ABB RER620, Technical Manual

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Section 4
Protection functions
RER620 161
Technical Manual
Figure 73: Phase angle classic 88 amplitude
4.1.4.9 Application
The directional ground-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 start levels and time delays.
67/51N and 67/50N consists of two different stages:
• low (67/51N and 67/50N-1)
• high (67/50N-2)
67/51N and 67/50N-1 contain several types of time delay characteristics. 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.
20
30
40
50
60
70
80
90
100
/ % of
0 90 45 30 15 75 60 -90 -45 -30 -15 -75 -60
Min forward angle
10
88 deg
Forward
area
Non-
operating
area
85 deg
73 deg
100%
of
20%
of
1% of
0
I
n
I
n
I
n
I
n
I