Step 1 of NS2PTOC can operate in the Definite Time (DT) or Inverse Time (IDMT)
mode depending on the selected value for the CurveType1 parameter. If CurveType1=
Definite, NS2PTOC operates with a Definite Time Delay characteristic and if
CurveType1 = Inverse, NS2PTOC operates with an Inverse Time Delay
characteristic. Step 2 is operating in an analogous way as Step 1.
Definite time delay is not dependent on the magnitude of measured negative sequence
current. Once the measured negative sequence current exceeds the set level, the
settable definite timer t1 or t2 respectively, starts to count and the corresponding trip
signal gets activated after the pre-set definite time delay has elapsed. Reset time in
definite time mode is determined by the setting parameters tResetDef1 or tResetDef2
respectively. If NS2PTOC has already started but not tripped and measured negative
sequence current drops below the start value, the start outputs remains active for the
time defined by the resetting parameters.
A BLOCK input signal resets NS2PTOC momentarily.
When the parameter CurveType1 is set to Inverse, an inverse curve is selected
according to selected value for parameter K1. The minimum trip time setting of
parameter t1Min and reset time parameter ResetMultip1 also influence step operation.
However, to match the heating characteristics of the generator, the reset time is
depending on the setting of parameter K1, which must be set according to the
generators negative sequence current capacity.
EQUATION2112 V1 EN
Where:
I
2
is negative sequence current expressed in per unit of the rated generator current
t is operating time in seconds
K is a constant [s], which depends on generator size and design
1MRK502052-UEN B Section 8
Current protection
439
Technical manual
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