ABB RELION REX640 Technical Manual page 512

Section 4
Protection functions
506
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
A
L R
U
L
0
I
L
GUID-9E30871C-AD03-4E45-986A-1B7834FD8E64 V1 EN
Figure 278: Typical radial compensated network employed with wattmetric
protection
The wattmetric function is activated when the residual active power component
exceeds the set limit. However, to ensure a selective operation it is also required that
the residual current and residual voltage also exceed the set limit.
It is highly recommended that core balance current transformers are used for
measuring Io when using the wattmetric method. When a low transformation ratio is
used, the current transformer can suffer accuracy problems and even a distorted
secondary current waveform with some core balance current transformers. Therefore,
to ensure a sufficient accuracy of the residual current measurement and consequently
a better selectivity of the scheme, the core balance current transformer should
preferably have a transformation ratio of at least 70:1. Lower transformation ratios
such as 50:1 or 50:5 are not recommended, unless the phase displacement errors and
current transformer amplitude are checked first.
It is not recommended to use the directional wattmetric protection in case of a ring or
meshed system as the wattmetric requires a radial power flow to operate.
The relay characteristic angle needs to be set based on the system earthing. In an
unearthed network, that is, when the network is only coupled to earth via the
capacitances between the phase conductors and earth, the characteristic angle is
chosen as -90º.
= I - I (I is the total earth-fault current of the network
Ctot Cfd Ctot
B C
ΣI
01
C
0
I
Cfd
ΣI
02
Ic = I
tot ef
1MRS759142 C
- U
0
R
0
ΣI
01
- U
0
ΣI
02
REX640
Technical Manual