en05000740_ansi.vsd
Direction
Element
4 step over current
element
One element for each
step
Harmonic
Restraint
Mode Selection
dirPhAFlt
dirPhBFlt
dirPhCFlt
harmRestrBlock
enableDir
enableStep1-4
DirectionalMode1-4
faultState
Element
faultState
I3P
V3P
I3P
PICKUP
TRIP
ANSI05000740 V1 EN
Figure 201: Functional overview of OC4PTOC (51/67)
A common setting for all steps, NumPhSel, is used to specify the number of phase
currents to be high to enable operation. The settings can be chosen: 1 out of 3, 2 out of
3 or 3 out of 3.
The sampled analog phase currents are processed in a pre-processing function block.
Using a parameter setting MeasType within the general settings for the four step phase
overcurrent protection 3-phase output function OC4PTOC (51/67), it is possible to
select the type of the measurement used for all overcurrent stages. It is possible to
select either discrete Fourier filter (DFT) or true RMS filter (RMS).
If DFT option is selected then only the RMS value of the fundamental frequency
components of each phase current is derived. Influence of DC current component and
higher harmonic current components are almost completely suppressed. If RMS option
is selected then the true RMS values is used. The true RMS value in addition to the
fundamental frequency component includes the contribution from the current DC
1MRK505222-UUS C Section 7
Current protection
383
Technical reference manual
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