reference residual voltage (-U
0
). In compensated networks, the phase angle criterion
with extended operating sector can also be used. When the relay characteristic angle
RCA is 0 degrees, the negative quadrant of the operation sector can be extended with
the Min forward angle setting. The operation sector can be set between 0 and -180
degrees, so that the total operation sector is from +90 to -180 degrees. In other words,
the sector can be up to 270 degrees wide. This allows the protection settings to stay
the same when the resonance coil is disconnected from between the neutral point and
earth.
System neutral earthing is meant to protect personnel and equipment and to reduce
interference for example in telecommunication systems. The neutral earthing sets
challenges for protection systems, especially for earth-fault protection.
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 (-U
0
).
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 (-U
0
)
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, I
0
cos(φ)
operation can be used.
In solidly earthed networks, the Characteristic angle is typically set to +60 degrees
for the phase angle. Alternatively, I
0
sin(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 I
0
sin(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 I
0
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
1MRS756378 D Section 5
Protection functions
REF615 153
Application Manual
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