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Engineering & Project ManagementEngineering & Project Management 1 of 54

LV & HV SwitchgearsLV & HV Switchgears

PRESENTATIONPRESENTATION

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Engineering & Project ManagementEngineering & Project Management2

of 54

1) Definitions

a) Switchgear

b) Disconnector

c) Switch

d) Circuit Breaker

2) Types of LV Circuit Breakers

3) Types of HV Circuit Breakers

4) Ring Main Unit (RMU)

Table of Contents

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A general term covering switching devices with associated control, measuring and

protection equipment, and assemblies of equipment with support structures, connections,enclosures.

Switchgear functions

Allow network control and disconnecting of circuits.

Protect the network from faults (Circuit Breaker)

Provide system monitoring (i.e. voltage and current)

Allow safe access to circuits

Switchgear

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Disconnector (Isolator)

A device used to create a safe, insulated gap in an electrical system and generally intended

to be operated off load.Able to withstand system over-voltages. Can carry rated fault

current when closed. Cannot be used to break fault or load current.

Switch

A device capable of breaking load current, up to a defined level of overload. Also able to CLOSE

fault current up to a defined current and time limit (rating) but not BREAKfault current. Often

used for the same safety function as a Disconnector when it is then referred to as a Switch

Disconnector.

Three position combined

disconnector and earthing

switch

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Circuit Breaker

A device capable of MAKING and BREAKINGfault current up to a defined limits of current and

time. Capable of automatic operation when the

protection system detects a system fault.

The maximum operating time for CB is 65

milliseconds

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Types of LV Circuit Breaker

1) Current Limiting

2) Withdrawable

3) Moulded Case CB (MCCB)

4) Miniature CB (MCB)

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Types of HV Circuit Breaker

1) Oil Circuit Breaker

2) Air Circuit Breaker

3) Air Blast Circuit Breaker

4) SF6 Circuit Breaker

5) Vacuum Circuit Breaker

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Oil Circuit Breaker

This is the basic

action of oil circuit

breakers

The arc is

stretched and

cooled until it

is extinguished

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Small oil volume

circuit breaker

action

The oil in the lower

compartment is

normally separate from

that in the uppercompartment, but it is

pumped through to the

upper compartment, to

clear the arc, when the

CB operates

Oil Circuit Breaker

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The Air Circuit

Breaker was once

popular, now

superseded by SF6or Vacuum for

applications above

1kV. This is the

magnetic blow out

principle

Air Circuit Breaker

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The Air Blast Circuit breaker is still widely used at transmission voltages

Arc

splitter

plates

Air tank

Blast

valve

Blast

nozzle

Air Blast Circuit Breaker

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This isolator

must be

capable of

closing fault

current

Method of operation

Air Blast CB

Air Blast Circuit Breaker

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SF6Circuit Breaker

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Vacuum Interrupter

Vacuum Circuit

Breaker

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Ring Main Unit (RMU)

A three element device combining two

Switch disconnectors and a Circuit

Breaker (sometimes a Switch Fuse) in the

same enclosure. Used widely in

distribution systems.

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CB

OPEN

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CLOSED

And a three

position switch-

disconnector/

earthing switch

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OPEN

And a three

position switch-

disconnector/

earthing switch

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EARTH

And a three

position switch-

disconnector/

earthing switch

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Important -

The circuit is

not earthed

until the CB is

also closed

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The vacuum

Circuit Breaker is

not a POINT of

ISOLATION; the

disconnector is the

POINT of

ISOLATION

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The Ring

Switches

comprise athree position

switch

disconnector/

earthing switch

CLOSED

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OPEN

The Ring

Switches


switch

disconnector/e

arthing switch

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EARTH

The Ring

Switches


switch

disconnector/e

arthing switch

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Test access to the

circuit may be

obtained by

removing the lower

panel, opening

these sockets andusing the operating

handle to remove

the star bar

(otherwise known

as the EARTH

LINK

) from theearth contacts

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Although network diagrams appearcomplex, they are made up of simple

elements

Consider the circuit shown in red

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Although network diagrams appearcomplex, they are made up of simple

elements

Consider the circuit shown in red

It comprises circuit breakers at A and B

And an open point at C

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The circuit breaker at A protects thenetwork shown in green

The circuit breaker at B protects the

network shown in brown

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The non automatic switches(disconnectors) out on network allow

different parts of the network to be

isolated, if faulty, and then repaired

This arrangement is known as

a RADIAL NETWORK

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Only the circuit breakers at A and Boperate automatically to disconnect

faults. The remainder of the network

switchgear is non automatic but it has

the capability to OPEN load current

and CLOSE fault current

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At each substation, the localtransformer is protected by a circuit

breaker or switch fuse

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In radial networks, the restorationtime after a fault or damage incident

is normally:

Formost of the substations, the time

needed to carry outmanual switching

In the worst case, the repair time on

the cable or overhead line

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WHY DO THE NETWORKSWITCHES NEED A FAULT

MAKING CAPABILITY?

Consider a fault occurring

on the network controlled

by circuit breaker A

At this stage, it is not

possible to knowwhere the

fault is located, only that

circuit breaker A hastripped and supplies are lost

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WHY DO THE NETWORK

SWITCHES NEED A FAULT

MAKING CAPABILITY?

Begin restoration by

opening at the firstsubstation and closing the

circuit breaker

Next close the switch to the

branch circuit

Close the switch towards theopen point C

At this point we have re-

energised the fault; the

network switch has closed

fault current, circuit

breaker A should open 1-2seconds later

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THANK

YOU

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