ABB REF 610 Technical Reference Manual page 54

REF 610 REF 610
54
Feeder Protection Relay
Technical Reference Manual
RD-type characteristic (RXIDG)
The RD-type characteristic is a special characteristic, which is principally used in
earth-fault protection and which requires a high degree of selectivity even at high
resistance faults. The protection can operate in a selective way even if it is not
directional. Mathematically, the time/current characteristic can be expressed as
follows:
⎛
[ ]
= − ×
t s 5 8 1 35 . . log
⎜
e
⎝
k I
t = operate time
I = phase (or earth-fault) current value
k (or k ) = time multiplier
0
I> (or I >) = set start value
0
The actual operate time of the relay includes an additional filter and
detection time and the operate time of the trip output contact. When the
operate time of the relay is calculated as above, approximately 30 ms
should be added to the result.
If the ratio between the current and the set start value is higher than 40,
the operate time will be the same as when the ratio is 40.
The RD-type characteristic is illustrated in Fig. 5.1.4.9.-9.
⎞
I
⎟
× >
⎠
1MRS755310
(7)