For cases where the generation is practically constant and discrimination with low tripping times is difficult to
obtain, because of the low impedance per line section, an extr
emely inverse relay can be very useful since only a
small difference of current is necessary to obtain an adequate time difference.
Another application for this relay is with auto reclosers in low voltage distribution circuits. As the majority of faults
are of a transient nature, the relay is set to operate before the normal operating time of the fuse, thus preventing
perhaps unnecessary blowing of the fuse.
Upon reclosure, if the fault persists, the recloser locks itself in the closed position and allows the fuse to blow to
clear the fault.
This characteristic is also widely used for protecting plant against overheating since overheating is usually an I2t
function.
This characteristic curve can be defined by the mathematical expression:
UK Long Time Inverse
This type of characteristic has a long time characteristic and may be used for pr
otection of neutral earthing
resistors (which normally have a 30 second rating). The relay operating time at 5 times current setting is 30
seconds at a TMS of 1.
This can be defined by:
In the above equations:
● t
op
is the operating time
● T is the time multiplier setting
● I is the measur
ed current
● I
s
is the current threshold setting.
The ratio I/I
s
is sometimes defined as ‘M’ or ‘PSM’ (Plug Setting Multiplier).
These curves are plotted as follows:
Chapter 6 - Current Protection Functions P24xM
86 P24xM-TM-EN-2.1
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