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component 3VGIGCosφ (φ is the angle between the polarizing quantity and operating
quantity compensated with a relay characteristic angle).
-VG (Polarizing quantity)
VG
Backward
area
Forward
area
IG (Operating quantity)
Zero torque line
(RCA = 0
˚)
Minimum
operate current
GUID-E5FD8983-CAD2-401F-A223-F6FB8AF0E121 V1 EN
Figure 202:
Characteristics of wattmetric protection
In a fully compensated radial network with two outgoing feeders, the ground-fault
currents depend mostly on the system ground 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-ground 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 ground capacitances of two healthy phases. The protection
relay in the healthy feeder tracks only the capacitive current flowing through its ground
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 ground-fault current of the network and I
Cfd
is the ground-fault current of the healthy feeder).
1MAC059074-MB A
Section 4
Protection functions
615 series ANSI
419
Technical Manual
Содержание Relion 615 series
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