In isolated networks, there is no intentional connection between the system neutral
point and earth. The only connection is through the line-to-earth capacitances (C
0
) of phases and leakage resistances (R
0
). This means that the residual current is
mainly capacitive and has -90 degrees phase shift compared to the residual voltage
(-Uo). The characteristic angle is -90 degrees.
In resonance-earthed networks, the capacitive fault current and the inductive
resonance coil current compensate each other. The protection cannot be based
on the reactive current measurement, since the current of the compensation coil
would disturb the operation of the relays. In this case, the selectivity is based on the
measurement of the active current component. This means that the residual current
is mainly resistive and has zero phase shift compared to the residual voltage (-Uo)
and the characteristic angle is 0 degrees. Often the magnitude of this component
is small, and must be increased by means of a parallel resistor in the compensation
equipment.
In networks where the neutral point is earthed through low resistance, the
characteristic angle is also 0 degrees (for phase angle). Alternatively, Iocos(φ)
operation can be used.
In solidly earthed networks, the
Characteristic angle is typically set to +60 degrees
for the phase angle. Alternatively, Iosin(φ) operation can be used with a reversal
polarizing quantity. The polarizing quantity can be rotated 180 degrees by setting
the
Pol reversal parameter to "True" or by switching the polarity of the residual
voltage measurement wires. Although the Iosin(φ) operation can be used in solidly
earthed networks, the phase angle is recommended.
Connection of measuring transformers in directional earth fault applications
The residual current Io can be measured with a core balance current transformer or
the residual connection of the phase current signals. If the neutral of the network is
either isolated or earthed with high impedance, a core balance current transformer
is recommended to be used in earth-fault protection. To ensure sufficient accuracy
of residual current measurements and consequently the selectivity of the scheme,
the core balance current transformers should have a transformation ratio of at least
70:1. Lower transformation ratios such as 50:1 or 50:5 are not recommended.
Attention should be paid to make sure the measuring transformers are connected
correctly so that DEFxPDEF is able to detect the fault current direction without
failure. As directional earth fault uses residual current and residual voltage (-Uo),
the poles of the measuring transformers must match each other and also the fault
current direction. Also the earthing of the cable sheath must be taken into notice
when using core balance current transformers. The following figure describes how
measuring transformers can be connected to the protection relay.
Protection functions
1MRS758755 C
318
REC615 & RER615
Technical Manual
