Switchgear& Switchyard

7/27/2019 Switchgear& Switchyard

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SWITCHGEAR

The apparatus used for Switching, Controlling and Protecting the

Electrical Circuits and equipment is called as a switchgear .

NEED OF SWITCHGEAR :

Switching during normal operating conditions for the purpose of

Operation and Maintenance.

Switching during Faults and Abnormal conditions and interrupting the

fault currents.

PARTS OF SWITCHGEAR:

1.Switching devices :o Circuit breakers / contactors

o Isolators

o Earthing switch

2. Control Circuit :o service / test / isolated position selectors

o Tripping and closing circuit

o Spring charging , anti pumping arrangement

o Supply monitoring , space heaters , indications

3. Measurement :o Ammeter

o voltmetero energy meter

4. Protection :o Relays

o Current Transformer ( CT )


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o Potential Transformer ( PT )

BASIC DESIGNING ASPECTS :

The Auxiliary power system in a power plant must form a RELIABLE

source of power to all unit and Station auxiliaries. The basic function

of Switchgear is to control supply of electric power and to protect the

equipment in the event of abnormal conditions. Hence the Switchgears

have to be RELIABLE , SAFE, and ADEQUATE .

Defining the reliability, safety aspects and adequacy aspects in terms

of Quantitative parameters forms the essential part in

SPECIFICATIONS

Indoor , metal clad single front and fully Compartmentalized , with

degree of protection IP42 and IP52 for metering compartments. For

33 KV the switchgears can be metal enclosed either.

Circuit Breakers of either SF6 or Vacuum type. They shall comprise of

three separate identical single pole interrupting units operated through

a common shaft by a sturdy mechanism.

Breakers are suitable for Switching transformers and motors at any

load and also for starting 3.3 KV - Above 200 KW to 1500 KW , 11

KV- above 1500 KW for 500 MW units and 6.6 KV- above 200 KW for

210 MW units.

Surge arresters are provided for all motor feeders to limit the over

voltages

For Coal handling plant Motors where frequent start/stop of motors is

called for HRC fuse backed contactors are provided.

Suitable Interlocks are provided to ensure that Breaker is off before

opening the rear doors/covers.


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SWITCHYARD

Switchyard is a switching station which includes the following points :

(i) Main link between Generating plant and Transmission system , which

has a large influence on the security of the supply.

(ii) Step-up &/or Step-down the voltage levels depending upon

the Network Node.

(iii) Switching ON/OFF Reactive Power Control devices , which

has effect on Quality of power .

SWITCHYARD

EQUIPMENTS FOUND IN SWITCHYARD :

Lightening arrestor

Current transformer

Voltage transformer

Power transformers / I.C.T.

Bus bar and clamp fitting


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Support structure

Isolators

Circuit Breaker

Wave traps

Earthing switch

FUNCTIONS OF EQUIPMENTS :

TRANSFORMER :

Transforms the voltage levels from higher to lower levelor viceversa , keeping the power constant .

TRANSFORMER

Circuit breaker :

It is a mechanical switching device used in electrical circuits

which makes or automatically breaks the electrical circuits

under loaded condition.


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Generally they are designed in accordance with the

national/international standards & assigned with standard rating;

but switchgears are tailor made to customers specific

requirements.

It is the most important component of an electrical powernetworks due to its function to protect other costly equipments

Such as transformers, generators, transmission lines, motor etc.

from any possible damage.

Rating assigned to a Circuit Breaker:

As per I.E.C. specify one A.C. ckt. Brkr. is assigned with following rating :

a. Rated voltage and no of phases, rated frequency.

b. Rated normal current

c. Rated breaking current capacities

d. Rated making Current capacities

e. Rated operating duty

f. Rated short time current & its maximum duration

g. Rated in selection level

h. Impulse withstand voltage.


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CIRCUIT BREAKER ISOLATOR

ISOLATOR :

It opens or closes the electrical circuits under No-load conditions.

The following interlocks are provided with isolators :

a. Isolator cannot operate unless the breaker is open.

b. Bus I & II isolators cannot be closed simultaneously.

c. This interlock can be by-passed in the event of closing of bus coupler

breaker.

d. No isolator can operate when corresponding earth switch is on.

e. Only one bay can be taken on by-pass bus .

WAVE TRAPS / LINE TRAPS :

It is used in PLCC circuits for Communication and telemetering .

LIGHTNING ARRESTORS :

It safeguards the equipments by discharging the high currents due to

lightning .


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WAVE TRAP LIGHTNING ARRESTOR

EARTH SWITCH :

It is used to connect the charged body to ground to discharge the

trapped charge to have a safe maintenance zone .

Features of earth switch are as follows

Used to ground sections required for maintenance .

Ground induction voltages .

Interlocked with breakers and isolators .

Can operate from local only .

Motor driven as well as hand driven . Safety driven .

OVERHEAD EARTH WIRE :

It protects the O/H transmission line from Lightning strokes .BUS BAR :

They are the conductors to which a number of circuits are connected .

VOLTAGE TRANSFORMER :To step-down the high magnitude of voltage to a safe value to

incorporate Measuring and Protection logics .

CURRENT TRANSFORMER :

To step-down the high magnitude of current to a safe value to

incorporate Measuring and Protection logics .

Factors affecting the selection of switching scheme :

System Security

Operational Flexibility

Simplicity of Protection arrangements


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Ability to limit the Short Circuit levels

Maintenance

Ease of extension

Total land area

Cost

Documents

Switchgear& Switchyard