Samiya Zafar
Samiya Zafar
Assistant Professor, EED NEDUET
Assistant Professor, EED NEDUET
M.Engg
M.Engg F
Fall Semester 2017
all Semester 2017
P!E" S#STEM P"TE$T%N
P!E" S#STEM P"TE$T%N
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Application of Inverse Definite Minimum
Application of Inverse Definite Minimum
Time Relay on a Distribution Feeder
Time Relay on a Distribution Feeder
Consider the same feeder with two buses A and B
Consider the same feeder with two buses A and B
where IDMT OC relays are to be used.
where IDMT OC relays are to be used.
The purpose of R
The purpose of R
BBis to provide primary protection to
is to provide primary protection to
line BC and The purpose of R
line BC and The purpose of R
AAis to provide primary
is to provide primary
protection to line AB and
protection to line AB and bac!up to line BC.
bac!up to line BC.
"e start settin# process from tail end
"e start settin# process from tail end of the system.
of the system.
$a% Decidin# the CT ratios and plu# settin#s for Relay A
$a% Decidin# the CT ratios and plu# settin#s for Relay A
and Relay B&
$i% At relay B' the ma(imum load current' assumin# )*+ overload is& ,- A $-.)* ( ,- A% / 0-- A
Assumin# 0 A relay to be used' the CT ratio can be selected to be 0-- & 0. The plu# settin# 12 can be done at 0oo+' i.e. 12 / 0.- A.
$ii% At relay A the ma(imum load current' assumin# )*+ overload is&
$03-,-% -.)* $03-,-% / 4-- A
Assumin# 0 A relay to be used' the CT ratio can be selected to be 4-- & 0. The plu# settin# can be done at 0oo+' i.e. 12 / 0.- A.
$b% Decidin# the time!multiplier settin#s& 2tartin# from the most remote relay RB
$i% 2ince RB does not have to maintain selectivity with any other relay' it can be made to operate the fastest. Thus the TM2 of RB can be selected as -.0.
$ii% 5ow' to maintain selectivity between RA and RB' the followin# constraint must be met&
The operatin# time of RB for ma(imum fault 6ust beyond bus B can be found from
7or ma(imum fault at B' fault current / 4--- A on primary side which becomes $4---80--% / 4- A secondary. 2ince plu# settin# is done at 0.- A'
The TM2 of RB has already been set at -.0. 2ubstitutin# these values' we #et&
9et TCB'B/ -.* 2. Then' TR'B TCB'B / -.: s
This value of -.: s is the desired operatin# time of RA. Assumin# overshoot time of RA to be 0-+ of -.: s' i.e. TO2'A / -.-: s.
The TM2 of RA can be found from the followin# e(pression for the operatin# time of RA&
"e have' for RA' for the above condition' 12M / Irelay812 / $4---84--%80 / 0-. ;ence for RA
This finishes the settin# of both the relays
"e can verify that the selectivity for minimum fault at bus B is automatically maintained.
Thus' we #et the re<uired operatin# time of RA for ma(imum fault at bus B&
"e can deduce the followin# #eneral rules from the above settin# e(ercise&
0%
2
tart the settin# from the relay at the tail end of the system.)%
1
lu# settin# should be such that& I9'ma( = 12 = If'min at the end of ne(t section i.e. for plu# settin# consider minimum fault current at the end of the ne(t section.4%
T
M2 should be decided such that the selectivity with the ne(t relay downstream is maintained for ma(imum fault current at the be#innin# of the ne(t section.7or the system shown below' desi#n the complete OC protection usin# the IDMT relays. Thus' decide the CT ratios' the plu# settin#s and the TM2 at all locations.
9oad 00* A ,- A 0-- A :: A :-A Minimum fault current 0*-- A 0--- A :,- A *,* A 4>- A Ma(imum fault current 3--- A *--- A 4--- A )--- A 0--- A
er $-rrent "elays &ont/*
1. Instantaneous Over current Relay
2. Defnite Time Over current Relay
3. Inverse Defnite Minimum
Time Relay
Very Inverse Type
As previously iscusse! T"ese are t"e types o# over current relay$
Example $ An IDMT over current relay "as a current se%n& o# 1'() an a *me mul*plier se%n& o# (.+. T"e primary o# relay is connecte to seconary o# ,T "avin& ra*o -(('. ,alculate t"e *me o# opera*on i# t"e circuit carries a #ault current o# '((( A. T"e *me/current c"aracteris*cs o# t"e relay is s"o0n in t"e f&ure.
Sol et us calculate M frst!
4ault current in relay coil 5 actual #ault current 6 ,T ra*o
5 '((( 7 '-(( 5 +2.' A
,urrent se%n& 5 1'()51.' M5 +2.' '7 1.' 5 8.333
4rom 4i&ure! t"e approximate *me o# opera*on #or M 8 is 2 secons
Actual opera*on *me5 2 7 *me se%n& mul*plier 5 2 7 (.+
Example$ T"e 4i& s"o0s a part o# a typical po0er system. I# #or t"e iscrimina*on ! t"e *me &rain& mar&in 9et0een t"e relays is (.+ sec! calculate t"e *me o# opera*on o# relay 1 an *me se%n& mul*plier #or relay 2 . Re#er to c"aracteris*cs &iven in 4i&. T"e *me se%n& mul*plier o# relay 1 is (.3.
