Substation, Distribution

1. K.S.R. POLYTECHNIC COLLEGE
TIRUCHENGODE 637 215
DISTRIBUTION AND UTILIZATION
Department of Electrical and Electronic Engineering
V A KUPPUSAMY, M.E., MISTE.,
Sr. Lecturer / EEE
K.S.R. POLYTECHNIC COLLEGE
TIRUCHENGODE – 637 215
Prepared by
DISTRIBUTION
Unit - I

2. DISTRIBUTION
UNIT – I

3. SUB – STATION
• The electrical power is generated, transmitted
and distributed in the form of alternating
current.
• Electrical power is produced at the power
houses which are far away from the consumers.
• It is delivered to the consumers through a large
network of transmission and distribution.
• At many places in the line of the power system,
it is desirable and necessary to change some
characteristics of electric supply.
• This is accomplished by suitable apparatus
called sub-station.
INTRODUCTION
Sub - Station 3

4. SUB – STATION
Sub - Station 4
• The assembly of apparatus used to change some
characteristic (voltage, ac to dc, frequency, p.f etc)
of electric supply is called sub station.
• Sub-stations are important part of power system.
The continuity of supply depends upon the
successful operation of sub-stations. The following
are the important points to be considered while
laying out a sub-station.
• It should be located at a proper site i.e., at the load
centre.
• It should provide safe and reliable arrangement.
• The capital cost should be minimum..

5. Classification of Sub – Station
• Sub stations may be classified according to
• Service requirement.
• Constructional features (or) design.
1. According to Service requirement
• A sub-station may be called upon to change voltage level or improve
power factor or convert a,c power into d.c power etc. according to the
service requirement substations may be classified into.
Sub - Station 5

6. a) Transformer sub-station
• The sub station which change the
voltage level of electrical supply are
called transformer sub stations.
• These sub stations receive power at
some voltage and deliver it at some
other voltage.
• Transformer will be the main
component in such sub stations.
• Most of the sub-stations in the power
system are of this type.
Sub - Station 6

7. b) Switching sub-station
• These sub station do not change the
voltage level ie incoming and
outgoing line have the same voltage.
• However they simply perform the
switching operations of power lines.
Sub - Station 7

8. c) Power factor correction sub-station
• These sub station improve the power
factor of the system.
• Such sub station are generally located
at the receiving end of transmission
lines.
• In these sub stations, synchronous
condensers are used for power factor
improvement.
Sub - Station 8

9. d) Frequency changer sub-station
• The sub station which change the
normal supply frequency are called
frequency changer sub station such a
frequency change may be required for
industrial utilisation.
Sub - Station 9

10. f) Industrial sub-station
• Big industrial consumer need bulk
power. The sub station which supply
power to individual industrial
concerns are known as industrial sub
station.
Sub - Station 10

11. 2. According to construction features
• A sub station has many components like circuit breakers, switches,
fuses, instruments etc.
• These components must be housed properly to ensure continuous and
reliable service.
• According to constructional features, the substations are classified as,
a. Indoor sub station.
b. Outdoor sub station.
c. Under ground sub station.
Sub - Station 11

12. a. Indoor sub station
• In these sub stations all the
equipment's are installed within the
sub station buildings. These sub
stations are usually designed for a
voltage range of 11KV, 33KV, and
66KV.
Sub - Station 12

13. b. Outdoor sub station
• For voltage beyond 66KV equipment's
are installed outdoor. It is because for
such voltages the clearances between
conductors and the space required for
switches, circuit breakers and other
equipment becomes so great.
• Hence in these substation all the
equipment's are mounted in out door.
• Outdoor sub stations are further
classified in to pole mounted substation
and foundation mounted sub station.
Sub - Station 13

14. a. Pole mounted substation
• This is an outdoor sub station with
equipment installed overhead on H-pole
or A-pole structure. It is the cheapest
form of substation for voltages not
exceeding 11KV.
• In pole mounted sub stations, upto
100KVA capacity transformers are
used.
Sub - Station 14

15. b. Foundation mounted substation
• In these sub stations above 100 KVA
transformers are mounted over a
concrete foundation.
• Suitable fences are arranged for safety.
Sub - Station 15

16. Underground substation
• In thickly populated areas, the space
available for equipment and building is
limited and the cost of land is high.
• Under such situations, the sub-station is
arranged in the underground.
Sub - Station 16

17. Gas Insulated substation
• A Gas insulated substation is an
electrical substation in which the major
structures are contained in a sealed
enclosure with SF6 gas as insulating
medium.
• High voltage conductor, switch gears,
instrument transformers, the bus bar
and all other equipment are housed in
metal enclosures.
• Filled with SF6 gas at 4 to 6 times the
atmospheric pressure.
Sub - Station 17

18. 11KV / 400V Distribution Substation
Sub - Station 18

19. SUB – STATION EQUIPMENT
• The equipment required for a transformer sub-station depends upon
• The type of sub-station
• Service requirement.
• The degree of protection required.
• A transformer sub – station has the following main equipment's.
• Bus Bar.
a) Single bus-bar arrangement.
b) Single bus-bar system with sectionalisation.
c) Double bus-bar arrangement.
Sub - Station 19

20. Bus - Bars
• Bus-bars are copper or aluminium bars and
operate at constant voltage.
• The incoming and outgoing lines in a sub-
station are connected to the bus-bars.
• The most commonly used bus-bar
arrangements in sub-station are:
• Single bus-bar arrangement.
• Single bus-bar system with sectionalisation.
• Double bus-bar arrangement.
• Number of lines operating at the same voltage have to be directly connected
electrically, bus bars are used as the common electrical component.
Sub - Station 20

21. Insulators
• The most commonly used material for the
manufacture of insulator is porcelain.
• There are several type of insulator (eg. Pin
type, suspension type, post insulator etc.)
• Their use in the sub-station will depend upon
the service requirements.
• The insulators serve two purposes. They support the conductors and provide
necessary insulation between conductors and supports to avoid leakage
current.
Sub - Station 21

22. Isolators
• This is accomplished by an isolating
switch or isolator.
• An isolator is essentially a knife switch. It
is designed to open a circuit under no
load.
• That is isolator switches are operated
when no current flows in the line in which
it is connected.
• It is often desired to disconnect a part of
the system for general maintenance and
repairs.
Sub - Station 22

23. Lightning arrestor
• In order to protect transmission line, transformer and other equipment from
lighting arrestors are used.
Sub - Station 23

24. Circuit Breaker
• It is so designed that it can be operated
manually (or by remote control) under
normal conditions and automatically
under fault conditions.
• For automatic operation, a relay circuit is
used with a circuit breaker.
• Equipment which can open or close a
circuit under normal as well as fault
conditions.
Sub - Station 24

25. Power Transformers
• Used in a substation to step-up or step-
down the voltage. Except at the power
station, all the subsequent substations use
step-down transformers to gradually
reduce the voltage
Sub - Station 25

26. Instrument Transformers
• Operate at high voltages and carry current
of thousands of amperes.
• Measuring instruments and protective
devices are designed for low voltages
(generally 110V) and currents (about 5A).
• Therefore, they will not work
satisfactorily of mounted directly on the
power lines.
• This difficulty is overcome by installing
instrument transformers on the power
lines.
a) Current transformer (C.T)
b) Potential transformer (P.T)
Sub - Station 26

27. Current Transformers (C.T)
• A current transformer is essentially a
step up transformer which steps down
the current in a known ratio.
• The primary of this transformer
consists of one or more turns of thick
wire connected in series with the line.
• The secondary consists of a large
number of turns of line wire and
provides for the measuring instruments
and relays.
Sub - Station 27

28. Potential Transformers (C.T)
• It is essentially a step down
transformer and steps down the
voltage in a known ratio.
• The primary of this transformer
consists of a large number of turns of
fine wire connected across the line.
• The secondary winding consists of a
few turns and provides for
measuring instruments and relays a
voltage which is a known fraction of
the line voltage.
Sub - Station 28

29. Metering and Indicating Instruments
• There are several metering and
indicating instruments (e.g ammeters,
voltmeters, energy meter etc.) installed
in a sub-station to maintain watch over
the circuit quantities.
• The instrument transformers are
invariably used with them for
satisfactory operation.
Sub - Station 29

30. Miscellaneous equipment
• In addition to above, there are
following equipment in a substation.
• Fuses.
• Carrier – current equipment.
• Sub-station auxiliary supplies.
Sub - Station 30

31. Carrier current equipment
• This equipment is installed in the substations for communication,
relaying telemetering or supervisory control.
• This equipment is suitably mounted in a room known as carrier room
and connected to the high voltage power circuit.
Sub - Station 31

32. Substation Auxiliary supply
• In addition to the above mentioned equipment’s, substation also contains
some auxiliary equipment’s and circuits. They are,
• Lighting in the switch yard, control rooms.
• Emergency lighting.
• Measuring instruments.
• Relays.
• Trip coils and closing coils.
• Protection system.
• Control Circuits.
• For giving supply to these equipments and circuits auxiliary supply
system is necessary. Sub - Station 32

33. Sub - Stationqassss 33

34. Sub - Station 34

35. Bus bars
• The bus bar is a bare conductor.
• Shape is rectangular, square, round tubes or solid bars
• Made up of aluminium.
• Main conductor from which a number of connections
are made.
• Bus bars are 5 to 6 meters of length.
• Aluminium is less weight and cheaper and excellent
corrosion resistance.
• Single bus-bar arrangement.
• Single bus-bar system with sectionalisation.
• Double bus-bar arrangement.
Sub - Station 35

36. Single bus-bar arrangement
• A single set of bus-bar is used for the
complete generating station.
• All the transformers, generators and feeders
were connected to this single bus bar.
• The generators are connected to bus bar
through isolators and circuit breakers.
• Similarly the outgoing feeders are also
connected to the bus-bar through isolators and
circuit breakers.
• This type of arrangement is used for D.C
stations and small A.C stations.
Sub - Station 36

37. Single bus-bar system with sectionalisation arrangement
• A single set of bus-bar is divided into
sections.
• Any two sections of the bus bar are connected
by a circuit breaker and isolators.
• If a fault occurs on any section of the bus, that
section can be isolated without affecting the
supply from other sections.
• Also repairs and maintenance of any section
of the bus-bar can be carried out by de-
energising that section only.
Sub - Station 37

38. Double bus-bar system
• This system consists of two bus bars, one
main bus bar and another auxiliary bus bar.
• The incoming and outgoing lines are
connected to the two bus bars through circuit
breaker and isolator.
• At normal condition, the incoming and
outgoing lines are connected to the Main bus
bar, and the auxiliary bus bar is kept as
reserve bus bar.
• In case of repair or maintenance of main bus
bar, the continuity of supply to the circuit can
be maintained by transferring the circuit to the
auxiliary bus bar.
Sub - Station 38

39. Sectionalised Double bus-bar system
• This scheme auxiliary bus bars are used with
the sectionalised main bus bar.
• In this method of connection, any section of
the bus bar can be isolated for maintenance
work.
• It will be noted that the auxiliary bus bar is
not sectionalised because this is not necessary
and is expensive.
Sub - Station 39

40. Ring bus-bar system
• The ends of the bus bars are returned to form
a ring. In this system each feeder is supplied
from two paths.
• By a doping this scheme of connections, the
alternators connected to any one of the bus
bar section can be used for supplying the load
to the feeders on any section.
Sub - Station 40

41. Distribution System
• A part of power system which distributes electric power from the substation to the consumers is
known as distribution system.
Sub - Station 41
Requirements of Distribution System
• A considerable amount of effort is necessary to maintain an electric power supply within the
requirements of various types of consumers.
• Some of the requirements of a good distribution system are proper voltage, availability of
power on demand and reliability.
1. Proper Voltage.
2. Availability of power on demand.
3. Reliability.
1. Inter connected system.
2. Reliable automatic control system.
3. Providing additional reserve facilities.

42. Parts of Distribution System
• A distribution system consists of three major parts. They are Feeder, Distributors and Service
mains
Sub - Station 42
Feeder
• Which connects the substation to the area where
power is to be distributed.
• No tappings are taken from the feeder. The current
in it remains the same throughout
• Main consideration in the design of feeder is the
current carrying capacity.

43. Distribution System
• A distribution is a conductor from which tappings are taken for supplying to the consumers.
• AB, BC, CD and DA are the distributors.
Sub - Station 43
• The current through a distributor is not constant
because tappings are taken ar various places along
its length. While designing distributor, voltage
drop along its length is the main consideration
since the permissible limit of voltage variations is
±5% of rated value at the consumer terminals.
Service Mains
• A service main is generally a small cable which
connects the distributor to the consumer terminals.
Sub
Station
Ring Main Distributor
A B
D C
Service Mains

44. Classification of Distribution Systems
• A distribution system may be classified according to
Sub - Station 44
1. Based on type of supply
a) D.C distribution system.
b) A.C distribution system.
2. Based on Character of service voltage
a) Low tension distribution (LT) (400V).
b) High tension distribution (HT) (11KV).
3. Based on type of construction
a) Overhead distribution system.
b) Underground distribution system.
4. Based on number of wires
a) Two wire distribution.
b) Three wire distribution.
c) Four wire distribution.
5. Based on Scheme of connection
a) Radial distribution system.
b) Ring distribution system.
c) Inter connected distribution system.

45. Various system of Power Distribution
• The power is distribution in two methods.
• 1. D.C system 2. A.C system
Sub - Station 45
1. D.C system
a) D.C two wire.
b) D.C two wire with mid point earthed.
c) D.C three wire.
2. A.C system
a) Single phase two wire.
b) Single phase two wire with mid point earthed.
c) Single phase three wire.
B. Two phase A.C system
a) Two phase four wire.
b) Three phase three wire.
C. Three phase A.C system
A. Single phase A.C system
a) Three phase three wire.
b) Three phase four wire.

46. Comparison of cost of conductors in A.C and D.C system
• While comparing the cost of conductors of various systems the following assumption to be
made.
• The power transmitted by each system is same.
• The distance over which the power transmitted is same.
• The line losses in the system are same.
• The maximum voltages between any conductor and earth is the same in all cases.
• In three wire system, the loads should be balanced.
Sub - Station 46

47. Sub - Station 47
D.C two wire system with one conductor earthed

48. Connection Scheme of Distribution System
• All distribution of electrical energy is done by constant voltage system. In practice the
following distribution circuits are generally used.
Sub - Station 48
1. Radial System
• In this system separate feeder radiate from a single
sub station and feed the distributors at one end
only.
• The radial system is employed only when power is
generated at low voltage and the sub station is
located at the centre of the load.

49. Sub - Station 49
2. Ring Main System
• In this system, the primaries of distribution transformers form a loop. The loop circuit starts
from the substation bus-bars, makes a loop through the area to be served, and returns to the
substation. The single line diagram of ring main system for a.c. distribution where substation
supplies to the closed feeder LMNOPQRS.
• The distributors are tapped from different points
M, O and Q of the feeder through distribution
transformers.
Advantages
• Less voltage fluctuations at consumer terminals.
• The system is more reliable because each
distributor is fed by two feeders.
• In the event of fault on any section of the feeder
the continuity of supply is maintained.

50. Sub - Station 50
3. Inter Connected System
• When the feeder ring is energised by two or more than generating stations or substations, it is
called inter-connected system. The single line diagram of interconnected system where the
closed feeder ring ABCD is supplied by two substations S1 and S2 at points D and C
respectively.
• Distributors are connected to points O, P, Q and
R of the feeder ring through distribution
transformers.
Advantages
• It increases the service reliability.
• Any area fed from one generating station during
peak load hours can be fed from the other
generating station. This reduces reserve power
capacity and increases efficiency of the system.

51. Sub - Station 51
1. A.C. DISTRIBUTION CALCULATIONS
A.C. distribution calculations differ from those of d.c distribution in the following respects :
• In case of d.c system, the voltage drop is due to resistance alone. However, in a.c. system, the
voltage drops are due to the combined effects of resistance, inductance and capacitance.
• In a d.c system, additions and subtractions of currents or voltages are done arithmetically but in
case of a.c system, these operations are done vectorically.
• In an a.c. system, power factor (p.f.) has to be taken into account. Loads tapped off form the
distributor are generally at different power factors. There are two ways of referring power
factor viz.
1. It may be referred to supply or receiving end voltage which is regarded as the reference
vector.
2. It may be referred to the voltage at the load point itself.
There are several ways of solving a.c. distribution problems. However, symbolic notation
method has been found to be most convenient for this purpose. In this method, voltages, currents
and impedances are expressed in complex notation and the calculations are made exactly as in d.c.
distribution.

52. Sub - Station 52
2. METHODS OF SOLVING A.C. DISTRIBUTION PROBLEMS
• In a.c. distribution calculations, power factors of various load currents have to be considered
Generally the power factors are referred in two ways.
• Power factor referred to receiving end voltage.
• Power factor referred to load voltage.
i). Power factor referred to receiving end voltage
• Consider an a.c. distributor A B with concentrated
loads of I1 and I2 tapped off at points C and B.
• Taking the receiving end voltage VB as the
reference vector,
• let lagging power factors at C and B be cos θ1 and
cos θ2 w.r.t. VB.
• Let R1 , X1 and R2 , X2 be the resistance and
reactance of sections AC and CB of the distributor.

53. Sub - Station 53
• Impedance of section AC
= ZAC = R1 + jX1
• Impedance of section CB
= ZCB = R2 + jX2
• Load current at point C = I1
I1 = I1 (cos θ1 – j sin θ1)
• Load current at point B = I2
I2 = I2 (cos θ2 – j sin θ2)
• Current in Section CB = ICB
I2 = I2 (cos θ1 – j sin θ2)
• Current in Section AC = IAC IAC = I1 + I2
= I1 (cos θ1 – j sin θ1) + I2 (cos θ2 – j sin θ2)
• Voltage drop in Section CB, VCB = ICB . ZCB
= I2 (cos θ2 – j sin θ2) (R2 – j X2)
R2 + jX2R1 + jX1
I1 cos θ1 I2 cos θ2
BCA

54. Sub - Station 54
• Voltage drop in Section CB, VCB = ICB . ZCB
= I2 (cos θ2 – j sin θ2) (R2 – j X2)
• Voltage drop in Section AC, VAC = IAC . ZAC
= [I2 (cos θ2 – j sin θ2) I2 (cos θ2 – j sin θ2)] (R2 – j X2)
• Sending end Voltage VA = VB + VAC + VCB
• Sending end Voltage IA = I1 + I2
• Sending end Voltage VA = cos θS

55. Sub - Station 55
I2X2
I1
I2
IAC
α
θ2
β
θs
θ1
R2 + jX2R1 + jX1
I1 cos θ1 I2 cos θ2
BCA

56. ii) Power factor referred to respective load voltages
• Let the power factors of the loads are referred to their respective load voltages.
• Then ∅1 is the phase angle between VC and I1 and ∅2 is the phase
angle between VB and I2.
• The vector diagram under these conditions is shown.
• Voltage drop in section CB.
VCB = I2 ZCB = I2 (Cos ∅2 – j Sin ∅2)(R2 + jX2 )
• Voltage at point C = VB + drop in section CB
= VC ∠𝛼
Now I1 = I1 ∠ − ∅1 w.r.t voltage VC
I1 = I1 ∠ − ∅1 − 𝛼 w.r.t VB
I1 = I1[cos ∅1 − 𝛼 - j sin ∅1 − 𝛼 ]
Sub - Station 56
I2X2
I1
I2
IAC
∅2
∅1
𝛼

57. ii) Power factor referred to respective load voltages
Now IAC = I1 + I2
I1 = I1[cos ∅1 − 𝛼 - j sin ∅1 − 𝛼 ]
+ I2 (Cos ∅2 – j Sin ∅2)
Voltage drop in section AC = IAC ZAC
Voltage at point A = VB + Drop in CB + Drop in AC.
Sub - Station 57
I2X2
I1
I2
IAC
∅2
∅1
𝛼

58. • Therefore, the neutrals are at the same potential and
voltage across each impedance is same and equal to
phase voltage whether the circuit is balanced or
unbalanced.
• The three phase currents or line currents can be
determined by dividing the phase voltage by the
impedance of the concerned phases.
Sub - Station 58
3 – Phase, 4 wire Star – Connected Unbalanced Load circuits
IB IY IYIB
IR
B Y
R
IN = IR + IY + IB
IR =
VP
ZP
B` Y`
R`
IB IY
VP
i.e., IR =
VR
ZR
; IY =
VY
ZY
; IB =
VB
ZB
• In 3 phase, 4 wire, star-connected load circuits the star points of load and the generator are tied
together through neutral wire of zero impedance.
• The current in neutral wire can be determined by applying Kirchhoff’s first law at star point N.
According which IN + IR + IY + IB = 0 or current in neutral wire, IN = - (IR + IY + IB)

59. • In case of a balanced three – phase, four – wire system when the neutral is disconnected, no
change is produced.
• But in case of unbalanced 3 – phase, 4 – wire system, when the neutral is disconnected, the loads
which are connected between any two conductors and the neutral are connected in series and
potential difference across the combined load become equal to line voltage. The potential
difference across each load is thus changed as per rating of the load.
Sub - Station 59
Consequence of Disconnection of Neutral in Three Phase Four wire system

60. Sub - Station 60