33. MDSP 805 Understanding Power Industry New

8/12/2019 33. MDSP 805 Understanding Power Industry New

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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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Understanding Power Industry

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UNIT 1 Power Scenario in India

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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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UNIT 2 Power Demand

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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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UNIT 2 Power Demand

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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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UNIT 2 Power Demand

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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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UNIT 2 Power Demand

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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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UNIT 3 Overview of Power Transmission Structure

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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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Documents

33. MDSP 805 Understanding Power Industry New