A
B
C
VG
C
0
Σ
IG
1
Σ
IG
2
- VG
Σ
IG
2
IG
f
R
0
L
R
L
- VG
Σ
IG
1
GUID-E2D023A9-1EDC-4C78-8541-2F92D3536F0B V1 EN
Figure 203:
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 trip, 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
IG 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 grounding. In an
ungrounded network, that is, when the network is only coupled to ground via the
capacitances between the phase conductors and ground, 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 has
zero phase shift compared to the residual voltage. In such networks, the characteristic
angle is chosen as 0º. Often the magnitude of an active component is small and must be
Section 4
1MAC059074-MB A
Protection functions
420
615 series ANSI
Technical Manual
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