ABB RELION REX640, Техническое руководство, Страница: 537 / 1960

4.2.7.7 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.
4.2.7.8 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).
-Uo (Polarizing quantity)
Uo
Backward
area
Forward
area
Io (Operating quantity)
Zero torque line
(RCA = 0 ˚)
Minimum
operate current
Figure 310: 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
0
) of the lines and the
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
0
). Often a
resistor (R
L
) in parallel with the coil is used for increasing the fault current.
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.
A typical network with the wattmetric protection is an undercompensated network
where the coil current I
L
= I
Ctot
- I
Cfd
(I
Ctot
is the total earth-fault current of the
network and I
Cfd
is the earth-fault current of the healthy feeder).
1MRS759142 F Protection functions
REX640
Technical Manual
537