Extra High Voltage AC (EHVAC) transmission lines are crucial components of electrical

power systems, designed to transmit electricity over long distances with minimal losses.
Here’s an overview of their necessity, advantages, limitations, applications, and their
status in India:

**Necessity:**

1. Growing Demand: With increasing urbanization and industrialization, the demand for
electricity continues to rise.

2. Long-Distance Transmission: EHVAC lines enable the efficient transmission of electricity

over long distances, connecting power generation facilities to distant load centers.

3. Grid Integration: They facilitate the integration of renewable energy sources, such as
wind and solar, which are often located far from populated areas.

4. Reliability and Stability: EHVAC lines enhance the reliability and stability of the power
grid by providing redundancy and alternative transmission paths.

**Advantages:**

1. Reduced Losses: EHVAC lines operate at higher voltages, resulting in lower transmission
losses compared to lower voltage lines.

2. Cost-Effectiveness: By reducing transmission losses, EHVAC lines help utilities save on

operating costs and improve overall efficiency.

3. Environmental Impact: The efficient transmission of electricity over long distances

reduces the need for additional power generation, resulting in lower greenhouse gas
emissions.

4. Increased Capacity: EHVAC lines have higher capacity ratings, allowing them to
accommodate the transmission of larger amounts of power.

**Limitations:**

1. High Initial Cost: The construction and installation of EHVAC transmission lines involve
significant capital investment due to the specialized equipment and infrastructure
required.
2. Maintenance Complexity: EHVAC lines require sophisticated monitoring and
maintenance procedures to ensure their safe and reliable operation.

3. Right-of-Way Challenges: Acquiring the necessary land rights for the construction of
EHVAC lines can be challenging, especially in densely populated areas.

4. Environmental Concerns: The construction of EHVAC lines may have environmental

impacts, including habitat disruption and visual pollution.

**Applications:**

1. Interconnected Grids: EHVAC lines are used to interconnect regional and national power
grids, enabling the transfer of electricity between different regions.

2. Bulk Power Transmission: They are employed for the long-distance transmission of bulk
power from large-scale power plants to urban and industrial centers.

3. Renewable Energy Integration: EHVAC lines facilitate the integration of renewable energy
sources by transporting electricity from remote wind and solar farms to demand centers.

4. Industrial Applications: EHVAC lines support the energy needs of large industrial
facilities, such as factories and refineries, located far from power generation sites.

**EHVAC Lines in India:**

In India, the implementation of EHVAC transmission lines has been instrumental in

modernizing the power infrastructure and meeting the growing demand for electricity.
Several key projects, such as the Western Region Strengthening Scheme (WRSS) and the
North-East-West Interconnection (NEWIC), have been undertaken to enhance the
transmission capacity and reliability of the power grid across different regions of the
country.

Despite facing challenges such as land acquisition and environmental concerns, India
continues to invest in expanding and upgrading its EHVAC transmission network to support
economic growth, improve energy access, and integrate renewable energy resources into
the grid.
(ii) This method overestimates the effects of line capacitance.

ii)Nominal T Method

In this method, the whole line capacitance is assumed to be concentrated at the middle point
of the line and half the line resistance and reactance are lumped on its either side as shown in
Fig.Therefore, in this arrangement, full charging current flows over half the line. In Fig. one phase of
3-phase transmission line is shown as it is advantageous to work in phase instead of line-to-line
values.

Let

IR = load current per phase ; R = resistance per phase

XL = inductive reactance per phase ; C = capacitance per phase

cos φR = receiving end power factor ( lagging) ; VS= sending end voltage/phase

V1 = voltage across capacitor C

The phasor diagram for the circuit is shown in Fig. Taking the receiving end voltage VR as the
reference phasor, we have,
iii) Nominal π Method

In this method, capacitance of each conductor ( i.e., line to neutral) is divided into two halves;
one half being lumped at the sending end and the other half at the receiving end as shown in Fig. It is
obvious that capacitance at the sending end has no effect on the line drop. However, its charging
current must be added to line current in order to obtain the total sending end current.
Let

IR = load current per phase R = resistance per phase

XL = inductive reactance per phase C = capacitance per phase

cos φR = receiving end power factor ( lagging) VS= sending end voltage per phase

The phasor diagram for the circuit is shown in Fig. Taking the receiving end voltage as the reference
phasor, we have,
ASSIGNMENT QUESTIONS

1. A single phase overhead transmission line delivers 1100 KW at 33 KV at 0.8

p.f. lagging. The total resistance and inductive reactance of the line are 10 Ω and
15 Ω respectively. Determine : i) sending end voltage ii) sending end power
factor and iii) transmission efficiency.
2. An overhead 3- phase transmission line delivers 5000 KW at 22 kV at 0.8 p.f.
lagging. The resistance and reactance of each conductor are 4 Ω and 6 Ω
respectively. Determine : i) sending end voltage ii) percentage regulation and
iii) transmission efficiency.
3. A medium single phase transmission line 100 km.long has resistance/km is 0.25
Ω, reactance/km is 0.8 Ω, susceptance/km 14× 10 -6 siemen and receiving end
line voltage 66000 V. Assuming that the total capacitance of the line is localized
at the receiving end alone, determine i) the sending end current, ii) sending end
voltage, iii) regulation, iv) supply power factor. The line is delivering 15000 kW
at 0.8 power factor lagging.
4. A 3-phase, 50 Hz overhead transmission line 100 km long has
resistance/km/phase is 0.1 Ω, inductive reactance/km/phase is 0.2 Ω, capacitive
susceptance/km/phase 0.04× 10-4 siemen. determine i) the sending end current,
ii) sending end voltage, iii) sending end power factor and iv) transmission
efficiency when supplying a balance load of 10000 kW at 66 kV, p.f. 0.8 lagging
using nominal T method.
5. A 100 km long, 3-phase, 50 Hz overhead transmission line has
resistance/km/phase is 0.1 Ω, reactance/km/phase is 0.5 Ω, suceptance/km/phase
10 × 10-6 S. If the line supplies load p.f. of 20 MW at 0.9 p.f. lagging at 66 kV at
the receiving end, determine by nominal π method i) sending end power factor
ii) regulation and iii) transmission efficiency.
1
130 Principles of Power System
Further, the structure of power system is shown by a single line diagram. The complete 3-phase circuit is
seldom necessary to convey even the most detailed information about the system. In fact, the complete diagram
is more likely to hide than to clarify the information we are seeking from the system viewpoint.

7.3 Compar ison of D

Comparison .C. and A.C. Transmission
D.C.
The electric power can be transmitted either by means of d.c. or a.c. Each system has its own merits
and demerits. It is, therefore, desirable to discuss the technical advantages and disadvantages of the
two systems for transmission of electric power.
1. D.C. transmission. For some years past, the transmission of electric power by d.c. has been
receiving the active consideration of engineers due to its numerous advantages.
Advantages. The high voltage d.c. transmission has the following advantages over high voltage
a.c. transmission :
(i) It requires only two conductors as compared to three for a.c. transmission.
(ii) There is no inductance, capacitance, phase displacement and surge problems in d.c. trans-
mission.
(iii) Due to the absence of inductance, the voltage drop in a d.c. transmission line is less than the
a.c. line for the same load and sending end voltage. For this reason, a d.c. transmission line
has better voltage regulation.
(iv) There is no skin effect in a d.c. system. Therefore, entire cross-section of the line conductor
is utilised.
(v) For the same working voltage, the potential stress on the insulation is less in case of d.c.
system than that in a.c. system. Therefore, a d.c. line requires less insulation.
(vi) A d.c. line has less corona loss and reduced interference with communication circuits.
(vii) The high voltage d.c. transmission is free from the dielectric losses, particularly in the case
of cables.
(viii) In d.c. transmission, there are no stability problems and synchronising difficulties.
Disadvantages
(i) Electric power cannot be generated at high d.c. voltage due to commutation problems.
(ii) The d.c. voltage cannot be stepped up for transmission of power at high voltages.
(iii) The d.c. switches and circuit breakers have their own limitations.
2. A.C. transmission. Now-a-days, electrical energy is almost exclusively generated, trans-
mitted and distributed in the form of a.c.
Advantages
(i) The power can be generated at high voltages.
(ii) The maintenance of a.c. sub-stations is easy and cheaper.
(iii) The a.c. voltage can be stepped up or stepped down by transformers with ease and effi-
ciency. This permits to transmit power at high voltages and distribute it at safe potentials.
Disadvantages
(i) An a.c. line requires more copper than a d.c. line.
(ii) The construction of a.c. transmission line is more complicated than a d.c. transmission line.
(iii) Due to skin effect in the a.c. system, the effective resistance of the line is increased.
(iv) An a.c. line has capacitance. Therefore, there is a continuous loss of power due to charging
current even when the line is open.
Supply Systems 151
7.9 Elements of a Transmission Line
For reasons associated with economy, transmission of electric power is done at high voltage by 3-
phase, 3-wire overhead system. The principal elements of a high-voltage transmission line are :
(i) Conductors, usually three for a single-circuit line and six for a double-circuit line. The
usual material is aluminium reinforced with steel.
(ii) Step-up and step-down transformers, at the sending and receiving ends respectively. The
use of transformers permits power to be transmitted at high efficiency.
(iii) Line insulators, which mechanically support the line conductors and isolate them electri-
cally from the ground.
(iv) Support, which are generally steel towers and provide support to the conductors.
(v) Protective devices, such as ground wires, lightning arrestors, circuit breakers, relays etc.
They ensure the satisfactory service of the transmission line.
(vi) Voltage regulating devices, which maintain the voltage at the receiving end within permis-
sible limits.
All these elements will be discussed in detail in the subsequent chapters.
7.10 Economics of Power Transmission
Po
While designing any scheme of power transmission, the engineer must have before him the commer-
cial aspect of the work entrusted to him. He must design the various parts of transmission scheme in
a way that maximum economy is achieved. The economic design and layout of a complete power
transmission scheme is outside the scope of this book. However, the following two fundamental
economic principles which closely influence the electrical design of a transmission line will be dis-
cussed :
(i) Economic choice of conductor size
(ii) Economic choice of transmission voltage
7.11 Economic Choice of Conductor Size
The cost of conductor material is generally a very considerable part of the total cost of a transmission
line. Therefore, the determination of proper size of conductor for the line is of vital importance. The
most economical area of conductor is that for which the total annual cost of transmission line is
minimum*. This is known as Kelvin’s Law after Lord Kelvin who first stated it in 1881. The total
annual cost of transmission line can be divided broadly into two parts viz., annual charge on capital
outlay and annual cost of energy wasted in the conductor.
(i) Annual charge on capital outlay. This is on account of interest and depreciation on the
capital cost of complete installation of transmission line. In case of overhead system, it will be the
annual interest and depreciation on the capital cost of conductors, supports and insulators and the
cost of their erection. Now, for an overhead line, insulator cost is constant, the conductor cost is
proportional to the area of X-section and the cost of supports and their erection is partly constant and
partly proportional to area of X-section of the conductor. Therefore, annual charge on an overhead†
transmission line can be expressed as :
Annual charge = P1 + P2 a ...(i)
* The question of voltage regulation is unimportant in a transmission line. Generally, the X-sectional area of
the conductor is decided on the basis of minimum annual cost.
† Underground system. A similar relationship exists for underground system. In this system, the annual
charge is on account of interest and depreciation on the cost of conductors, insulation and the cost of laying
the cables. Now, the cost of insulation is constant and the cost of conductor is proportional to area of X-
section of conductor.
∴ Annual charge = P1 + P2 a
204 Principles of Power System
where q = charge on the line in coulomb
v = p.d. between the conductors in volts
The capacitance is uniformly distributed along the whole length of the line and may be regarded
as a uniform series of capacitors connected between the conductors as shown in Fig. 9.2(i). When an
alternating voltage is impressed on a transmission line, the charge on the conductors at any point
increases and decreases with the increase and decrease of the instantaneous value of the voltage
between conductors at that point. The result is that a current (known as charging current) flows
between the conductors [See Fig. 9.2(ii)]. This charging current flows in the line even when it is
open-circuited i.e., supplying no load. It affects the voltage drop along the line as well as the effi-
ciency and power factor of the line.

9.2 Resistance of a Transmission Line

The resistance of transmission line conductors is the most important cause of power loss in a trans-
mission line. The resistance R of a line conductor having resistivity ρ, length l and area of cross-
section a is given by ;
l
R = ρ
a
The variation of resistance of metallic conductors with temperature is practically linear over the
normal range of operation. Suppose R1 and R2 are the resistances of a conductor at t1ºC and t2ºC
(t2 > t1) respectively. If α1 is the temperature coefficient at t1°C, then,
R2 = R1 [1 + α1 (t2 − t1)]
α0
where α1 =
1+ α 0 t1
α0 = temperature coefficient at 0º C
(i) In a single phase or 2-wire d.c line, the total resistance (known as loop resistance) is equal to
double the resistance of either conductor.
(ii) In case of a 3-phase transmission line, resistance per phase is the resistance of one conduc-
tor.
9.3 Skin Effect
Effect
When a conductor is carrying steady direct current (d.c.), this current is uniformly distributed over
the whole X-section of the conductor. However, an alternating current flowing through the conductor
does not distribute uniformly, rather it has the tendency to concentrate near the surface of the conduc-
tor as shown in Fig. 9.3. This is known as skin effect.
The tendency of alternating current to concentrate near the surface of a conductor is known as
skin effect.
Due to skin effect, the effective area of cross-section of the con-
ductor through which current flows is reduced. Consequently, the re-
sistance of the conductor is slightly increased when carrying an alter-
nating current. The cause of skin effect can be easily explained. A solid
conductor may be thought to be consisting of a large number of strands,
each carrying a small part of the current. The *inductance of each strand
will vary according to its position. Thus, the strands near the centre are
surrounded by a greater magnetic flux and hence have larger induc-
tance than that near the surface. The high reactance of inner strands
* For a direct current, inductance is zero and hence the current distributes uniformly over the entire X-
section of the conductor.