ABB REL650 Applications Manual page 202

Section 7
Current protection
196
overcurrent protection. The minimum fault current Iscmin, to be detected by the
protection, must be calculated. Taking this value as a base, the highest pick up current
setting can be written according to equation 107.
£ ×
Ipu 0.7 Isc min
EQUATION1263 V2 EN
where:
• 0.7 is a safety factor
• Iscmin is the smallest fault current to be detected by the overcurrent protection
Im ax
× £ £ ×
1.2 Ipu 0.7 Isc min
k
EQUATION1264 V2 EN
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 1.2 k I
t sc max
high
EQUATION1265 V1 EN
where:
• 1.2 is a safety factor
• kt 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 1.1
• 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
plotted in a 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.
94 shows how the time-versus-current curves are
1MRK 506 334-UUS A
(Equation 112)
(Equation 113)
(Equation 114)
Application manual