Module v,EHV Transmission

7/27/2019 Module v,EHV Transmission

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1MODULE - V- EHV TRANSMISSION

Introduction

In 1915, the total installed capacity in India was 105 M.W and by 1947, theindependence year, it was 2300 MW. Now it has grown over 125,000 MW. As far astransmission voltage is concerned the first 100 KV line was commissioned in Maharasthra

in 1911. The first 132 KV line in1932 and first-220KV line in 1962. The first 400 KV linewas commissioner in 1978. All 400 KV lines use bundle conductors of 2,3 or 4 subconductors. The next higher Voltage chosen was 765/800 KV. Some of the linesconstructed with this voltage are Anapara- Unnao and Tehri-Meerut in U.P,Mohakishenpura in Punjab and Vidyachal - Bina- Nagda in M.P. High voltage D.C.transmission line also has been introduced in India. Voltage selected is 500 KV.Richand- Dadri and Vidyachal back to back systems are already commissioned. TheRichand - Dadri line transmits more than 1000 mw of power.

Need for EHV Transmisssion

Voltages above 300 KV is termed as EHV and above 750 KV are termed asultra high voltage (uttv). Necessity for EHV transmission are the following:-

(1) For a given amount of power to be transmitted for a given distance, thetransmission efficiency increases as the transmission voltage increases.

(2) In EHV/ UHV lines, the per unit resistance drop reduces and volume of conductormaterial decreases.

(3) Power transmission capacity of a line is proportional to the square of voltage. Hencethe transmission capacity increases tremendously as the voltage is increased to theEHV/UHV range. But cost of towers terminal equipments also increases, but this increasein proportional to the voltage only. As a result over all capital cost on transmissiondecreases as the voltage is increased.

(4) Large power stations are economically more viable. EHV/UHV lines were requiredto transmit this large block of power from these large stations.

(5) The load carrying capacity of lines are expressed in terms of surge impedance loador natural loading. At this loading reactive VA consumed by the inductive part of the line isequal to copacitive VA in the line.

PSIL =ZsV

2

Where sZ = Surge impedance =CL

(SIL - Surge impedance loading)

Hence Surge impedance loading increases as the square of voltage.

6 It is not possible to interconnect power systems without EHV Transmission lines

Problems associated with EHV Transmission

(1) Corona and Radio interferences - Corona is largely depending on the system voltageand radio interferences is the consequence of corona. During bad weather corona loss is

very lugh. To reduce corona spacing between the conductors or size of the conductorshas to be increased or bundled conductors are to be used.


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2(2) Line supports

EHV line tower has to with stand heavy mechanical loading due to bundledconductors. Large air and ground clearance has to be provided. Hence the cost on towerswell increase and it will be approximately 30% to 50% of the total cost of the line.

(3) Erection difficulties - It is necessary to evolve and employ new erection and

conductor handling techniques.(4) Insulation Requirements : - The insulation level has to be increased to with standlightning surges and switching surges. Lightning surges are taken care of mostly by theearth wires.

(5) Power station and sub station Equipment :- The power station and sub stationequipments have to handle the high voltage and high fault current. Modern transformersand circuit breakers are capable of handling these high values. But the cost increases.

Limitations of EHV, A.C. Power Transmission

1 Natural loading

Consider a line with negligible.......................resistance and reactance Landcapacitance C, per unit length Load current is Iamperes. To keep the sending end voltage andreceiving end voltage same, lagging voltamberesshould be equal to the leading voltamperers.

ie (I x L) I = v

Xc

Vor I 2 wL = wcV2

SZ

C

L

I

V== ohm SZ is called the surge impedance of the line. SZ behaves like a

pure resistance. Power transmitted per phase is

V x I =SS

Z

V

Z

VxV2

= and this is called the surge impedance loading or natural loading of the

line. But normally line is not operated at its natural loading. While transmitting power at alagging power factor there will br voltage drop, but at leading p. f. there well be voltagerise.

Voltage K.V. 132 220 380 500

Surge Impedance ZS ohm 350 320 295 276

Natural loading MW 50 150 490 900

Current amp A 220 385 740 1000

The natural loading do not impose any restriction on the distance of transmission

line. But it restricts the amount of power that can be transmitted for a particular voltage orit fix up an voltage for a certain amount of power.


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3(ii) Stability considerations:- If resistance and shunt leakage of a transmission line(Rand g) is neglected the power that can be transmitted within stability limit is

P =X

VVrS sin SV = Sending end Voltage

rV = Receiving and voltage

X = Series reactance in ohm/plan

= Load angle - angle between Vs andVr

When == rS VV V

P =X

SinV 2

=( )

sin

2

X

Kvfor 3 phase where

KV = line to line voltage.

If

= 90

0

maximum power can be transmitted. But

is not allowed to vary more than200 to 300. Other wise, the machines will loose synchronism under transient conditions.Consider a case, when x= 0.3475 ohm per km.

030= S = length in km of line. We have

P =( ) ( ) ( )

MWS

kvx

kv

x

kv222

43.15.034755.0

sin ==

If the line in to operate at natural load then

PSIL =( ) 2

SZ

KVEquating the two

( ) ( )

SZ

kv

S

kv22

43.1= or S= 1.43 Z S km.

This means that the length of the line that can operate at natural loading withoutloosing stability is 1.43 SZ (For the assumed value of x, For any other value of X these is

another fixed value of S). This imposes a limit of on the length of transmission line in anA.C. system. In EHV line exceeding 200 km addtitional equipment like series capacitanceand staun reactance is to be installed.

3.Current carrying capacity of conductors

In an overhead system maximum power transmitted is never dependant on thermalcondition. But due to other consideration as explained in 2 and 3 above. Hence thecurrent carrying capacity is not fully utilized.

4 Ferrants effect

If the line is loaded with a leading, reactance volt amperers, there will be a rise ofvoltage at receiving end. The rise is of the order of 1.5% for 160 Km, 13% for 500km and100% for 960 km. There is also rise of voltage at the sanding end when the load is thrownoff suddenly. This may agravate the problem. Usually shunt reactors are used at the load

end to control the voltage rise due to Ferranti effect.


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68 Stability problems do not arise since it

is in asynchronous operation of thegenerators

- Due to stability problem distance islimited.

9 Contribution of D.C. Line to short circuitcurrent is less.

In AC system short current is more

10 Power control is fast and accuratesince there is less inertia due to theabsence of rotating parts

- Power control is slow

11 Unlimited power can be transmitted

Disadvantages

12 The converters required at both endsare very expensive. Hence D.C linesare economically viable only of thedistance of the line is more.

Investment in terminal equipment isless.

13 The converters absorb reactive powerwhich must be supplied locally. SinceD.C. blocks the transmission of reactiveVA. the receiving end must be capableof supplying the whole of reactivecomponent of power required by load.

- No such problem

14 Suitable for point to point transmissiononly

- Suitable for inter connected system

Module v,EHV Transmission

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