ABB RELION Series Technical Manual page 375

1MRS758755 A
REC615 and RER615
Technical Manual
Minimum
operate current
GUID-4E73135C-CEEF-41DE-8091-9849C167C701 V1 EN
Figure 193: 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.
A typical network with the wattmetric protection is an undercompensated network
where the coil current I
L
and I is the earth-fault current of the healthy feeder).
Cfd
-Uo (Polarizing quantity)
Forward
area
Backward
area
Uo
= I - I (I is the total earth-fault current of the network
Ctot Cfd Ctot
Section 4
Protection functions
Io (Operating quantity)
Zero torque line
(RCA = 0˚)
) of the lines and the
0
). Often a resistor
0
369