Measurement Modes; Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.2.7.7
4.2.7.8
REX640
Technical Manual
Figure 276: Operation time curves for wattmetric IDMT for S

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-5FFA4516-E2E2-4631-9662-718099F70B3A V1 EN
Figure 277: 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
-Uo (Polarizing quantity)
Forward
area
Backward
area
Uo
Section 4
Protection functions
set at 0.15 xPn
ref
Io (Operating quantity)
Zero torque line
(RCA = 0˚)
) of the lines and the
0
). Often a resistor
0
505