TRANSMISSION LINES

CHAPTER 2

TRANSMISSION LINES

• FUNCTION - to transfer bulk of electrical energy from generating power plants to electrical substations located near demand centres.

• When interconnected with each other, they become transmission networks (Malaysia-National Grid).

• FOUR (4) basic parameters : resistance (R), inductance (L), capacitance (C), and conductance (G).

• Most transmission lines use HVAC.• HVDC used for greater efficiency for long distances

including submarine power cable.• Electricity is transmitted at HIGH VOLTAGE to reduce

energy lost in long-distance transmission.

Diagram of electrical power system

INDUCTANCE (L)

• Inductance is the number of flux linkages produced per ampere of current flowing through the line.

• The greater the spacing between the phases of a transmission lines, the greater the inductance of the line.

• The greater the radius of the conductors in a transmission line, the lower the inductance of the line.

EXAMPLE 1

• Calculate the loop inductance per km of a single-phase transmission line consisting of 2 parallel conductors 1.5m apart and 1.5cm in diameter. If it is operating at 50Hz frequency, calculate the reactance of the transmission line.

D=distance (m) r=radius (m)

27788.0'

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ln104

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ln104

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mHr

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D=1.5m

A B

r=1.5cmr=1.5cm

SOLUTION 1

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EXAMPLE 2

• A 3-phase transmission line 100km has its conductors of 0.6cm diameter spacing at the corners of an equilateral triangle of 100cm. Find the inductance per phase of the system.

100cm

100cm

100cm

A

CB

SOLUTION 2

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lineontransmissilongkmFor

kmHr

DL

mcmD

mr

A

A

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CAPACITANCE (C)

• The charge deposited on the conductors is proportional to the applied voltage. The constant of proportionality is the capacitance.

• The greater the spacing between the phases of a transmission lines, the lower the capacitance of the line.

• The greater the radius of the conductors in a transmission line, the higher the inductance of the line.

EXAMPLE 3

• A single phase transmission line has two parallel conductors 5m apart, radius of each conductor is 1.5cm. Calculate the capacitance of the line per km. Given that ε0=8.854 x 10-12 F/m.

kmFC

mFC

mFCrD

/107883.4

/107883.4

105.15

ln

/ln

9

12

2

0

0

EXAMPLE 4

• A 3-phase, 50Hz, 66kV transmission line are placed in horizontal plane. The conductor diameter is 1.25cm and distance between conductors is 2m. If the line length is 100km, calculate capacitance per phase.

2m 2m

A B C

SOLUTION 4

FC

lengthlinekmFor

mF

r

DC

r

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TYPES OF OVERHEAD TRANSMISSION LINETYPES OF OVERHEAD TRANSMISSION LINE

Short transmission line

Medium transmission line

Long transmission line

SHORT TRANSMISSION LINE

• When the length of the transmission line is up to 80km and the line voltage is less than 20kV.

• Due to smaller length and lower voltage, the capacitance (C) effects are small and hence can be NEGLECTED.

• Only resistance (R) and inductance (L) are considered.

where, Z = series impedance

r = per-phase resistance

L = per-phase inductance

l = line length

lLjrjXRZ )(

SHORT TRANSMISSION LINE

• Circuit diagram

Vs Vr Load

R XIs

Ir

Vs - sending end voltage Vr - receiving end voltage I - load currentR - Loop resistance () X - Loop inductance ()

+

-

MEDIUM TRANSMISSION LINE

• When the length of the line is about 80km to 250km and the line voltage is moderately high between 20kV to 100kV.

• Due to sufficient length and line voltage, capacitance (C) is considered.

MEDIUM TRANSMISSION LINE

• Circuit diagram

Vs Vr load

R XIS

Vs- sending end voltage Vr- receiving end voltage Is- sending end currentIr- receiving end current Ic- capacitance current R- loop Capacitance ()X- loop Inductance () C- capacitance (farad)

C

line

Neutral

Ir

IC

LONG TRANSMISSION LINE

• When the length of the line is more than 250km and line voltage is very high which is more than100kV.

• The line constants (R,L,C,G) are uniformly distributed over the whole length of the line.

• Resistance (R) and inductance (X) are serial elements of transmission line.

• Capacitance (C) and conductance (G) are shunt elements of transmission line. It caused the power losses and corona effects.

LONG TRANSMISSION LINE

• Circuit diagram

Vs

IS

LoadVr

Ir

B/n G/n

R/n X/n

Vs- sending end voltage Vr- receiving end voltage Is- sending end currentIr- receiving end current Ic- capacitance current R- loop Capacitance ()X- loop Inductance () C- capacitance (F) G – loop conductance

SHORT TRANSMISSION LINE

(i) Equivalent circuit(ii) Phase diagram for LAGGING power factor

VOLTAGE REGULATION

• Voltage regulation (VR) is the percentage change in voltage at the receiving-end of the line in going from no-load to full-load.

• Lagging power factor – %VR is positive • Leading power factor - %VR is negative (capacitive

load)

100%

100%

R

RS

loadFull

loadFullloadNo

V

VVVR

V

VVVR

TRANSMISSION EFFICIENCY

• Efficiency is defined as the ratio of receiving-end power to the sending-end power.

• where I2R is line losses.

100

100cos

cos

100

2

RIP

P

IV

IV

Ppowerendsending

Ppowerendreceiving

R

R

SSS

RRR

s

R

EXAMPLE 1

• A single phase transmission line delivered 1,100 kW power to a factory at 11 kV in 0.8 p.f. lagging. This line have a resistance of 2 and inductance coil of 3. Calculate:

 

i) Sending-end voltage [11,426V]

ii) Regulation Percent [3.873%]

iii)Transmission Line Efficiency [97.24%]

EXAMPLE 2

• An 11 kV,3-phase transmission line has resistance of 1.5 and inductance of 4 for each phase. Calculate regulation percent and efficiency if total end receiver load, 5000 kVA in 0.8 p.f. lagging.

[I=262.4A ; Vs=7,295.8V ; Regulation= 14.88%]

POWER FLOW (3-phase)• Real input power (watt)

• Real output power (watt)

• Apparent input power (VA)

• Apparent output power (VA)

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CORONA

• Corona was electrical discharge emerge around overhead line conductor, due to air flow where would disturb radio waves and creating lost power.

• When a normal ac voltage is applied across two conductors with enough spacing between them, there is no change in the atmospheric conditions surrounding the conductors.

• But if the voltage exceeds a particular limiting value, then the air surrounding the conductors will gets ionized and luminous glow (weak purple color) will rise with hissing sound.

• This phenomena is called corona.

The corona discharge around a high voltage coil.

Large corona discharge (white) around conductors energized by a 1.5 million volt transformer in a laboratory.

CORONA EFFECTS

1. Power loss

2. The 3rd harmonic components makes the current non-sinusoidal and this increase the corona loss.

3. The ozone gas formed chemically reacts with the conductor and can cause corrosion.

4. Light (faint violet glow).

5. Audible noise (hissing or cracking).

6. Insulation damage

7. Radio, television and computer interference.

METHODS TO REDUCE CORONA EFFECTS

a. Increase the diameter of the conductor

-i.e. ACSR conductors

b. Increase the space between the conductors

c. Using bundled conductors

-produced less resistances and reduce losses

INSULATORS

• Functions:

- to provide perfect insulation between the live conductors and the supports.

- to prevent any leakage current from the live conductors to earth through the supports.

INSULATOR MATERIALS

1. Porcelain (ceramic)

- most commonly used material for the insulators

- the dielectric strength is about 60 kV/cm

- has a particular shape and covered with glaze

2. Glass

- cheaper but less stronger than the porcelain

- the dielectric strength is about 140 kV/cm

3. Synthetic resin

- consist of the compounds of silicon, rubber, resin etc.

- light weight and comparatively cheaper

- high leakage current and short life

DESIGN PRINCIPLE & CONSTRUCTION

1) Physical strength - able to withstand loads suitable with the weight of a conductor.

2) Have high insulation resistance to prevent current leakage to earth.

3) High resistance ratio of rupture due to surge voltage.

4) The insulator’s material used must be water-proof and does not affected by changes in temperature.

5) Construction must be free from any impurities and cracks as well as non-transparent to liquids and gases from materials from space.

TYPES OF INSULATORS

Types

Pin type insulators

Suspension type insulators

Tension insulators

PIN TYPE INSULATOR

Schematic design

• Small, simple in construction and cheap.• Used for transmission and distribution of electrical

power up to 33kV.• For lower voltage up to 11kV – one piece is used.• For higher voltage – two or more pieces are used.• It becomes more heavy and costly for higher voltages.• Mounted on the cross-arm of the pole.• The line conductor is placed in the groove at the top

of insulator and is tied down with binding wire of the same material as the conductor.

PIN TYPE INSULATOR

SUSPENSION TYPE INSULATOR

Schematic design

• Used for voltages above 33kV.• Have no. of porcelain disc units which are connected

to one another in series by using metal links to form a string of porcelain discs.

• The top of insulator is connected to the cross-arm of the tower while the lowest insulator holds the line conductor.

• Each unit is designed for the low voltage about 11kV.• No. of units depend on the operating voltage i.e. if

operating voltage is 132kV , the no. of units required is 12.

SUSPENSION TYPE INSULATOR

• Two types of suspension type insulators:

(1) Cemented cap type

(2) Hewlett @ inter-linking type

• In case of failure of any of the units,

the replacement work done on that unit

and entire string need not be replaced.• Just add additional units to the string if the line voltage

is required to be increased at some later stage.

SUSPENSION TYPE INSULATOR

• Used for handling the mechanical stresses at angle positions of the line :

- corner/ sharp curve

- end of lines

- intermediate anchor towers

- long river-crossings• Low-tension (LT) line – shackle insulators are used• High-tension (HT) line - assembly of the suspension

insulators is used as ‘strain insulator’ but are arranged on a horizontal plane.

• On extra long spans (river crossings) two or more strings of strain insulators are used in parallel.

TENSION TYPE INSULATOR

ADVANTAGES OF SUSPENSION INSULATORS

• For higher voltages, these are cheaper than the pin insulator.

• Each unit is designed for low voltage (11kV) but by connecting such units in series to form a string, insulator for higher voltage level can be designed.

• In case of any failure, it is sufficient to replace the damaged disc and do not need to replace the entire string.

• Provide greater flexibility to the line. The string is suspended and is free to swing in any direction.

• The line conductors are less affected by lighting because the conductor is lower than the tower cross-arm and the string acts as lighting arrestor.

Type of tests

Flashover test

Performance test

Routine test

TYPE OF INSULATOR TESTS

FLASHOVER TESTS

•Voltage is applied between the electrodes of the insulators and is gradually increased over the specified limit.•Insulator must sustain the minimum voltage for 1 minute.

Dry flashover test

•Similar to dry test but in addition to the applied voltage, the water is sprayed over the surface at an angle of 450 (raining condition).•Insulator must sustain the minimum voltage for 30 seconds under wet condition.

Wet flashover test

•A generator develops a very high voltage at a frequency of several hundred kilohertz.•This voltage is applied to the insulator and ‘spark-over voltage’ is noted.

Impulse frequency

flashover test )50( Hzfrequencypowervoltageoverspark

voltageoversparkimpulseratioimpulse

PERFORMANCE TESTS

•Insulator is suspended in insulating oil and applied voltage is increased gradually until puncture occurs.•The voltage at which puncture starts is called ‘puncture voltage’ and it is 30% greater than dry flashover voltage.

Puncture voltage test

•Determine mechanical strength of pin type insulator.•Insulator is mounted on a steel pin and 250% of working load is applied for 1 minute.

Mechanical strength test

•Insulator sample is taken and broken into pieces and immersed in a 1% alcohol under pressure of 150kg/cm2.•After 1 hour, the pieces are removed and are observed for the penetration of the dye.•This gives the degree of porosity indication.

Porosity test

ROUTINE TESTS

•Insulator are inverted and are placed in water up to the neck.•The spindle hole is also filled with water and a high voltage is applied for 5 min.•After the test, the insulator should remain undamaged.

High voltage test

•All types of testing insulators are assembled and a tensile load of 20% in excess of the working load is applied for 1 min.•After the test, the insulator should remain undamaged.

Proof-load test

•The insulator with its fitting is suspended in a copper sulphate (CuSO4) solution at 15.20 for 1 min.•Then, it is removed, wiped, cleaned, and put again in (CuSO4) solution. •This procedure is repeated 4 times which results in zero metal deposits over the insulator.

Corrosion test

• ‘A string of insulators’ or ‘network insulator’ is the unit formed by connecting several discs in a series with help of metal links.

• The capacitance due to two metal fittings on either side of an insulator is known as ‘mutual capacitance’.

• The capacitance between the metal fittings of each unit and the earth/tower is known as ‘shunt capacitance’.

• The capacitance between the conductor and the metal link is neglected.

VOLTAGE DISTRIBUTION IN INSULATOR NETWORK

• Due to shunt capacitance, ‘the charging current’ in all discs of a string is not equal.

• So, the voltage across each unit will be different.• The discs nearer to the line conductor will have

maximum voltage and minimum voltage across the top unit (near the cross-arm).

VOLTAGE DISTRIBUTION IN INSULATOR NETWORK

VOLTAGE DISTRIBUTION IN INSULATOR NETWORK

• The voltage across the unit nearer to the conductor is more than the voltage in the unit nearer to the tower.

• 100% efficiency means that the voltage across the disc will be exactly same.

NETWORK EFFICIENCY

nVT

E

conductorlinethetonearinsulatortheacrossVoltagen

stringtheacrossVoltage

efficiencyNetwork

• Cross-arm-increase the length of cross-arms by increasing the distance between insulator and tower.

- the ratio of shunt capacitance to mutual capacitance (k=C1/C) will reduce to 0.1.

- the network efficiency increases and the voltage distribution is more uniform.

- only suitable for high and large tower post to support long bar weight and network insulator.

METHODS TO IMPROVE NETWORK EFFICIENCY

METHODS TO IMPROVE NETWORK EFFICIENCY

D

D = Bar length

Conductor

Tower Bar

Figure 2.15 Cross arm schematic

• Guard ring

-ring way obstruction can be done with use static shield.

- this static shield assembled on end lower part insulator unit connected by using joining of metal in suspension insulator and then connects to line conductor.

- reduce the earth capacitance and create capacitance between insulator line and cap.

- higher capacitance in nearby unit with guard ring and this will reduce voltage fall in the insulator.

- the same voltage in per unit is impossible to obtain practically.

METHODS OF INREASING STRING EFFICIENCY

METHODS OF INREASING STRING EFFICIENCY

C

C

C

C1

C1

C1 Cx

I1

I2

I3

i1

i2

i3

Ix

CyIy

V3

V2

V1

(b) Equivalent circuit

Obstruction Ring

Tower post

Conductor

CzIz Obstruction Ring

Tower Post

(a) Construction

Arc Horn