This document provides an introduction to electrical distribution systems. It discusses the history of electrical power systems and how they evolved from direct current to alternating current systems. It also describes the key components of electrical power systems, including generation, transmission, distribution, control, conversion, and measurement. Distribution system planning is discussed, with the goal being to satisfy growing electricity demand in an optimal way through technically adequate and economically feasible additions to the distribution system. Customer load characteristics must be determined and factored into distribution planning.

1. INTRODUCTION TO DISTRIBUTION SYSTEMS.
DISTRIBUTION SYSTEM PLANNING
ELECRTICAL DISTRIBUTION SYSTEMS
IV B.TECH II SEMESTER EEE
S K B PRADEEPKUMAR CH
ASSISTANT PROFESSOR
EEE

2. BRIEF HISTORY OF ELECTRICAL POWER SYSTEMS
Electrical power systems have been in existence for many years. The applications of power
systems have expanded rapidly since their development. At the present time, applications
continue to increase, placing additional requirements on power production, distribution
systems and associated systems. Thomas Edison is given credit for developing the concept of
widespread generation and distribution of electrical power. He performed developmental
work on direct-current (DC) generators which were driven by steam engines. Edison’s work
with electrical lights and power production led the way to development of electric motors,
distribution systems and associated control equipment. Most early discoveries related to
electrical power dealt with direct current (DC) systems. Alternating-current power generation
became widespread a short time later. The primary reason for converting to AC power
production and distribution was that transformers could be used to increase AC voltage levels
for long-distance distribution of electrical power.

5. Thus the discovery of transformers allowed the conversion of electrical power from DC to AC
systems. Presently, almost all electrical power systems produce and distribute three-phase
alternating current. Transformers allow the voltage produced by AC generators to be increased
while decreasing current level by a corresponding amount. This allows long-distance
electrical power distribution at a reduced current level, reduces power losses, and increases
overall power system efficiency. The increased use of electrical motors for home appliances
and industrial and commercial equipment has increased the need for electrical
power to be distributed to various locations. In the early days of electrical power, the
distribution systems were only an extension of the power generating plant. There was little
planning for the efficient transfer of energy from the generating plant to the limited number of
consumers. The expansion of electrical energy use has placed greater demands on the
distribution system. Not only are more customers served, but today’s equipment requires
closer attention to voltage variation and little toleration of service interruption. The design and
operation of electrical power distribution systems has become a very important science..

6. Well-engineered power systems of today are connected together in such a way that if a
problem occurs in one system, it can be supplemented by another system. Electrical loads can
be transferred easily from one system to another. The United States has a very reliable “grid”
system which maintains electrical power to customers at the proper voltage level without
interruption. It is extremely rare for “blackouts” or “brownouts” to occur. These conditions
are avoided by proper planning for situations of extremely high demand. A blackout is a
complete interruption of electrical power, while a brownout is a reduction of voltage level to
the consumer. A brownout could be purposely done in order to deliver available power at a
reduced voltage to avoid a blackout during a problem of extremely high demand. High
demand usually occurs during abnormally hot or cold temperatures over an extended period
of time Early power distribution systems supplied direct current (DC) at low voltage levels
over relatively short distances. The invention of the transformer and the problems associated
with delivering power over long distances brought about a change to the use of alternating
current (AC) power systems.

7. Today, greater electrical power demand can be supplied with long-distance, high voltage
transmission. Voltage levels may be easily increased and reduced by transformers in order to
supply electrical energy. Not only has the efficiency of the electrical power distribution
system been improved, but also the materials, equipment, and associated control systems have
been continually updated. Examples of such improvement include the quality of steel towers,
wood poles of long lasting design, better conductors and insulators, and more reliable
computer systems for monitoring and controlling the electrical distribution
system.

8. THE ELECTRICAL POWER SYSTEM
The block diagram of an electrical power system is shown in Figure 1-3. The first block or
the electrical power production section is an important part of the complete electrical power
system. However, once electrical power is produced, it must be distributed to the location
where it will be used, so electrical power distribution systems (block 2) transfer electrical
power from one location to another. Electrical power control systems (block 3) are probably
the most complex of all the parts of the electrical power system as there are unlimited types
of devices and equipment used to control electrical power. Then, the electrical power
conversion systems (block 4), also called loads, convert the electrical power into some
other form of energy, such as light, heat, or mechanical energy. Thus, conversion systems
are an extremely important part of the electrical power system. Another part of the
electrical power system is power measurement (block 5). Without electrical power
measurement systems, control of electrical power would be almost impossible.

9. Each of the blocks shown in Figure 1-3 represents one important part of the electrical power
system. Thus, we should be concerned with each part of the electrical power system rather
than only with isolated parts. In this way, we can develop a more complete understanding of
how electrical power systems operate. This type of understanding is needed to help us solve
our energy problems that are related to electrical power. We cannot consider only the
distribution aspect of electrical power systems. We must understand and consider each pan of
the system. The “Electrical Power System” model will be used in this book to help
understand electrical distribution systems. Refer to Figure 1-3 as a reference as you study the
chapters of this book. Figure 1-4 shows the generation and transmission of electrical power
as an example. Power is produced at a generating plant (source). Distribution occurs between
the plant and the consumer by power lines. Transformers are used to control the voltage and
current levels. Conversion of electrical power to another form (light, heat, mechanical)
occurs at the home.

11. An understanding of the terms energy, work, and power is necessary in the study of
electrical power systems. The first term, energy, means the capacity to do work. For
example, the capacity to light a light bulb, to heat a home, or to move something requires
energy. Energy exists in many forms, such as electrical, mechanical, chemical, and heat. If
energy exists because of the movement of some item, such as a ball rolling down a hill, it
is called kinetic energy. If energy exists because of the position of something, such as a
ball that is at the top of the hill but not yet rolling, it is called potential energy. Energy has
become one of the most important factors in our society. A second important term is work.
Work is the transferring or transforming of energy. Work is done when a force is exerted to
move something over a distance against opposition, such as when a chair is moved from
one side of a room to the other. An electrical motor used to drive a machine performs
work.
ENERGY, WORK, AND POWER

12. Work is performed when motion is accomplished against the action of a force that tends
to oppose the motion. Work is also done each time energy changes from one form into
another. A third important term is power. Power is the rate at which work is done. It
considers not only the work that is performed but the amount of time in which the work
is done. For instance, electrical power is the rate at which work is done as electrical
current flows through a wire. Mechanical power is the rate at which work is done as an
object is moved against opposition over a certain distance. Power is either the rate of
production or the rate of use of energy. The watt is the unit of measurement of electrical
power.

14. INTRODUCTION TO DISTRIBUTION SYSTEMS
To achieve a good understanding of electric distribution systems, it is necessary to first get
acquainted with the appropriate background. A description of the main concepts of electric
distribution systems is given in this chapter followed by a more detailed discussion of the
various aspects in the following chapters.
Power System Arrangements
A power system contains all electric equipment necessary for supplying the consumers
with electric energy. This equipment includes generators, transformers (step - up and step
- down), transmission lines, sub transmission lines, cables and switchgear [1] . As shown
in Figure 1.1 , the power system is divided mainly into three parts. The first part is the
generation system in which the electricity is produced in power plants owned by an
electric utility or an independent supplier. The generated power is at the generation
voltage level. The voltage is increased by using step - up power transformers to transmit
the power over long distances under the most economical conditions.

15. The second part is the transmission system that is responsible for the delivery of power to
load centers through cables or overhead transmission lines. The transmitted power is at
extra high voltage (EHV) (transmission network) or high voltage (HV) (sub transmission
network). The third part is the distribution system where the voltage is stepped down at the
substations to the medium voltage (MV) level. The power is transmitted through the
distribution lines (or cables) to the local substations (distribution transformers) at which the
voltage is reduced to the consumer level and the power lines of the local utility or
distribution company carry electricity to homes or commercial establishments. The physical
representation given in Figure 1.1 needs to be expressed by a schematic diagram adequate
for analyzing the system. This is done by drawing a single - line diagram (SLD) as shown
in Figure 1.2 . This figure illustrates two power systems connected together by using tie -
links as they exist in real practice to increase system reliability and decrease the probability
of load loss. The voltage values shown in this figure are in accordance with the standards of
North American power systems.

17. INDUSTRY 4.O

18. Each system contains generators delivering power at generation voltage level, say 13.8 kV.
By using step - up transformers, the voltage is stepped up to 345 kV and the power is
transmitted through the transmission system. The transmission lines are followed by 138
kV sub transmission lines through terminal substations. The sub transmission lines end at
the zone substations where the voltage is stepped down to 13.8 kV to supply the MV
distribution network at different distribution points (DPs) as primary feeders. Then the
electricity is delivered to the consumers by secondary feeders through local distribution
transformers at low voltage (LV) [3, 4] . To get a better understanding of the physical
arrangement of the power system, consider how electricity is supplied to a big city. In the
first part of the arrangement, the power stations are often located far away from the city
zones and sometimes near the city border. According to how big the city is, the second part
of the arrangement (transmission and sub transmission systems) is determined.

19. Overhead transmission lines and cables can be used for both systems. They are spanned
along the boundary of the city where the terminal and zone substations are located as well.
This allows the planner to avoid the risk of going through the city by lines that operate at
HV or EHV. For the third part, the distribution system, the total area of the city is divided
into a number of subareas depending on the geographic situation and the load (amount and
nature) within each subarea. The distribution is fed from the zone substation and designed
for each subarea to provide the consumers with electricity at LV by using local transformers.
As an illustrative example, consider the total area of a big city is divided into three
residential areas and two industrial areas as shown in Figure 1.3 . Power station #1, terminal
substations #2 (345/138/69 kV), and the zone substations #3 (138/69/13.8 kV) are located at
the boundary of the city. The transmission system operates at 138 and 69 kV. Both of these
systems are around the city and do not go through the city subareas.

20. Of course, the most economical voltage for the transmission and sub transmission systems
is determined in terms of the transmitted power and the distance of power travel. Also, the
supply network to the industrial zones is operating at 69 kV because of the high power
demand and to avoid the voltage drop violation at the MV level [5] . Substation #4
(69/13.8 kV) is located at a certain distance inside the city boundary where the distribution
system starts to feed the loads through DPs. The outgoing feeders from DPs are connected
to local distribution transformers to step down the MV to LV values.

21. System planning is essential to assure that the growing demand for electricity can be
satisfied by distribution system additions that are both technically adequate and
reasonably economical. Even though considerable work has been done in the past on the
application of some types of systematic approach to generation and transmission system
planning, its application to distribution system planning has unfortunately been
somewhat neglected. In the future, more than in the past, electric utilities will need a fast
and economical planning tool to evaluate the consequences of different proposed
alternatives and their impact on the rest of the system to provide the necessary
economical, reliable, and safe electric energy to consumers.
Distribution System Planning

23. The objective of distribution system planning is to assure that the growing demand for
electricity, in terms of increasing growth rates and high load densities, can be satisfied in an
optimum way by additional distribution systems, from the secondary conductors through the
bulk power substations, which are both technically adequate and reasonably economical. All
these factors and others, for example, the scarcity of available land in urban areas and
ecological considerations, can put the problem of optimal distribution system planning
beyond the resolving power of the unaided human mind.
Distribution system planners must determine the load magnitude and its geographic location.
Then the distribution substations must be placed and sized in such a way as to serve the load
at maximum cost effectiveness by minimizing feeder losses and construction costs, while
considering the constraints of service reliability. In the past, the planning for other portions
of the electric power supply system and distribution system frequently has been authorized
at the company division level without the review of or coordination with long-range plans.

24. As a result of the increasing cost of energy, equipment, and labor, improved system planning
through use of efficient planning methods and techniques is inevitable and necessary. The
distribution system is particularly important to an electrical utility for two reasons: (1) Its
close proximity to the ultimate customer and (2) its high investment cost. Since the
distribution system of a power supply system is the closest one to the customer, its failures
affect customer service more directly than, for example, failures on the transmission and
generating systems, which usually do not cause customer service interruptions. Therefore,
distribution system planning starts at the customer level. The demand, type, load factor, and
other customer load characteristics dictate the type of distribution system required. Once the
customer loads are determined, they are grouped for service from secondary lines Connected
to distribution transformers that step down from primary voltage.

28. The distribution transformer loads are then combined to determine the demands on the
primary distribution system. The primary distribution system loads are then assigned to
substations that step down from transmission voltage. The distribution system loads, in
turn, determine the size and location, or siting, of the substations as well as the routing and
capacity of the associated transmission lines. In other words, each step in the process
provides input for the step that follows. The distribution system planner partitions the total
distribution system planning problem into a set of sub problems that can be handled by
using available, usually ad hoc, methods and techniques. The planner, in the absence of
accepted planning techniques, may restate the problem as an attempt to minimize the cost
of sub transmission, substations, feeders, laterals, etc., and the cost of losses. In this
process, however, the planner is usually restricted by permissible voltage values, voltage
dips, flicker, etc., as well as service continuity and reliability.

29. In pursuing these objectives, the planner ultimately has a significant influence on additions
to and/or modifications of the sub transmission network, locations and sizes of substations,
service areas of substations, location of breakers and switches, sizes of feeders and laterals,
voltage levels and voltage drops in the system, the location of capacitors and voltage
regulators, and the loading of transformers and feeders. There are, of course, some other
factors that need to be considered such as transformer impedance, insulation levels,
availability of spare transformers and mobile substations, dispatch of generation, and the
rates that are charged to the customers. Furthermore, there are factors over which the
distribution system planner has no influence but which, nevertheless, have to be
considered in good long-range distribution system planning, for example, the timing and
location of energy demands; the duration and frequency of outages; the cost of equipment,
labor, and money; increasing fuel costs; increasing or decreasing prices of alternative
energy sources;

30. changing socioeconomic conditions and trends such as the growing demand for goods and
services; unexpected local population growth or decline; changing public behavior as a
result of technological changes; energy conservation; changing environmental concerns of
the public; changing economic conditions such as a decrease or increase in gross national
product (GNP) projections, inflation, and/or recession; and regulations of federal, state, and
local governments.