ABB 615 series Technical Manual page 357

1MAC059074-MB A
615 series ANSI
Technical Manual
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 (-VG) 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 grounded through low resistance, the characteristic
angle is also 0 degrees (for phase angle). Alternatively, Iocos(φ) operation can be used.
In solidly grounded 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 grounded networks, the
phase angle is recommended.
Connection of measuring transformers in directional ground fault
applications
The residual current IG 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 grounded with high impedance, a core balance current transformer is
recommended to be used in ground-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 67/51N, 67/50N is able to detect the fault current direction without failure. As
directional ground fault uses residual current and residual voltage (-VG), the poles of the
measuring transformers must match each other and also the fault current direction. Also
the grounding 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.
Section 4
Protection functions
351