ABB 615 Series ANSI, Техническое руководство, Страница: 193 / 713

Section 4
1MAC050144-MB C
Protection functions
186 615 series ANSI
Technical Manual
System neutral grounding is meant to protect personnel and equipment and to reduce
interference for example in telecommunication systems. The neutral grounding sets
challenges for protection systems, especially for ground-fault protection.
In isolated networks, there is no intentional connection between the system neutral point
and ground. The only connection is through the line-to-ground capacitances (C
0
) of phases
and leakage resistances (R
0
). This means that the zero sequence current is mainly
capacitive and has –90 degrees phase shift compared to the zero sequence voltage (-V
0
).
The characteristic angle is -90 degrees.
In resonance-grounded 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 zero sequence current is mainly resistive and has zero
phase shift compared to the zero sequence voltage (-V
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 grounded through low resistance, the characteristic
angle is also 0 degrees (for phase angle). Alternatively, I
0
cos( φ ) operation can be used.
In solidly grounded 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 zero sequence voltage measurement
wires. Although the I
0
sin( φ ) operation can be used in solidly grounded networks, the phase
angle is recommended. In some applications, negative sequence polarization is preferred
over zero sequence polarization. The IED also offers negative sequence polarization
option where users can set the angle between V
2
and I
2
based on their application and
practice.
Connection of measuring transformers in directional ground fault
applications
The zero sequence 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 grounded with high impedance, a core balance current transformer is
recommended to be used in ground-fault protection. To ensure sufficient accuracy of zero
sequence 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 and 67/50N is able to detect the fault current direction without failure. As
directional ground fault uses zero sequence current and zero sequence voltage (-V
0
), the
polarities 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 consideration when
using core balance current transformers. The following figure describes how measuring
transformers can be connected to the IED.