electrical transmission line

ELECTRICAL TRANSMISSION

Line---By Dhananjay Jha, Engineer (E), SJVN

Electrical transmission system is the

means of transmitting power from

generating station to different load

centres.

---By Dhananjay Jha, Engineer (E), SJVN

gridmap\powergrid_map.pdf


gridmap/powergrid_map.pdf

Transmission Network

Conductor

Earthwire

Inuslator

Tower


Network

Transmission line model

Characteristics

Conductors

Insulators

Earthwire


Generating Station

Transmission Line

Sub-Stations


Associated system

Voltage magnitudes & angle

Active and reactive power


A transmission line can be represented by a 2-port network – a

network that can be isolated from the outside world by two

connections (ports) as shown:


If the network is linear, an elementary circuits theorem (analogous to

Thevenin’s theorem) establishes the relationship between the sending and

receiving end voltages and currents as

S R R

S R R

V AV BI

I CV DI

Here constants A and D are dimensionless, a constant B has units of ,

and a constant C is measured in siemens. These constants are sometimes

referred to as generalized circuit constants, or ABCD constants.

•Upto 80 Km

•Shunt capacitance is neglected and resistance & inductance are lumped

together.

•Therefore, IS = IR = I

Hence the ABCD constants for the short transmission line model, are

1

0

1

A

B Z

C

D

Considering medium-length lines (80 to 250 Km-

long).

•The shunt admittance is also included for

calculations. However, the total admittance is

usually modeled ( model) as two capacitors of

equal values (each corresponding to a half of total

admittance) placed at the sending and receiving

ends.

The current through the receiving end capacitor can be found as :

22

C R

YI V

And the current through the series impedance elements is :

2ser R R

YI V I


From the Kirchhoff’s voltage law, the sending end voltage is

2 12

ser RS RC R RRZI V Z I I VYZ

V V ZI

The source current will be

1 1 2 1 122 2 4

S R RC ser C C R S R R

Y YI I I I I V

ZYV

ZYI Y II V

Therefore, the ABCD constants of a medium-length transmission line are

12

14

12

ZYA

B Z

ZYC Y

ZYD

If the shunt capacitance of the line is

ignored, the ABCD constants are the

constants for a short transmission line.

• For long lines, it is not accurate enough to approximate the shunt

admittance by two constant capacitors at either end of the line.

Instead, both the shunt capacitance and the series impedance must be

treated as distributed quantities.

• The voltages and currents on the line is found by solving differential

equations of the line.

sinh'

tanh 2'

2

dZ Z

d

dY Y

d

Here, Z is the series impedance of the line,

Y is the shunt admittance of the line,

d is the length of the line,

is the propagation constant of the line.

yz where y is the shunt admittance per kilometer and z is the

series impedance per km.

The ABCD constants for a long transmission line are

' '1

2

'

' '' 1

4

' '1

2

Z YA

B Z

Z YC Y

Z YD

AC voltages are usually expressed as phasors.

Load with lagging power factor.

Load with unity power factor.

Load with leading power factor.

For a given source voltage VS and

magnitude of the line current, the

received voltage is lower for

lagging loads and higher for

leading loads.

In overhead transmission lines, the line reactance XL is normally much larger than

the line resistance R; therefore, the line resistance is often neglected. We consider

next some important transmission line characteristics…

1. The effect of load changesAssuming that a single generator

supplies a single load through a

transmission line.

Assuming that the generator is ideal, an increase of load will increase a

real and (or) reactive power drawn from the generator and, therefore,

the line current.

1) If more load is added with the same lagging power factor, the

magnitude of the line current increases but the current remains at the

same angle with respect to VR as before.

The voltage drop across the reactance increases but stays at the same

angle.Assuming zero line resistance and source voltage to

be of constant magnitude:

voltage drop across reactance jXLI will stretch

between VR and VS.

Therefore, when a lagging load increases, the received voltage

decreases.

2) An increase in a unity PF load,

on the other hand, will slightly

decrease the received voltage at

the end of the transmission line.

Vs = VR + jXLI

3) Finally, an increase in a load with

leading PF increases the received

(terminal) voltage of the transmission line.

In a summary:

1. If lagging (inductive) loads are added at the end of a line, the

voltage at the end of the transmission line decreases .

2. If leading (capacitive) loads are added at the end of a line, the

voltage at the end of the transmission line increases.


The voltage regulation of a transmission

line is

VReg = (Vnl - Vfl )/ Vfl

where Vnl and Vfl are the no-load and full

load voltages at the line output.


2. Power flow in a transmission line

The real power input to a 3-phase transmission line can be computed as

,3 cos 3 cosin S S S LL S S SP V I V I

where VS is the magnitude of the source (input) line-to-neutral voltage and VLL,S is

the magnitude of the source (input) line-to-line voltage.

Similarly, the real output power from the transmission line is

,3 cos 3 cosout R R R LL R R RP V I V I

The reactive power input to a 3-phase transmission line can be computed as

,3 sin 3 sinin S S S LL S S SQ V I V I

The apparent power input to a 3-phase transmission line can be

computed as

,3 sin 3 sinout R R R LL R R RQ V I V I

And the reactive output power is

,3 3in S S LL S SS V I V I

And the apparent output power is

,3 3out R R LL R RS V I V I

A simplified phasor diagram of a transmission line indicating that IS = IR = I.

Further it can be observed that the vertical segment bc can be expressed as

either VS sin or XLIcos. Therefore:

sincos S

L

VI

X

Therefore, the power supplied by a transmission line depends on the angle between

the phasors representing the input and output voltages.

The maximum power supplied by the transmission line occurs when = 900:

max

3 S R

L

V VP

X

This maximum power is called the steady-state stability limit of the transmission

line. The real transmission lines have non-zero resistance and, therefore, overheat

long before this point. Full-load angles of 250 are more typical for real

transmission lines.

Then the output power of the transmission line equals to its input

power:3 sinS R

L

V VP

X

Few interesting observations can be made from the power

expressions:

The maximum power handling capability of a transmission

line is a function of the square of its voltage. For instance, if

all other parameters are equal, a 220 kV line will have 4 times

the power handling capability of a 110 kV transmission line.

Therefore, it is beneficial to increase the voltage. However,

very high voltages is limit by other factors.


The maximum power handling capability of a transmission line is

inversely proportional to its series reactance, which may be a

serious problem for long transmission lines. Some very long lines

include series capacitors to reduce the total series reactance and

thus increase the total power handling capability of the line.

In a normal operation of a power system, the magnitudes of

voltages VS and VR do not change much, therefore, the angle

basically controls the power flowing through the line. It is

possible to control power flow by placing a phase-shifting

transformer at one end of the line and varying voltage phase.

3. Transmission line efficiency

The efficiency of the transmission line is

100%out

in

P

P

4. Transmission line ratings

One of the main limiting factors in transmission line operation is its resistive

heating. Since this heating is a function of the square of the current flowing through

the line and does not depend on its phase angle, transmission lines are typically

rated at a nominal voltage and apparent power.

5. Transmission line limits

Several practical constrains limit the maximum real and reactive power that a

transmission line can supply. The most important constrains are:

1. The maximum steady-state current must be limited to prevent the overheating in

the transmission line. The power lost in a line is approximated as

23loss LP I R

The greater the current flow, the greater the resistive heating losses.

2. The voltage drop in a practical line should be limited to approximately 5%. In

other words, the ratio of the magnitude of the receiving end voltage to the

magnitude of the sending end voltage should be greater than 95%.

This limit prevents excessive voltage variations in a power system.

3. The angle in a transmission line should typically be 300 ensuring that the

power flow in the transmission line is well below the static stability limit and,

therefore, the power system can handle transients.

Any of these limits can be more or less important in different

circumstances.

In short lines, where series reactance X is relatively small, the

resistive heating usually limits the power that the line can supply.

In longer lines operating at lagging power factors, the voltage

drop across the line is usually the limiting factor.

In longer lines operating at leading power factors, the maximum

angle can be the limiting f actor.


Standard Voltage - 66,110,132, 220, 400 KV

or above

Selection Criterion of Economic Voltage –

Quantum of power to be evacuated

Length of line

Voltage regulation

Power loss in Transmission

Surge impedance level


Economic Voltage of Transmission of Power

Nc

KVALV

*1506.15.5

V = Transmission voltage (KV) (L-L).

L = Distance of transmission line in KM

KVA=Power to be transferred

Nc= Number of circuits

Empirical Formula


Aluminium Conductor Steel Reinforced consists of a

solid or stranded steel core surrounded by one or more

layers of strands of 1350 aluminium. The high-strength

ACSR 8/1, 12/7 and 16/19 standings', are used mostly

for overhead ground wires, extra long spans, river

crossings, etc. The inner core wires of ACSR is of zinc

coated (galvanized) steel.


Nomenclature :- Al/Steel/dia.

Panther :- 30/7/3.0 mm

Zebra :- 54/7/3.18mm

Snowbird :- 42/3.98 + 7/2.21mm

Moose :- 54/7/3.53mm


AAC (All Aluminium Conductor)

AAAC (All Aluminium Alloy Conductor)

AL-59

ACCC (Aluminium conductor composite core)


ACSR Moose AAAC Moose Al-59

Dia. (mm) 31.77 31.05 31.50

Cross sectional area

(sq-mm)

597 570 586.59

Ambient

Temperature

(deg.C)

40 40 40

Current carrying

capacity(A) at 75 C

728 699 759

At 95 C NA 952 976

SAG (m) at 85 C 13.26 14.15 14.52


Mechanical Requirement

Tensile Strength(For Tension)

Strain Strength(For Vibration)

Mechanical Requirement

Electrical Requirement


o Continuous current rating.

o Short time current carrying rating.

o Voltage drop

o Power loss

o Minimum dia to avoid corona

o Length of line


According to short time rating conductor size is given

by-

Where A=area of conductor(mm2)

IF= fault current(KA)

t= fault duration(1 sec.)

tIA F **58.7


Visual corona voltage in fair weather condition is

given by-

V0= corona starting voltage, KV(rms)

r= radius of conductor in cm

D= GMD equivalent spacing between conductors in cm

m= roughness factor

= 1.0 for clean smooth conductor

=0.85 for stranded conductor

r

Dn

r

rmV log

)3.01(1.210


Voltage gradient at the surface of conductor at operating voltage-

r

DLog

V

n

g 30

Corona discharge form at the surface of conductor if g0≥ corona

starting gradient i.e.

rrmg

)3.01(1.21

0

(rms kv/cm)


River crossing

Weight/ Dia. - Less Weight/Dia ratio conductor swing more.


In overhead transmission lines, the conductors are suspended

from a pole or a tower via insulators.

Insulator are required to support the line conductor and

provide clearance from ground and structure.

Insulator material-

High grade Electrical Porcelain

Toughened Glass

Fiber Glass


System over voltage factors shall be

evaluated.

Due to switching

Power frequency

Lightning


Switching over-voltage :

Switching off of long lines on no load

Energizing lines of no load


Power frequency over voltage

Loaded line interrupted at one end

Occurrence of fault

Open line suddenly connected to load


Type of Insulator-

Pin type insulator

Suspension insulator

Strain insulator


Pin Type Insulator : Used for transmission and

distribution of electric power at voltages up

to 33 kV


Suspension : For voltages greater than 33 kV,

it is a usual practice to use suspension type

insulators


Strain : A dead end or anchor pole or tower is used

where a straight section of line ends, or angles off

in another direction.


Disc insulator are joint by their ball pins and

socket in their caps to form string.

No of insulator disc is decided by system

voltage, switching and lighting over voltage

amplitude and pollution level.

Insulator string can be used either suspension

or tension.

Swing of suspension string due to wind has to

be taken into consider.

Fig. single string

Fig. Double string


Earth wire provided above the phase conductor across the line

and grounded at every tower.

It shield the line conductor from direct strokes

Reduces voltage stress across the insulating strings during lightning

strokes

Design criterion:

Shield angle

25°-30° up to 220 KV

20° for 400 KV and above

Earth wire should be adequate to carry very short duration lightning

surge current of 100 KA without excessive over heating


tIA 5A= Area(in mm2) of conductor

I =current in KA

t = Time in second

Area of Steel Wire = 3*A(mm2)

For EHV line it is suggested as 70 mm2 (7/3.66 mm).

ACSR is used as earth wire (12/3.0 mm AL+7/3.0 mm steel)

OPGW

EARTH WIRE


Optical Ground Wire

Advantages :

Serves the dual purpose of ground wire and

communication.

High speed data transmission.


OPGW


A Smart Grid is an electricity network that can

intelligently integrate the actions of all users connected

to it – generators, consumers and those that do both –

in order to efficiently deliver sustainable, economic

and secure electricity supplies. European Smart Grid

Technology Platform

SG3 defines Smart Grids as the concept of

modernizing the electric grid. The Smart Grid is

integrating the electrical and information technologies

in between any point of generation and any point of

consumption. Smart Grid Working Group 3


The aim of smart grid is to provide real-time

monitoring and control, and thus improve the

overall efficiency of the entire system apart from

inclusion of renewable energy resources into the

system.


Presently, the Indian electricity system faces

a number of challenges:

Shortage of power

Power Theft

Poor access to electricity in rural areas

Huge losses in the grid

Inefficient power consumption

Poor reliability


The Smart Grid is a transition of the present

energy system into a new era of reliability,

availability and efficiency.

The smart grid vision involves a uniformly

integrated communication system with the present

power system.


The India Smart Grid Task Force (ISGTF) is an

inter-ministerial group set up under the

chairmanship of Shri Sam Pitroda in September

2010 to serve as Government's focal point for

activities related to Smart Grid and to evolve a

road map for Smart Grids in India.


Indian Smart Grid Forum (ISGF) was set up in

2010 to provide a mechanism through which

academia, industry, utilities and other stakeholders

could participate in the development of Indian

smart grid systems and provide relevant inputs to

the government’s decision-making.


UHBVN,Haryana

CESC, Mysore

TSECL, Tripura

KSEB, Kerala

Electricity department-Govt. of Puducherry

UGVCL, Gujrat

AP CPDCL, Andhra Pradesh

APDCL, Assam

MSEDCL, Maharashtra

CSPDCL, Chattisgarh

HPSEB, Himachal Pradesh

PSPCL, Punjab

WBSEDCL, West Bengal

JVVNL, Rajasthan


HPSEB, Himachal Pradesh

Project Area- Kala Amb

No. of Consumers-650

Total Cost- 17.85 Cr.

MoP Share-Rs. 8.92 Cr.


Benefits Envisaged

Shifting peak load

Reduction in penalties

Reduction in outages

Status- RFP (Request for Proposal) issued on

25.08.2014.


electrical transmission line

Engineering

transmission line characteri

short transmission line

engineer e

dhananjay jha

sjvnindian transmission

shunt admittance

shunt capacitance

generalized circuit