This document discusses various aspects of distribution systems including Kelvin's Law, AC and DC distributions, methods of voltage control and power factor improvement, distribution losses, types of substations, grounding techniques, and trends such as HVDC, FACTS, EHVAC transmission. HVDC transmission provides advantages over AC such as requiring less space, allowing ground return conductors, and providing asynchronous operation and controllable power flow. FACTS devices like TCSC, SVC, STATCOM, SSSC, and UPFC are used to control power flows and improve system stability and power transfer capability.

1. UNIT V DISTRIBUTION SYSTEMS
• Distribution Systems – General Aspects –
Kelvin’s Law – AC and DC distributions –
Techniques of Voltage Control and Power
factor improvement – Distribution Loss –
factor improvement – Distribution Loss –
Types of Substations -Methods of
Grounding – Trends in Transmission and
Distribution: EHVAC, HVDC and FACTS
(Qualitative treatment only).

8. (a) Monopolar

10. (b) Bipolar

12. (c) Homopolar

13. ADVANTAGES OF HVDC TRANSMISSION
• Requires less space compared to ac for same voltage rating and size
• Ground can be used as return conductor
• Less corona loss and radio interference (RI)
• No charging current
• Cheaper for long-distance transmission
• Cheaper for long-distance transmission
• No skin and Ferranti effect
• Asynchronous operation possible
• No switching transient

14. ADVANTAGES OF HVDC TRANSMISSION
• No transmission of short-circuit power in case of fault
• Power control possible
• No compensation problem
• No stability problem
• No reactive power-loss
• No reactive power-loss
• Low short-circuit current
• No technical limit for transfer of power except thermal limit
• Fast fault clearing time.

15. DISADVANTAGES OF HVDC TRANSMISSION
• High cost of terminal equipments
• Introduction of harmonics
• Blocking of reactive power
• Point-to-point transmission not possible
• Point-to-point transmission not possible

16. Cost comparison of ac and dc lines
• Intersection of the cost curves of DC & AC transmission at a
distance, known as, breakeven distance.
• This distance is about 600–800 km.

17. Components of HVDC transmission

18. Components of HVDC transmission
• Converters
• Converter transformers
• Smoothing reactors
• Harmonic filters
• Harmonic filters
• Overhead lines
• Reactive power source
• Earth electrodes

19. Applications of HVDC Transmission System
• Long-distance bulk power transmission
• Underground or submarine cables
• Asynchronous connection of ac system with different frequencies
• Control and stabilize the power system with power flow control
• Control and stabilize the power system with power flow control

20. FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)
OBJECTIVES

24. BASIC TYPES
1. Series controllers
2. Shunt controllers
3. Combined series–series controllers
4. Combined series–shunt controllers
4. Combined series–shunt controllers

32. Thyristor Controlled Series Capacitor (TCSC)
TCR
Series Capacitor
TCR

33. Static VAr Compensator (SVC)

34. Static Synchronous Compensator (STATCOM)
STATCOM based on
(a) a voltage-sourced converter (b) a current-sourced converter

35. Static Synchronous Series Compensator (SSSC)

36. Unified Power Flow Controller (UPFC)

37. KELVIN’S LAW
• Kelvin’s law gives the cross-section of a feeder conductor based on
an economic balance between capital cost (interest and depreciation,
cost of conductors, insulators, supports and their erection) and
operating cost (running cost).
• The cost of support and erection may be partly constant and partly
proportional to the cross-section
• Thus the annual capital cost can be represented as (C1 + C2a), where
• Thus the annual capital cost can be represented as (C1 + C2a), where
C1 and C2 are the constants and a is the cross-section of conductor.
• The running cost of a feeder is the cost associated with the energy
wasted in the Conductor
• Since ohmic loss is inversely proportional to the area of cross
section, the annual cost of energy wasted can be represented as C3/a,
where C3 is a constant.
• Thus the total annual cost (C) will be sum of the annual capital cost
and the cost of energy wasted per annum:

38. • For economical area of cross-section of the feeder, the total annual
cost must be minimum.
• This shows that ‘the variable part of annual capital charges must
be equal to the cost of annual losses due to energy wastage in
conductor for the most economical working’. It is Kelvin’s Law.

39. DISTRIBUTION SYSTEMS

45. RADIAL SYSTEM
Radial System for DC Distribution Radial System for AC Distribution

47. RADIAL SYSTEM
Radial System for DC Distribution Radial System for AC Distribution

50. RING MAIN SYSTEM

53. INTERCONNECTED SYSTEM

55. INTERCONNECTORS
• Sometimes, between the two points of a ring main, the potential
difference becomes very high which can be reduced by joining
those points which have high potential difference by an
interconnector.
• Interconnected distribution system and system with
• Interconnected distribution system and system with
interconnectors are different.
• In the interconnected distribution system, different feeding stations
(more than one) are connected by interconnected feeders. However
interconnectors are used in distributors to reduce the potential
difference between any two points.

57. VARIOUS TRANSMISSION AND DISTRIBUTION SYSTEMS
(a) DC 2-wire system
(b) DC 3-wire system
(c) AC 1-phase, 2-wire
(d) AC 3-phase, 3-wire
(e) AC 3-phase, 4-wire
(e) AC 3-phase, 4-wire

65. Comparison of various Transmission & Distribution Systems
(a) The dc transmission is economical compared to the ac
transmission.
(b) For the distribution system, normally 3-phase, 3-wire system is
the best, which is preferred due to greater efficiency of polyphase
generation and convenience.
generation and convenience.
(c) Although, in the ac distribution, power factor is involved
whereas its voltage can be stepped up to down as per need through
transformers.

66. Methods of Voltage Control
i. By excitation control
ii. By using tap changing transformers
iii. Auto-transformer tap changing
iv. Booster transformers
iv. Booster transformers
v. Induction regulators
vi. By synchronous condenser

67. Excitation Control
The voltage of the alternator can be kept constant by changing the
field current of the alternator in accordance with the load.

68. Tap-Changing Transformers
(i) Off load tap-changing transformer
• Number of tappings have been provided on the secondary.
• As the position of the tap is varied, the effective number of
secondary turns is varied and hence the output voltage of the
secondary can be changed.

69. Disadvantages
• It cannot be used for tap-changing on load.
• Suppose for a moment that tapping is changed
from position 1 to position 2 when the
transformer is supplying load. If contact with stud
1 is broken before contact with stud 2 is made,
there is break in the circuit and arcing results.
On the other hand, if contact with stud 2 is made
before contact with stud 1 is broken, the coils
connected between these two tappings are short
circuited and carry damaging heavy currents.
For this reason, the above circuit arrangement
cannot be used for tap-changing on load.

70. (ii) On-load tap-changing transformer
• The secondary consists of two equal parallel
windings which have similar tappings ‘1a...... 5a’
and ‘1b.........5b’.
• In the normal working conditions, switches a, b
and tappings with the same number remain
closed and each secondary winding carries one-
half of the total current.
• Referring to Figure, the secondary voltage will
be maximum when switches a, b and 5a, 5b are
closed. However, the secondary voltage will be
minimum when switches a, b and 1a, 1b are
closed.

72. Disadvantages
• During switching, the impedance of
transformer is increased and there will be
a voltage surge.
• There are twice as many tappings as the
• There are twice as many tappings as the
voltage steps.

73. Auto-Transformer Tap-changing

74. • Here, a mid-tapped auto-transformer or reactor is used. One
of the lines is connected to its mid-tapping.
• One end, say a of this transformer is connected to a series of
switches across the odd tappings and the other end b is
connected to switches across even tappings.
• A short-circuiting switch S is connected across the auto-
transformer and remains in the closed position under normal
operation.
• Referring to Figure, it is clear that with switch 5 closed,
minimum secondary turns are in the circuit and hence the
minimum secondary turns are in the circuit and hence the
output voltage will be the lowest. On the other hand, the
output voltage will be maximum when switch 1 is closed.
• Suppose now it is desired to alter the tapping point from
position 5 to position 4 in order to raise the output voltage.
For this purpose, short-circuiting switch S is opened, switch 4
is closed, then switch 5 is opened and finally short-circuiting
switch is closed. In this way, tapping can be changed without
interrupting the supply.

75. Booster Transformer
• Sometimes it is desired to control the voltage of a transmission line
at a point far away from the main transformer. This can be
conveniently achieved by the use of a booster transformer as shown
in Fig.

76. • The secondary of the booster transformer is
connected in series with the line whose voltage is
to be controlled. The primary of this transformer is
supplied from a regulating transformer fitted with
on-load tap-changing gear.
• The booster transformer is connected in such a
way that its secondary injects a voltage in phase
with the line voltage.
Disadvantages
• More expensive than the on-load tap-changing
transformer.
• Less efficient owing to losses in the booster
• More floor space is required.

77. Induction Regulators
• An induction regulator is essentially a constant voltage
transformer, one winding of which can be moved w.r.t. the
other, thereby obtaining a variable secondary voltage.
• The primary winding is connected across the supply while
the secondary winding is connected in series with the line
the secondary winding is connected in series with the line
whose voltage is to be controlled.
• When the position of one winding is changed w.r.t. the
other, the secondary voltage injected into the line also
changes.

78. (i) Single-phase induction regulator
• In construction, it is similar to a single phase induction motor
except that the rotor is not allowed to rotate continuously but
can be adjusted in any position either manually or by a small
motor.
• The primary winding AB is wound on the stator and is
connected across the supply line. The secondary winding CD
is wound on the rotor and is connected in series with the line
whose voltage is to be controlled.

79. • The primary exciting current produces an
alternating flux that induces an alternating voltage
in the secondary winding CD. The magnitude of
voltage induced in the secondary depends upon its
position w.r.t. the primary winding. By adjusting the
rotor to a suitable position, the secondary voltage
can be varied from a maximum positive to a
maximum negative value. In this way, the regulator
maximum negative value. In this way, the regulator
can add or subtract from the circuit voltage
according to the relative positions of the two
windings.
• Owing to their greater flexibility, single phase
regulators are frequently used for voltage control of
distribution primary feeders.

80. (ii) Three-phase induction regulator.

81. • In construction, a 3-phase induction
regulator is similar to a 3-phase induction
motor with wound rotor except that the
rotor is not allowed to rotate continuously
but can be held in any position by means
of a gear.
• The primary windings either in star or delta
• The primary windings either in star or delta
are wound on the stator and are
connected across the supply. The
secondary windings are wound on the
rotor and the terminals are brought out to
be connected in series with the line.

82. Voltage Control by Synchronous Condenser
• By changing the excitation of a synchronous motor, it can be
made to take a leading power factor. A synchronous motor at
no load and taking a leading power factor is known as a
synchronous condenser. It is so called because the
characteristics of the motor resemble with that of a
condenser.
• The voltage at the receiving end of a transmission line can be
controlled by installing specially designed synchronous
controlled by installing specially designed synchronous
motors called synchronous condensers at the receiving end of
the line.
• The synchronous condenser supplies wattless leading kVA to
the line depending upon the excitation of the motor. This
wattless leading kVA partly or fully cancels the wattless
lagging kVA of the line, thus controlling the voltage drop in the
line. In this way, voltage at the receiving end of a transmission
line can be kept constant as the load on the system changes.