ABB RELION 620 Series Technical Manual page 470

Section 4
Protection functions
4.2.7.6
4.2.7.7
464
Measurement modes
The function operates on three alternative measurement modes: "RMS", "DFT" and
"Peak-to-Peak". The measurement mode is selected with the Measurement mode
setting.
Application
The wattmetric method is one of the commonly used directional methods for detecting
the earth faults especially in compensated networks. The protection uses the residual
power component 3UoIoCosφ (φ is the angle between the polarizing quantity and
operating quantity compensated with a relay characteristic angle).
Minimum
operate current
GUID-4E73135C-CEEF-41DE-8091-9849C167C701 V1 EN
Figure 234: Characteristics of wattmetric protection
In a fully compensated radial network with two outgoing feeders, the earth-fault
currents depend mostly on the system earth capacitances (C
compensation coil (L). If the coil is tuned exactly to the system capacitance, the fault
current has only a resistive component. This is due to the resistances of the coil and
distribution lines together with the system leakage resistances (R
(R ) in parallel with the coil is used for increasing the fault current.
L
When a single phase-to-earth fault occurs, the capacitance of the faulty phase is
bypassed and the system becomes unsymmetrical. The fault current is composed of
the currents flowing through the earth capacitances of two healthy phases. The
protection relay in the healthy feeder tracks only the capacitive current flowing
through its earth capacitances. The capacitive current of the complete network (sum
of all feeders) is compensated with the coil.
-Uo (Polarizing quantity)
Forward
area
Backward
area
Uo
1MRS757644 F
Io (Operating quantity)
Zero torque line
(RCA = 0˚)
) of the lines and the
0
). Often a resistor
0
620 series
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