13.8.4.1 Introduction
Sudden change in current variation (SCCVPTOC, 51) function is a fast way of finding
any abnormality in line currents. When there is a fault in the system, the current
changes faster than the voltage. SCCVPTOC (51) finds abnormal condition based on
phase-to-phase current variation. The main application is as a local criterion to increase
security when transfer trips are used.
13.8.4.2 Principle of operation
Sudden change in current variation (SCCVPTOC, 51) function calculates the variation
in phase-to-phase current and gives the RI output when this variation crosses the sum
of start level and float threshold for a time of tDelay. The variation is calculated for all
the three phase-to-phase currents.
The variation in the current is calculated using the following equation:
( ) ( ) ( )
2 2i i t i t T i t TD = - × - + -
EQUATION2252 V1 EN
Where:
i(t)
Amplitude of the current at the present instant
i(t-T)
Amplitude of the current at the instant exactly one cycle time before
i(t-2T)
Amplitude of the current at the instant exactly two cycle time before
Criteria:
Di > i (1.8 · DI
T
+ IPickup )
EQUATION2253-ANSI V1 EN (Equation 158)
IPickup: pickup level
It is the full-circle integral of the phase-to-phase current variation
( )
2 1
1
T
T
n T
I i t n
T
-
=
D = D -
å
EQUATION2254 V1 EN
(Equation 159)
T: the count of the sampling value in one cycle
Section 13 1MRK505222-UUS C
Scheme communication
840
Technical reference manual
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