Chapter I

BACKGROUND OF THE STUDY

Imagine life without the existence of electricity. Almost everything in this modern

society is connected and associated with electricity. It brought us to a world with an

extraordinary and unexpected technologies which changed our lives to the extent and became a

part of modern life and one cannot think of a world without it.

From the generation, electricity is being produced or generated, transmitted and

distributed to the consumers. But beyond that, electricity cannot be produced, transmitted and

distributed to the customers without the head of the power system, the switchgear.

Switchgear plays a vital role in power distribution. Switchgear is used both to de-energize

equipment to allow work to be done and to clear faults downstream. It can be operated manually

as when required and it is also operated during over current and short circuit or any other faults

in the system by sensing the abnormality of system. The circuit breaker senses the faulty

condition of system through protection relay and this relay is again actuated by faulty signal

normally comes from current transformer or voltage transformer.

Protected relays or switch gears includes protected current transformers, voltage

transformers, protective relays, time delay relays, auxiliary relays, secondary circuits, trip

circuits, etc. Each component plays its own role, which is very important in the overall operation

of the scheme. The protective relaying is the team work of all this components. The protective

relaying also provides the indication of the location and type of the fault.

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Fuses, isolators, contactors and circuit breakers are the main components of switchgears.

Fuse is a device which protects the electrical wires, cables and other electrical equipment against

overloading and short circuit. It contains a fuse element which breaks the circuit by melting it.

Isolator is a simple switchgear device with limited application and basically used to visibly

isolate a section of electric circuit or installation from the rest. It is also known as disconnect or

isolating switch. Lastly, circuit breaker is an automatic device capable of making and breaking

an electric circuit under normal and abnormal condition in the event of overloading or short

circuits.

Switchgear has a proven record of reliability and performance failures are rare, but there

are precautions that should be taken to ensure the safety of personnel working with or in the area

of switchgear. Older types of switchgear have with time been proven to be deficient in some

areas. This together with changing system fault levels and poor or no maintenance can lead to

high safety risks to personnel in aged switchgear installations. This risk can be largely eliminated

by performing system studies followed by remedial actions such as retrofitting aged installations

with modern circuit breakers, high speed protection systems, remote switching or ultimately the

Internal Arc Fault feature.

OBJECTIVES OF THE STUDY

This study aims to:

1. Make a clearer understanding of switchgears and enumerate its types and

classifications.

2. Identify and describe the parts and functions of the switchgear.

3. Determine the classification, application and operation of switchgear.

4. Differentiate and identify the applications of low, medium, and high voltage

switchgears.

5. Give details and know the functions of current transformer, voltage transformers and

net metering.

SIGNIFICANCE OF THE STUDY

Switchgears, being the overall head of the power system, by knowing it thoroughly it

may be a great help to the following:

1. This study will mainly benefit the students with an electrical-related courses and

electrical practitioners especially students enrolled in Bachelor of Science in Electrical

Engineering to practice awareness on their own field and to expand their knowledge on

how switchgear works.

2. The study also welfares those engineers or people who is assigned in such industry. It

may help them in practicing safety to avoid damage or destruction.

SCOPE AND LIMITATION

The study only focused on medium voltage switchgear and limits only on the topics

included in the objectives which are: the types and classifications of switchgear, parts of

switchgears, application and operation of switchgear and the functions of current and voltage

transformers and net metering.

DEFINITION OF TERMS

Electrical circuit breaker - is a switching device which can be operated manually as well as

automatically for controlling and protection of electrical power system respectively.

Figure 1.1 Circuit Breaker

Current transformer (CT) - is used for measurement of alternating electric currents. When

current in a circuit is too high to apply directly to measuring instruments, a current transformer

produces a reduced current accurately proportional to the current in the circuit, which can be

conveniently connected to measuring and recording instruments. A current transformer isolates

the measuring instruments from what may be very high voltage in the monitored circuit. Current

transformers are commonly used in metering and protective relays in the electrical power

industry.

Figure 1.2 Current Transformer

Potential transformers (PT) (also called voltage transformers (VT)) - are a parallel connected

type of instrument transformer. They are designed to present negligible load to the supply being

measured and have an accurate voltage ratio and phase relationship to enable accurate secondary

connected metering.

Figure 1.3 Potential Transformer/ Voltage Transformer