Switchgear and Protection.ppt

8/20/2019 Switchgear and Protection.ppt

1/51

Power SystemProtection

EEE Department

Kazi Firoz Ahmed


2/51

Kazi Firoz Ahmed

Switchgear and Protection


3/51



4/51



5/51

Switchgear and ProtectionSwitchgear:

It is a general term covering a wide range of equipment concerned with switchingand protection. All equipment associated with fault clearing process are covered !the term switchgear. Switchgear includes switches" fuses" circuit rea#er" isolator"control panels" lighting arrestor" $%" P% and various associated equipment


6/51



7/51

Control Gear:

It is used for switching and controlling power consuming devices. It is a form ofswitching device capale of ma#e and rea# the current in one or more electriccircuit such as rela!.

Circuit Breaker (CB):

A device capale of ma#ing" rea#ing an electric circuit under normal and anormalconditions such short circuits.

It has & 'asic function(• switching during normal operating condition for operation and maintenance• switching during anormal condition for interrupting the fault current caused !short circuits.



8/51

Switchgear are necessar! at ever! switching point in a power s!stem. 'etween thegenerating station and final load point" there are several voltages and fault levels.)ence in various application" the requirement of the switchgear var! dependingupon the location" ratting and local requirement.

Single pole CB:

In this rea#er fault current interrupting mechanism is involved with one phase. It isused in an isolated s!stem.

Triple Pole CB:

Fault interrupting is involved with three phase in the s!stem. It is used in aintegrated s!stem ut has a disadvantage as it creates uns!mmetrical situation in thes!stem inherentl!.



9/51


$' has two states onl!• $lose• *pen

+nder normal operating condition its operation can e manual or can e automaticremotel!.

During fault condition its operation must e automatic. *pening mechanism is

responsile for this automatic operation


10/51

%he sequence of function during anormal condition

Summar!



11/51

Switchgear and ProtectionWhen the fault occurs

• the rela! ,&- connected to $% ,- andP% ,/- actuates and close its contact ,0-• $urrent flows from the atter! ,1- inthe trip circuit ,2-• %rip coil ,3- is energized and $' ,4-operating mechanism is actuated.

• Au5iliar! contact ,a- can e used forinterloc#ing the $6 with otherequipment to avoid enormousoperation.


12/51

Switchgear and ProtectionTrip free feature of CB:

Suppose a $' has een instructed to close ! manual instruction ! pushing of push utton. %he operating mechanism will start operatingfor closing operation. 7eanwhile a fault has een ta#en place and therela! closes the trip circuit of the $'. %hen the mechanism which

permits the $' to e tripped ! the protective repla! even if it isunder the process of closing. It means the $' is giving the priorit! to

the rela! response or automatic response. It is #nown as the trip freefeature of $'


13/51



14/51

Switchgear and ProtectionWhat is arc?

Arc Flash is the result of a rapid release of energ! due to an arcing fault etween a phase us ar and another phase us ar" neutral or a ground.

An electric arc is an electrical rea#down of a gas which produces an ongoing plasma discharge" resulting from a current through normall! nonconductive mediasuch as air.

Why arc is create?

Sudden contact rea# down in high voltage line

http://en.wikipedia.org/wiki/File:Arcing_pickup_shoe.jpghttp://en.wikipedia.org/wiki/File:Lichtbogen_3000_Volt.jpg


15/51

Switchgear and Protection!oe of "rc e#tinction

$%&igh resistance interruption:

In this process the resistance of arc is increased ! lengthening andcooling it to such an e5tent that the s!stem can8t maintain the arc and thus

e5tinguishes. %his is used in the Air 'rea# $' and D.$. $'.

'%ow esistance or *ero point interruption:In this process the arc gets e5tinguished at the natural current zero of the

alternating current wave and is prevented from restricting again ! rapidincreasing the dielectric strength of the contact space. It is used in almost all )9 accircuit rea#er

arc

arc

arc

I

V r =


16/51



17/51

Switchgear and ProtectionTechnical particulars of a CB:

I.%!pe of medium for arc e5tinction

II.:ated voltage

III.:ated rea#ing current

I9.*ther rated characteristic , %ransient :ecover! 9oltage ,%:9-" restrictingvoltage" ma#e current etc-

9.%!pe of construction• Indoor metal clad• *utdoor t!pe

• 7etal clad SF0 gas insulated t!pe

9I.%!pe of operating mechanism• Spring opened" spring closed mechanism• Solenoid closed" spring opened mechanism• )!draulic mechanism• Pneumatic mechanism


18/51

Switchgear and ProtectionTechnical particulars of a CB:

9II.%otal 'rea# time

%his is , rela! time ; $' time-

< , Instant of fault to closure of trip circuit- ; ,closure of trip circuit to finalarc e5tinction time-

Spee of CB an elay

'efore 4=3>( $' interrupting time( 41?3> c!cles@ rela! interrupting time( 0?4&>c!cles

4=3>( $' interrupting time( c!cles

4=31( $' interrupting time( 3 c!cles

Present da!s rela! time ecomes as small as 4?3 c!cles


19/51

Switchgear and ProtectionTransient in series circuit:

Selection of $' for the power s!stem depends not onl! on the normal current ut also upon the ma5imum current it ma! have to carr! momentaril! and thecurrent it ma! have to interrupt at the voltage of the line in which it is placed. Inorder to approach the prolem for calculating the initial current when a

s!nchronous generator is short circuited.

$onsider a : circuit

9m sin, Bt ;C - < :i ; didt

where < G: & ; ,B-&H@ < tan?4,B:-

-sin,-sin,-, θ α θ α ω −−−+=− t

L

R

mm

e Z

V

t Z

V

t i

ac component dc component


20/51

Switchgear and ProtectionTransient in series circuit:

-sin,-sin,-, θ α θ α ω −−−+=− t L

R

mme

Z

V t

Z

V t i

ac component dc component

i,t- will e ma5imum when dc

components will e ma5imum at C? < ?J

i,t- will e minimum when dc

components will e minimum at C <


21/51

Switchgear and ProtectionIn the case of short circuit at the terminal of a S!nchronous generator.

+ , - ./g .01, 2 .+3. - ./g .013 2 .+. - ./g .012


22/51

Switchgear and Protection• %he fault current attains the pea# value at c!cle after the fault inception which

is the rea# value of the fault current under su transient period• )aving the fault current een sensed ! $% the operation of the protective rela!starts to. %he $' contact start to apart after the operation of the protective rela!which generall! ta#es little time. So the contacts of the $' generall! starts toseparate when the initial current reduces to lower value in the transient period•

%he rms value of the current at the instant of the contact separation is called the4reaking current of the $' and is e5pressed in #A• in other words the rea#ing current is equal to the rms value of the fault currentunder transient period.• if a $' closes its contact on the e5isting fault the current would increase to a pea# value during the first half c!cle from the instant of closing the $' on the

fault. %he fault current is reached to the pea# value at the c!cle from theinception of the fault. %his pea# value of the fault current is called the makingcurrent of the $'.• %he rated short circuit ma#ing current of a $' is the pea# value of the firstcurrent loop ,i.e. c!cle- of the short circuit current which the $' is capale of

ma#ing at the rated voltage.


23/51

Switchgear and Protection• %he e5pression of instantaneous current of a phase for a three phase solid fault at the terminal

of an unloaded generator(

< iac ;idc

• %he rms rea#ing current ,s!mmetrical- < iac &

• :7S rea#ing current ,as!mmetrical- <

• :ated short circuit rea#ing capacit! , :upturing $apacit!- of a $'

< 3 L #9 ,:ated 9oltage- L #A ,as!mmetrical rea#ing current-• :ated short circuit ma#ing current

< 4. L & L rated short circuit rea#ing current ,s!mmetrical-

< &.11 L rated short circuit rea#ing current ,s!mmetrical-

•

7a#ing $apacit! < &.11 L s!mmetrical rea#ing capacit!

-cos,-4

,&-cos,M4

-44

,-44

,N&-,O

P

P

O

PO θ α θ α ω −−−++−+−=

−−−

ad d T

t

d d

T

t

d d

T

t

d d

e x

E t x

e x x

e x x

E t i

&&

ac -&D, dcii +


24/51

Switchgear and Protection!athematical Pro4lem 5 $

" 67ϕ CB is rate $'89 " '999 !;" 66k; < sec% =in the symmetrical

4reaking current making current an short time rating

Solution:

• :ated s!mmetrical rms rea#ing current < &>>> 3 L 33 < 31 #A

• :ated short circuit rea#ing ma#ing current

< &.11 L rated short circuit rea#ing current ,s!mmetrical-

< &.11 L 31 < = #A

• Short time rating < 31 #A for 2 sec


25/51

Switchgear and ProtectionCurrent +nterruption in ac CB:

• Suppose a 3?ϕ short circuit is applied on an unloaded alternator at the instantwhen voltage of phase Q with respect to neutral is zero.•In such case the short circuit current in phase ' will have the ma5imum dccomponent and the waveform of I' will e ma5imum uns!mmetrical aoutnormal zero a5is as shown


26/51



27/51


28/51

Switchgear and Protection• Rhen the arc gets e5tinguished at fault current zero" a high frequenc! transientvoltage appears across the contact. %his transient vanishes within a ver! short

time in the order of less than >.>>4 sec•%his high frequenc! transient voltage tries to re?stri#e the arc. %his is calledrestriking >oltage or transient reco>ery >oltage (T;)

•%he power frequenc! s!stem voltage appearing etween the poles after arce5tinction is called eco>ery ;oltage• %he current that would flow in the circuit if the $' were replaced ! solidconductor is called Prospecti>e Current%• For successful arc interruption" the rate of uilding up of dielectric strength must e higher than the rate of rise of %:9.• %he rate of rise of %:9 depends on

o circuit parameterso t!pes of switching dut! involved• :ate of uilding up of dielectric strength depends onoEffective design of interruptiono $'


29/51

Switchgear and Protection• Due to switching a capacitive current" a high voltage appearing across thecontact can cause reignition of the arc after its final e5tinction. If the space rea#sdown within a period of of a c!cle of the initial arc e5tinction" the phenomenonis called reignition%• If the rea# down after of of a c!cle of the initial arc e5tinction" the phenomenon is called restrike%•

%:9 has a power frequenc! component and an oscillating transient component.%he oscillating transient component is developed due to the presence ofinductance and capacitive circuit. Power frequenc! component is due to s!stemvolt• the frequenc! of oscillating transient component is

Rhen f n < frequenc! of %:9 < equivalent inductance

c < equivalent capacitance•In actual s!stem the waveform of %:9 has several component frequenciesranging from a few )z to several thousand )z depending upon the circuit parameter

Hz LC

f nπ &

4=


30/51

Switchgear and Protection/ffect of natural freuency on T;:

• Rith the increase in the natural frequenc! of %:9 the rate of rise of %:9increases the rate of rise of %:9 across the rea# pole causes the voltage stresson the contact gap tending to continue the arc with higher frequenc! , sa! f 2n -"relativel! less time is availale for the uilding up of dielectric strength of the

contact gap. )ence higher frequenc! is associated with greater stress.• the rea#ing capacit! of a $' is related with the rate of rise of %:9 and

therefore with natural frequenc! of %:9. %he rea#ing capailit! is reducedwith the increase in natural frequenc!


31/51

Switchgear and Protection/ffect of Power factor on T;:

• %he voltage appearing across the $' pole at the instant of final current zero isinfluenced ! the power the factor of the current

• the instantaneous value of the voltage developed across the pole at the instantof current zero , when the arc is e5tinguished- depends on the phase on angle

etween the voltage and current i.e. power factor.• for unit! p.f load" oth current and voltage are in phase and oth are zero at the

instant of current zero.


32/51

Switchgear and Protection/ffect of Power factor on T;:

• for zero p.f current" the pea# value of the voltage ema5 is impressed across the pole at the instant of current zero. Suc# sudden application of voltage give riseto severe transient and has a high rate of rise of %:9. )ence interrupting currentof low p.f. is a difficult switching dut!

• :ate of rise and pea# value of %:9 depends upon several factors such as networ# configuration t!pe of fault t!pe of neutral earthing

,i.e solid grounding@Trounding through resistor-


33/51

Switchgear and Protectionate of ise of T; (%%%;) :

• :ate of rise of restri#ing voltage or %:9 is usuall! areviated ! :.:.:.9 andis e5pressed in voltUsec.

• :ate of rise of %:9 and natural frequenc! of the %:9 are closel! associated.:.:.:.9. depends on the s!stem parameters.

• %he $' should e capale of interrupting its rated rea#ing current under thespecific condition of %:9.

• %he following characteristic of %:9 are significant(

Pea# value of %:9 %ime to reach the pea#" hence the rate of rise Frequenc! of %:9 Initial rate of rise


34/51

Switchgear and Protectionate of ise of T; (%%%;) :

•

if e is the restri#ing voltage" then :.:.:.9< dedt voltsUsec• Pea# restri#ing voltage is defined as the ma5imum instantaneous value attained

! the restri#ing voltage ,em -

• Em < pea# recover! voltage

• :.:.:.9< emtm voltsUsec@ Amplitude factor < em Em• 6atural Frequenc! < 4>3 &tm #iloc!clesec@ since f < 4t < 4 &tm for an!

sinusoidal wave form


35/51

Switchgear and Protection@eri>ation of estriking ;oltage:

$onsider the following circuit" when current reaches zero at final arc e5tinction" avoltage e is suddenl! impressed across capacitor and there across the $'contact. %he current i which would flow to the fault is now inVected in thecapacitor and the inductance. %hus

-&,........

-4,........4

&

&

dt

ed c

L

e

dt

di

dt

decedt

Liii c L

+=

+=+= ∫


36/51


• S!stem voltage" e < Em cosBt i < ,Em B- sinBt Nsince the current lags the voltage ! =>> efore

opening of $'M

Putting Equation ,2- in equation ,&- we get@

Solving this equation for e" we get" e < Em N4?cos,t$-MWW..,1-

Em < Pea# value of recover! voltage" phase to neutral@ t< time in sec

< inductance in henr!@ $< capacitance in farad@ e< restri#ing voltage in volts

-2......,@>

-3.......,cos

L

E

dt

dit at

t L

E

dt

di

m

m

==

= ω ω ω

&

&

dt

ed c

L

e

L

E m +=


37/51


:.:.:.9 is ma5imum when

-0.........,sin

-Mcos4,N...

LC

t

LC

E

LC

t E dt d

dt deV R R R

m

m

=

−==

LC t LC

t

LC

t

&&

4sin

π π =⇒=

=

LC

E

LC

LC

LC

E V R R R

equationinequation Putting

mm==

&sin-...,

-0,-/,

ma5

π


38/51


Further" pea# restri#ing voltage occurs when e is m5imum.

Rhen dedt < >

Pea# restri#ing voltage is equal to , from equation 1-

LC t LC

t LC

t

LC

E

dt

de m

π π

π

=⇒=

=== sin>sin

mm peak E LC

LC E E &-cos4, =−=

π


39/51

Switchgear and ProtectionSummary:


40/51

Switchgear and Protection!athematical Pro4lem 5 ': A 1> )z 3X alternator with grounded neutral hasinductance of 4.0 m) per phase and is connected to us ar through a $'. %hecapacitance to earth etween the alternator and the $' is >.>>3YF per phase. %he $'opens when r.m.s value of the current is /1>>A. Determine(

a- Frequenc! of oscillation

- %he e5pression of %.:.9

c- 7a5imum :.:.:.9.d- %ime for ma5imum :.:.:.9.

e- 7a5imum voltage across the contents of the $' after the instant when arc isformed.

f- Average rate of rise of voltage up to the first pea# of the oscillation.



41/51


S itchgear and Protection


42/51

Switchgear and Protection!athematical Pro4lem 5 6: A 1> )z" 00#9" 3X generator has an earthed neutral. %he

inductance and capacitance per phase of the s!stem are /.1m) and >.>41YFrespectivel!. In a short circuit test the p.f of the fault current was >.&1 and the fault

current was s!mmetrical and the recover! voltage was oserved as =>Z of the fullline voltage. $alculate the rate of rise of restri#ing voltage ,phase to neutralvalue-. Assume that fault is isolated from the ground.

Solution(



43/51




44/51

Switchgear and Protectionesistance Switching @amping of T%%;:

A delierate connection of resistance in parallel with contact space ,arc- is called resistanceswitching. :esistance switching is use in $' having high post zero resistance of contactspace ,Air last $'-.

It is used to

[ 9anish the frequenc! of restriping voltage

[ :.:.:.9 is #ept within the limit[ Divert the arc current

%he value of resistance at which the frequenc! of %.:.9 ecomes zero is called thecritical amping resistance%



45/51

Switchgear and Protection!athematical Pro4lem 5


46/51

Switchgear and Protection">antages of "ir 4last Circuit Breaker:

\ Air which is free of cost can e used at high pressure.\ :eliale operation due to e5ternal source of e5tinguishing energ!.\ Air is free from decomposition.\ Air is clear and non?inflammale.\ Fresh medium is used ever! time.\ At high pressure" small contact travel is enough.\ %he same air serves the purpose of moving the contact and arc e5tinction.\ )igh speed operation.\ It is ver! eas! to incorporate the rapid auto?reclosing facilit!.\ ess maintenance is required compared to *il $ircuit 'rea#er.\ 'etter stailit!.

@isa>antages of "ir 4last Circuit Breaker:

[ )ave to maintain a compressed air.[ ea#age from the compressed air s!stem.[ $urrent chopping.[ 9er! nois!.



47/51

Switchgear and ProtectionSwitching of a Capacitor 4ank:

During opening of a capacitor an# the reignition and restri#ing can occur in an interrupter. %he current

drawn or supplied to the capacitor is generall! of small order and the $' can interrupt such smallcurrent invarial! at first current zero. Due to =>] phase difference" the voltage across thecapacitor is at ma5imum value ,ec- at this instant t4 and the capacitor remains charged at thisvoltage till t&. After another c!cle t&" the recover! voltage of appro5imate magnitude er ,ma5- appearsacross the $' and the total voltage across the contact gap is the sum of the two voltages.



48/51


%hus the transient recover! voltage of the order of & Ema5 might appear across the$' at the instant of t&" after the c!cle from current zero. %herefore" a restri#e ma!occur and this can cause damage to the s!stem.



49/51

Switchgear and Protection"uto eclosure:

It is a special t!pe of $' in which a mechanism is provided so that the contacts ofthe $' will open when there is a fault on the s!stem and will automaticall! tr! toclose after a short dela!. If the fault is removed within this dela! time" the contactwill e ale to close ! itself" otherwise it will e opened again. In this wa! the$' will tr! to close its contact three times. If the fault still persists" the $' willremain opened.

@ea time of "uto reclosure:

A minimum of >.&s must e allowed to enale the contact space to ecome

deionized completel!. )ence" a dead time of >.3s can e ta#en as a safe reclosure time.



50/51




51/51

Switchgear and ProtectionA Commissioning Test: %his test is done after installation at site to verif! the

operational readiness and proper functioning.

o 7echanical operationo Simultaneous touching of contact

o Insulation resistance

o *peration of opening and closing

o 7anual operation signalo Automatic operation ! energizing the rela!

A Short circuit Test: Stress on $' during short circuit test.

o %hermal stress" in turn stress on insulation.

o Electromagnetic stress.

Date post:	07-Aug-2018
Category:	Documents
Upload:	ahad-silve
View:	236 times
Download:	3 times

Switchgear and Protection.ppt

Documents