ABB Relion REC670 Applications Manual page 118

Section 7
Current protection
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary the operating current shall be chosen within the interval stated in equation 31.
Im ax
×
1.2
k
EQUATION1264 V2 EN-US
The high current function of the overcurrent protection, which only has a short delay of the
operation, must be given a current setting so that the protection is selective to other
protection in the power system. It is desirable to have a rapid tripping of faults within as large
portion as possible of the part of the power system to be protected by the protection (primary
protected zone). A fault current calculation gives the largest current of faults, Iscmax, at the
most remote part of the primary protected zone. Considerations have to be made to the risk
of transient overreach, due to a possible DC component of the short circuit current. The lowest
current setting of the most rapid stage, of the phase overcurrent protection, can be written
according to
³
I
high
EQUATION1265 V1 EN-US
where:
1.2
k
t
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate times of the phase overcurrent protection has to be chosen so that the fault time
is so short that protected equipment will not be destroyed due to thermal overload, at the
same time as selectivity is assured. For overcurrent protection, in a radial fed network, the
time setting can be chosen in a graphical way. This is mostly used in the case of inverse time
overcurrent protection. Figure
diagram. The time setting is chosen to get the shortest fault time with maintained selectivity.
Selectivity is assured if the time difference between the curves is larger than a critical time
difference.
112
×
is a safety factor
£ £ ×
Ipu 0.7 Isc min
× ×
1.2 k I
t sc max
is a safety factor
is a factor that takes care of the transient overreach due to the DC component of the fault current
and can be considered to be less than 1.05
46 shows how the time-versus-current curves are plotted in a
1MRK 511 358-UEN A
(Equation 30)
(Equation 31)
(Equation 32)
Application manual