differentiating the 1st equation we get
and using in the 2ndequation we get,
This is a linear differential equation whose general solutioncan be written as follows:
where
and C1 & C2 are the constants to be evaluated.
Using the boundary conditions the value of C1& C
2 can be
obtained and then substituting them in the original equationwe get,
where Zc is the characteristic impedance and is the
propagation constant.
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and
Surge Impedance Loading
A line terminated in its characteristic impedance is called
the infinite line. The incident wave under this condition
cannot distinguish between a termination and an infinite
continuation of the line. Power system engineers normally
call Zc the surge impedance. It has a value of about 400
ohms for an overhead line and its phase angle normally varies
from 0o to -15o. For underground cables Zc is roughly one-
tenth of the value for overhead lines. The term surge
impedance is, however, used in connection with surges (due
to lightning or switching) on transmission lines, where the
line loss can be neglected such that,
is a pure resistance.
Surge Impedance Loading (SIL) of a transmission line is
defined as the power delivered by a line to purely resistive
load equal in value to the surge impedance of the line. Thus
for a line having 400 ohms surge impedance,
where is the line-to-line receiving-end voltage in kV.
Sometimes, it is found convenient to express line loading in
per unit of SIL, i.e. as the ratio of the power transmitted to
surge impedance loading.
Ferranti Effect
The effect of the line capacitance is to cause the no-load
receiving-end voltage to be more than the sending-end
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voltage. The effect becomes more pronounced as the line
length increases. This phenomenon is known as the Ferranti
effect.
A simple explanation of the Ferranti effect on an approximate
basis can be advanced by lumping the inductance and
capacitance parameters of the line. As shown in Figure 9 the
capacitance is lumped at the receiving-end of the line.
Figure 9: Simple circuit demonstrating the Ferranti effect
Now,
since C is small compared to L, Ll can be neglected in
comparison to 1/ Cl. Thus,
Now,
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The magnitude of voltage rise:
where is the velocity of propagation of the
electromagnetic wave along the line, which is nearly equal
to the velocity of light.
Tuned Power Lines
For an overhead line, shunt conductance G is always
negligible and it is sufficiently accurate to neglect line
resistance R as well. With this approximation,
It simplifies to,
now if where n = 1, 2, 3..
i.e. the receiving-end voltage and current are numerically
equal to the corresponding sending-end values, so that there
is no voltage drop on load. Such a line is called a tuned line.
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For 50 Hz , the length of line for tuning is,
is the velocity of light.
Therefore, we have
It is too long a distance of transmission from the point of
view of cost and efficiency (note that line resistance was
neglected in the above analysis). For a given line, length and
frequency tuning can be achieved by increasing L or C, i.e.
by adding series inductances or shunt capacitances at several
places along the line length. The method is impractical and
uneconomical for power frequency lines and is adopted for
telephone lines where higher frequencies are employed. A
method of tuning power lines which is being presently
experimented with, uses series capacitors to cancel the effectof the line inductance and shunt inductors to neutralize line
capacitance. A long line is divided into several sections which
are individually tuned. However, so far the practical method
of improving line regulation and power transfer capacity is
to add series capacitors to reduce line inductance; shunt
capacitors under heavy load conditions; and shunt inductors
under light or no-load conditions.
Power Flow Through A Transmission Line
So far the transmission line performance equation waspresented in the form of voltage and current relationships
between sending and receiving-ends. Since loads are more
often expressed in terms of real (watts/KW) and reactive
(VARs/kVAR) power, it is convenient to deal with
transmission line equations in the form of sending and
receiving-end complex power and voltages. The principles
involved are illustrated here through a single transmission
line (2-node 2-bus system) as shown in Figure 10,
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Figure 10: Two bus system
Let us take receiving-end voltage as a reference phasor
and let the sending-end voltage lead it
by an angle . The angle is known as the
torque angle. The complex power leaving the receiving-end
and entering the sending-end of the transmission line can
be expressed as (on per phase basis),
Receiving and sending-end currents can, however, be
expressed in terms of receiving and sending-end voltages
as,
by solving we get,
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Similarly,
In the above equations SRand S
Sare per phase volt amperes,
while VR and V
S are expressed in per phase volts. [PSE,
Nagrath & Kothari].
Conductor Types
Transmission lines consisting of single solid cylindrical
conductors for forward and return paths are rarely used. Toprovide the necessary flexibility for stringing, conductors
used in practice are always stranded except for very small
cross-sectional areas. Stranded conductors are composed of
strands of wires electrically in parallel, with alternate layers
spiraled in opposite direction to prevent unwinding. The total
number of strands (N) in concentrically stranded cables with
total annular space filled with strands of uniform diameter
(d) is given by,
N= 3x2 3x+ 1
Where xis the number of layers wherein, the single central
strand is counted as the first layer. The overall diameter (D)
of a stranded conductor is,
D = (2x 1)d
Aluminium is now the most commonly employed conductor
material. It has the advantages of being cheaper and lighter
than copper though with less conductivity and tensile
strength. Low density and low conductivity result in larger
overall conductor diameter which offers another incidental
advantage in high voltage lines. Increased diameter results
in reduced electrical stress at conductor surface for a given
voltage so that the line is corona free. The low tensile
strength of aluminium conductors is made up by providing
central strands of high tensile strength steel. Such a
conductor is known as aluminium conductor steel reinforced
(ACSR) and is most commonly used in overhead transmission
lines. Figure 3.11 shows the cross-sectional view of an ACSR
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conductor with 24 strands of aluminium and 7 strands of
steel.
Figure 11: Cross-sectional view of ACSR-7 steel strands, 24
aluminium strands
In extra high voltage (EHV) transmission line, expanded
ACSR conductors are used. These are provided with paper
or hessian between various layers of strands so as to increasethe overall conductor diameter in an attempt to reduce
electrical stress at conductor surface and prevent corona.
The most effective way of constructing corona-free EHV lines
is to provide several conductors per phase in suitable
geometrical configuration. These are known as bundled
conductorsand are a common practice now for EHV lines.
Bundled Conductors
It is economical to transmit large chunks of power over long
distances by employing EHV lines. However, the linevoltages that can be used are severely limited by the
phenomenon of corona. Corona, in fact, is the result of
ionization of the atmosphere when certain field intensity
(about 3,000 kV/m at NTP) is reached. Corona discharge
causes communication interference and associated power loss
which can be severe in bad weather conditions. Critical line
voltage for formation of corona can be raised considerably
by the use of bundled conductors i.e., a group of two or more
Aluminium
strands
Steel strands
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conductors per phase. This increase in critical corona voltage
is dependent on number of conductors in the group, the
clearance between them and the distance between the groups
forming the separate phases. The bundle usually comprises
two, three or four conductors arranged in configurations
illustrated in Fig 12.
Figure 12: Configurations of conductors in bundled
conductors
Circuit Breaker
Figure 13 is illustrative of a 3-phase symmetrical short-
circuit on a generator with an intervening circuit breaker
having three circuit opening poles, one in each phase. The
short circuit current would comprise two components-DCoffset current and symmetrical short-circuit current. The DC
offset current is maximum in the phase whose voltage is zero
at the instant of short circuit (say in phase B).
Figure 13: Phase short-circuit and circuit breaking
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Because of the time-varying synchronous reactance of the
synchronous generator, the symmetrical short-circuitcurrent decays reaching steady state after passing through
subtransient and transient phases. The short-circuit current
of phase B is shown in Figure 13.
The heavy short-circuit current is sensed by protective
relaying, which energizes the trip circuit of the circuit
breaker (CB) causing its moving poles to separate from the
fixed poles at high speed. This is accomplished by a
mechanical toggle mechanism. As the poles separate electricarc is struck across the intervening air-gap feeding the
current. The arc would extinguish at current zero (of the AC
current) and, if it does not restrike, the circuit opens
successfully. The voltage across the poles is almost constant
(about 80 V) during the arcing phase (nonlinear nature of
the arc phenomenon). After the arc is extinguished, AC
voltage appears across the poles which builds up with
passage of time as the air-gap flux in the generator recoverswith the vanishing armature reaction.
The waveforms of iBand V
Bare shown in Figure 3.14. These
phenomena also occur in other phases with a time phase
difference of 120. The voltage Vn will not be the phase voltage
during the time phases when R and Y have not yet opened.
The short-circuit current has an initial major loop (called
making current), whose peak value is known as the
maximum momentary current. The mechanical parts of
the circuit breaker must be capable of withstanding forces
released by this current (these are proportional to square of
the current). The voltage appearing across the poles (va)
when the arc extinguishes is known as recovery voltage.
The current which would have flown if the breaker did not
open is called the prospective current.
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Figure 14: Short circuit current and recovery voltage
At the instant of current interruption (arc extinction) an LC
transient occurs involving generator inductance and stray
capacitance causing high frequency damped oscillations as
shown in Figure 15. The recovery voltage with this transient
is known as transient recovery voltage (TRV). Thus the
voltage VB across the breaker poles has a fast rate of rise
and a peak value almost double the maximum voltage of thepower-frequency component of the recovery voltage. These
two phenomena in the recovery voltage tend to restrike the
arc so that the breaker would then open at a later current
zero when larger pole separation has occurred. Restriking
is detrimental to circuit breaking as it would damage the
poles and delay the fault clearing in the power system.
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Figure 15: Transient recovery voltage (TRV)
Power System Transients
In this chapter we will discuss the abnormal situation,
wherein the power system is in dynamic state with largescale perturbation caused by a fault, or opening or closing of
a switch, or other large scale disturbances. This is the study
of power system transients.
Transient phenomenon lasts in a power system for a very
short period of time, ranging from a few s up to 1s. Yet the
study and understanding of this phenomenon is extremely
important, as during these transients, the system is subjected
to the greatest stress from excessive over-currents or
voltages which, depending upon their severity can causeextensive damage. In some extreme cases, there may be a
complete shutdown of a plant, or even a blackout of a whole
area. Because of this, it is necessary that a power system
engineer should have a clear understanding of power system
transients, to enable him to find out their impact on the
system, to prevent them if possible, or at least control their
severity or mitigate the damage caused. This chapter is
devoted to the study of power system transients.
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Types of System Transients
The main causes of momentary excessive voltages andcurrents are:
(i) Lightning
(ii) Switching
(iii) Short-circuits and
(iv) Resonance conditions.
Out of these, lightning and switching are the most common,
and usually the most severe causes. Transients caused by
short-circuits or resonance conditions usually arise as
secondary effects, but may well lead to the plant breakdown
in EHV (500-765 kV) systems. Also in EHV systems the
voltage transients or surges caused by switching, i.e. opening
and closing of circuit breakers, are becoming increasingly
important. On cable systems, of course, lightning transients
rarely occur and the other causes become more important.
Depending upon the speed of the transients, these can be
classified as:
Surge phenomena (extremely fast transients)
Short-circuit phenomena (medium fast transients)
Transient stability (slow transients)
Surge Phenomena
This type of transient is caused by lightning (atmospheric
discharges on overhead transmission lines) and switching.
Physically, such a transient initiates an electromagnetic wave
(surge) travelling with almost the speed of light (3 108m/s)
on transmission lines. In a 150 km line, the travelling wave
completes a round trip in 1 ms. Thus the transient
phenomena associated with these travelling waves occur
during the first few milliseconds after their initiation. The
ever-present line losses cause pretty fast attenuation of
these waves, which die out after a few reflections.
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The reflection of surges at open line ends, or at transformers
which present high inductance, leads to multiplicative effect
on voltage buildup, which may eventually damage the
insulation of high-voltage equipment with consequent short-
circuit (medium fast transient). The high inductance of the
transformer plays a beneficial role of insulating the generator
windings from transmission line surges. The travelling
charges in the surges are discharged to ground via lightning
arresters without the initiation of a line short-circuit,
thereby protecting the equipment.
Selection of insulation level of various line equipment and
transformers is directly related to the overvoltages causedby surge phenomena. Hence the importance of studying this
class of transients.
Short-circuit Phenomena
About more than 50% short-circuits take place on exposed
overhead lines, owing to the insulation failure resulting from
overvoltages generated by surge phenomena described
earlier, birds and other mechanical reasons. Short-circuits
result from symmetrical (3-phase) faults, as well as
unsymmetrical (LG, LL, LLG) faults. The occurrence of asymmetrical fault brings the power transfer across the line
to zero immediately, whereas the impact is only partial in
case of unsymmetrical faults. Like surge phenomena, short-
circuits are also fully electric in nature. Their speed is
determined by the time constants of the generator windings,
which vary from a few cycles of 50 Hz wave for the damper
windings to around 4s for the field winding. Therefore, these
transients will be sufficiently slower than the surge
phenomena. The time range that is of practical importance
to power system analyst is from 10 to 100 ms, i.e. the first
few (5-10) cycles of the short-circuit currents.
The short-circuit currents may attain such high values that,
if allowed to persist, they may result in thermal damage to
the equipment. Therefore, the faulty section should be
isolated as quickly as possible. Most of the short-circuits do
not cause permanent damage. As soon as the fault is cleared,
short-circuit path is deionized, and the insulation is restored.
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Reclosing breakers are, therefore, used in practice which
automatically close periodically to find out if the line has
recovered. If the fault continues for some time, then of course,the breaker has to open permanently. This whole operation
of successive closing-opening cycle may last for a second or
so.
Transient Stability
Whenever a short-circuit takes place at any part of the
integrated system, there is an instantaneous total or partial
collapse of the bus voltages of the system. This also results
in the reduction of the generator power output. Since initially
for some instants the input turbine power remains constant,as there is always some time delay before the controllers
can initiate corrective actions, each generator is subjected
to a positive accelerating torque. This condition, if sustained
for some time, can result in the most severe type of transients,
namely the mechanical oscillations of the synchronous
machine rotors. These electromechanical transients may,
under extreme conditions, lead to loss of synchronism for
some or all of the machines, which implies that the power
system has reached its transient stability limit. Once this
happens, it may take several hours for an electric system
engineer to resynchronize such a "blacked-out" system. Thus,
it is quite necessary to simulate this phenomenon on the
computers and use the switching and load-management
strategies that will avoid or minimize, the ill effects of short-
circuits.
The rotor swings are quite slow, as they are mechanical in
nature. A transient stability study, thus, may confine itself
for the time period of a few milliseconds to one minute in
most of the cases. [PSE, Nagrath & Kothari]
Generation of Overvoltages on Transmission Lines
Transmission lines and power apparatus have to be protected
from over voltages. The over voltages in' a power system fall
under three categories:
Resonance overvoltages
Switching overvoltages
Lightning overvoltages
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