ANSI05000740-2-en.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
PICKUP
TRIP
ANSI05000740 V2 EN-US
Figure 247: Functional overview of OC4PTOC (51/67)
M12883-16 v10
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 1 and 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 component as well as from higher current harmonic. The
selected current values are fed to OC4PTOC (51/67).
In a comparator, the DFT or RMS values are compared to the set operation current value of the
function
(Pickup1, Pickup2, Pickup3, Pickup4)
for each phase current. If a phase current is larger
than the set operation current, outputs PICKUP, PU_STx, PU_A, PU_B and PU_C are activated
without delay. Output signals PU_A, PU_B and PU_C are common for all steps. This means that the
lowest set step will initiate the activation. The PICKUP signal is common for all three phases and all
Section 8 1MRK 502 066-UUS B
Current protection
460
Technical manual
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