ELECTRICAL DISTRIBUTION SYSTEMS

Electric power is normally generated at 11-25 kV in a power station. To transmit

over long distances, it is then stepped-up to 220-kVor 400kV as necessary. Power
is carried through a transmission network of high voltage lines. Usually, these lines
run into hundreds of kilometers and deliver the power in to a common power
pool called the grid.
Grid – a network of synchronized power providers and consumers that are
connected by transmission and distribution lines and operated by one or more
control centers. When most people talk about power grid, they’re referring to the
transmission system for electricity.
Electric grids perform three functions: power generation, transmission and
distribution.
The electricity sector is fully privatized, with one major utility, Meralco,
holding 80% market share. The remaining 20% is made up of a few
regional players, and 100+ electric cooperatives serving the island
configuration.J
The electricity sector in the Philippines provides electricity through power
generation, transmission, and distribution to many parts of the country. The
Philippines is divided into three electrical grids, one each for Luzon,
the Visayas and Mindanao.[1] As of June 2016, the total installed capacity in
the Philippines was 20,055 megawatts (MW), of which 14,348 MW was on the
Luzon grid. As of June, 2016, the all-time peak demand on Luzon was 9,726
MW at 2:00 P.M. on May 2, 2016; on Visayas was 1,878 MW at 2:00 P.M. on
May 11, 2016; and on Mindanao was 1,593 MW at 1:35 P.M. on June 8, 2016.
[1]
However, about 12% of Filipinos have no access to electricity. The
Philippines is also one of the countries in the world that has a fully functioning
electricity market since 2006 called the Philippine Wholesale Electricity
Spot Market (WESM) and is operated by an independent market operator.
The grid is connected to load centers (cities) through a sub-transmission network
normally 132kV (or sometimes 66kV)lines. These lines terminate into a 132kV (or
66kV) substation, where the voltage is stepped-down to 33kV or11kV for power
distribution network of lines at 11kV and lower. T&D System Consists of Several
Levels of Power Delivery Equipment The power network, which generally concerns
the common man, is the distribution network of 11kV lines or feeders
downstream of the 33kV substations. Each 11kV feeder, which emanates from the
33kV substation branches further into several subsidiary 11kV feeders to carry
power close to the load points (localities, ELECTRICAL DISTRUBUTION SYSTEMS
LECTURE NOTES ON Page 4 industrial areas, villages, etc..). At these load points, a
transformer further reduces the voltage from 11kV to 415V to provide the last-
mile connection through 415v feeder(also called as Low Tension(LT)feeders)to
individual customers, either at 240v(as single-phase supply)or at 415v(as three-
phase supply).A feeder could be either an overhead line or an underground cable.
In urban areas, owing to the density of customers, the length of an 11kV feeder is
generally up to 3km.On the other hand ,in rural areas, the feeder length is much
larger(up to 20km).A415v feeder should normally be restricted to about 0.5-1.0
km unduly long feeders lead to low voltage at the consumer end.
A feeder is a conductor which connects the sub-station (or. localized
generating station) to the area where power is to be distributed.
Feeder. Short description. A circuit, such as conductors in conduit or a
busway run, which carries a large block of power from the service
equipment (or generator switchboard) to a subfeeder panel or a
branch circuit panel or to some point at which the block power is
broken into smaller circuits.
Power transmission lines are used to connect power stations and
substations, and for connections between substations, in order to efficiently
transmit large amounts of electricity at high voltage without loss, and
therefore play a crucial role in providing electricity.

Subtransmission lines carry voltages reduced from the major

transmission line system. Typically, 34.5 kv to 69 kv, this power is sent
to regional distribution substations. Sometimes the subtransmission
voltage is tapped along the way for use in industrial or large
commercial operations.

Single Line Diagram of Power Supply System

The electrical energy is produced at generating stations, and
through the transmission network, it is transmitted to the
consumers. Between the generating stations and the distribution
stations, three different levels of voltage (transmission, sub-
transmission and distribution level of voltage) are used.

The high voltage is required for long distance transmission and,

the low voltage is required for utility purposes. The voltage level
is going on decreasing from the transmission system to the
distribution system.The
electrical energy is generated by the three-phase synchronous
generator (alternators) as shown in the figure below. The
generation voltage is usually 11kV and 33 KV.

This voltage is too low for transmission over long distance. It is,
therefore, stepped up to 132, 220, 400 KV, or more by step-
up transformers. At that voltage, the electrical energy is
transmitted to the bulk power substation where energy is
supplied from several power substations.

The voltage at these substations is stepped down to 66KV and fed

to the sub-transmission system for onward transmission to the
distribution sub-stations. These substations are located in the
region of the load centres.

The voltage is further stepped down to 33KV and 11KV. The large
industrial consumers are supplied at the primary distribution level
of 33KV while the smaller industrial consumer is supplied at 11KV.

The voltage is stepped down further by a distribution transformer

located in the residential and commercial area, where it is
supplied to these consumers at the secondary distribution level of
400V three phase and 230V single phase.

Busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or
bar, typically housed inside switchgear, panel boards, and
busway enclosures for local high current power distribution. They are also used to
connect high voltage equipment at electrical switchyards, and low voltage equipment
in battery banks.

Single Line Diagram of 11kV Substation

Substation provides the energy supply for the local area in which
the line is located. The main function of the substation is to
collect the energy transmitted at high voltage from the
generating station and then reduce the voltage to an appropriate
value for local distribution and gives facilities for switching. The
substation is of two types one is the simple switching type where
the different connection between transmission line are made and
the other is the converting stations which convert AC to DC or
vice versa or convert frequency from higher to lower or lower to
higher.

The substation has an additional function like they provide points

where safety devices may be installed to disconnect equipment or
circuit in the event of the fault. The synchronous condenser is
placed at the end of the transmission line for improving the power
factor and for measuring the operation at the various part of
the power system. Street lighting, as well as the switching control
for street lighting, can be installed in a substation.

The single line diagram of an 11 KV substation is shown in the

figure below. The single line diagram makes the system easy and
it provides the facilitates reading of the electrical supply and
connection.
Main Components of 11kV Substation
The working of the electrical equipment used in the substation is
explained below in details.

1. Isolator – The isolator connects or disconnects the

incoming circuit when the supply is already interrupted. It
is also used for breaking the charging current of the
transmission line. The isolator is placed on the supply
side of the circuit breaker so that the circuit breaker
isolated from the live parts of the maintenance.
2. Lightning Arrester – The lightning arrester is a
protective device which protects the system from
lightning effects. It has two terminals one is high voltage
and the other is the ground voltage. The high voltage
 terminal is connected to the transmission line and the
ground terminal passes the high voltage surges to earth.
3. CT Metering – The metering CT measure and records
the current when their secondary terminal is connected
to the metering equipment panel.
4. Step-down Transformer – The step-down transformer
converts the high voltage current into the low voltage
current.
5. Capacitor Bank – The capacitor bank consists series or
parallel connection of the capacitor. The main function of
the capacitor bank is to improve the power factor of the
line. It draws the leading current to the line by reducing
the reactive component of the circuit.
6. Circuit Breaker – The circuit breaker interrupts the
abnormal or faults current to flow through the line. It is
the type of electrical switch which open or closes the
contacts when the fault occurs in the system.
The outgoing feeder supplies the input power to the consumer
end.

Single Line Diagram for Substation – Detailed

Explanation

What is a Single Line Diagram?

A single line diagram also called the one-line diagram is a symbolic or graphical
representation of a three-phase power system. It has a diagrammatic representation of
all the equipment and connections. The electrical elements such as circuit breakers,
transformers, bus bars, and conductors, are represented using standardized schematic
symbols so that they can be read and understood easily. In a single line diagram,
instead of representing each of three phases with separate lines, only a single
conductor is represented using a single line. A single line diagram makes it easy to
understand an electrical system, particularly in the case of complicated systems in
substations. It helps in a detailed study and evaluation of the system and its efficiency.
Advantages of Single Line
Diagram:
– Gives an overall understanding of the system and eases evaluation.

– It simplifies the troubleshooting process and makes it faster.

– It further ensures the safety of personnel and makes maintenance more convenient.

– It ensures a safer and more reliable operation of the system.

Important symbols for a Single

Line Diagram:
Isolating switch: In power substations, it is required to disconnect a part of the system
for general maintenance and repairs. This is accomplished by an isolating switch or an
isolator. An isolator is essentially a switch designed to open a circuit under no load. For
example, if the entire substation is divided into five sections. Each section can be
disconnected with the help of an isolator for maintenance.

Busbar: A busbar is an assembly of bus conductors with associated connection joints

and insulating supports. It is a grounded metal enclosure containing factory-mounted,
bare or insulated conductors, usually copper or aluminium bars, rods or tubes.
Circuit breaker: A circuit breaker is a circuit component that can open or close a circuit
under normal and fault conditions. It is designed such that it can be operated manually
under normal conditions and automatically under fault conditions. It is a special type of
switching device which can be operated safely under huge current carrying conditions. It
is used for timely disconnecting and reconnecting different parts of the power system for
protection and control.

Transformers: Transformers are essential components in power transmission and

distribution. They are used to step up or step down the voltage. Mostly at a power
station, a step-up transformer is used to increase the generated voltage to a higher
value. At subsequent substations, a step-down transformer is used to reduce the supply
voltage and then finally deliver it at the utilization end.

A current transformer is a step-up or step-down transformer that multiplies the current to

a known ratio. For example, if a current transformer has a rating of 100/5A, the current
on the primary side is 100A and the secondary is 5A. It is a type of instrument
transformer. Another type of instrument transformer is the voltage transformer or
potential transformer.
A potential transformer is an instrument transformer used for protection and
measurement purposes. It measures the high alternating voltage in a power system. It
is usually a step-down transformer with a lesser number of windings on the secondary
side.

Protective relays: The primary function of protective relays in substations is to cause

prompt removal of any element from the service when it suffers a short circuit. Also, it
protects when part of a system starts to operate in an abnormal manner that might
cause damage or interfere with the normal operation of the complete system. There are
different types of protection relays mainly based on their characteristics, logic, actuating
parameters and operation mechanism.

Above, we have discussed some of the important components we see in a single line
diagram. There are more components used in an electrical system that have various
applications.

Now we shall see an example of a single line diagram of an 11kV/400V indoor

substation and its explanation.
The 3-phase, 3-wire 11 kV line is tapped and brought to the gang operating switch
installed near the substation. The gang operated switch (G.O. switch) consists of
isolators connected to each phase of the 3-phase line. From the G.O. switch, the 11
kV line is brought to the indoor substation as an underground cable. It is then connected
to the high voltage or primary side of the transformer (11 kV/400 V) via the 11 kV Oil
Circuit Breaker. The transformer steps down the voltage to 400 V, 3-phase, 4-wire.

A single-phase residential load can be connected between any one phase and neutral.
A 3-phase, 400 V motor load is to be connected across 3-phase lines directly. Current
Transformers are located at suitable places in the substation circuit and supply for the
metering and indicating.

The secondary of the main transformer supplies to the busbars through the main circuit
breaker. From the busbars 400V, 3-phase, 4 wire supply is given to customers via 400V
Circuit breaker. The voltage between any two phases is 400V, and that between one
phase and one neutral is 230V.
Primary Substation in a Power System

There are different classifications of power substations, which might be used in

network. They might be classified by their function, amount of transformers,
total power and other parameters. Generally, power substations are used to
control the power flow and supply quality in the grid.

Design and
electrical calculations for 110(220)/35/10 kV power substation
The main purpose of the equipment, which is used on substation, is to
transform the voltage, protect the grid, and make all necessary switchings.

Depending on the purpose served, power substations might be classified

as:
 Step-up substations – This type of substations steps up the
generated voltage to the voltage level, which is used to transmit the
electric power.
 Primary substations – These substations receive the electric
power, which is transmitted by three-phase overhead system. The
transmitted voltage is then stepped down to appropriate voltage
level.
  Secondary substations – These substations receive energy from
the primary substation and step down the voltage level until the
level, which is used at distribution substations.
 Distribution substations – This type of substations is constructed
not far from consumers. The main function of these substations is to
step down the voltage level to three-phase voltage, which is used in
distribution network.
Figure 1 represents the structure of a power supply system with all mentioned
above substations. Not all power supply schemes may include all these types of
substations, and some of substations could be neglected.

An example of power supply system

Primary substations in a network are used to step down a high voltage level in
order to supply secondary substations by lower voltage. Usually they use 110
kV or 220 kV voltage level. Generally, a primary substation includes a high-
voltage busbar system , medium-voltage busbar system, auxiliary system, and
one or several main transformers.
In order to provide operational flexibility and to have more than one supply
alternative, there might be several incoming radial lines.

Drivers for a new primary substation

Often there are several reasons for a new substation investment. Thus, when
people or business move to a new location, it produces a load growth in the
region.

However, it might be inefficient to supply new loads with a power from distant
substations. If existing substations, even reconstructed, could not provide new
loads with a power, the need in new substation will appear.
Moreover, a new primary substation construction could bring some benefits in
reliability improvement. The main reason for that is reduction of the line length
downstream from the circuit breaker. Reliability is usually considerably
improved, as the line length per circuit breaker may be cut into a half in many
places.
Thus, the primary substation is shown to the end-consumers by a reduced
number of faults. Also the duration of faults per fault reduces slightly, as the
time required for isolating the faults and switching on the backup supplies can
be somewhat reduced.

8 STEPS TO FOLLOW IN POWER SUBSTATION DESIGN AND ENGINEERING

Building a new substation or retrofitting the old one is a complex process full of
design and engineering tasks to be worked on. The main steps in substation
design and engineering are as follows:
 8
steps to follow in power substation design and engineering (photo credit: Matt
Alsup via Flickr)

Step 1 – switching system

Selection of a substation switching system: ring bus, breaker-and-a-half,
etc. based on reliability requirements.
SUBSTATION SWITCHING SCHEMES

Step 2 – key plan, location of

components
Preparation of a key plan which should show the location of all components
of a substation and their interconnections, as well as steel structures, control
house, fire walls, driveways, fence and property line.
Step 3 – equipment selection and
ordering
Selection and ordering of equipment, which is usually done in a utility company
by a designated group of equipment experts.
They specify transformers, breakers, etc., request bids
form approved vendors, evaluate the bids, place the
order with a winning bidder, and participate in testing and
commissioning of equipment.

Step 4 – engineering support

Engineering support for licensing and permitting which includes preparation
of necessary drawings sealed by professional engineers, testifying at public
hearings at the municipalities where a new substation is planned to be built,
ordering of noise studies and selecting means of noise mitigation if needed.

Step 5 – civil and structural design

Civil and structural design which includes:
1. Pile design
2. Foundations
3. Steel structures
4. Control house
Step 6 – electrical layout design
Electrical layout design which includes:
1. Positioning of equipment
2. Bus design
3. Design of manhole and conduit system
4. Design of auxiliary A.C. power system
5. Selection of D.C. batteries and battery chargers
6. Layout of control house
7. Grounding and lightning protection design
Step 7 – relay protection, SCADA
Control design which includes:
 1. Relay protection and instrumentation system schematics and
wiring diagrams
2. Relay racks or panels
3. Remote control and metering (SCADA – system control and
data acquisition)
Step 8
Construction support which includes a resolution of technical
problems discovered during construction, ordering of additional materials,
etc.

Further details of major

equipment
Because selection of the major equipment is one of the most critical tasks in
substation engineering, there are a lot of details. These are just the part of
some major equipment ratings:

Power transformer ratings

 Capacity including overload capability
 Cooling class
 Frequency
 Primary and secondary voltage
 Phase relation between primary and secondary voltages
 BIL for both high and low voltage sides
 Voltage regulation requirements: load and no-load taps
 Transformer impedance
 Sound level

Circuit breaker ratings

 Rated maximum voltage
 Rated continuous current
 BIL
 Rated short circuit current
  Interrupting time
 Rated frequency

Current transformer (CT) ratings

 BIL
 Rated current
 Rated frequency
 Number of taps and ratio for each tap
 Accuracy class
 Type (bushing CT, free standing, etc.)

BIL – Basic Insulation Level is an important aspect of electrical engineering that ensures the safe
and dependable operation of electrical equipment. Transformer circuit , circuit breakers and
power cables for example , are designated to operate at specific voltage levels.

Voltage transformer (VT) ratings

 Rated voltage factor
 Rated primary voltage (Up)
 Rated secondary voltage
 Accuracy power
 Accuracy class
 Voltage ratio error
 Phase or phase displacement error
 Rated thermal limiting output

Disconnect switch ratings

 Rated voltage
 Rated frequency
 Rated current
 Rated short-time withstand current and duration
 Rated peak withstand current
 Rated short-time power frequency withstand voltage (rms) – To
earth between breaks
 Rated lightning impulse withstand voltage (peak) – To earth between
breaks
 Rated busbar transferring current
 Type of motor operation mechanism
 Switching capacitive current
 Switching inductive current
 Switching busbar transferring current Bus-transfer current