EE207 Electrical Power - Lecture 6

7/30/2019 EE207 Electrical Power - Lecture 6

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EE207 Electrical Power

Lecture 6

Power Transmission and Distribution


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Rajparthiban Kumar EE207 Electrical Power 2

Generation of Electrical Power

There are three main types of generating stations:

Thermal Generating Stations Hydropower Generating Stations

Nuclear Generating Stations

Thermal Stations

Thermal generating stations produce electricity fromthe heat released by the combustion of coal, oil ornatural gas.

Thermal stations are rated between 200-1500MW andusually located near a river or lake because of the largequantity of cooling water required.


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The efficiency of thermal generating station is

always low. The maximum efficiency of anymachine that converts heat energy into

mechanical energy is given as:

=efficiency of the machine %

T1=Temperature of gas entering the turbine [K]

T2=Temperature of gas leaving the turbine [K]

10011

2 ==== )TT(


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Note that T2/T1 should be as small as possible

to obtain high efficiency.

The highest feasible value for T1 is 550oC

(823K). Because we can not exceed thetemperature that steel and other metals can

withstand. Also T1 is usually in range of 20oC

(ambient Temperature).


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MP

P4

G

HP

LP

P1

2

S3

S2

S1

9

10

P3

8

111

34 5

6S

4

7

CoolingWater in

CoolingWater out

Thermal Power Plant


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The basic structure of a thermal generating stationconsists of the following components:

(1) A huge boiler: Transferring heat from theburning fuel to row of water tubes (S1)surrounded by flames. Pump P1 keeps the water

circulating. (2) Drum: Containing water and steam under high

pressure. Steam races towards the High pressure

pump HP after passing through Superheater S2.Superheater S2 ensures that the steam is dry toimprove the station overall efficiency.


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(3) HP: converts thermal energy into mechanicalenergy by letting the steam expands as it movesthrough the turbine blades. In order to preventpremature condensation the steam passes througha reheater S3.

(4) MP: Medium Pressure turbine is similar to theHP turbine except it is bigger so that the steammay still expand more.

(5) LP: Low Pressure turbine consists of twoidentical sections. Removes the available energyfrom the steam.


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(6) Condenser: Causing the steam to condense by

passing it over the cooling pipes S4. A condensatepump P2 removes the lukewarm condensed steam

and drives it through a reheater (7) towards a

feedwater pump (8).

(7) Heat Exchanger: Receives hot steam bled from

the HP to raise the temperature of the feedwater.

This will improve the efficiency of the station.

(9) Burners: supply and control the amount of

gas,oil, or coal injected into the boiler.


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(10) Forced-draft fan: supplies enormous

quantities of air needed for combustion. (11) Induced-draft fan: carries the products of

combustion and gases towards cleansing apparatus

then to the outside air. (12) G: generator directly coupled to all three

turbines converts mechanical energy into electrical

energy.


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Thermal Stations and the Environment.

The main combustion products when oil, coal, gas are

burned: CO2 (Carbon dioxide), SO2 (Sulfur dioxide) and

Water.

Water and CO2 produce no immediate environmental

effects, but SO2 creates substances that give rise to acidrain.

Natural gas produces only Water and CO2. This explains

why Natural gas is preferable.

Usually filters are used to remove particles from the boiler-

gas flue stream.


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Hydropower Generating Stations

These stations converts the energy of movingwater into electrical energy by means of hydraulic

turbines coupled to synchronous generators.

The power that can be extracted from a waterfalldepends on the height and the rate of flow and is

given by:

where,q: water rate of flow m3/s h: head of water m

P: available water power kW

qh.P 89====


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Types of Hydropower stations

High Head developments: have head in excess of300m and high speed Pelton turbines are used. Theamount of impounded water is usually small.

Medium Head developments: have heads between 30and 300 m, and medium speed Francis turbines areused. A dam is usually built across a river bed inrelatively mountainous area. The amount of impoundedwater is huge.

Low Head developments: have heads under 30m andlow speed Kaplan or Francis turbines are used. Thesestations usually extract power from flowing rivers.


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Makeup of a Hydropower Plant

A hydropower installation consists of:

Dams: made of earth and concrete are built across river

beds to create a storage reservoirs. Dams permit us to

regulate the water flow throughout the year.

Spillways adjacent to the dam are provided to dischargewater whenever the reservoir level is too high

Conduits, Penstock, and Scroll-Case:

Conduits: lead the water from the dam site to thegenerating plant.

Penstock: huge steel pipes that channels the water into a

scroll-case that surrounds the turbine.


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Scroll-Case: distribute the water evenly around the

turbines circumference.Guide vanes and wicket gates

control the water so that it flows smoothly into the

runner blades. Wicket gates open and close in response

to a powerful hydraulic mechanism controlled by the

respective turbine governors.

Draft Tube and Tailrace: Carefully designed verticalchannels to remove water from the turbine. The water is

led to a tailrace which channels the water to a

downstream river bed.

Powerhouse: Contains Synchronous generator,

transformers, circuit breakers and control apparatus.


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Transmission of Electrical Energy

A transmission and Distribution must satisfy

the following basic requirements: Provide at all times the power that consumers need.

Maintain a stable, nominal voltage that does not vary

by more than 10%. Maintain a stable frequency that does not vary by more

than 0.1%. Supply energy at an acceptable price.

Meet standards of safety.

Respect environmental standards.


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An Elementary Transmission and Distribution

system is depicted below.

G1

G2

Generating

Stations

Transmission

Substations

Interconnection

Substations

Transmission

Substations

Distribution

Substations

Small industry

CommerceResidences

Medium EHV HV MV LV

DistributionTransmissionGeneration

Heavy

industryMedium

industry

345kV

to

765kV

115kV

to

230kV

Tie-line

2.4kV

to

69kV

120/240Vsingle phase

600V three

phase


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Transmission substations: Change the line voltage bymeans of a step up/step down transformer and regulate

it by means of static var compensators, synchronous

condensers.

Distribution substations: Change medium voltage to

low voltage by means of step down transformers whichmay have automatic tap-changing capabilities to

regulate the low voltage.

Power distribution systems are divided into twomajor categories:


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Transmission Systems: The line voltage is roughlybetween 115kV and 800kV.

Distribution Systems: The voltage is generally in therange of 120V and 69kV. This is subdivided into:

Medium Voltage Distribution Systems: 2.4kV to 69kV, and

Low Voltage Distribution Systems: 120 to 600V.

The design of a power line depends on the followingfactors:

The amount of active power it has to transmit

The distance over which the power must be carried. The cost of the power line.

Esthetic considerations, urban congestion, ease ofinstallation, and expected growth.


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Types of Power lines according to voltage class:

Low voltage (LV) lines: Provide power to buildings,factories, commercial establishments. These are usuallyinsulated Aluminium conductors (as overhead orunderground cables). often transfer power from localpole-mounted distribution transformer to the serviceentrance of the customer.

Medium Voltage (MV) lines: Tie the load centres(high rise buildings, shopping centres, campusesetc.)to one of the substations of the utility company. Thevoltage level is 2.4kV to 69 kV.

High Voltage (HV) lines: connect the main substationto the generating station. The lines are either aerial


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conductors or underground cables. The voltage level is

below 230kV.

Extra High Voltage (EHV) lines: are used when thegenerating stations are very far from the load centres.

These lines operate at voltage levels up to 800kV and

may be as long as 1000km.

Components of a HV Transmission Line: Conductors: conductors for HV transmission are

always bare. The are made of stranded copper or Steel

Reinforced aluminium Cables (ACSR). ACSR areusually preferred because they are lighter and more

economical.


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Insulators: made of Porcelain and serve to support and

anchor the conductors and to insulate them from

ground.

Pin-type insulators for voltages below 70kV.

Suspension-type insulators for HV.

Pin type Insulator

Suspension type


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Supporting Structure: must keep the conductors at a

safe height from ground and at an adequate distance

from each other. For voltages below 70kV woodenpoles can be used and steel towers made of galvanised-

angle-iron pieces are used for very high voltages.

Equivalent Circuit of a Power Transmission Line

Generally an ac PTL posses a resistance (r), an

inductive reactance (xL), and a capacitive

reactance (xC) uniformly distributed over the entire

length of the line.


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The line equivalent circuit can be simplified by

lumping the individual resistances, inductancesand capacitances to give a total resistance (R) andtotal inductance and capacitance (jXL) and (jXC)respectively.

Thus the simplified equivalent circuit of a PTLbecomes:

r jxL

-jxC

N

L


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Note that the total Capacitances is divided into

two parts (each equal to 2XC) at both ends of the

line.

R jXL

N

L

-j2XC-j2XC


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The circuit can be simplified further by omitting

one or more of the equivalent circuit parameters

based on the amount of active, reactive power

associated with the line.

For LV lines, the distance is short and and the voltage

is low, thus capacitive reactive power QC associated

with the line is very small and negligible. Thus the

capacitive component can be omitted.

C

CX

EQ

2====


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HV and EHV lines are always long, and so the reactive

powers associated with the line inductances and

capacitances become more important. Also the

efficiency of the line is high so the I2R associated with

the line resistance become small, thus HV and EHV

simplified equivalent cct. becomes:

R jXL

E

PTL Low & Medium voltage

Simplified Equivalent cct.


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Example: a PTL delivers 300MW to 3-phase load

(see figure). If the line voltage at both ends

(source and Load) is 230kV, determine thefollowing:

jXL

-j2XC

-j2XCE1 E2

HV and EHV voltage

Simplified Equivalent cct.


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Line parameters: 0.065/km

Inductance: 0.5 /km

Capacitance: 300k.km

Active, Reactive power associated with the line.

The approximate equivalent circuit, per phase.

230kV

300MW

load

1000k cmil

50km


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Solution

R=50km x 0.065 /km =3.25XL=50km x 0.5 /km = 25XC= 300000 .km /50km = 6000

The line to neutral voltage E=230kV/3=133kVThe active power per phase P=300MW/3=100MWThe load current I=100MW/133kV=750A

If we temporarily neglect the presence of thecapacitor in parallel with the load, then line I2R loss


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Pline=I2R=(750)2x 3.25=1.83MW (1.8% of total P)

The absorbed reactive power of the line: QLn=

I2XL=(750)2 x 25=14.1 Mvar (14% of total P).Reactive power generated by the capacitor at eachend: E2/XC=(133)

2/12000=1.47Mvar

Total reactive power generated by the capacitorsQCln =2 x 1.47Mvar = 2.94 Mvar ( 3 % of total P)Comparing Pline,QLn and QCln shows that QLn is thedominant component, and thus the line is inductive.The resistance and Capacitance of the line can beneglected.


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Voltage Regulation and Power Capability of TL

Four types of lines will be examined in terms ofvoltage regulation and Active power handling

capabilities.

1. Resistive lines

R

ERP

ES

Source TL EquivalentImpedance

Variable Load

I


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The maximum power that can be transmitted by the line tothe load is obtained when the load impedance is equal the

complex conjugate of the line impedance.

ES

0.95ES

0.5ES

019% 100%

ER

P

P=E2S/4R

Characteristics of Resistive line


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Under this condition the load voltage

ER=(1/2)Es

and the maximum power transferred to the load

Pmax= Es2/4R

However this voltage level at the load is not acceptable. If

we consider 5% voltage regulation (i.e. ER=0.95ES)is anacceptable limit, the the power that can be transferred by

the line to the load:

P= (0.95Es)2

/19Rthis power is only 19% of the maximum power (Pmax)!


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This can be shown as:

%R/E

R/E.

P

P

R

E.R)

R

E.(P

,Powerloadand,RI

ER

,thusandR

E.

R

E.EI

E.E

s

s

max

load

ssload

Rload

sss

sR

194

04750

0475019

050

19

050950950

2

2

22

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EE207 Electrical Power - Lecture 6