ABB REQ650 - Four Step Residual Overcurrent Protection (EF4 PTOC)

Operating current
Current I
The IED does not reset
Line phase current
Time t
Reset current
IEC10000274-1-en.vsd
IEC10000274 V1 EN
Figure 53: Operating and reset current for an overcurrent protection
The lowest setting value can be written according to equation 15.
Imax
Ipu 1.2
k
³ ×
EQUATION1262 V2 EN (Equation 20)
where:
• 1.2 is a safety factor,
• k is the resetting ratio of the protection, and
• Imax is the maximum load current.
The maximum load current on the line has to be estimated. There is also a demand that
all faults, within the zone that the protection shall cover, must be detected by the phase
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
16.
Ipu 0.7 Iscmin
£ ×
EQUATION1263 V2 EN
(Equation 21)
where:
• 0.7 is a safety factor
• Iscmin is the smallest fault current to be detected by the overcurrent protection
1MRK 505 291-UEN A Section 6
Current protection
109
Application manual