ABB Relion 670 Series Technical Reference Manual page 172

Section 5
Differential protection
166
Phase-to-phase current variation
Phase-to-phase current variation one is main startup element. It covers most of the
abnormal status of the system. The phase-to-phase current variation fails in high
impedance faults, three-phase fault on weak side and switch onto fault on unloaded line
because of low sensitivity in these cases.
Phase-to-phase current variation takes the current samples as input and the signal is
evaluated using the sampling value based algorithm.
The phase-to-phase current variation criterion is shown below:
D > D + D
i 1.8 I I
FF
T ZD
EQUATION2255 V1 EN
Where:
Δi
sampling value of phase-to-phase current variation
ФФ
ΔI
setting of fixed threshold, which corresponds to setting
ZD
setting is 0.2·
ΔI
float threshold
T
It is the full-circle integral of the phase-to-phase current variation
-
1 2 T 1
å
D = D
I | i ( t n
FF
T
T
=
n T
EQUATION2256 V1 EN
Where:
T count of sample values in one cycle
Δi is calculated using the below formula:
ФФ
D = - -
i k ( ) [ ( ) i k i k N (
= - -
i k ( ) 2 ( i k N
EQUATION2257 V1 EN
N is the number of samples in one cycle.
IBase , where IBase is the base current.
-
) |
- - -
)] [ ( i k N ) i k (
+ -
) i k ( 2 ) N
1MRK505222-UUS C
ICV> . The default value for the
-
2 )] N
Technical reference manual