ABB RELION REF615R, Техническое руководство

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4.1.5.9 Application
The directional earth-fault protection 67/51N and 67/50N is designed for protection and
clearance of ground faults and for ground-fault protection of different equipment
connected to the power systems, such as shunt capacitor banks or shunt reactors, and for
backup ground-fault protection of power transformers.
Many applications require several steps using different current pickup levels and time
delays. 67/51N and 67/50N consist of two different stages.
• Low 67/51N
• High 67/50N-1 and 67/50N-2
67/51N contains several types of time delay characteristics. 67/50N-1 and 67/50N-2 is
used for fast clearance of serious ground faults.
The protection can be based on the phase angle criterion with extended operating sector.
It can also be based on measuring either the reactive part I
0
sin(φ) or the active part
I
0
cos(φ) of the zero sequence current. In isolated networks or in networks with high
impedance grounding, the phase-to-ground fault current is significantly smaller than the
short-circuit currents. In addition, the magnitude of the fault current is almost independent
of the fault location in the network.
The function uses the zero sequence current components I
0
cos(φ) or I
0
sin(φ) according to
the grounding method, where φ is the angle between the zero sequence current and the
reference zero sequence voltage (-V
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 ground.
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
Section 4
1MRS240050-IB C
Protection functions
196 REF615R
Technical Manual