ABB RET650 Technical Manual page 154

Section 8
Current protection
faultState
I3P
U3P
IEC05000740 V2 EN
Figure 64: Functional overview of OC4PTOC
148
dirPh1Flt
Direction
Element
dirPh2Flt
dirPh3Flt
Harmonic
harmRestrBlock
Restraint
Element
enableDir
Mode Selection
enableStep1-4
DirectionalMode1-4
The sampled analogue 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, 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.
In a comparator, for each phase current, the DFT or RMS values are compared to the
set operation current value of the function (I1>, I2>, I3> or I4>). If a phase current
is larger than the set operation current, outputs START, STx, STL1, STL2 and STL3
are, without delay, activated. Output signals STL1, STL2 and STL3 are common for
all steps. This means that the lowest set step will initiate the activation. The START
1MRK 504 135-UEN A
4 step over current
element
One element for each
step
en05000740-2-en.vsd
faultState
START
TRIP
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