INDEX

Introduction 2

Electrical substation 2

Elements of a substation 2

Transmission substation 3

Distribution substation 4

Grid Substation –Horana 5

Substation main equipment 6

Surge Arresters 6

Substation Current Transformers 8

Circuit breakers 9

Types of insulations used in Overhead lines 10

Insulators and Fittings 11

Conductors 12

Cut out fuse 13

Aerial Bundled Conductors 14

Minimum Factor of Safety 15

1

Introduction

We were trained last two month in CEB, at Horana Area office and Construction

Branch at Panadura. We mainly studied about Distribution System and Transmission System and

about construction.

Electrical substation

An electrical substation is a subsidiary station of an electricity generation, transmission and

distribution system where voltage is transformed from high to low or the reverse using

transformers. Electric power may flow through several substations between generating

plant and consumer, and may be changed in voltage in several steps.

A substation that has a step-up transformer increases the voltage while decreasing the

current, while a step-down transformer decreases the voltage while increasing the current

for domestic and commercial distribution. The word substation comes from the days before

the distribution system became a grid. The first substations were connected to only one

power station where the generator was housed, and were subsidiaries of that power station.

Elements of a substation

Substations generally have switching, protection and control equipment and one or more

transformers. In a large substation, circuit breakers are used to interrupt any short-circuits

or overload currents that may occur on the network. Smaller distribution stations may use

re closer circuit breakers or fuses for protection of distribution circuits. Substations do not

usually have generators, although a power plant may have a substation nearby. Other

devices such as capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-

purpose buildings. High-rise buildings may have several indoor substations. Indoor

substations are usually found in urban areas to reduce the noise from the transformers, for

reasons of appearance, or to protect switchgear from extreme climate or pollution

conditions.

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Transmission substation

A transmission substation connects two or more transmission lines. The simplest case is

where all transmission lines have the same voltage. In such cases, the substation contains

high-voltage switches that allow lines to be connected or isolated for fault clearance or

maintenance. A transmission station may have transformers to convert between two

transmission voltages, voltage control/power factor correction devices such as capacitors,

reactors or static VAr compensators and equipment such as phase shifting transformers to

control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small "switching station"

may be little more than a bus plus some circuit breakers. The largest transmission

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substations can cover a large area (several acres/hectares) with multiple voltage levels,

many circuit breakers and a large amount of protection and control equipment (voltage and

current transformers, relays). Modern substations may be implemented using International

Standards such as IEC61850.

Distribution substation

A distribution substation transfers power from the transmission system to the distribution

system of an area. It is uneconomical to directly connect electricity consumers to the high-

voltage main transmission network, unless they use large amounts of power, so the

distribution station reduces voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or sub

transmission lines. Input voltage may be, for example, 33kV, or whatever is common in the

area. The output is a number of feeders. Distribution voltages are typically medium

voltage, between 230V depending on the size of the area served and the practices of the

local utility.

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The feeders will then run overhead, along streets (or under streets, in a city) and eventually

power the distribution transformers at or near the customer premises.

Besides changing the voltage, the job of the distribution substation is to isolate faults in

either the transmission or distribution systems. Distribution substations may also be the

points of voltage regulation, although on long distribution circuits (several km/miles),

voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities,

with high-voltage switching, and switching and backup systems on the low-voltage side.

More typical distribution substations have a switch, one transformer, and minimal facilities

on the low-voltage side.

Grid Substation –Horana

Horana Grid Substation placed between Pannipitiya and Mathugama substations. It’s

132Kv side is a outdoor Grid and 33Kv side is a indoor system. Whole system is fully

computer control and also manually control. There were two main transformers and those

are on load tap changing machines. Rated power is 31500 KVA .

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Horana Grid Substation

Pannipitiya Substation

Mathugama Substation

Ace Power Plant

8*33Kv Feeders

Substation main equipment

Surge Arresters

The Back flash

On shielded transmission lines or under-built distribution circuits, the arrester prevents

tower-to-phase insulator back-flashovers during a lightning strike. On unshielded sub

transmission or distribution circuits, the arrester prevents phase-to-ground flashovers.

How Transmission Line Arresters Work

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Transmission Line Surge Arresters conduct lightning surges around the protected insulator

so that a lightning flashover is not created. They are designed to be installed functionally in

parallel with the line insulator. The arrester conducts the lightning surges around the

protected insulator so that a subsequent 60 Hz fault on the circuit is not created. The

arrester becomes a low ohmic path for the surge as voltage across it increases. When the

voltage returns to normal, the arrester once again returns to a high ohmic device with only

micro amps of leakage current.

Capacitor voltage transformer

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer

(CCVT) is a transformer used in power systems to step down extra high voltage signals

and provide a low voltage signal, for measurement or to operate a protective relay. In its

most basic form the device consists of three parts: two capacitors across which the

transmission line signal is split, an inductive element to tune the device to the line

frequency, and a transformer to isolate and further step down the voltage for the

instrumentation or protective relay. The device has at least four terminals: a terminal for

connection to the high voltage signal, a ground terminal, and two secondary terminals

which connect to the instrumentation or protective relay.

I here CVT is used for communication system . CVTs in combination with wave traps are

used for filtering high frequency communication signals from power frequency. This forms

a carrier communication network throughout the transmission network.

Substation Current Transformers

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Current transformers in electrical substations measure the system currents at predetermined

measuring points of the switchgear with a certain measurement inaccuracy. The measuring

points are typically located at all incoming and outgoing lines and possibly also within the

system, e.g. for the busbar protection. The current measurement signals are used for

protective functions, for monitoring the substation, for calculating performance data for

operating purposes or for consumption billing and for the representation on a display. The

output of the current transformer provides a representation of the current flowing through

the assembly that is being monitored. Associated monitoring and control instrumentation in

combination with the current transformer may provide critical system functions such as

overload protection and power usage monitoring.

Electrical power distribution systems may require the use of a variety of circuit condition

monitoring devices to facilitate the detection and location of system malfunctions. Current

transformers and current sensors are well known in the field of electronic circuit breakers,

providing the general function of powering the electronics within the circuit breaker trip

unit and sensing the circuit current within the protected circuit. Ground fault circuit

breakers for alternating current distribution circuits are commonly used to protect people

against dangerous shocks due to line-to-ground current flow through someone's body.

Ground fault circuit breakers must be able to detect current flow between line conductors

and ground at current levels. Upon detection of such a ground fault current, the contacts of

the circuit breaker are opened to de energize the circuit. Current transformers are an

integral part of ground fault circuit breakers.

Circuit breakers

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Electrical power transmission networks are protected and controlled by high-voltage

breakers. The definition of high voltage varies but in power transmission work is usually

thought to be 11 kV or higher. High-voltage breakers are nearly always solenoid-operated,

with current sensing protective relays operated through current transformers. In substations

the protective relay scheme can be complex, protecting equipment and busses from various

types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Bulk oil Minimum oil Air blast Vacuum SF6

High-voltage circuit breakers used on transmission systems may be arranged to allow a

single pole of a three-phase line to trip, instead of tripping all three poles; for some classes

of faults this improves the system stability and availability.

Main Failure Problems.

Lightning Insulation Damages Falling trees Short circuit problems Accidents

Types of insulations used in Overhead lines

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Overhead line insulators, as the name suggests, are used to electrically insulate pylons from

live electrical cables.

Overhead line insulators may consist of a string of insulator units, depending on insulator

type and application. The higher the line voltage insulated, the more insulator units used in

the string. Different types of line insulators are used, depending on voltage and mechanical

strain (tension) requirements.  The more widely used types are as follows.

Disc type

where insulation discs (also called insulation units) are strung together depending on the

insulation level desired.

Each disc is typically rated at 10-12kV, with a capacitance of 30-40pF . Discs are strung

together via their caps and pins. Locking mechanisms may be ball-socket or clevis-tongue

type. The cap is insulated form the pin via the porcelain (or glass) disc which adheres to

the cap and pin via adhesive cement.

Long rod type

These may also be strung together for higher insulation and may have similar ball-socket

and clevis-tongue locking mechanisms used among the disc types . Their longer length

makes them applicable for phase-to-phase insulation to reduce line galloping during strong

winds . Both disc and long rod-type insulators are commonly used in suspension or strain

(tension) insulator applications .

Pin type

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Pin types are screwed onto a bolt shank secured on the cross-arm of the transmission pole

or pylon. The pin type does not take main transmission line strain (tension) ,and functions

as a jumper line insulator.

Shackle type

insulators. These are mostly applied to support line strain (tension), such as at changes of

transmission line direction.

Insulators and Fittings

132 kV Line - 400mm2 ACSR

Suspension Insulator Discs, 120kN

Jumper Suspension Insulator Discs,70kN

Suspension Insulator Hardware

Tension Insulator Discs, 160kN

Tension Insulator Hardware

Suspension Clamps - Conductor

Tension Clamps – Conductor

Most used LT conductors in CEB

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Fly 7/3.40 mm

Wasp 7/4.39 mm

AAAC 7/4.25 mm

AAAC 7/3.10 mm

Most used LT conductors in CEB

AAAC (ELM) 19/3.76 mm

ACSR (Racoon) 7/4.09 mm

Earth Conductors

Steel Wires 7/3.25mm

Stranding Steel Wires 7/3.25mm

Earth conductor fittings

Clamps

Tension Clamps

Mid-span Joints

Line Conductor fittings

Mid-Span Joints - Conductor

Repair Sleeves

Vibration Dampers

Cut out fuse

12

In electrical distribution, a fuse cutout or cut-out fuse is a combination of a fuse and a

switch, used in primary overhead feeder lines and taps to protect stepdown transformers

from current surges and overloads.A cutout consists of three major components:

* The cutout body, an open "C"-shaped frame that supports the "fuse holder" and a

porcelain insulator that electrically isolates the conductive portions of the assembly from

the support to which the insulator is fastened.

* The fuse holder, often called the "fuse tube" or "door", which contains the

interchangeable fuse element and also acts as a simple knife switch. When the contained

fuse operates or blows, the fuse holder will drop open, disengaging the knife switch, and

hang from a hinge assembly. This hanging fuse holder provides a visible indication that the

fuse has operated and assurance that the down-stream circuit is electrically isolated.

* The fuse element, or "fuse link", is the replaceable portion of the assembly that

operates due to high electrical currents.

The fuse elements, or fuse links used in most distribution cutouts are tin or silver alloy fuse

links that melt (or operate) when exposed to high current conditions. Ampere ratings of

fuse elements vary from 1 ampere to 200 amperes.

Aerial Bundled Conductors

13

Aerial bundled cables (also aerial bundled conductors) are overhead power lines using

several insulated phase conductors bundled tightly together, usually with a bare neutral

conductor. This contrasts with the traditional practice of using uninsulated conductors

separated by air gaps.

In moister climates, tree growth is a significant problem for overhead power lines. Aerial

bundled cables will not arc over if touched by tree branches. Although persistent rubbing is

still a problem, tree-trimming costs can be reduced.

Note that bundled cables are used only for low voltages (1000 V or less), in the distribution

portion of the electrical grid, as the required insulation thickness would be impractical at

higher voltages

i) Number of strands 12 or 19

ii) Nominal cross sectional area 70 sq. mm

iii) Max. linear resistance at 20°C 0.443 Ohms/Km

iv) Minimum breaking strength 840 da N

v) Diameter of compacted bare conductor Max. 10.2mm Min. 9.7mm

vi) Thickness of insulating sheath Max. 1.8mm Min. 1.52mm at 1 point

vii) Insulated cable outside diameter Max. 14.2mm Min. 13.3mm

One, two, and three Ribs to distinguish the three phase cores from each other and the

neutral shall be plain without any Ribs.

Minimum Factor of Safety

14

Description Factor of Safety

Conductors, Earth wires and OPGW at Maximum Working

Tension based on Ultimate Strength 2.5

Conductors and Earth wires at Everyday Temperature still Air

Tension, based on Ultimate Strength 4.5

Anchor Clamps and Mid-span Joints, based on Ultimate

Strength of Conductor and Earth wire 0.95

Insulator Strings and Fittings at Maximum Working Tension

based on Failing Load 3.0

Straight Line Supports and Foundations under Normal Working

Loads 2.0

Angle, Section and Terminal Supports and Foundations under

Normal Working Loads 2.5

Towers under Broken Wire Loads 1.25

Foundations under Broken Wire Loads 1.5

Cross arms of straight line support under broken wire condition 2.0

Cross arms of angle, section and terminal support under

broken wire condition 2.5

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