Power Transmission Unit-1

1UNIT 1 Overview of Power Transmission Structureu

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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, intenseexploration is being undertaken in various regions of thecountry. India has immense water power resources also; ofwhich only around 20% have so far been utilised, i.e., only21000 MW has so far been commissioned up to the end of 7thplan. As per a recent report of the CWPC (Central WaterPower Commission), the total potential of hydro power is89800 MW at 60% load factor. As regards nuclear power, Indiais deficient in uranium, but has rich deposits of thoriumwhich can be utilised at a future date in fast breeder reactors.Since independence, the country has made tremendousprogress in the development of electric energy and today ithas the largest system among the developing countries.

When India attained independence, the installed capacitywas as low as 1900 MW. In the early stages of the growth ofpower system, the major portion of generation was throughthermal stations. But due to economical reasons, hydrodevelopment received attention in areas like Kerala, TamilNadu, Uttar Pradesh and Punjab.

Objectives

After studying this unit you should be able to:

y Get an overview of power systems in India

y Understand the problems Indian power sector is facing

y Get a technical overview of Power Transmission

Unit 1

Overview of PowerTransmission Structure

2 Power Transmissionu


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In the beginning of the First Five Year Plan (1951-56), thetotal installed capacity was around 2300 MW (560 MW hydro,1004 MW thermal, 149 MW through oil stations and 587 MWthrough non-utilities). For transporting this power to theload centers, transmission lines of up to 110 KV voltage levelwere constructed.

The emphasis during the Second Plan (1956-61) was on thedevelopment of basic and heavy industries and thus therewas a need to step up power generation. The total installedcapacity which was around 3420 MW at the end of the FirstFive Year Plan became 5700 MW at the end of the SecondFive Year Plan. The introduction of 230 KV transmissionvoltage came up in 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) transmissiongrowth took place very rapidly, with a nine-fold expansionin voltage level below 66 KV. Emphasis was on ruralelectrification. A significant development in this phase wasthe emergence of an interstate grid system. The country wasdivided into five regions, each with a regional electricityboard, to promote integrated operation of the constituentpower systems. Figure 1.1 shows these five regions of thecountry with projected installed capacity in MW for the year1989-90.

During the Fourth Five Year Plan, India started generatingnuclear power. At the Tarapur Nuclear Plant 2 x 210 MWunits were commissioned in April-May 1969. This stationuses two boiling water reactors of American design.

By August 1972, the first unit of 220 MW of the RajasthanAtomic Power Project, Kota (Rajasthan), was added to thenuclear generating capability. The total generating capacityat Kota is 430 MW with nuclear reactors of Canadian designwhich use natural uranium as fuel and heavy water as amoderator and coolant. The third nuclear power station of 2x 235 MW has been commissioned at Kalpakkam (TamilNadu). This is the first nuclear station to be completelydesigned, engineered and constructed by Indian scientistsand engineers. A reactor research centre has been set up



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near the Madras Atomic Power Station to carry out study infast breeder reactor technology. The fourth nuclear powerplant has been set up at Narora in Uttar Pradesh. It has twounits of 235 MW each. The fifth will be in Kaiga in Karnatakaand sixth in Gujarat near Surat.

Figure 1.1: Map of India showing five regions with projected installed capacityin MW for the year 1989 (Reprinted with permission of Central ElectricityAuthority of India, Super Grid Directorate, New Delhi from Power SystemPlanning Studies, Phase I, 1978-1989 System)

The growth of generating capacity so far and future projectionfor 2000-2001 AD are given in Table 1.1.

Table 1.1: Growth of Installed Capacity in India (in MW)

Year Hydro Nuclear Thermal Diesel Total

1970-71 6383 420 7503 398 14704

1978-79 11378 890 16372 - 28640

1984-85 14271 1095 27074 - 42240

2000-01 49000 10000 78859 - 137859



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To be self-sufficient in power, BHEL has plants spread outallover the country and these turn out an entire range ofpower equipment, viz. turbo sets, hydro sets, turbines fornuclear plants, high pressure boilers, power transformers,switch gears, etc. Each plant specializes in a range ofequipment. BHEL's first 500 MW turbo-generator has beencommissioned at Singrauli. Today BHEL is consideredamong the top ten manufacturers in the world with an annualproduction capacity of 4000-5000 MW.

Problems Facing Indian Power Industry and its Choices

The electricity requirements of India have growntremendously and the demand has been running ahead ofsupply. Electricity generation and transmission processesin India are very inefficient in comparison with those of someadvanced countries. Generating capacity is utilised on anaverage for 3,600 hours out of 8,760 hours in a year, while inJapan it is used for 5,100 hours. If the utilization factor couldbe increased, it should be possible to avoid power cuts. Thetransmission loss in 1991 on a national basis was 22%,consisting of both technical losses in transmission lines andtransformers, and also nontechnical losses caused by energythefts and meters not being read. It should be possible toachieve considerable saving by reducing this loss to 15% bythe end of the Eighth Five Year Plan by using well knownways and means and by adopting sound commercial practices.Further, every attempt should be made to improve systemload factors by flattening the load curve, by giving propertariff incentives and taking other administrative measures.As per the Central Electricity Authority's (CEA) fourteenthannual report, the all India load factor likely to prevail by1992-93 is 62.4%. By 1991, 4.9 lacs of villages (85%) have beenelectrified and 88 lacs of pumpsets have been energized.

Assuming a very modest average annual energy growth of5%, India's electrical energy requirement in the year 2000will be enormously high. A difficult and challenging task ofplanning, engineering and constructing new power stationsis imminent to meet this situation. The government has builtseveral super thermal stations such as at Singrauli (UttarPradesh), Farakka (West Bengal), Korba (Madhya Pradesh),



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Ramagundam (Andhra Pradesh) and Neyveli (Tamil Nadu),all in coal mining areas, each with a capacity in the range of2000 MW. Many more super thermal plants would be builtin future. Intensive work must be conducted on boilerfurnaces to burn coal with high ash content. National ThermalPower Corporation (NTPC) is in charge of these large scalegeneration projects.

Hydro power will continue to remain cheaper than othertypes for the next decade. As mentioned earlier, India hasso far developed only around 18% of its estimated total hydropotential of 89000 MW. The utilization of this perennialsource of energy would involve massive investments in dams,channels and generation-transmission system. The CentralElectricity Authority, the Planning Commission and theMinistry of Energy are coordinating to work out a perspectiveplan to develop all hydroelectric sources by the end of thiscentury, to be executed by the National Hydro PowerCorporation (NHPC).

Nuclear energy assumes special significance in energyplanning in India. Because of the limited coal reserves andits poor quality, India has no choice but to keep going onwith its nuclear energy plans. According to the Atomic EnergyCommission, India's nuclear power generation will increaseto 10000 MW by year 2000. Everything seems to be set for atake off in nuclear power production using the country'sthorium reserves in breeder reactors.

In India, concerted efforts to develop solar energy and othernon-conventional sources of energy need to be emphasized,so that the growing demand can be met and depleting fossilfuel resources may be conserved. To meet the energyrequirement, it is expected that the coal production will haveto be increased to more than 450 million tons in 2004-2005 ascompared to 180 million tons in 1988.

A number of 400 kV lines are operating successfully asmentioned already. This is the first step in working towardsa national grid. There is a need in future to go in for evenhigher voltages (750/1000 kV). It is expected that by the year1995, 48964ckt km of 400 kV lines and 86571 ckt km of220 kV lines would be in operation.



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Also lines may be in series and shunt compensated to carryhuge blocks of power with greater stability. There is a needfor constructing HVDC (High Voltage DC) links in thecountry since DC lines can carry considerably more powerat the same voltage and require fewer conductors. A 400 kVSingrauli Vindhyachal line of 500 MW capacity is the firstHVDC back-to-back scheme that has been commissioned byNPTC (National Power Transmission Corporation), followedby first point-to-point bulk EHVDC transmission of 1500 MWat 500 kV over a distance of 915 km from Rihand to Delhi.

At the time of writing, the whole energy scenario is soclouded with uncertainty that it would be unwise to try anyquantitative predictions for the future. However, certaintrends that will decide the future developments of electricpower industry are clear.

Generally, unit size will go further up from 500 MW. A highervoltage (765/1200 kV) will come eventually at thetransmission level. There is a little chance for six-phasetransmission becoming popular though there are few suchlines in USA. As the population touches the 1000 million markin India, we may see a trend to go toward undergroundtransmission in urban areas.

Public sector investment in power has increased from Rs2,600 million in the First Plan (1951-56) to Rs 2,42,330 millionin the Seventh Plan (1985-90). Shortfall in the Sixth Plan hasbeen around 26%. There have been serious power shortagesand generation and availability of power in turn have laggedtoo much from the industrial, agricultural and domesticrequirements. Huge amounts of funds (of the order of Rs.18,93,200 million) will be required if we have to achieve powersurplus position by the time we reach the terminal year tothe IX Plan (1999-2000). Otherwise achieving a target of 580billion units of electric power will remain a utopian dream.

Because of power shortages, many of the industries,particularly power-intensive ones, have installed their owncaptive power plants. Currently 7% of the electricitygenerated in India comes from the captive power plants andthis is bound to go up in the future. Consortium of industrial



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consumers should be encouraged to put up coal-based captiveplants. Import should be liberalized to support this activity.

With the ever increasing complexity and growth of powernetworks and their economic and integrated operation, it isplanned to establish central automatic load dispatch centerswith real time computer control.

Energy Conservation

Energy conservation is the cheapest new source of energy.We should resort to various conservation measures such ascogeneration (discussed earlier), and use energy-efficientmotors to avoid wasteful electricity uses. We can achieveconsiderable electric power savings by reducing unnecessaryhigh lighting levels, oversized motors etc. Everyone shouldbe taught how consumption levels can be reduced withoutany essential lowering of comfort. Rate restructuring canhave incentives in this regard. There is no consciousness onenergy accountability yet and no sense of urgency as indeveloped countries.

Load Management

By various "load management" schemes, it is possible to shiftdemand away from peak hours. A more direct method wouldbe the control of the load either through modified tariffstructures that encourage the individual customers toreadjust their own electric use schedules or direct electricalcontrol of appliances in the form of remote timer controlledon/off switches with least inconvenience to the customer.Systems for load management are varied. Ripple control hasbeen tried in Europe. Remote kWh meter reading by carriersystems is being tried. Most of the potential for load controllies in the domestic sector. In USA, power companies areplanning the introduction of system-wide load managementschemes.

Maintenance

Management and plant utilization factors of existing plantsmust be improved. Maintenance must be on schedule ratherthan an emergency. Maintenance manpower training shouldbe 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 powersystem. Generating stations and a distribution system areconnected through transmission lines, which also connectone power system (grid, area) to another. A distributionsystem connects all the loads in a particular area to thetransmission lines.

For economical and technological reasons (which will bediscussed in detail in later chapters), individual powersystems are organized in the form of electrically connectedareas or regional grids (also called power pools). Each areaor regional grid operates technically and economicallyindependently, but these are eventually interconnected toform a national grid (which may even form an internationalgrid) so that each area is contractually tied to other areas inrespect to certain generation and scheduling features. Indiais now heading for a national grid.

Interconnection has the economic advantage of reducingthe reserve generation capacity in each area. Underconditions of sudden increase in load or loss of generation inone area, it is immediately possible to borrow power fromadjoining interconnected areas. Interconnection causeslarger currents to flow on transmission lines under faultycondition with a consequent increase in capacity of circuitbreakers. Also, the synchronous machines of allinterconnected areas must operate stably and in asynchronized manner. The disturbance caused by a shortcircuit in one area must be rapidly disconnected by circuitbreaker openings before it can seriously affect adjoiningareas. It permits the construction of larger and moreeconomical generating units and the transmission of largechunk of power from the generating plants to major loadcentres. It provides capacity savings by seasonal exchangeof power between areas having opposing winter and summerrequirements. It permits capacity savings from time zonesof random diversity. It facilitates transmission of off-peakpower. It also gives the flexibility to meet unexpectedemergency loads.



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The siting of hydro stations is determined by the naturalwater power sources. The choice of site for coal fired thermalstations is more flexible. The following two alternatives arepossible.

1. Power stations may be built close to coal mines (calledpit head stations) and electric energy is evacuated overtransmission lines to the load centres.

2. Power stations may be built close to the load centresand coal is transported to them from the mines by railroad.

In practice, however, power station siting will depend uponmany factors---technical, economical and environmental. Asit is considerably cheaper to transport bulk electric energyover extra high voltage (EHV) transmission lines than totransport equivalent quantities of coal over rail road, therecent trend in India is to build super (large) thermal powerstations near coal mines. Bulk power can be transmitted tofairly long distances over transmission lines of 400 kV andabove. However, the country's coal resources are locatedmainly in the eastern belt and some coal fired stations willcontinue to be sited in distant western and southern regions.

As nuclear stations are not constrained by the problems offuel transport and air pollution, a greater flexibility existsin their siting. So these stations are located close to loadcentres, avoiding high density pollution areas to reduce therisks, however remote, of radioactivity leakage.

In India, as of now, about 65% of electric power used isgenerated in thermal plants (including nuclear). Theremaining 35% comes from hydro stations. Coal is the fuelfor most of the steam plants; the rest depends upon oil/naturalgas and nuclear fuels.

Electric power is generated at a voltage of 11 to 25 kV whichis then stepped up to the transmission levels in the range of66 to 400 kV (or higher). As the transmission capability of aline is proportional to the square of its voltage, research iscontinuously 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 grid

level 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 1.2 depicts schematically the structure

of a power system.



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Figure.1.2: Schematic diagram depicting power systemstructure

Though the distribution system design, planning andoperation are subjects of great importance, we are compelled,for reasons of space, to exclude them from the scope of thisbook except for a short appendix (M) which gives elementarydescription 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 shuntadmittance (jwCl) is small enough to be negligible resultingin the simple equivalent circuit as shown in figure 1.3.



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__________________ Figure 1.3: Simple equivalent circuit

This being a simple series circuit, the relationship betweensending-end receiving-end voltages and currents can beimmediately written as:

The phasor diagram for the short line is shown in Figure 1.2for 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, weget

or,



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Figure 1.4: Phasor Diagram for the short line for the lagging

current

Voltage regulation

Voltage regulation of a transmission line is defined as the

rise in voltage at the receiving-end; expressed as percentage

of full load voltage, when full load at a specified power factor

is thrown off, i.e.

Percentage voltage regulation =

Medium Transmission Lines

For lines more than 100km long, charging currents due to

shunt admittance cannot be neglected. For lines in range

100km to 250km length, it is sufficiently accurate to lump all

the line admittance at the receiving-end, resulting in the

equivalent diagram shown in Figure 1.5



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Figure 1.5: Medium line, localized load end capacitance

Starting from fundamental circuit equations, it is fairlystraight forward to write the transmission line equations inthe ABCD constant form given below:

Nominal T Representation

If all the shunt capacitance is lumped at the middle of theline, it leads to the nominal-T circuit shown in Figure 1.6.

Figure 1.6: Medium line nominal T representation



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For the nominal T circuit, the following circuit equations canbe written,

Substituting for Vc and Is in the last equation, we get

Rearranging the results , we get

Nominal-ppppp Representation

In this method the total line capacitance is divided into twoequal parts which are lumped at the sending and receiving-ends resulting in the nominal- p representation as shown inFigure 1.7.

Figure 1.7: 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 lineare not lumped but distributed uniformly throughout itslength must be considered.

Figure 1.8: Schematic diagram for a long transmission line

Figure 1.8 shows one phase and the neutral return (of zeroimpedance) of a transmission line. Let dx be an elementalsection of the line at a distance x from the receiving-endhaving a series impedance zdx and a shunt admittance ydx.The rise in voltage to neutral over the elemental section inthe 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 2nd equation 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 & C2 can beobtained and then substituting them in the original equationwe get,

where Zc is the characteristic impedance and l is thepropagation constant.



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and

Surge Impedance Loading

A line terminated in its characteristic impedance is calledthe infinite line. The incident wave under this conditioncannot distinguish between a termination and an infinitecontinuation of the line. Power system engineers normallycall Zc the surge impedance. It has a value of about 400ohms for an overhead line and its phase angle normally variesfrom 0o to -15o. For underground cables Zc is roughly one-tenth of the value for overhead lines. The term surgeimpedance is, however, used in connection with surges (dueto lightning or switching) on transmission lines, where theline loss can be neglected such that,

is a pure resistance.

Surge Impedance Loading (SIL) of a transmission line isdefined as the power delivered by a line to purely resistiveload equal in value to the surge impedance of the line. Thusfor 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 inper unit of SIL, i.e. as the ratio of the power transmitted tosurge impedance loading.

Ferranti Effect

The effect of the line capacitance is to cause the no-loadreceiving-end voltage to be more than the sending-end



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voltage. The effect becomes more pronounced as the linelength increases. This phenomenon is known as the Ferrantieffect.

A simple explanation of the Ferranti effect on an approximatebasis can be advanced by lumping the inductance andcapacitance parameters of the line. As shown in Figure 1.9the capacitance is lumped at the receiving-end of the line.

Figure 1.9: Simple circuit demonstrating the Ferranti effect

Now,

since C is small compared to L, wLl can be neglected incomparison to 1/ wCl. 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 equalto the velocity of light.

Tuned Power Lines

For an overhead line, shunt conductance G is alwaysnegligible and it is sufficiently accurate to neglect lineresistance 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 numericallyequal to the corresponding sending-end values, so that thereis 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 ofview of cost and efficiency (note that line resistance wasneglected in the above analysis). For a given line, length andfrequency tuning can be achieved by increasing L or C, i.e.by adding series inductances or shunt capacitances at severalplaces along the line length. The method is impractical anduneconomical for power frequency lines and is adopted fortelephone lines where higher frequencies are employed. Amethod of tuning power lines which is being presentlyexperimented with, uses series capacitors to cancel the effectof the line inductance and shunt inductors to neutralize linecapacitance. A long line is divided into several sections whichare individually tuned. However, so far the practical methodof improving line regulation and power transfer capacity isto add series capacitors to reduce line inductance; shuntcapacitors under heavy load conditions; and shunt inductorsunder 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 relationshipsbetween sending and receiving-ends. Since loads are moreoften expressed in terms of real (watts/KW) and reactive(VARs/kVAR) power, it is convenient to deal withtransmission line equations in the form of sending andreceiving-end complex power and voltages. The principlesinvolved are illustrated here through a single transmissionline (2-node 2-bus system) as shown in Figure 1.10,



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Figure 1.10: 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 d is known as the

torque angle. The complex power leaving the receiving-endand entering the sending-end of the transmission line canbe expressed as (on per phase basis),

Receiving and sending-end currents can, however, beexpressed in terms of receiving and sending-end voltagesas,

by solving we get,



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Similarly,

In the above equations SR and SS are per phase volt amperes,while VR and VS are expressed in per phase volts. [PSE,Nagrath & Kothari].

Conductor Types

Transmission lines consisting of single solid cylindricalconductors for forward and return paths are rarely used. Toprovide the necessary flexibility for stringing, conductorsused in practice are always stranded except for very smallcross-sectional areas. Stranded conductors are composed ofstrands of wires electrically in parallel, with alternate layersspiraled in opposite direction to prevent unwinding. The totalnumber of strands (N) in concentrically stranded cables withtotal annular space filled with strands of uniform diameter(d) is given by,

N = 3x2 – 3x + 1

Where x is the number of layers wherein, the single centralstrand 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 conductormaterial. It has the advantages of being cheaper and lighterthan copper though with less conductivity and tensilestrength. Low density and low conductivity result in largeroverall conductor diameter which offers another incidentaladvantage in high voltage lines. Increased diameter resultsin reduced electrical stress at conductor surface for a givenvoltage so that the line is corona free. The low tensilestrength of aluminium conductors is made up by providingcentral strands of high tensile strength steel. Such aconductor is known as aluminium conductor steel reinforced(ACSR) and is most commonly used in overhead transmissionlines. Figure 1.11 shows the cross-sectional view of an ACSR



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conductor with 24 strands of aluminium and 7 strands ofsteel.

Figure 1.11: Cross-sectional view of ACSR-7 steel strands, 24aluminium strands

In extra high voltage (EHV) transmission line, expandedACSR conductors are used. These are provided with paperor hessian between various layers of strands so as to increasethe overall conductor diameter in an attempt to reduceelectrical stress at conductor surface and prevent corona.The most effective way of constructing corona-free EHV linesis to provide several conductors per phase in suitablegeometrical configuration. These are known as bundledconductors and are a common practice now for EHV lines.

Bundled Conductors

It is economical to transmit large chunks of power over longdistances by employing EHV lines. However, the linevoltages that can be used are severely limited by thephenomenon of corona. Corona, in fact, is the result ofionization of the atmosphere when certain field intensity(about 3,000 kV/m at NTP) is reached. Corona dischargecauses communication interference and associated power losswhich can be severe in bad weather conditions. Critical linevoltage for formation of corona can be raised considerablyby the use of bundled conductors i.e., a group of two or more

Aluminiumstrands

Steel strands



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conductors per phase. This increase in critical corona voltageis dependent on number of conductors in the group, theclearance between them and the distance between the groupsforming the separate phases. The bundle usually comprisestwo, three or four conductors arranged in configurationsillustrated in Fig 1.12.

Figure 1.12: Configurations of conductors in bundledconductors

Circuit Breaker

Figure 1.13 is illustrative of a 3-phase symmetrical short-circuit on a generator with an intervening circuit breakerhaving three circuit opening poles, one in each phase. Theshort circuit current would comprise two components-DCoffset current and symmetrical short-circuit current. The DCoffset current is maximum in the phase whose voltage is zeroat the instant of short circuit (say in phase B).

Figure 1.13: Phase short-circuit and circuit breaking



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Because of the time-varying synchronous reactance of the

synchronous generator, the symmetrical short-circuit

current decays reaching steady state after passing through

subtransient and transient phases. The short-circuit current

of phase B is shown in Figure 1.13.

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 electric

arc 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 recovers

with the vanishing armature reaction.

The waveforms of iB and VB are shown in Figure 1.14. 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 1.14: 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 1.15. 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 the power-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 1.15: 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 ofa switch, or other large scale disturbances. This is the studyof power system transients.

Transient phenomenon lasts in a power system for a veryshort period of time, ranging from a few µs up to 1s. Yet thestudy and understanding of this phenomenon is extremelyimportant, as during these transients, the system is subjectedto the greatest stress from excessive over-currents orvoltages which, depending upon their severity can causeextensive damage. In some extreme cases, there may be acomplete shutdown of a plant, or even a blackout of a wholearea. Because of this, it is necessary that a power systemengineer should have a clear understanding of power systemtransients, to enable him to find out their impact on thesystem, to prevent them if possible, or at least control theirseverity or mitigate the damage caused. This chapter isdevoted 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 byshort-circuits or resonance conditions usually arise assecondary effects, but may well lead to the plant breakdownin EHV (500-765 kV) systems. Also in EHV systems thevoltage transients or surges caused by switching, i.e. openingand closing of circuit breakers, are becoming increasinglyimportant. On cable systems, of course, lightning transientsrarely occur and the other causes become more important.

Depending upon the speed of the transients, these can beclassified as:

n Surge phenomena (extremely fast transients)

n Short-circuit phenomena (medium fast transients)

n Transient stability (slow transients)

Surge Phenomena

This type of transient is caused by lightning (atmosphericdischarges on overhead transmission lines) and switching.Physically, such a transient initiates an electromagnetic wave(surge) travelling with almost the speed of light (3 × 108 m/s)on transmission lines. In a 150 km line, the travelling wavecompletes a round trip in 1 ms. Thus the transientphenomena associated with these travelling waves occurduring the first few milliseconds after their initiation. Theever-present line losses cause pretty fast attenuation ofthese waves, which die out after a few reflections.



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The reflection of surges at open line ends, or at transformerswhich present high inductance, leads to multiplicative effecton voltage buildup, which may eventually damage theinsulation of high-voltage equipment with consequent short-circuit (medium fast transient). The high inductance of thetransformer plays a beneficial role of insulating the generatorwindings from transmission line surges. The travellingcharges in the surges are discharged to ground via lightningarresters without the initiation of a line short-circuit,thereby protecting the equipment.

Selection of insulation level of various line equipment andtransformers is directly related to the overvoltages causedby surge phenomena. Hence the importance of studying thisclass of transients.

Short-circuit Phenomena

About more than 50% short-circuits take place on exposedoverhead lines, owing to the insulation failure resulting fromovervoltages generated by surge phenomena describedearlier, birds and other mechanical reasons. Short-circuitsresult from symmetrical (3-phase) faults, as well asunsymmetrical (LG, LL, LLG) faults. The occurrence of asymmetrical fault brings the power transfer across the lineto zero immediately, whereas the impact is only partial incase of unsymmetrical faults. Like surge phenomena, short-circuits are also fully electric in nature. Their speed isdetermined by the time constants of the generator windings,which vary from a few cycles of 50 Hz wave for the damperwindings to around 4s for the field winding. Therefore, thesetransients will be sufficiently slower than the surgephenomena. The time range that is of practical importanceto power system analyst is from 10 to 100 ms, i.e. the firstfew (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 tothe equipment. Therefore, the faulty section should beisolated as quickly as possible. Most of the short-circuits donot 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 whichautomatically close periodically to find out if the line hasrecovered. If the fault continues for some time, then of course,the breaker has to open permanently. This whole operationof successive closing-opening cycle may last for a second orso.

Transient Stability

Whenever a short-circuit takes place at any part of theintegrated system, there is an instantaneous total or partialcollapse of the bus voltages of the system. This also resultsin the reduction of the generator power output. Since initiallyfor some instants the input turbine power remains constant,as there is always some time delay before the controllerscan initiate corrective actions, each generator is subjectedto a positive accelerating torque. This condition, if sustainedfor some time, can result in the most severe type of transients,namely the mechanical oscillations of the synchronousmachine rotors. These electromechanical transients may,under extreme conditions, lead to loss of synchronism forsome or all of the machines, which implies that the powersystem has reached its transient stability limit. Once thishappens, it may take several hours for an electric systemengineer to resynchronize such a "blacked-out" system. Thus,it is quite necessary to simulate this phenomenon on thecomputers and use the switching and load-managementstrategies that will avoid or minimize, the ill effects of short-circuits.

The rotor swings are quite slow, as they are mechanical innature. A transient stability study, thus, may confine itselffor the time period of a few milliseconds to one minute inmost of the cases. [PSE, Nagrath & Kothari]

Generation of Overvoltages on Transmission Lines

Transmission lines and power apparatus have to be protectedfrom over voltages. The over voltages in' a power system fallunder three categories:

n Resonance overvoltages

n Switching overvoltages

n Lightning overvoltages



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Overvoltages due to the latter two causes, though transientin nature, constitute the basis for selection of insulation levelof lines and apparatus, and of devices for surge protection.Resonance overvoltages on the other hand decide the steadyvoltage rating of such devices. Resonance and switchingovervoltages are directly related to the system operatingvoltage, but the level of lightning overvoltages caused by thenatural phenomenon are independent of it. At transmissionline voltages up to around 230 kV, the insulation level isdictated by the requirement of protection against lightning.For voltages from 230 kV to 700 kV, both switching transientsand lightning overvoltages must be accounted for in decidingthe insulation levels. In EHV (> 700 kV) switching surgescause higher overvoltages than lightning, and are thereforemainly responsible for insulation level decision. Fortunatelycables are not exposed to lightning, and are automaticallyimmune to the line surges which attenuate, upon entering acable. However, lines are preferred to cables for economicand technical reasons.

Resonance Overvoltages

Though it is unlikely that resonance in a supply network beobtained at nominal supply frequencies, it is possible to havethis condition at harmonic frequencies. Near resonanceconditions may occur under certain type of unsymmetricalfaults. Temporary overvoltages are also caused by inrushcurrent, when transformers or reactors are energized (ferro-resonance). Such overvoltages are important in choosinglightning arresters, which are not supposed to operate atthese voltages. Thus they indirectly determine the insulationlevel of the network.

Switching Overvoltages

These overvoltages are caused by normal switch-gearoperation and/or power system faults, and their magnitudesare related to the system operating voltages. Further, theseovervoltages have a very wide range of magnitudes and wave



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shapes and last for durations ranging from a few MS (MicroSeconds) to several seconds. At EHV levels the mostimportant causes of switching overvoltages are classified as:

n Sustained earth fault on phase conductors.

n Energisation or reclosure of long lines.

n Load rejection at receiving end.

n Fault initiation and reclosure.

Switching transients are also classified as single-energy ordouble energy transients. In a single-energy transient, energyis redistributed in the circuit inductance or capacitance whilein a double-energy transient, one transient is interchangedbetween system inductance and capacitance, giving rise tonatural frequency (fn = 1/2p (VL/C) with R = 0) voltages andcurrents. Closing a circuit may result in excessive currents,and perhaps voltages also, while its opening normally resultsonly in excessive transient voltages.

Among factors which decide the switching behaviour of powersystems are the nature of source, characteristics of thetransmission circuit, its length, the termination condition,the characteristic of earthing and shunt compensation. Whenthe terminations are such that the energy is entirely or almostentirely reflected, high surge voltages are likely to build up.

The most important switching operations to be consideredare line energisation and reclosing. With the improvementof arc restriking performance of circuit breakers, theconsequent surges--interruption of line charging current andchopping of magnetizing current are no longer of significance.

Attenuation of surges caused by line losses; corona andreflection at the far end of the line from a loaded transformerhelp reduce the switching overvoltages, but mutual effectsof sequential reclosing of the three phases tend to accentuatethese.

The cost of EHV transmission system may be lowered bydecreasing the switching overvoltages. This can be achievedby employing a circuit breaker, filled with a closing resistor



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of the order of the line surge impedance, in series with thebreaker and the line which is subsequently short-circuited.The system is energised in two stages, producing-twoovervoltages, both of which are smaller than the overvoltageproduced without the resistor.

Lightning Overvoltages

Lightning is a naturally occurring phenomenon whereinclouds get charged to several thousand kilovolts, and adischarge (stroke) can occur to high ground objects, or evento the ground. Transmission lines and towers being highobjects attract lightning stroke, the underground cables beinginherently immune to strokes. Lightning transients to whichpower system (lines, towers, substations and generatingstations) are susceptible may occur on account of:

n Indirect strokes

n Direct strokes to phase conductors

n Direct strokes to towers

n Direct strokes to earth wires

Direct Stroke A direct stroke occurs when a thunder clouddirectly discharges on to transmission lines, tower or earthwires. This is the most severe and rarest form of stroke.

Indirect Stroke When a thunder cloud passes over groundobjects, it induces a positive charge in them. Over a periodof hundreds of seconds, positive charges leak from the toweralong the string insulators to the line conductors. Thishappens due to high field gradients involved. In the eventthe cloud discharges to some earth object, the line is left witha huge free concentration of positive charge, which cannotleak suddenly, but instead travels in the form of two identicalsurges in either line direction. This is called an indirectstroke.

Typical lightning voltage surge in wave form and amplitudethat may be injected in direct stroke on line conductors inabsence of ground wire is shown in Figure 1.16 (a). Typicallightning current on transmission line tower is shown in



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Figure 1.16 (b). High voltages of the form of Figure 1.16 (a)are known as impulse voltages. The standard test impulsevoltage will be defined later in this section.

(a) Typical lightning stroke voltage (b) Typical lightning current on a

on transmission line without transmission line tower

ground wire

Back Flashover A direct stroke to tower causes a high

voltage to be set up across the tower inductance and tower

footing resistance by the fast changing lightning current (say

10 KA/µs). This appears as an overvoltage between the tower

top and conductors which are at lower voltage and can cause

a flashover from tower to line conductor across the line

insulator, called back flashover. The voltage wave caused on

the line because of back flashover has a very high rate of rise

which can cause damage to terminal equipment.

The amplitudes of voltages induced indirectly by lightning

strokes to a tower, earth wire or nearby ground object, are

normally much less than those caused by direct stroke to a

line conductor. This voltage depends upon electrical nature

of tower footing resistance and stroke characteristics. They

are of significance at lower voltages, such as 33 kV and below,

and may even be important above this voltage.



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Protection of Transmission Lines Against Lightning

Surges due to lightning are mostly injected into the powersystem through long cross-country transmission lines.Substation apparatus is always well shielded against directlightning strokes. The protection of transmission linesagainst direct strokes requires a shield to prevent lightningfrom striking the electrical conductors. Adequate drainagefacilities and adequate insulation structures must beprovided so that the discharge can drain to ground withoutaffecting the conductors. This prevents any arc from lineconductor to ground.

Protection Using Shielding Wires or Ground Wires

The ground wire is a conductor run parallel to the mainconductors of the transmission line. It is placed higher thanthe main conductors, is supported on the same towers and isearthed at equally and regularly spaced towers. It acts intwo ways to protect the main conductors.

n The ground wire helps to increase the effectivecapacitance between the line conductor and ground, suchthat the voltage appearing between conductor andground because of static cloud charge is reduced. Thisis illustrated by the capacitor equivalent of the cloud-conductor system shown in Figure 1.17.

Figure 1.17: The capacitor equivalent of the cloud-conductorsystem



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n Being higher than the ground wire shields the mainconductor against direct strokes, though it increases theprobability of a direct stroke to itself (more than whatit would be for the main conductor if the ground wirewere absent). The protection (or shielding) angle of aground wire is found to be 30° for tower heights of 30mor less. The protection zones of one and two groundwires are shown in Figures 1.18 (a) and (b), while Figure1.18 (c) shows a double circuit line protected by a singleground wire. The height of the ground wire above thehighest line conductor can be easily determined by theprotection zone geometry. However, the present trendin fixing tower height and the shielding angle is byconsidering flashover rates and failure probabilities.

Figure 1.18: (a) Protection zone of one ground wire (b)Protection zone of two ground wires (c) A double circuit line

n The presence of ground wire(s) helps reduce the rise ofback flashover in the event of direct stroke to tower, asthe instantaneous potential to which the tower top israised is reduced by the fact that half the surgeimpedance (Zg/2) of the ground wire appears in parallel



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to the tower surge impedance (ZT). It follows fromFigure 1.19 that the tower top voltage is

n where Ii is the impulse current injected into the tower.It is easily seen that Zg (as low as possible) reduces VT.

Figure 1.19: Half the surge impedance (Zg/2) of ground wireappears in parallel to the tower surge impedance (Zr)

If the surge impedance of the tower which is the effectivetower footing resistance, is reduced, the tower top surgevoltage reduces to a considerable extent. Towers aregrounded by providing driven ground rods and counterpoisewires connected to tower legs at its foundations.

The standard value of this resistance is around 10 ohms for66 kV lines, and increases with the operating voltage. For400 kV it is approximately 80 ohms. The tower footingresistance is the value of the footing resistance whenmeasured at 50 Hz. It is made as low as economicallyjustifiable.

Power System Protection

Introduction

A fail-free power system is neither economically justifiablenor technically feasible. Faults can occur in any power system



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components generators, transformers, buses, lines - thoughtransmission lines being exposed to environment are themost vulnerable. Faults fall into two general categories -short-circuit faults and open circuit faults. Short circuit faultsare the most severe kind, resulting in flow of abnormallyhigh currents. If allowed to persist even for a short period oftime, short circuits can lead to extensive damage toequipment. Undesirable effects of short circuits faults areenumerated below:

n Arcing faults (most common) can vaporize in the vicinityleading to, possibly, fire and explosion, e.g. intransformers and circuit breakers.

n Power system components carrying abnormal currentsget over heated, with consequent reduction in the lifespan of their insulation.

n Operating voltages can go above or below theiracceptable values, leading to development of anotherfault or damage to utilization equipment.

n Consequent unbalanced system operation causes overheating of generator rotors.

n Power flow is severely restricted, or even completelyblocked, while the short circuit lasts.

n As a consequence of blockage of power flow, powersystem areas can lose synchronism. The longer a faultlasts, the more is the possibility of loss of synchronism.

Open circuit faults cause abnormal system operation anddanger to personnel. Voltages tend to rise well beyondacceptable values in certain parts of the system withpossibility of insulation failure and development of a shortcircuit fault. While open circuit faults can be tolerated forlonger periods of time than short circuit faults, these cannotbe allowed to persist , and must be removed. We shall devoteour attention to the more severe type faults, i.e. the shortcircuit faults. There are also other abnormal operatingconditions which require remedying, but do not fall into thetwo categories of faults mentioned. Two such importantconditions are heavily unbalanced generator operation andloss of generator excitation.



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Faults should be instantly detected and the faulty sectionisolated from the rest of the system in the shortest possibletime. It is obviously not possible to do this manually, and itmust therefore be accomplished automatically. Faults aredetected automatically by means of relays and the faultysection (say a line, a transformer or generator). Thecombination of relays and circuit breakers is known as theprotective system. The salient features of power systemprotection are:

n Speed: Faults at any point in the system must bedetected and isolated in the shortest possible time. Thistime is of the order of 30 - 100 ms, depending upon thefault level of the section involved.

n Sensitivity: Relaying equipment must be sufficientlysensitive to operate reliably when required underconditions that produce the least operating tendency.

n Selectivity: Relaying equipment must clearlydiscriminate between normal and abnormal systemconditions, so that it never operates unnecessarily.

Protective system must isolate a fault keeping as much ofthe system interconnected as possible.

n Reliability: Relaying equipment must be found inhealthy operating condition, when called upon to act,as years might elapse between two consecutiveoperations of relays at a particular station.

n On important lines the protective system, after onceisolating the fault, must try to recluse the breakersrestoring the system to its original configuration. Thisis necessary as many faults (arcing faults) are self-clearing and the system must be healthy in this respect.

The above objectives of a protective system are quite stringentand some of these may conflict. Systems have however beendevised and installed that work quite satisfactorily. Thepriorities of protective schemes differ from one organizationto another, giving rise to a wide variation in relay application.



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Protective Zones

In order to demerit the number of elements disconnected bythe protective system during a fault, the protective systemis divided into a number of zones. Each protective zone hasthe primary responsibility to disconnect the element orelements in the zone in the event of a fault. For this purposecircuit breakers and relays are located at the zoneboundaries. The protective zone concept is illustrated bymeans of Figure 1.20 below. Certain features are observedhere.

Figure 1.20: Protective Zones of a power system

n A separate zone of protection is established around eachsystem element. Any failure within a zone will cause"tripping" (i.e. opening) of all circuit breakers of thatzone, and only those breakers.

n The generator and transformer are lumped together inmodern unit generation system (210 MW and above) andare protected by a single zone. However, separateprotective schemes must be provided for the generatorand transformer and both these schemes control the zonebreaker. In older schemes, separate protective zones areemployed for generator and transformer, necessitatingtwo or more circuit breakers and a low voltage bus.



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n Adjoining protective zones are made to overlap. For afault in the overlapping area of two zones, more circuitbreakers will open than the minimum necessary todisconnect the faulty elements. Instead if there is nooverlap, a small region between adjoining zones willremain unprotected. This is not acceptable.

The protection provided by each zone to its element(s) isknown as primary protection. There may arise situations,however rare, that some components of zone protectionscheme failed to operate when called upon to do so. In orderto almost 100% protect the power system elements and toprevent extensive damage, backup protection is providedwhich takes over only in the event of primary protectionfailure. Backup relays should not employ or control anythingthat is in common with primary relays that are to be backedup. This requires that back up relays should be located atdifferent physical locations (relaying station). Because of thisrequirement, a larger chunk of power system getsdisconnected when backup relays operate. The principle ofbackup protection is illustrated by means of Fig 1.21. Certainobservations can be made immediately from this figure.

Figure 1.21: Backup Protection

n For a fault on line 1-2, if the primary protection fails tooperate, backup relays will trip circuit breakers ABIJopening five lines in place of one.

n For a fault on bus 1, backup protection is provided byrelays located at ABF.

While remote backup concept is dealt above, mention needsto be made about local backup. This is widely used in present



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day 132 kV and above lines. Local backup relay trips all thebreakers connected to the bus is not isolated by its breakersbeyond a certain time. With reference to Figure 1.21, a faultin the line 1-2 will be mainly isolated by E and F. If any ofthem fails e.g. E, a local backup relay will isolate C and Dafter some time (obviously before the opening of A and B).

In general we conclude:

n Backup relaying should function with sufficient timedelay so that first opportunity is given to primary relaysto function in the event of a fault.

n When back up relaying functions, a larger part of thesystem is disconnected than when primary relayingoperates correctly.

n Backup relaying is a must but not a substitute for goodmaintenance.

n Backup relaying need to be provided for only the mostsevere kind of faults, i.e. short circuits. No backuprelaying is employed for other abnormal conditions.

In the event of a fault on the line AC (Figure 1.19), the backuprelay at A, which controls the breaker A provides a backupprotection of sorts. In spite of this line AC must be providedwith its independent backup protection whose relays andbreakers are located physically away from the stations A andC.

Relaying Elements and Quantities

Currents and voltages at the two ends of a protected elementare the basic quantities which are employed to recognize ifthe fault is in the protected zone. These quantities are fed tothe relay which suitably processes these to produce a binaryoutput-"trip" or "not trip (block)"- in the circuit breaker underits control. In order that the relay (which indeed is a signalprocessor) be of small size and low-expense element, it mustnot be fed directly by the system currents and voltages whoselevel is tremendously high. This is further necessitated bythe fact that the personnel working with the relay must beprovided with a safe environment. Low-level samples of



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power system currents and voltages must, therefore, beextracted by means of transducers which are nothing butcurrent and voltage transformers. It is seen with referenceto Figure 1.22 that at each relaying station, the protectionsystem comprises three elements.

n Circuit breaker (CB) - to open the line

n Transducers (T) - to provide low-level current andvoltage samples to the relay

n Relay (R) - to process the current and voltage signals toproduce binary logic signal-'trip' or 'not trip'.

The power supply needed to trip the circuit breaker or toprovide the biasing signal in case of electronic relays mustbe provided by an independent battery source, which mustbe regularly and thoroughly maintained. This is a must asduring a fault, power system voltage would dip to very lowlevels. In case of a fault at the point P of line 1-2 inFigure 1.22, both relays RI2 and R21 must recognize or seethis fault and proceed to trip the respective breakers undertheir control. The area of responsibility of a relay is knownas the reach of the relay.

Figure 1.22: Relay system

The signal processing time to arrive at the logical decisionis typically 8-40 ms, depending upon the type of relay



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employed. The total time that may elapse from the initiationof a fault to opening of the circuit breaker is between 30-100ms depending upon the type of relay and circuit breakeremployed. Intentional time delays may be included forcoordination purposes.

For every type and location of a fault, there is some distinctivedifference in the attributes of power system currents andvoltages. Each relay is designed to recognize a particulardifference and to operate in response to it. The differencesare possible in one or more of the following attributes leadingto various kinds of relays.

n Magnitude (of current or voltage).

n Direction (large change in phase angle).

n Ratio (impedance).

n Duration.

n Rate of change.

n Order of change.

n Frequency.

n Harmonics in wave shape.

[PSE, Nagrath & Kothari]

Power Transmission Cables & Transformers

The motivation for applying superconducting materials to apower transmission and distribution system is the promiseof power delivery and conversion without the electric lossesthat result from I2R or Joule heating. The period of 25 yearsfrom 1961 to 1986 saw considerable activity in developmentof power transmission cables using metallic or lowtemperature superconductors (LTS). Had it not been for theenergy crisis of the early 1970s and the subsequent declinein energy demand, today there might be superconductingpower transmission cables in use throughout the world.Target power ratings per circuit for superconducting cablesystems dropped from 5,000-10,000 MW in the 1970s to 1,000MW by the early 1980s (Engelhardt, Von Dollen, and Samm



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1992). Although economic considerations continue todominate the criteria for deciding whether asuperconducting solution to electric power problems isappropriate, other factors are becoming increasinglyimportant in the minds of decision makers. These includegrowing public concern over environmental issues and safetyand the uncertain effects of deregulation on the generationand distribution of electric power. The responses to many ofthese issues will be known only after lengthy debate and nodoubt countless pages of legislation. While the actual needfor superconducting cables and transformers will bedetermined by local market conditions, aided perhaps byvarying legislative requirements, the technology ofsuperconducting systems is being developed globally andcompetitors in the United States, Europe and Japan who arelooking for a stake in an anticipated multi-billion dollarbusiness are making excellent progress. When leaders in thefield of superconductivity convened in Japan in May 1996for the Fifth International Superconductivity IndustrialSummit, they agreed that the world market for electric powerdevices based just on superconductivity will exceed $10billion by the year 2010.

In spite of worldwide efforts to develop superconductingcables and transformers using LTS materials, the expenseof cryogenic cooling systems for liquid He operation at 4.2 Kwith the strict operational reliability demanded by electricutilities, and the difficulty of developing a suitable low lossAC superconductor, presented seemingly unsurmountablebarriers to their introduction into the network. The discoverysince 1986 of high temperature superconducting (HTS)materials in oxide-based systems with increasingly hightransition temperatures has rekindled an interest insuperconductivity in everyone in the power delivery chain,from generator to consumer. The operating temperature ofHTS materials of up to 77 K (liquid nitrogen temperature) isconsiderably higher than the 4.2 K (liquid heliumtemperature) on which design of the LTS power systems ofthe 1970s and early 1980s was based. With highertemperatures come not only reduced refrigeration costs butalso enhanced reliability.



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Superconducting Power Transmission Cables � Overview

Although the energy crisis of the 1970s is now past anddemand has increased considerably, the motivation fordeveloping HTS power transmission cables comes primarilyfrom the need to increase the power-handling capabilities ofexisting underground circuits, which are filled to capacity.HTS cables not only offer a doubling of the power per circuit,they also provide an environmentally attractive solution,because a leak in an underground HTS system would causethe benign release of nitrogen, whereas a leak in existingoil-filled high voltage cables could result in devastating soilcontamination. Where oil-filled cables are used underwater,such leaks could produce even greater environmentaldamage.

Upgrading a power system by retrofitting existing ducts withHTS cables is most likely to occur in dense urban areas wherethe costs of trenching to install higher-capacity conventionalsystems would be prohibitive. In Tokyo, for instance, wheredemand for electric power is increasing at a rate of 2-3% peryear, use of HTS cables is attractive since space is extremelylimited and most underground ducts are filled to capacity.The opportunity in Tokyo alone provides a tremendousdevelopment incentive. There are ten large cable tunnels inTokyo, each 20 km long and each containing three cables. Ifthese cables were replaced with HTS cables at the rate ofonly one of the three cables in several tunnels each year, theproject would require 600 km of cable and last ten years.The HTS conductor alone needed for such a venture wouldexceed 100 million meters and represent a businessopportunity of several billion dollars. And if the relativeeconomic value of the joints and terminations requiredfor the cable follows today's pattern, then the businessopportunity for these cryogenic components is at leastten times greater than that of the conductor businessitself.

Development of LTS cables and cable concepts in the 1960swas pursued by industrial giants like Siemens, GE-France,BICC, and Westinghouse, and by several academic andgovernment laboratories, including important contributions



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from the Technical University of Graz, Austria, andBrookhaven National Laboratory (BNL) in the United States(Giese 1993). In Japan, members of the MITI's Electro-technical Laboratory carried out an economic study andconcluded that superconducting cables were especiallyattractive for high power DC applications. Early testbedsused LTS materials in a variety of configurations.

In Germany, Linde studied the AC loss characteristics ofrigid Nb tubes and built a 7 meter-long cryostat to measurethese losses. Later the Linde team proposed a compositeconductor of Nb, copper, and invar. In a collaborative programbetween the Technical University of Graz, AEG, Kabelmetal,and Linde (Munich), the superconductor was formed bycoating the inner and outer walls of concentric corrugatedtubes with a layer of Nb so that the layer on the outside ofthe smaller tube faced the Nb layer on the inside of the largertube.

The BNL project employed Nb3Sn superconducting tapes, andfor the demonstration cable Intermagnetics GeneralCorporation (IGC) manufactured a composite tape that hadlayers of copper, Nb3Sn, Nb, and stainless steel. This workresulted in design of a 1,000 MVA, 138 kV, 4,200 Amp systemand in a preliminary solution to the problem of terminations.Progress in phase two of this work went well; however, theproject was terminated for economic reasons.

HTS Power Transmission Cables - An Overview

The major players in development of power transmissioncables using high temperature superconductors are Pirelliand Southwire Corporation in the United States, Siemens,Pirelli, and BICC in Europe, and Sumitomo ElectricCorporation, Furukawa and Fujikura in Japan. Each of themajor Japanese corporations manufactures its own HTStapes. Siemens also manufactures its own HTS tapes, but ithas also purchased material from others for use in earlierexperiments. Pirelli has an arrangement with AmericanSuperconductor Corporation (ASC) that gives it exclusiveaccess to ASC's tapes for use in power transmission cables.IGC also supplies high performance HTS tapes to this



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marketplace (Beales et al. 1996). All experimental andprototype HTS cables have been manufactured with multi-filamentary tape containing the BSCCO-2223 compound.

The German government is providing half the funds (DM 20million over three years) necessary to complete the cableprogram carried out and cost-shared by Siemens. In both theUnited States and Japan, on the other hand, the utilities areplaying a major role in promoting the development of HTScables. Tokyo Electric Power Company (TEPCO) providesnearly a million dollars annually to both Sumitomo andFurukawa to develop HTS cable prototypes and terminations,respectively. Chubu Electric Power Company has alsoworked with both Fujikura (Kume et al. 1995) and Sumitomo(Masuda et al. 1995) to develop related technology. In theUnited States, the Electric Power Research Institute, withsome financial assistance from the Department of Energy,has invested heavily in power cable technology by forging aclose alliance with Pirelli and ASC. The total program costsare estimated to be $6 million over three years. Pirelli,Siemens, and BICC Cables have benefited from an earlierEuropean collaboration, which included GEC, ABB, andAlcatel Cable, and which was financially supported by theEuropean Commission under both the JOULE and BRITE-EuRAM initiatives.

The European project resulted in a very useful techno-economic study (Ashworth, Metra, and Slaughter 1993) thatallowed individual members of the consortium to decidewhether there was sufficient incentive to pursue furtherdevelopment of cable technology on their own. The significantconclusions of this study were

1. that for transmitted powers greater than 1 GVA, theHTS conductor's critical current density must exceed200,000 A/cm2 at liquid nitrogen temperatures in orderfor the overall costs to be comparable to conventionalcables and

2. that in a 150 mm fixed diameter duct, an HTS cable cantransmit up to seven times more power (to 700 MVA at66 kV) at the same transmission cost.



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HTS conductors, like so many other materials in theirembryonic development phases, must exhibit improvedperformance and become less expensive if they are to gainwidespread acceptance as articles of commerce. The Joulestudy presented a target performance-price window for HTSconductors (Figure 1.23) for transmission of 400 MVA in afixed diameter duct (believed to be an early application ofHTS cables), the superconductor must carry in excess of150,000 A/cm2 if the price is about $40/meter. If the pricefalls to say, $5/meter, the superconductor performance maybe as low as 50,000 A/cm2. For a given conductor price, say$5/meter, the "economic" critical current density is greaterfor application in a high power link than in a fixed diameterduct. This seems reasonable as HTS must compete with thebest available conventional cable solution in the former case,whereas HTS can be considered enabling in the latter.

HTS Power Transmission Cable Development in Japan

Of the major players in Japan that are developing HTS powertransmission cables – Sumitomo, Furukawa, and to a lesserextent, Fujikura and Mitsubishi (Yuhya, Hosotani, andHiraoka 1995), Sumitomo Cable working with TEPCO hasdemonstrated the best performance in both fundamentalmaterials development and cable construction. Theconfiguration of its 7-meter cable prototype is schematicallyillustrated in Figure 1.23, and the corresponding dimensionsare shown in Table 1.2 (Shibata et al. 1995). Note that in thisdesign the HTS tapes are used not only for transmission ofthe primary current, but also for shielding the external pipefrom the magnetic fields generated by the tapes transmittingthe power. This design increases the needed quantity ofcostly HTS conductor, but the lower electrical losses placeless strain on the cryogenic systems, which reduces coolingcosts. Several characteristics of this cable are as follows:

n 3-phase, 66 kV / 1 kVArms (114 MVA)

n 7 m length, 130 mm diameter

n magnetic shielding layer, PLPP insulated

n 3-phase continuous current test ( 1 kArms, 7 hours)



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Figure 1.23: Schematic illustration of 7-meter HTS cableprototype (Sumitomo)

Table 1.2: Sumitomo/TEPCO Cable Prototype

Table 1.3 gives characteristics of the Sumitomo HTSconductor used in assembling the 50-meter cabled conductorthat is shown in Figure 1.24.



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Table 1.3: Sumitomo/TEPCO HTS Conductor

Figure 1.24: Fifty-meter-long cabled conductor coil

The Sumitomo HTS tapes have a high cross-sectional aspectratio. The self-field critical current densities are not as highas those cited as being "economic" in the Joule study;however, they are presently the best in the world andrepresent the state of the art for long-length HTS conductors.At 10-12 ohm * m and 10-13 ohm * m criteria, the Ics of the



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conductor are 2,900 A and 2,200 A, respectively. It should benoted that it was the requirement of the utility sponsor,TEPCO, that the critical current density for its application(fixed diameter retrofit) be 100,000 A/cm2. Lower criticalcurrent densities could only be tolerated if the attendantAC losses were reduced substantially. At the time of thisWTEC study, AC losses were ten times higher thanacceptable. Recent measurements on each of the fourindividual layers constituting this cable confirm that the ACloss is described by a self-field loss of a single cylindricalbulk superconductor based on the Bean model (Saga et al.1996).

Based on Sumitomo's cabled conductor characteristics, onlyfour tape layers are needed to carry nearly 3,000 amps (dccritical current) at 77 K. With a tape width of 4.1 mm and acable diameter of 23 mm, one can estimate that just over 60tapes have been used to wind this cabled conductor, whichmeans that the average Ic of one tape is approaching 50 A at77 K! This is nearly ten times the Ic of the Furukawa cabledconductor.

Characteristics of Furukawa cabled conductor are as follows:

n 1-phase, 66 kV / 1.4 kArms (38 MVA)

n 5 m length, 124 mm diameter

n 66 kV - class terminations

n load test (1.4 kArms, 15 minutes)

In the Furukawa case, the Jc and the overall tape cross-section are nearly half of the Sumitomo values; consequently,one would expect an Ic at least four times smaller. Since tentape layers were wound at a larger diameter than in theSumitomo cable to produce a much smaller DC criticalcurrent at 77 K, the Ic is much smaller and is estimated to beabout 5 A.

Although there is not much published detail about the cableprototype proposed and assembled by Fujikura, it is uniquein that it has HTS tapes lying axially along the cable lengthinstead of being wound with a pitch (Kakimoto et al. 1995;Kume et al. 1995).



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Superconducting Transformers � An Overview

Transformers represent one of the oldest and most matureelements in a power transmission and distribution network.From the point of electricity generation at a power plant,where extremely high voltages are needed to "push" largeamounts of power into the grid, to the end user of electricityin a home or office, where typical appliances operate at muchlower voltages (100-200 volts), transformers are needed foreffective voltage conversions. At each conversion point,energy is lost, primarily in the form of wasted heat fromchanging electrical and magnetic fields in the copper (coil),iron (core), tank, and supporting structure. Even when thetransformer is "idling," so-called "no-load losses" (NLL) aregenerated in the core. Research over the last 50 years hassucceeded in reducing NLL by a factor of three whileincreasing core costs by a factor of two. Recent substitutionin distribution transformers (ratings below about 100 kVA)of amorphous metals for silicon iron core material hasreduced NLL further, but this material has not been used inthe cores of power transformers (ratings greater than 500kVA). When a transformer is under a loaded condition, Jouleheating (I2R losses) of the copper coil adds considerably tothe amount of lost energy. In spite of the fact that today'sutility power transformer loses less than 1% of its total ratingin wasted energy, any energy saved within this one percentrepresents a tremendous potential savings over the expectedlifetime of the transformer.

In a conventional power transformer, load losses (LL)represent approximately 80% of total losses. Of this load loss,80% are I2R losses. The remaining 20% consists of stray andeddy current losses. To date, efforts to reduce load losseshave been directed toward the latter. Unlike copper andaluminum, superconductors present no resistance to the flowof DC electricity, with the consequence that I2R lossesbecome essentially zero, thereby creating the potential for adramatic reduction in overall losses. In AC operation, thesuperconductor in an HTS transformer experiences a typeof eddy current loss: both the heat produced by this loss(although extremely small in comparison to the energy lost



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in conventional materials) and heat conducted into the lowertemperature regions of the superconducting transformerneed to be removed through refrigeration. Even with theadded cost of refrigeration, HTS transformers in the 10 MVAand higher range are projected to be substantially moreefficient and less expensive than their conventionalcounterparts.

Motivation for developing superconducting transformers isnot based solely on economic considerations of lowering totalowning costs (initial capital cost + capitalized cost of loadand no-load losses over the transformer's effective life). Withlimited new siting availability in urban areas, the anticipated2% annual growth in power demand means that existing sitesmust be upgraded with higher power capabilities. Manyexisting sites are indoors or adjacent to buildings, whichrestricts the use of most oil-filled transformers. The inherentdangers of oil-filled devices are totally eliminated byapplication of superconducting technology where the onlycoolant required is benign (nitrogen as opposed to oil).Consequently, superconducting transformers operatingeither with a refrigerated coil or one cooled with liquidnitrogen pose no fire hazards and no threat to theenvironment comparable to that posed by leaks of flammableoils and toxic chemicals such as PCBs.

Serious interest in superconducting transformers began inthe early 1960s as reliable low temperature superconductorsbased on Nb-Ti and Nb3Sn became available. Analysis of thefeasibility of such LTS transformers concluded that the highrefrigeration loads required to keep the LTS materials at4.2 K made the LTS transformers uneconomical. A majorreduction in refrigeration costs and/or the discovery ofmaterials that superconduct at much higher temperatureswould be required to improve the economic attractivenessof these electric power applications. In the mid-1970sWestinghouse conducted an exhaustive design study of a1,000 MVA, 550/22 kV generator step-up unit; it found thatcurrent transfer, over-current operation, and protectionremained persistent problems.



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Since 1980, development of LTS transformers has beenconducted primarily by ABB and GEC-Alsthom in Europeand by various utilities, industries, and universities in Japan.Advances in production of long-length ultrafine multi-filamentary Nb-Ti conductor and high resistivity Cu-Nimatrix materials have assisted in the reduction of AC losses.Feasibility of weight reduction and higher efficiencies hasbeen demonstrated on smaller devices with ratings smallerthan 100 kVA: single-phase 80 kVA (Alsthom), 30 kVA(Toshiba), and a three-phase 40 kVA (Osaka University).Larger units have also been constructed and testedsuccessfully. A single-phase 330 kVA transformer built byABB included provisions for fault-current limiting andquench protection. Kansai Electric Power Company reportedthe development of an LTS transformer utilizing Nb3Snconductor. One phase of this three-phase 2,000 kVA unitoperated at 1,379 kVA without quenching and transferredfault current to parallel coils under quench condition.

Power Transmission Unit-1

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Transcript of Power Transmission Unit-1