ABB REL650 series Technical Manual page 206

Section 7
Current protection
200
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, for each phase current, the DFT or RMS values are compared to the set
operation current value of the function (Pickup1, Pickup2, Pickup3, Pickup4). If a phase
current is larger than the set operation current, outputs PICKUP, PU_STx, PU_A, PU_B
and PU_C are, without delay, activated. 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 steps. It shall be noted that the
selection of measured value (DFT or RMS) do not influence the operation of directional
part of OC4PTOC (51/67) .
Service value for individually measured phase currents are also available on the local HMI
for OC4PTOC (51/67) 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:
= -
V V V
ref _ AB A B
GUID-4F361BC7-6D91-47B5-8119-A27009C0AD6A V1 EN
= -
V V V
ref BC _ B C
ANSIEQUATION1450 V1 EN
= -
V V V
ref CA _ C A
ANSIEQUATION1451 V1 EN
Table continues on next page
= -
I I I
dir AB _ A B
= -
I I I
dir BC _ B C
= -
I I I
dir CA _ C A
1MRK 506 335-UUS A
(Equation 41)
(Equation 42)
(Equation 43)
Technical manual