Protection Functions
PRS-7367 219
The judgment condition of CT disconnection is that the maximum value of three-phase current
on one side is more than 0.12In and the minimum value of three-phase current is less than 0.04In.
1. After being judged as CT disconnection: a delay of 5 seconds will result in a CT
disconnection alarm signal.
2. After being judged as CT disconnection: if the differential current exceeds the limit and all
channel currents do not increase, the decision to block the ratio differential protection is based on
the control word.
XOR
&
&
&
FLG Diff Offlimit and No Current Up
87MP_CTS_EnableSET
Min(I1a,I1b,I1c)<0.04*InANA
Max(I1a,I1b,I1c)>0.12*InANA
Max(I2a,I2b,I2c)>0.12*InANA
Min(I2a,I2b,I2c)<0.04*InANA
87MPCTS_CTAlm
SIG
87MPCTS_CTBrk
SIG
0 5s
&
Figure 3.46-7 The Logic of CT supervision alarm and blocking
3.46.6 CT saturation blocking
To prevent differential protection from mis-operation due to CT saturation during external
faults, the device has a CT saturation detection element to determine whether the CT is saturated
and whether the relevant differential protection is locked. CT saturation determines the second
harmonic, as detailed in Three phase inrush function OCR_INR.
3.46.7 Overall Logic
SIG 87MP_HS_StrA
SIG 87MP_HS_StrB
SIG 87MP_LS_StrB
SIG 87MP_LS_StrA
SIG 87MP_HS_StrC
SIG 87MP_LS_StrC
1
1
1
1
87MP_Str
SIG
Figure 3.46-8 The Logic of overall start up
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