Distribution Systems.

2. UNIT –III
DISTRIBUTION SYSTEMS
CO-3 Explain basic elements of distribution
system , types distribution lines, calculate
voltage drop in feeders and explain the
functions of load dispatch station.

3. • Distribution system.
That part of power system which
distributes electric power for local use is
known as distribution system.

4. • Feeders : A feeder is a conductor which
connects the sub-station (or localized
generating station) to the area where power is
to be distributed.
• Generally, no tappings are taken from the feeder
so that current in it remains the same
throughout. The main consideration in the
design of a feeder is the current carrying
capacity.

6. • Distributor : A distributor is a conductor from
which tappings are taken for supply to the
consumers.
The current through a distributor is not constant
because tappings are taken at various places
along its length. While designing a distributor,
voltage drop along its length is the main
consideration since the statutory limit of voltage
variations is ± 6% of rated value at the
consumers’ terminals.

7. • Service mains :
A service mains is generally a small cable
which connects the distributor to the
consumers’ terminals.

8. • Classification of distribution system.
(i) Nature of current. According to nature of current, distribution
system may be classified as-
(a) d.c. distribution system
(b) a.c. distribution system.
Now-a-days, a.c. system is universally adopted for distribution
of electric power as it is simpler and more economical than
direct current method.
(ii) Type of construction. According to type of construction,
distribution system may be classified as –
(a) overhead system
(b) underground system.
The overhead system is generally employed for distribution as
it is 5 to 10 times cheaper than the equivalent underground
system. In general, the underground system is used at places
where overhead construction is impracticable or prohibited by
the local laws.

9. • (iii) Scheme of connection. According to
scheme of connection, the distribution system
may be classified as-
(a) radial system
(b) ring main system
(c) inter-connected system.

10. Single line Diagram of AC distribution system.

11. • The necessary electrical power for the distribution
network is transformed at the distribution
substations. A distribution substation consists of
transformers, high voltage and low voltage bus bars,
feeders, circuit breakers, instrument transformers,
different types of relays (such as over current,
differential & earth fault relays) etc.
Main functions of a distribution substation are:
Stepping down the transmission voltage up to the
distribution voltage level.
Distribution of power in multiple directions.
LV distribution feeders using circuit breakers.
• Figure shows a schematic diagram of a distribution
substation. The voltages of lines, which leave the low
voltage bus bars, are further stepped down.

12. • A.C. Primary distribution system

13. • It is that part of a.c. distribution system which operates at
voltages somewhat higher than general utilization and handles
large blocks of electrical energy than the average low-voltage
consumer uses.
• The voltage used for primary distribution depends upon the
amount of power to be conveyed and the distance of the
substation required to be fed. The most commonly used
primary distribution voltages are 11 kV, 6·6 kV and 3·3 kV. Due
to economic considerations, primary distribution is carried out
by 3-phase, 3-wire system.
• Fig. shows a typical primary distribution system. Electric power
from the generating station is transmitted at high voltage to
the substation located in or near the city. At this substation,
voltage is stepped down to 11 kV with the help of step-down
transformer. Power is supplied to various substations for
distribution or to big consumers at this voltage. This forms the
high voltage distribution or primary distribution.

14. • A.C Secondary distribution system.

15. • It is that part of a.c. distribution system which includes
the range of voltages at which the ultimate consumer
utilizes the electrical energy delivered to him. The
secondary distribution employs 400/230 V, 3-phase, 4-
wire system.
• Fig. shows a typical secondary distribution system. The
primary distribution circuit delivers power to various
substations, called distribution substations. The
substations are situated near the consumers’ localities
and contain step-down transformers. At each
distribution substation, the voltage is stepped down to
400 V and power is delivered by 3-phase,4-wire a.c.
system. The voltage between any two phases is 400 V
and between any phase and neutral is 230V. The single
phase domestic loads are connected between any one
phase and the neutral, whereas 3-phase 400 V motor
loads are connected across 3-phase lines directly.

16. Connection Schemes of Distribution System
1. Radial System.
2. Ring main system.
3. Interconnected system.

17. (i) Radial System

18. • In this system, separate feeders radiate from a
single substation and feed the distributors at
one end only. Fig. (i) shows a single line diagram
of a radial system for d.c. distribution where a
feeder OC supplies a distributor AB at point A.
Obviously, the distributor is fed at one end only
i.e., point A is this case.
• Fig. (ii) shows a single line diagram of radial
system for a.c. distribution. The radial system is
employed only when power is generated at low
voltage and the substation is located at the
centre of the load.

19. • This is the simplest distribution circuit and has the
lowest initial cost. However, it suffers from the
following drawbacks :
(a) The end of the distributor nearest to the feeding
point will be heavily loaded.
(b) The consumers are dependent on a single feeder
and single distributor. Therefore, any fault on the
feeder or distributor cuts off supply to the
consumers who are on the side of the fault away
from the substation.
(c)The consumers at the distant end of the distributor
would be subjected to serious voltage fluctuations
when the load on the distributor changes.

20. (ii) Ring main system

21. • 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. Fig. shows 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. The ring main
system has the following advantages :
• (a) There are less voltage fluctuations at consumer’s terminals.
• (b) The system is very reliable as each distributor is fed via two
feeders.
• In the event of fault on any section of the feeder, the
continuity of supply is maintained. For example, suppose that
fault occurs at any point F of section SLM of the feeder. Then
section SLM of the feeder can be isolated for repairs and at
the same time continuity of supply is maintained to all the
consumers via the feeder SRQPONM.

22. (iii)Interconnected systems

23. • When the feeder ring is energized by two or more
than two generating stations or substations, it is
called inter-connected system. Fig. shows 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.
• The interconnected system has the following advantages :
• (a) It increases the service reliability.
• (b) 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.

24. D.C. Distribution
• It is a commonly electric power is generated,
transmitted and distributed as a.c. However, for
certain applications, d.c. supply is absolutely
necessary. For instance, d.c. supply is required for
the operation of variable speed machinery (i.e., d.c.
motors), for electrochemical work and for
congested areas where storage battery reserves are
necessary. For this purpose, a.c. power is converted
into d.c. power at the substation by using
converting machinery e.g., mercury arc rectifiers,
rotary converters and motor-generator sets. The
d.c. supply from the substation may be obtained in
the form of (i) 2-wire or (ii) 3-wire for distribution.

25. (i) 2-wire d.c. system.
As the name implies, this system of distribution consists
of two wires. One is the outgoing or positive wire and the
other is the return or negative wire. The loads such as
lamps, motors etc. are connected in parallel between the
two wires as shown in Fig. This system is never used for
transmission purposes due to low efficiency but may be
employed for distribution of d.c. power.

26. (ii) 3-wire d.c. system.
It consists of two outers and a middle/Neutral wire. which is
earthed at the substation. The voltage between the outers is twice
the voltage between either outer and neutral wire as shown in Fig
The principal advantage of this system is that it makes available
two voltages at the consumer terminals viz., V between any outer
and the neutral and 2V between the outers. Loads requiring high
voltage (e.g., motors) are connected across the outers, whereas
lamps and heating circuits requiring less voltage are connected
between either outer and the neutral.

27. Types of D.C. Distributors
The most general method of classifying d.c.
distributors is the way they are fed by the
feeders. On this basis, d.c. distributors are
classified as:
(i) Distributor fed at one end
(ii) Distributor fed at both ends
(iii) Distributor fed at the centre
(iv) Ring distributor.

28. Concept of voltage drop in feeders &
distributors
OR The design considerations in distribution
system.
(i) Feeders. A feeder is designed from the point of view of its
current carrying capacity while the voltage drop
consideration is relatively unimportant. It is because
voltage drop in a feeder can be compensated by means of
voltage regulating equipment at the substation.
(ii) Distributors. A distributor is designed from the point of
view of the voltage drop in it. It is because a distributor
supplies power to the consumers and there is a statutory
limit of voltage variations at the consumer’s terminals (±
6% of rated value). The size and length of the distributor
should be such that voltage at the consumer’s terminals is
within the permissible limits.

29. Requirements of a distribution system
1. Proper voltage. One important requirement of a distribution
system is that voltage variations at consumer’s terminals
should be as low as possible. Therefore, a good distribution
system should ensure that the voltage variations at
consumers terminals are within permissible limits. The
statutory limit of voltage variations is ± 6% of the rated value
at the consumer’s terminals.
2. Availability of power on demand. Power must be available
to the consumers in any amount that they may require from
time to time This necessitates that operating staff must
continuously study load patterns to predict in advance those
major load changes that follow the known schedules.
3. Reliability. Modern industry is almost dependent on electric
power for its operation. However, the reliability can be
improved to a considerable extent by (a) interconnected
system (b) reliable automatic control system (c) providing
additional reserve facilities

30. • Simple problems on DC distributor fed at one end.
1. A 2-wire d.c. distributor cable AB is 2 km long and supplies loads
of 100A, 150A,200A and 50A situated 500 m, 1000 m, 1600 m
and 2000 m from the feeding point A. Each conductor has a
resistance of 0·01 Ω per 1000 m. Calculate the p.d. at each load
point if a p.d. of 300 V is maintained at point A.
Solution :

31. Fig. shows the single line diagram of the distributor with its tapped currents.
Resistance per 1000 m of distributor = 2 × 0·01 = 0·02 Ω
Resistance of section AC, RAC = 0·02 × 500/1000 = 0·01 Ω
Resistance of section CD, RCD = 0·02 × 500/1000 = 0·01 Ω
Resistance of section DE, RDE = 0·02 × 600/1000 = 0·012 Ω
Resistance of section EB, REB = 0·02 × 400/1000 = 0·008 Ω
Referring to Fig. , the currents in the various sections of the distributor are :
IEB = 50 A
IDE = 50 + 200 = 250 A
ICD = 250 + 150 = 400 A
IAC = 400 + 100 = 500 A
P.D. at load point C, VC = Voltage at A − Voltage drop in AC
= VA − IAC RAC
= 300 − 500 × 0·01 = 295 V
P.D. at load point D, VD = VC − ICD RCD
= 295 − 400 × 0·01 = 291 V
P.D. at load point E, VE = VD − IDE RDE
= 291 − 250 × 0·012 = 288 V
P.D. at load point B, VB = VE − IEB REB
= 288 − 50 × 0·008 = 287·6 V

32. Objectives / Need of distribution automation.
In the conventional distribution system the abnormal conditions
are detected manually which costs lots of time and money to
both consumers and power industry. In order to maintain high
service quality and reliability and minimize loss in revenues,
automation is required. Automation may be applied to the power
distribution system so that problems on the distribution network
may be detected and operated upon so as to minimize the outage
time.
1. Increased performance and reliability of electrical protection.
2. Advanced disturbance and event recording capabilities, aiding in
detailed electrical fault analysis.
3. Display of real time substation information in a control center.
4. Remote switching and advanced supervisory control.
5. Increased integrity and safety of the electrical power network
including advanced interlocking functions.
6. Advanced automation functions like intelligent load-shedding.

33. characteristics of Distribution automation
1. The system should be adaptable to any vendor’s
hardware.
2. It should incorporate distributed architecture to
minimize wiring.
3. It should be flexible and easily set up by the user.
4. The substation unit should include a computer to store
data and pre-process information

34. • Functions of Distribution Automation.
Bus voltages and frequencies, line loading, transformer loading, power
factor, real and reactive power flow, temperature, etc. are the basic
variables related with substation control and instrumentation.
• (a) Control System : The task of control system in a substation
includes data collection, scanning, event reporting and
recording; voltage control, power control, frequency control,
other automatic and semiautomatic controls etc. The various
switching actions like auto reclosing of line circuit breakers,
operation of sectionalizing switches, on-load tap changers are
performed by remote command from control room. The other
sequential operations like load transfer from one bus to
another, load shedding etc. are also taken care by control
center.
• (b) Protective System : The task of protective system includes
sensing abnormal condition, annunciation of abnormal
condition, alarm, automatic tripping, back-up protection,
protective signaling.

35. • The above two systems work in close co-operation
with each other. Most of the above functions i.e.
automatic switching sequences, sequential event
recording, compiling of energy and other reports,
etc. are integrated in software in the substation
computer.
• This software is of modular design, which
facilitates addition of new functions. The
communication between circuit breakers, auto re-
closers and sectionalizing switches in the primary
and secondary distribution circuits located in the
field and the PC in distribution substation control
room is through radio telecontrol or fibre optic
channel or power line carrier channel as is
feasible.

36. • Benefits of Distribution Automation.
(a) Reduced line loss :
The distribution substation is the electrical hub for the
distribution network. A close coordination between the
substation equipment, distribution feeders and
associated equipment is necessary to increase system
reliability. Volt/VAR control is addressed through expert
algorithms which monitors and controls substation
voltage devices in coordination with down-line voltage
devices to reduce line loss and increase line throughout.
(b) Power quality :
Mitigation equipment is essential to maintain power
quality over distribution feeders. The substation RTU in
conjunction with power monitoring equipment on the
feeders monitors, detects and corrects power-related
problems before they occur, providing a greater level of
customer satisfaction.

37. (c) Deferred capital expenses :
A preventive maintenance algorithm may be integrated
into the system. The resulting ability to schedule
maintenance, reduces labour costs, optimizes
equipment use and extends equipment life.
(d) Energy cost reduction :
Real-time monitoring of power usage throughout the
distribution feeder provides data allowing the end user
to track his energy consumption patterns, allocate
usage and assign accountability to first line supervisors
and daily operating personnel to reduce overall costs.

38. (e) Optimal energy use :
Real-time control, as part of a fully-integrated, automated
power management system, provides the ability to perform
calculations to reduce demand charges. It also offers a load-
shedding/ preservation algorithm to optimize utility and
multiple power sources, integrating cost of power into the
algorithm.
(f) Economic benefits :
Investment related benefits of distribution automation came
from a more effective use of the system. Utilities are able to
operate closer to the edge to the physical limits of their
systems. Distribution automation makes this possible by
providing increased availability of better data for planning,
engineering and maintenance. Investment related benefits
can be achieved by deferring addition of generation
capacity, releasing transmission capacity and deferring the
addition, replacement of distribution substation equipment.

39. (g)Improved reliability :
On the qualitative side, improved reliability adds
perceived value for customer and reduce the number of
complaints. Distribution automation features that
provide interruption and customer service related
benefits include load shedding and other automatic
control function.
(h) Compatibility :
Distribution automation spans many functional and
product areas including computer systems, application
software, RTUs, communication systems and metering
products. No single vendor provides all the pieces.
Therefore, in order to be able to supply a utility with a
complete and integrated system, it is important for the
supplier to have alliances and agreements with other
vendors

40. SCADA (Supervisory Control and Data Acquisition)
SCADA system provides supervisory control,
monitoring and management of various
Electrical utility / industrial automation systems
(such as manufacturing and process control
automation systems) by acquiring and analyzing
the data from remote devices.

41. Block diagram of SCADA.

42. components of SCADA and their functions.
Master Terminal Unit (MTU)
It is the heart of the SCADA system, which can be a
dedicated computer, a Programmable Logic Controller
(PLC), or a network server that communicates with
remote field side RTUs. It initiates all communication,
collects the data, stores the data in database, provides
interfaces to operators and sends the information to
other systems.
It allows the users to perform controlling functions on
field devices such as breakers, switches and other
actuators depending on the gathered data. It
continuously communicates with other devices in master
station so as to facilitate data logging, alarm processing,
trending and reporting, graphical interface and security
system.

43. Remote Terminal Units (RTUs)
RTUs gathers the information from various field sites
in which they are employed. Each RTU is connected
with various sensors and actuators that manage local
process or field equipments.
It collects the information from various sensors and
sends the information to the MTU. Also, it receives
the control commands from MTU and
correspondingly controls the various actuators. Many
RTUs store the data in their database and waits for a
request from the MTU to send or transmit the data.
In sophisticated systems, PLCs are used as RTUs
which directly transfers the field data and controls
the parameters without a request from the MTU. It
uses a local area network to communication with
various field intelligent devices.

44. Communication Equipment/Network
It provides the link between RTUs (in the field) to
MTU (in the control center). The communication
can be wired or wireless or through internet which
provides bidirectional and uninterrupted
communication between RTU and MTU.
SCADA systems can be connected using various
communication mediums including twisted pair
cables, coaxial metal cables, fiber optic cables,
satellites, high frequency radio, telephone lines,
and microwave radio. The topology of the SCADA
system network depends on the type of system or
application it is intended for. Mostly redundant
topology is recommended for critical control
applications.

45. SCADA Software
It is an important aspect of every SCADA
system which presents the information to the
user and also allows the user to intervene in the
process control. Many SCADA systems use
commercial proprietary software upon which
SCADA system is developed.
This software comprises a computer
operating system which controls the central
host computer hardware, communication
network management, graphical generation
tool for HMI, database management and report
generation tools.

46. • Functions of SCADA system in power system
1. Data acquisition
2. Networked data communication
3. Data presentation
4. Control
These functions are performed by four kinds of SCADA components:
1. Sensors (either digital or analog) and control relays that directly interface
with the managed system.
2. Remote Terminal units (RTUs). These are small computerized units deployed
in the field at specific sites and locations. RTUs serve as local collection
points for gathering reports from sensors and delivering commands to
control relays.
3. SCADA master units. These are larger computer consoles that serve as the
central processor for the SCADA system. Master units provide a human
interface to the system and automatically regulate the managed system in
response to sensor inputs.
4. The communications network that connects the SCADA master unit to the
RTUs in the field.

47. Advantages of SCADA.
1. The system provide facility to store large amount of data.
2. The data can be displayed in various formats as per user
requirements.
3. It provides interface to connect thousands of sensors
across wide region for various monitoring and controlling
operations.
4. It is possible to obtain real data simulations with the help
of operators.
5. Many types of data can be gathered from RTUs (Remote
Terminal Units) connected with the master unit.
6. The redundancy of units are incorporated in the SCADA
system in order to have backup in the event of faults or
failures. This makes system more robust.
7. It is fast in obtaining response.
8. It is scalable and flexible in adding additional resources.

48. Disadvantages of SCADA
1. PLC based SCADA system is complex in terms
of hardware units and dependent modules.
2. As the system is complex, it requires skilled
operators, analysts and programmers to
maintain SCADA system.
3. Installation costs are higher.
4. The system increases unemployment rates.
5. The system supports use of restricted
software's and hardware equipments.

49. MODEL QUESTIONS PAPER BANK
1. Explain the single line diagram of low tension
distribution system.
2. Explain the different classes of distribution systems.
3. Explain with sketch the AC Primary distribution system.
4. Draw the AC Secondary distribution system.
5. Explain the AC Secondary distribution system.
6. Explain the different forms of DC distribution system.
7. Explain the 2 wire dc system.
8. Explain the 3 wire dc system.
9. Compare overhead versus underground system.
10. Explain briefly the different connection schemes of
distribution system.

50. 11. Explain with sketch Radial distribution system.
12. Explain with sketch Ring main distribution system.
13. Explain with sketch Interconnected distribution
system.
14. Explain briefly the requirements of a distribution
system
15. Explain the design considerations in distribution
system.
16. State the need for Distribution automation.
17. List the characteristics of Distribution automation.
18. List the functions of Distribution automation.
19. List the benefits of Distribution automation.
20. Explain the block diagram of SCADA.
21. List the advantages of SCADA.
22. List the functions of SCADA.