ABB RELION REX640, Техническое руководство, Страница: 538 / 1960

A B C
U
0
C
0
ΣI
01
ΣI
02
- U
0
ΣI
02
R
0
L R
L
- U
0
ΣI
01
I
L
I
Cfd
Ic
tot
= I
ef
Figure 311: 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 -90°.
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 0°. Often the magnitude of an active component
is small and must be increased by means of a parallel resistor in a compensation
coil. In networks where the neutral point is earthed through a low resistance, the
characteristic angle is always 0°.
As the amplitude of the residual current is independent of the fault location, the
selectivity of the earth-fault protection is achieved with time coordination.
Protection functions 1MRS759142 F
538 REX640
Technical Manual