ABB Relion 670 Series Technical Reference Manual page 157

1MRK 505 208-UEN B
Technical reference manual
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.
Phase-phase short circuit:
= -
U U U
refL L 1 2 L 1 L 2
EQUATION1449 V1 EN
= -
U U U
refL L 2 3 L 2 L 3
EQUATION1450 V1 EN
= -
U U U
refL L 3 1 L 3 L 1
EQUATION1451 V1 EN
Phase-earth short circuit:
Table continues on next page
= -
I I I
dirL L 1 2 L 1 L 2
= -
I I I
dirL L 2 3 L 2 L 3
= -
I I I
dirL L 3 1 L 3 L 1
Section 6
Current protection
(Equation 4)
(Equation 5)
(Equation 6)
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