Transformers Prot.sett.

8/20/2019 Transformers Prot.sett.

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Coordination Setting Guidelines Transformers

presented in this application guide is for reiew! approal! interpretation and application by a registered professional engine

ponsibility and liability resulting from the use and interpretation of this information"

his material is permitted proided proper ac$nowledgement is gien to SKM Systems %nalysis &nc"

tion and coordination of protectie deices is mandated in article 110"10 of the 'ational (lectrical )ode" To ful*ll this re+ui

dination study is re+uired" The electrical engineer is always responsible for this analysis" &t is an unfortunate fact of life tha

eci*ed and purchased the e+uipment will not set the deices" Therefore! compromises are ineitable"

undamental ob,ecties to oercurrent coordination that engineers should $eep in mind while selecting and setting protecti

ectie is life safety" ife safety re+uirements are met if protectie deices are rated to carry and interrupt ma.imum aailab

stand and interrupt ma.imum aailable fault currents" ife safety re+uirements are neer compromised"ob,ectie is e+uipment protection" /rotection re+uirements are met if oercurrent deices are set aboe load operating le

mage cures" eeder and transformer damage cures are de*ned in applicable e+uipment standards" Motor and generator

chine speci*c! and are normally proided in the endor data submittal pac$age" 3ased on system operating and e+uipmen

ection is not always possible"

ectie is selectiity" Selectiity re+uirements are met if in response to a system fault or oerload! the minimum area of the

ed from serice" %gain! based on system operating and e+uipment selection practices selectiity is not always possible"

his guide is to proide oercurrent protectie deice setting guidelines for transformers to meet the ob,ecties listed aboe

r Switchgear Feeder Unit

d oercurrent protection schemes for M transformers fed from switchgear circuit brea$ers include an instantaneous oercu

1 relay characteristics are plotted on a phase T)) along with the transformer and feeder damage cures"

he phase oercurrent relay is to allow for full use of the transformer! and to protect the transformer and cable from oerloa

the relay cure should be to the right of the transformer % rating and inrush point! and to the left of the transformer and

able amp rating"

ns are listed below that hae historically allowed for safe operation of the transformer and cable while reducing instances

7eice unction #ecommendations )omments

)T Sie 009 of % % on base rating"

51 /ic$up 110-1409 of % Set below the transformer damage cure"

Set at or below cable ampacity"

Time 7iallet-thru current : 1"0

second

Set below the transformer damage cure"

Set at or aboe low oltage main deice"

50 /ic$up 009 of let-thru Set below cable damage cure"


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or

009 of inrush

)able damage cure must be aboe the ma.imum

fault current at 0"1 seconds"

ig" 1 M transformer switchgear feeder unit - one line


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ig" M transformer switchgear feeder unit - phase T))

r Fused Switch Feeder Unit

ses are typically used in fused switches sering M transformers" uses rated 100( or less must trip in ;00 seconds at curr

r ( ratings" uses aboe 100( must trip in


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50 use Sie (-rating = %

% based on the top rating of the unit"

Sie at or below cable ampacity"

Sie aboe transformer inrush point of 1 .

nominal % at 0"1 seconds"





ig" ; M transformer fused switch feeder unit - one line


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ig" 4 M transformer fused switch feeder unit - phase T))

CB Feeder Unit

d oercurrent protection schemes for transformers fed from circuit brea$ers e+uipped with long-time! short-time and ins

cuit brea$er characteristics are plotted on a phase T)) along with the transformer and feeder damage cures"

he circuit brea$er is to allow for full use of the transformer! and to protect the transformer and cable from oerloads and fa

rea$er cure should be to the right of the transformer % rating and inrush point! and to the left of the transformer and ca


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51 T/> 100-159 of %Set below the transformer damage cure"

Set at or below cable ampacity"

T7! ST/> ?ST7

Set to coordinate withdownstream deices

Set below the transformer damage cure"

50&nstantaneou

s009 of inrush

Set below cable damage cure"

)able damage cure must be aboe the

ma.imum fault current at the )3 total clear

cure"

ig" 5 transformer )3 feeder unit - one line


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ig" < transformer )3 feeder unit - phase T))

Fused Switch Feeder Unit

nd )lass =


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50 use Sie 100-159 of %

% based on the top rating of the unit"

Sie at or below cable ampacity"

Sie aboe transformer inrush point of 1 .nominal % at 0"1 seconds"





ig" B transformer fused switch feeder unit - one line


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ig" 8 transformer fused switch feeder unit - phase T))

ation Duides oEered by SKM Systems %nalysis at www"s$m"com

ansmission and 7istribution #eference 3oo$! %33 /ower T?7 )ompany! #aleigh! 'orth )arolina! 1FFB

elaying Theory and %pplications! nd (dition! Marcel 7e$$er! 'ew Gor$! 004

! &((( #ecommended /ractice for /rotection and )oordination of &ndustrial and )ommercial /ower Systems &((( 3uE 3oo

B"F1! &((( Duide for /rotectie #elay %pplications to /ower Transformers


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Transformers Prot.sett.

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