GEK-34054
APPLICATION
Ti me ove r c ur rent r e lays a r e used e xtensively for the protecti on
of uti li ty and i nd us tr ial d i str i buti on systems , and frequen tly for
ove r load ba ckup pr otecti on at other locati on s .
The i r ve r y - i n ve rs e
ti me-cur rent characte r i s ti c makes
the s e
re la y s
we ll
s ui te d for
a pplica tion
i n
locati ons
where
the
fault-c urren t mag n itude
i s
dependent ma i n ly upon the locati on of the fault r e lati ve to the relay
a nd only slightly upon the system generating setup at the ti me of the
fault.
Use of relays havi ng the ve ry- i n ve r se chara c te r i s ti c i n this
s i tuation wi ll us ua lly r e s ult· i n fa s te r clea r ing ti me s tha n if the
i nve r s e r e lay we r e us ed.
The gene r a l pr acti ce is to use a set of two or thr e e r elay s f or
i nte r phase faults a nd a sepa rate r e lay, r e s i d ua lly c onnec ted, f or
s i ng le-pha se-to-g r ound
fa ults.
Typi cal c onnec ti ons
for
s uch
a n
a ppli cati on are shown i n Figure 1 5.
Use of a s e pa ra te g round re lay
i s advantageous becaus e i t can be adjusted to provide more s e n s i t i ve
pr otecti on on g r ound faults.
In
the a pplicati on
of
these relays wi th
a utomati c
rec los ing
devi c e s , the r es e t ti me sh ould be cons i d e r ed.
The res e t ti me of a ll
IAC53 a nd IAC54 re lays c ove r ed by these ins tr uc tions i s a pproximately
60 seconds , fr om the f ully closed to the f ully open pos i ti on , whe n
s e t a t the n umbe r 10 time d i a l.
When s e tti ng the se r e lays to c oor d i na te wi th " d owns tr ea m" rela y s ,
a
coord i na ti on
time
of
f r om 0. 25 t o . 040 sec onds
i s
gene r a lly
a llowed, depend i ng on the clea r ing ti me of the breake r i nvolved.
These c oor d i nation ti mes i nclude,
in add i ti on to breake r- c learing
ti me , 0. 1 0 second for r e lay ove r tr avel and 0. 17 s ec ond f or s a f e ty
fac tor .
For example, i f the br eake r -clea r i ng time i s 0. 1 3 s e c ond (8
cycles ) , the c oor d i nati on ti me would be 0. 40 secon d .
If the relay
ti me i s set by te s t at the c ur r ent level i n que s ti on , the s a f e ty
factor may be r educed f r om 0. 1 7 to 0. 07 second.
Then, wi th relay
overtravel of
.10
second,
if the
"downstream"
breaker time is
5
cycle s (0. 08 s ec onds ) a min i mum
of
0.25 seconds c ould be allowed f or
coor d i nati on.
CONSTRUCTION
The
induction unit
is
the basic unit in all
Type
IAC
r elays.
Fi g ur e s
3
a nd 4 s h ow the inducti on un i t mounted
in
the cradle.
The s e
un i ts a re of the i nduction-di s k-c ons truc ti on type.
The
disk i s
actuated by a curre nt-ope rati ng c oi l on a lami nated U-mag n e t.
The
d i s k s haft ca r r i e s the movi ng c ontact that complete s the a la r m or
tr i p c i r cui t when i t touches the stati ona ry c ontact or c on ta c ts .
The
d i s k s haft i s r e s tra i ned by a spira l s pri ng to g i ve the proper
c ontact-closing current, and its moti on i s r eta r ded by a perman e n t
magnet acti ng on the d i s k to g i ve the c or r ect time d e lay.
The r e is a target a nd seal-i n un i t mounted on the fr ont, to the
left of the shaft of the ti me-ove r cur rent unit.
The seal-i n un i t has
its c oi l i n s e r i e s and i ts contacts in pa r a l l el wi th the c on ta c ts of
the ti me - ove r cur r ent un i t, s uch tha t when the i nducti on -un i t c onta c ts
close, the seal- i n un i t pi c ks up and seals i n.
When the s ea l- i n un i t
4
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