ABB REL 356

ABB REL 356 Current Differential Protection
4-2
Measuring Elements and Operational Logic
IA
IB
IC
A/D
IT
DFT cosine
filter
Im(ITL) to transmitter
LDT
OP = |ITL + ITR|
RES = |ITL| + |ITR|
IF [OP - 0.7RES] > OTH
THEN trip
OTH
Im(IRL) from receiver
PLT (trip command)
Figure 4-2. Current Differential Logic
Operate quantity
[][ ]
22
)()()()( ITRimITLimITRreITLreITRITLOP +++=+=
Restraint quantity
[][][][]
2222
)()()()( ITRimITRreITLimITLreITRITLRES +++=+=
Trip decision
IF [(OP – 0.7RES) > OTH] THEN TRIP
Where
ITL = local quantity
ITR = remote quantity
OTH = set operating threshold