4.2.2.9
Application
The directional ground-fault protection 67/51N, 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, 67/50N consist of two different stages.
•
Low 67/51N
•
High 67/50N
67/51N contains several types of time delay characteristics. 67/50N 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 Iosin(φ) or the active part
Iocos(φ) of the residual 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 residual current components Iocos(φ) or Iosin(φ) according to the
grounding method, where φ is the angle between the residual current and the reference
residual voltage (-VG). 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 residual current is mainly
capacitive and has -90 degrees phase shift compared to the residual voltage (-VG). 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
Section 4
1MAC059074-MB A
Protection functions
350
615 series ANSI
Technical Manual
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