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1. Introduction to Switchgear and Protection

Function of Switchgear and Protection
+ Why do we need system protection
+ Detect fault
+ Isolate faulted component
+ Restore faulted component

» Aims

+ Continued supply for rest of the system
+ Protect faulted part from damage

2. Introduction to Switchgear and Protection

Essential qualities of protection scheme
Reliability
Operate dependably and in healthy
operating condition when fault conditions
occur, even after remaining idle for months
or years
Selectivity
Clearly discriminate between normal and
abnormal system condition to avoid
unnecessary, false trips
Sensitivity
Ability to distinguish the fault condition,
although the different between fault and
normal condition is small

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3. Introduction to Switchgear and Protection

Essential qualities of protection scheme

Speed
Fault at any point in the system must be
detected and isolated rapidly to minimize
fault duration and equipment damage. Any
intentional time delays should be precise

Economy
Provide maximum protection at minimum
cost

Simplicity

Minimize protection equipment and circuitry

4. Introduction to Switchgear and Protection

Switchgear and Protection
> Switch gear is an essential part of power system and also any of electric circuit
equipment associate with switching and fault clearing process are termed as Switchgear
+ Switch gear covers wide range of equipment concerned with switching and protection
schemes
For example: Switches, Fuses, Circuit-breaker, Isolator, Relays, Lightning arresters,
Control panels, Current transformer and Isolators

420 kV Gas insulated 145 kV, 40 kA Compact
switchgear switchgear assembly

5. Introduction to Switchgear and Protection

System Protection Flow

Voltage/current rise from normal condition
Voltage/current is reduced to match with relay rating

Activates circuit breaker
Circuit isolation

6. Equipment Used in Switchgear and Protection

Circuit Breaker
» Protect electrical power system circuits from damage due to either over loads or faults
Fuse

» Used in circuit for protecting electrical equipment against overloads and/or short circuit by!

melting of fuse element when an excessive current flows

245 KV Circuit Breaker

Mini Circuit Breaker Substation

7. Equipment Used in Switchgear and Protection

Protection Scheme

+ When a fault occurs in the protected circuit,
the relay connected to the CT and PT actuates
and closes its contacts

+ Flow of Current from the battery is through trip
coil in the trip circuit

+ Trip coil of the circuit breaker is energized and
actuates operating mechanism of circuit
breaker for the opening operation

Circuit Breaker Control for the
Opening Operation

8. Circuit Breakers
Requirements

» Circuit breaker must safely interrupt the short-circuit
currents and non responsive for normal operating
currents

» After the occurrence of fault, the circuit breaker must
isolate the fault circuit

» It should isolate only the faulty circuit without

effecting the healthy one.

Circuit Breaker

» It should not operate whenever over current flows
under healthy conditions

9. Circuit Breakers

Basic Principle of Operation

> "A mechanical switching device, capable of making, carrying and breaking currents
under normal circuit conditions. Also capable of making and carrying for a specified
time and breaking currents under specified abnormal circuit conditions, such as those
of a short circuit.”

Supply

Elementary Schematic Diagram of a Circuit Breaker

Moving
contact

10. Circuit Breakers

ion Time Sequence

> Time sequence for various operation at the time of fault occurrence

> fem mors

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1 —+ CA contacts open

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ty m+ Contact cose

Circuit Breaker
Time |

11. Types of Circuit Breakers

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un oa or
Pressure Puffer

12. Type of Breaker

Arc Extinguished by

Arc contacts and arcing
horns operating in air

A blast of compressed air

Medium of Arc Quenching

Remarks

Used in Next Generation
Data for Voltages up to and
including 13.8 kv

Used in Next Generation
Data for high
115kV, 230kV and 500kV

Used in Next Generation
Data at 115kV, 230kV
Used at Bull Hill Wind Power
at 2.4 kv

Gaining acceptance by Over

13. There are 3 types of Air Blast Circuit Breakers, They are:

Air Blast Circuit Breaker

|
Y

Pal Bast

|

Cross*Blast

|

Radial-Blast or Double Blast

In this type, the air blast is
directed along the arc path,

Fixed
contact

Moving
contact

|
y

In this type, the air blast is
directed at right angles to the
arc path,

Fixed
contact

Moving
contact

In this type, the air blast is
directed radially.

|
T

Fixed Moving
contact contact

14. Axial-Blast Air Circuit Breaker - Construction and Working

© The contacts of the Axial-Blast Air
Circuit Breaker (i.e., fixed and
moving contacts) are arranged in
closed by means of a spring
pressure as shown in the image
under normal conditions

0 An air valve connects the air
reservoir with the arcing chamber

0 Under normal conditions, this
valve is in closed position, But it
automatically opens by the
tripping impulse during the
occurrence of a fault

Ar valve
/ Normal air

clearance for
working voltage

Air Reservoir

Normal Air Clearance for Working Voltage

15. Axial-Blast Air Circuit Breaker - Construction and Working

0 The high pressure air entering the
arcing chamber through the air
valve, forces the moving contact
to move in a direction which is
opposite to the spring pressure

0 As the moving contact separates
from the fixed contact, an arc is
struck

0 At this instant, air at high
pressure blown along the arc and
the air takes away the ionized
gases along with it

0 Consequently, the arc
extinguishes and current does not
flow

Arvake |]
AN Normal air

Clearance for
working voltage

Air Reservoir

Normal Air Clearance for Working Voltage

16. Cross-Blast Air Breaker - Construction and Working

0 In this type of circuit breaker, a blast
of air normal to the arc is supplied,
This cross-blast lengthens the arc
and forces it into a chute favorable
for extinction of arc

© The major parts of a basic cross-
blast air circuit breaker are shown
in the image displayed

0 When the moving contact is
separated from the fixed contact,
an arc is struck between them.
This arc, by means of a high
pressure cross-blast, is forced into
a chute comprising of arc splitters

Fixed
and baffles alee

Cross-Blast Air Circuit Breaker

17. Cross-Blast Air Breaker - Construction and Working

© The splitters lengthens of the arc and
baffles give improved cooling

© As a result, the arc gets extinguished
and the current ceases to flow. As the
blast pressure for all currents is the
same, the inefficiency at low currents
is eliminated

@ The final gap for interruption is great
enough to give normal insulation
clearance so that a series isolating
switch is not necessary

© It is used to interrupt high currents
upto 100 kA at lower voltages

Fixed
Contact

Cross-Blast Air Circuit Breaker

18. Radial-Blast Air Breaker - Construction and Working

0 It works on the double blast breaking
principle. In this case, air blast is
admitted radially as shown in the
image

© While the flow in the arcing region is
axial i.e., along the direction of the arc
formation, increase in the number of
breaks increases the efficiency of
breaker at EHV and super high voltage
applications

© Resistance switching is employed in
this circuit breakers to reduce
transient over voltages

0 Axial blast circuit breakers are suitable
for extra high voltages

Blast
voy
A À
Fixed Moving
contact contact

Radial-Blast Air Circuit Breaker

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Advantages of Air Blast Circuit Breaker

By connecting several breaker heads in series, the voltage rating of the breaker
can be increased

By careful design, the interrupting capacity rating can be increased to over 50,000
MVA

High speed of operation

Negligible maintenance

Elimination of fire hazards

Reduced size

20. >
>

>

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v

Limitations of Air Blast Circuit Breaker

Expensive

Complicated construction, requiring air receivers and high pressure pipe work
Maintenance is time consuming, as access is difficult

A supply of very dry, compressed air is required to ensure no condensation or ice
formation on the insulators or contacts

The breaker, when opened, unless fitted with silencers, is very noisy and

consequently cannot be used in built-up residential areas

21. Applications

> The air blast circuit breakers find many applications in high voltage installations

> Majority of the circuit breakers for voltages beyond 11kV to 1100kV and are
placed in outdoor

> They are suitable for all EHV applications

22. SF Circuit Breakers are constructed in three ways:

23. vw

Properties of SF, Gas

SF6 gas is colorless, odorless, non-toxic and non-inflammable gas

It is extremely stable and inert

Its density is 5 times higher than air

Its thermal conductivity (heat transfer ability) is about (2 to 2.5 times) higher than air
and assists in better cooling of current carrying parts, so heat will be dissipated

Its thermal time constants are low and as a result the pressure it should be stored and

used are relatively smaller as compared to the air

It is chemically stable up to 5000C and does not react with the structural material up to
5000C

24. Advantages and Limitations

> Due to the superior arc quenching property of
SF6, such circuit breakers have very short arcing
time

> Since the dielectric strength of SF6 gas is 2to3 > SF6 breakers are costly due to

times that of air, such breakers can interrupt the high cost of SF6
much larger currents > Since SF6 gas has to be
> The SF6 circuit breaker gives noiseless operation reconditioned after every
due to its closed gas circuit and no exhaust to operation of the breaker,
atmosphere unlike the air blast circuit breaker additional equipment is

> The closed gas enclosure keeps the interior dry so required for this purpose
that there is no moisture problem

> There is no risk of fire in such breakers because
SF6 gas is non-inflammable

25. Applications

> Atypical SF6 circuit breaker consists of interrupter units each capable of dealing
with currents up to 60 kA and voltages in the range of 50-80 kV

> A number of units are connected in series according to the system voltage

> SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power
ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles

26. Non-Puffer Type

@ The image shows the constructional details of the
SF circuit breaker

@ It contains an arc interruption chamber, In the arc
| interruption chamber fixed and moving contacts
are enclosed and is filled with SFs gas

@ The chamber is connected to a reservoir. Valve
mechanism passes SF; gas at very high pressure
during separation of the contacts of the breaker.
The gas flows from the reservoir with a high
pressure towards the arc interruption chamber

@ The fixed contact is hollow and cylindrical in shape.
It is a current carrying contact fitted with the arc
horn. The moving contact is also a hollow cylinder
and contains holes on its sides for the SFs gas to
pass through them after flowing across the arc

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Murder

Me Chamber

Non-Puffer Type

27. Non-Puffer Type

Working

© When the breaker is closed, SFs gas at a pressure of
about 2.8 kg/cm? encloses the contacts

@ When the breaker operates, the moving contact gets
separated from the fixed contact and an arc is struck
between the contacts

@ The movement of the moving contact is synchronized
with the opening of a valve which permits SFs gas at
14 kg/cm? pressure from the reservoir to the arc
interruption chamber

© The SF gas flowing at high pressure, absorbs the
free electrons present in the arc path rapidly. Then it
forms immobile negative ions which do not serve as
charge carriers

Non-Puffer Type

28. Non-Puffer Type

Working

© Asa result, the dielectric strength of the medium
between the contacts rapidly raises high and the arc
gets extinguished

@ After the breaker operation (that is, after arc
extinction), the valve is closed by the action of a set
of springs

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Non-Puffer Type

29. Construction

Insulated rods for
operating moving
member

+*—— SF, gas inlet

E a +—— Arcing horn
a Fixed

member

Gas Outlet

Contacts
Arc Chamber

Non-Puffer Type

30. Single Pressure Puffer Type

© The puffer type SF; breakers of single pressure type are the
most Favoured types prevalent in power industry

working
@ The piston is fixed but the cylinder is movable

© The cylinder is fixed to the moving contact because the
cylinder along with the moving contact moves away from the
fixed contact when the breaker is operated

@ The SF; gas in the cylinder is compressed as the fixed piston is
present

© The compressed SF; gas flows through the nozzle and over the
electric arc in axial direction

@ Owing to the convection and radiation of heat, the arc radius
reduces gradually and finally gets extinguished at current
zero

Moving

SF, Gas Compresses —YIinder

Single Pressure Puffer Type
SFs Circuit Breaker

31. Single Pressure Puffer Type x

working

@ The dielectric strength of the medium existing between the
separated contacts increases rapidly and is restored quickly Nozzle Fixed Piston
as a new SFs gas occupies the space

© During the quenching of arc, a small quantity of SF; gas
breaks down to some other fluorides of sulphur which mostly
recombine to form SFs again

0 A filter is used in the interrupter for absorbing the
decomposed by product that is remaining

@ The gas pressure inside the cylinder is maintained at around
5 kg per sq cm. At higher pressure, the dielectric strength of
the gas increases. But at higher pressure, the SFs gas

4 a Single Pressure Puffer Type
liquefies at higher temperature which is undesired SFs Circuit Breaker

SF, Gas Compresses

32. Single Pressure Puffer Type
working

Nozzlı Fixed Piston À

© Hence, heater is required to be arranged for automatic
control of the temperature for circuit breakers where higher
pressure is utilized. If the SFs gas will liquefy then it loses the
ability to quench the arc

SF, Gas Compresses

Single Pressure Puffer Type
SFs Circuit Breaker

33. Construction

Fixed Piston

Nozzole

j

Moving

daras Contact

Contact

| osos] |

Moving
SF, Gas Compresses treylinder

Single Pressure Puffer Type SFs Circuit Breaker

34. Double Pressure Puffer Type

@ During the process of arc extinction, gas from a high pressure
system flows into a low pressure system through the nozzle

© In this circuit breaker, gas is made to flow from Zone P, to Pa
À i |

through a convergent divergent nozzle. The nozzle is located |
such that flow of gas covers the arc

© Gas flow attains almost supersonic speed in divergent portion
of nozzle, thereby taking away the heat from periphery of arc
causing reduction in diameter of the arc

© Finally, the diameter of the arc becomes zero at current zero
and the arc gets extinguished

@ Arc space is filled with SF; gas which increases the dielectric
strength of contact space L

Double Pressure Puffer type SF.
Circuit Breaker

35. Zone P q a po
e ae D. Zone P,

28 , ® ‘ Contact
Contact ¿

Nozzle

Double Pressure Puffer Type SFs Circuit Breaker

36. Vacuum Circuit Breakers

> A vacuum circuit breaker consists of a sealed

vacuum pot or flask, which contains the
contacts

> A vacuum provides the insulation and arc
extinguishing medium. The moving contact is
moved by some moving mechanism and the
arc is extinguished at the first "crossing of
current” through zero amplitude.

In such breakers, vacuum (degree of vacuum
being in the range from 10-7 to 10-5 torr) is
used as the arc quenching medium

> Since vacuum offers the highest insulating
strength, it has far superior arc quenching
properties than any other medium

Representation of Vacuum

a

37. Vacuum Circuit Breakers

Production of Arc and Arc Extinction

» When the contacts of the breaker are opened in
vacuum an arc is produced between the
contacts by the ionization of metal vapors of
contacts

> However, the arc is quickly extinguished
because the metallic vapors, electrons and ions
produced during arc rapidly condense on the

surfaces of the circuit breaker contacts,

resulting in quick recovery of dielectric strength Production of Arc

38. Vacuum Circuit Breakers

Working of Vacuum Circuit Breakers

> When the breaker operates, the moving contact
separates from the fixed contact and an arc is
struck between the contacts

> The production of arc is due to the ionization of
metal ions and depends very much upon the
material of contacts

> The arc is quickly extinguished because the
metallic vapors, electrons and ions produced
during arc are diffused in a short time and seized
by the surfaces of moving and fixed members
and shield

Representation of Vacuum

39. Advantages of Vacuum Circuit Breakers

> Small in size, reliable and have longer life

> Require little maintenance as they are "sealed for life"

> Can be operated many tens of thousands of times before replacement is required

> No fire hazards

> No generation of gas during and after operation

> Can interrupt any fault current

> Can break any heavy fault current perfectly just before the contacts reach the
definite open position

> Require little maintenance and are quiet in operation

> Can successfully withstand lightning surges

> Have low arc energy

> Have low inertia and hence require smaller power for control mechanism

40. Limitations of Vacuum Circuit Breakers

> Loss of vacuum can be dangerous and difficult to detect
> Normally, no repairs can be done to the breaker
> Faulty units are usually discarded

41. Applications

> Industrial applications, where voltages are high and current to be interrupted is low.
Because of the short gap and excellent recovery characteristics of vacuum breakers,
they are very useful as very high speed making switches

> Rural areas as they are suitable for a majority of applications with limited rating of
say 60 to 100 MVA. Vacuum circuit breakers are being employed for outdoor
applications ranging from 22 kV to 66 kVO

42. Oil Circuit Breaker

> Because of the noise problem associated with air blast circuit breakers, oil circuit
breakers tend to be installed near built up areas and air blast breakers tend to be
installed in rural areas.

ixed

Fi
Jens Gas Bubble Contact

Insulation
Moving

Contact

Arc

43. Oil Circuit Breaker

Tripping and Closing
> Tripping:
* When the trip coil is energized, the latch is released and due to spring action, the
contacts rapidly move downward
> Closing:
+ As soon as the arcs have extinguished, the gases escape through the vents and the
pots re-fill with oil

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LA

A

1% Z

Arc
Oil —E

(a) Breaker Pot With Contacts in the Closed Position
(b) How the Arc is Extinguished In An Oil Circuit Breaker

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Z

44. Advantages and Limitations of Oil Circuit Breaker

> Quiet in operation :

1 . > Contains flammable oil
> High interrupting E B

e Ñ . > Requires an oil treatment plant
> High interrupting capacity can be

: : > Oil must be kept at a very high
achieved by series connecting

. standard
several pots interrupt the current

45. Low oil Circuit Breaker

Construction:
® Fig shows the cross section of a single
phase low oil circuit breaker.

° Separated from each other but both

=
filled with oil. =
| ==
* The upper chamber is the circuit ==
breaking chamber. ==
EE
° The lower one is the supporting
chamber.

+ The two chambers are separated by a
partition and oil from one chamber is
prevented from mixing with the other
chamber.

° This arrangement permits two
advantages. Firstly, the circuit breakin,
chamber requires a small volume of oi
which is just enough for arc extinction.
Secondly, the amount of oil to be
replaced is reduced as the oil in the
supporting chamber does not get
contaminated by the arc.

46. Low oil Circuit Breaker

Operation:
© Under normal operating conditions,

the moving contact remains engaged
with the upper fixed contact.

e When a fault occurs, the moving
contact is pulled down and an arc is
struck.

e The arc energy vaporises the oil and
produces gases under high pressure.

° This action constrains the oil to pass
through a central hole in the moving
contact and results in forcing series of
oil through the respective passages of
the turbulator.

e The process of turbulation is orderly
one, in which the sections of the arc
are successively quenched by the effect
of separate streams of oil moving
across each section in turn and bearing
away its gases.

47. Low oil Circuit Breaker

Advantages: A low oil circuit breaker has the following advantages over a
bulk oil circuit breaker:
(i) It requires lesser quantity of oil.
(ii) It requires smaller space.
(iii) There is reduced risk of fire.
(iv) Maintenance problems are reduced.

Disadvantages: A low oil circuit breaker has the following disadvantages as
compared to a bulk oil circuit breaker :
(i) Due to smaller quantity of oil, the degree of carbonisation is
increased.
(ii) There is a difficulty of removing the gases from the contact space in
time.
(iii) The dielectric strength of the oil deteriorates rapidly due to high

degree of carbonisation

48. BULK OIL CIRCUIT BREAKER

U Bulk oil circuit breaker or BOCB is such types
of circuit breakers where oil is used as arc
quenching media as well as insulating media
between current carrying contacts and
earthed parts of the breaker. The oil used here
is same as transformer insulating oil.

49. CONSTRUCTION of BOCB

* The basic construction of bulk oil circuit breaker is quite simple.
Here all moving contacts and fixed contacts are immerged in oil
inside closed iron vessel or iron tank. Whenever the current
carrying contacts are being open within the oil the arc is produced
in between the separated contacts.

° The large energy will be dissipated from the arc in oil which
vaporizes the oil as well as decomposes it. Because of that a large
gaseous pressure is developed inside the oil which tries to displace
the liquid oil from surrounding of the contacts. The inner wall of the
oil tank has to withstand this large pressure of the displaced oil.

* Thus the oil tank of bulk oil circuit breaker has to be sufficiently
strong in construction.

50. Air cushion

Fixed
contacts

Moving
contacts

Fig, 19.3

Conceptual view of Bulk Oil Circuit Breaker

51. Working Principle or Arc Quenching in
BOCB

When the current carrying contacts in the oil are separated an arc is established in
between the separated contacts.
This arc will produce rapidly growing gas bubble around the arc.

As the moving contact move away from fixed contact the length of arc is increased
as a result the resistance of the arc increases. The increased resistance causes
lowering the temperature and hence reducing the formation of gasses surround
the arc.

The arc quenching in bulk oil circuit breaker takes place when current passes
through zero crossing.

As the gas bubble is enclosed by the oil inside the totally air tight vessel, the oil
surround it will apply high pressure on the bubble, which results highly
compressed gas around the arc. As the pressure is increased the de-ionization of
gas increases which helps the arc quenching. The cooling effect of hydrogen gas
also helps in arc quenching in oil circuit breaker.

52. * Advantages e Disadvantages

Y” Oil has high dielectric Y” Long arcing time

strength Y Do not permit high speed of
v Oil absorbs arc energy while interruption

decomposing Y Arc interruption control can
y Good cooling property of be obtained only by

the gas formed due to increasing the length of arc

decomposition

Ÿ It acts as an insulator
between the live parts and
earth