A common setting for all steps,
StartPhSel
, 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 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
signal is common for all three phases and all steps. It shall be noted that the selection
of measured value (DFT or RMS) do not influence the operation of directional part of
OC4PTOC.
Service value for individually measured phase currents are also available on the local
HMI for OC4PTOC function, which simplifies testing, commissioning and in service
operational checking of the function.
A harmonic restrain of the function can be chosen. A set 2nd harmonic current in
relation to the fundamental current is used. The 2nd harmonic current is taken from the
pre-processing of the phase currents and the relation is compared to a set restrain
current level.
The function can be directional. The direction of the fault current is given as current
angle in relation to the voltage angle. The fault current and fault voltage for the
directional function is dependent of the fault type. To enable directional measurement
at close in faults, causing low measured voltage, the polarization voltage is a
combination of the apparent voltage (85%) and a memory voltage (15%). The
following combinations are used.
Section 8
1MRK502052-UEN B
Current protection
346
Technical manual
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