eee457 class lecture 28.02.10 -...

Isolator (Disconnecting Switch)Isolator (Disconnecting Switch)

Isolator in a substation

Isolator (cont )Isolator (cont……)

Isolator operates under no load conditionIt does not have any making or breakingcapacity as well as is not provided with any arc

ti i hi t lik CBextinguishing arrangement like CBThey are provided on with CB to assist CB inmaking isolation and enable maintenancemaking isolation and enable maintenance.They are put on each side of a CB in anincoming or outgoing transmission lineincoming or outgoing transmission line.They are necessary on supply side of CB inorder to ensure isolation of CB from live partsorder to ensure isolation of CB from live parts

Isolator (cont……)

Interlocking between CB and IsolatorsWhile opening:

CB i d fi t- CB is opened first- then isolator is opened

While closing:While closing: - isolator is closed first- then CB is closed

Earthing switchEarthing switch

Earthing switch is connected between lineEarthing switch is connected between lineconductor and earthNormally it is openNormally, it is openWhen line is disconnected, the earthingswitch is closed so as to discharge theswitch is closed so as to discharge thetrapped voltage on the lineEarthing switch are mounted on the frame ofEarthing switch are mounted on the frame ofisolators

Sequence of operation of CB, Isolator and SE/S in transmission line.

To break a circuit:To break a circuit:1. Open CB, 2. Open isolator, 3. Close E/S

T k i itTo make a circuit:1. Open E/S, 2. Close isolator, 3. Close CB

Surge Arrester Surge Arrester

Surges in a power system and how fast they react:g p y y- Line Overvoltage (<70ms)- Lightning surge (1.2us)- Switching surge (100us)

f fWithstand capacity of a busbar for high voltage surge:For 132KV nominal voltage-Line Overvoltage 300KV rmsLine Overvoltage - 300KV rms Lightning Surge – 450KV peakSwitching Surge – 380KV peakSwitching Surge 380KV peak

Surge Arrester (cont ) !!!Surge Arrester (cont……) !!!

Surge ArresterSurge Arrester andand

Earth SwitchEarth Switch are not sameare not same

Operation of a surge arresterOperation of a surge arrester

Connected between line and earthConsists of a resistive element (SiC) in serieswith a gap elementResistor element offers a non-linear resistanceFor normal power frequency system voltageth i t i hi hthe resistance is highWhen a surge appears in line, the resistance ofSiC becomes lowSiC becomes lowAs a result the gap breaks down and surge isdiverted to the groundg

Operation of a surge arrester (cont….)

When surge vanishes normal frequencyWhen surge vanishes, normal frequencyvoltage is set up across the arrester.The resistance offered by resistor to theThe resistance offered by resistor to thevoltage is very high.Arc current reduces and voltage across theArc current reduces and voltage across thegap is no more sufficient to maintain arc.Current flowing to the earth is automaticallyCurrent flowing to the earth is automaticallyinterrupted and normal condition is restored.

Surge arrester connected on both side of a transformer

Surge arrester connected intransmission line