insulation coordination of ais and gis

7/21/2019 Insulation Coordination of Ais and Gis

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Introduction – stress at the equipment – insulation

strength and its selection – standard BILs – Applicatiosimplied method – Comparison with IEEE and IEC gu

INSULATION COORDINATION

OFAIS AND GIS



NECESSIT



Introduction

!ith the incoming surge and the arrester ratingselected" the process of selecting the BILs of theequipment can #e doneBIL of transformer – $%& 'alues

BIL of circuit #rea(er – $ 'alue

BIL of #us#ar" disconnectors" etc – ) 'alues

NE*A standard S+, e-plains the procedure to to the station apparatus



NE*A Standard S+,$. Evaluate the need and type of opened crea!er protection

The need for opened circuit rea!er protection i" evaluated #r"t$ Li%htnin% ter&inate" on the "hield 'ire or pha"e conductor of the line( re"ultin%

The "ur%e travel" into the "tation( ut ecau"e the rea!er i" clo"ed( arre"ter""tation provide protection to the rea!er$

Ne*t( the relay "en"e" the fault( the rea!er" open +in aout ,- &".( and in aorea!er reclo"e"$

0o'ever( a li%htnin% )a"h i" co&po"ed of one or &ore "tro!e"( and a "u"e1uen

occur et'een the /- and /-- &" and produce a "ur%e volta%e that e*ceed" thein"ulation "tren%th2'hen the rea!er i" opened and unprotected$

• Three alternate "olution" to thi" prole& have een propo"ed3

• +4. apply arre"ter" on the line "ide of all rea!er"

• e"t ut e*pen"ive

• +5. apply rod %ap" on the line "ide 6

• di7cult to "et %ap "pacin% 8

• Le"" e*pen"ive and le"" e9ective

•



NE*A Standard S+,). Select the inco&in% "ur%e The inco&in% "ur%e &ay ori%inate fro& Shieldin% failure :ac!)a"h

Su"1uent "tro!e" of the )a"h that &ay endan%er the rea!er 'hen open or durin% t

• In "i&ple "tation" to evaluate any e1uip&ent on the tran"for&er u"( the inco&in% "a"ed on n ti&e" the ;T:F 'here n i" the nu&er of line"$

• Thi" "ur%e i" called the ;T:S( the &ean ti&e et'een "ur%e"$

• For e*a&ple( for an ;T:F of 4-- year" and a three2line "tation( the e1uip&ent on theu" i" evaluated 'ith an ;T:S of /-- year"( i$e$( a /--2year "ur%e$ The e1uip&ent in"tren%th on other u"e" i" evaluated 'ith the ;T:S < ;T:F( e$%$( a 4--2year "ur%e$

• For air2in"ulated "tation"( ;T:F" of ,- to 4-- year" have een u"ed( althou%h va,-- year" are u"ed in the IEC %uide$

• For %a"2in"ulated "tation"( ecau"e of the con"e1uence of failure( lar%er value" ai$e$( /-- to 4--- year" i" "u%%e"ted( ut =-- year" ha" een u"ed

• The "elected reliaility criterion i" pri&arily a function of the con"e1uence of the faillife of the e1uip&ent$

• For e*a&ple( the reliaility criterion for a "tation &ay e &ore "trin%ent than thaecau"e a )a"hover in a "tation i" of %reater con"e1uence$

• Even 'ithin a "tation( the reliaility criterion &ay chan%e accordin% to the type o

• For e*a&ple( ecau"e of the con"e1uence" of failure of a tran"for&er( the t

&ay e provided 'ith a hi%her order of protection$



NE*A Standard S+,

&. Select the candidate BILsApparatus BILs are selected carefull/. 0or circuit #rea(

used 1&,)(2" ) BSL 'alues 3$% closed condition 4condition5 are listed against $ BIL 'alue.

6. E'aluate contingenc/ conditions

The normal condition of a station during thunconditions is usuall/ with all lines or some lines in ser'i

If contingenc/ conditions are important" then the pro#athe contingencies should #e e'aluated to arri'appropriate incoming surge.



NE*A Standard S+,7. Select arrester rating and preliminar/ location of arresters

• To determine the arrester rating" the rules are*C82 must #e equal to or greater than the ma-imum line%to%ground s/stem '

Switching surge energ/ discharged #/ the arrester must #e less than the ene

The temporar/ o'er'oltage across the arrester must #e less than the arrestercapa#ilit/.

The continuous am#ient temperature must #e less than 69:C and the tempormust #e less than $97:C.

Altitude limit; $<99 m 3,999 feet5. The pressure relief current must #e equal to or greater than the fault current.

• Location of arrestors line entrance

Near to transformer

Near #us



NE*A Standard S+,,. Set up model on =igital Transient program

•

A single phase model of the station is required.• Station #uses – distri#uted lines descri#ed #/ th

surge impedances and lengths

• Transformers – surge capacitances #etween $ 4

• 8ther equipments – surge capacitances

• Incoming surge



NE*A Standard S+,>. Surge 'oltages? e'aluationselect BILs and Clearances

BILs;

• The magnitude and wa'eshapes of incoming surge 'oltages are measurethe station" usuall/ at equipment locations and at opened points on the #

• If the/ are not standard" some e'aluation method is required to change'oltages to equi'alent crest 'oltages for a $.)?79@s• 0or non self restoring insulations" a safet/ factor of $7 to )9 is recomm

con'entional BIL.

• 0or self restoring insulations" -ing margins is dicult.

Clearances;• Estimated using the highest equi'alent crest 'oltage of the $.)?79@s wa'es

#/ 3%gradient of C085

• 0oe altitudes a#o'e sea le'el" the insulation strength decreases as a linear f

relati'e air densit/.



NE*A Standard S+,<. ee'aluation•

If the required BILS and clearances are e-cessi'e#e decreased #/ two methods• Additional arrestors can #e used

• =ecreasing tower footing resistance

0or unshielded lines" o'erhead ground wires or gaps caused for the second method.



Stress at the equipment

S – steepness of th

surgeE – Crest of the inc2D0 – power frequen

TA – arrester lead le

T T – time ta(en ?dis

#etween the transfarrester

TB – time ta(en?dist#etween the arrest



2oltage at the transformer with capacitance

Assumptions; arrester discharge 'oltage is

constant Independent of current

discharged #/ the arrestor Tail of the incoming surge was

constant Falf of the innite 'alue



>olta%e at the Tran"for&er andArre"ter2:u" ?unction 'ith Capacita

*odeling the transformer with its surge capacitance toground dramaticall/ increases the 'oltage at thetransformer

Surge 'oltages at thearrester Surge 'oltages at thetransformer

eGected 'oltatransformer



>olta%e :ehind the Arre"ter

•

Sin%le2Line Station



E"ti&atin% the Arre"terDi"char%e >olta%e and Curre



eferences• Andrew . Fileman" HInsulation coordination for power s/stems" Ta/lor

0rancis"$JJJ.

insulation coordination of ais and gis

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