ABB RELION 620 Series Technical Manual page 471

1MRS757644 F
620 series
Technical Manual
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-A524D89C-35D8-4C07-ABD6-3A6E21AF890E V1 EN
Figure 235: 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.
In compensated networks, the capacitive fault current and inductive resonance coil
current compensate each other, meaning that the fault current is mainly resistive and
= 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
Section 4
Protection functions
- U
0
R
0
ΣI
- U
0
ΣI
02
01
465