G EK-49849
The choi ce between very i nverse and extremel y i nverse t i me re
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ays i s more
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i m i ted than between them and the i nver s e t ime re
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ay as they are more near
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y a
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i ke
i n thei r t i me-current character i st i c curves .
For grad i ng wi th fuses, the extremel y
i n verse t i me rel ay shou l d be chosen as the t i me-current curves more near l y match the
fuse curve .
Another advant age of the extreme l y i nverse rel ay i s t h at i t i s better
s u i ted than both the i nverse and very i nverse re l ays for pi cki ng up co l d l oad .
For
any g i ven co l d l oad pi ckup c apabi l i ty, the resu l t i ng sett i ngs wi l l provi de f aster
pro tect i on at h i gh f au l t currents wi th the extremel y i nverse rel ay t h an wi th the
l es s i nvers e rel ays .
The operat i ng t i me of the t i me overcurrent u n i t for any g i ven val ue of current
and t ap setti ng i s determ i ned by the t i me d i al sett i ng .
The operat i ng t i me i s
i nverse l y proport i on a l to the current magn i tu de as i l l u s tr ated by the t i me curves i n
F i g . 12, 13 and 1 4 .
Note that the current val ues on these curves are g i ven as
mu l t i pl es of the tap sett i ng . That i s, for a g i ven t i me d i a l s ett i ng, the t i me wi l l
be the s ame for 80 amperes on the e i ght ampere t ap as for 50 amperes on the f i ve
ampere tap, s i nce i n both cases, the current i s ten t imes tap s ett i ng .
If s e l ecti ve act i on of two or more rel ays i s requ i red, determ i ne the m ax i mum
pos s i bl e short-c i rcu i t current of the
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i ne and then choose a t i me val ue for e ach
re l ay that d i ffers suff i c i en t l y to i n sure the proper sequence i n the operat i on of
severa l c i rcu i t bre akers .
A l l owance must be made for the t i me i nvo l ved i n ope n i n g
each bre aker after t h e rel ay cont acts c l os e .
The i nstantaneous cup un i t i s torque contro l l ed by the d i rect i on a l u n i t . When
it is used for d i rect tri ppi ng, i t wi l l on l y be necess ary when determ i n i ng a sett i ng
to con s i der the maxi mum current that the i n stantaneous u n i t wi l l see for a f au l t at
the remote term i n a l .
The i nstantaneous cup u n i t h as l ow tran s i en t overreach .
It
shou l d be set wi th a mar g i n of at l east ten percent above the max imum current for a
remote f au l t negl ect i ng tran s i ent overre ach .
The Y1A re l ays contai n a H i -Sei smi c i nstantaneous overcurrent u n i t . Th i s u n i t
may be s et h i gh to provide d i rect tr i ppi ng for heavy i nter n a l f au l ts .
In
determ i n i ng the sett i n g for thi s u n i t when i t i s used for d i rect tri ppi ng, i t wi l l
be necess ary to con s i der the maximum extern al f au l t for f au l ts at e ach end of the
l i ne because the u n i t i s non - d i rect i on a l .
The u n i t s hou l d be set wi th a s u i tabl e
margin above the maxi mum
external fault taking into
account the effects of trans i en t
overreach a s i l l ustrated i n F i g . 17 .
The red jumper l eads between studs 18 and 1 9 and 19 and 20 are l ocated i n s i de
the c ase on the crad l e bl ock and may be removed to provi de exter n a l torque contro l
of the t i me overcurrent u ni t (TOC) and the i nstantaneous overcurrent u n i t ( IOC)
respe cti ve l y .
If external torque control of e i ther or bot h u n i ts i s requ i red,
remove the red j umper l ead assoc i ated wi th the u n i t (s ) to be contro l l ed, and connect
the extern a
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control contacts between the appropr i ate studs .
Note that the u n i t s
wi l l sti l l be torque contro l l ed by the d i rect i onal u n i t i n add i t i on t o the external
contro l .
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