In a comparator, the DFT or RMS values are compared to the set operation current
value of the function (I1>, I2>, I3> or I4>) for each phase current. If a phase
current is larger than the set operation current, outputs START, STx, STL1, STL2
and STL3 are activated without delay. 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 values for individually measured phase currents are 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 function can be directional.The direction of a fault is given as the current angle
in relation to the voltage angle. The fault current and fault voltage for the
directional function are dependent on the fault type. The selection of the measured
value (DFT or RMS) does not influence the operation of the directional part of
OC4PTOC. To enable directional measurement at close-in faults, causing a 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:
1 2 1 2 1 2 1 2
= - = -
refL L L L dirL L L L
U U U I I I
EQUATION1449 V1 EN-US (Equation 1)
2 3 2 3 2 3 2 3
= - = -
refL L L L dirL L L L
U U U I I I
EQUATION1450 V1 EN-US (Equation 2)
3 1 3 1 3 1 3 1
= - = -
refL L L L dirL L L L
U U U I I I
EQUATION1451 V1 EN-US (Equation 3)
Phase-earth short circuit:
1 1 1 1
= =
refL L dirL L
U U I I
EQUATION1452 V1 EN-US (Equation 4)
2 2 2 2
= =
refL L dirL L
U U I I
EQUATION1453 V1 EN-US (Equation 5)
3 3 3 3
= =
refL L dirL L
U U I I
EQUATION1454 V1 EN-US (Equation 6)
1MRK 505 384-UEN A Section 6
Current protection
Breaker protection REQ650 2.2 IEC 95
Technical manual
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