20
30
40
50
70
80
90
100
Io / % of I
n
0
90
45
30
15
75
60
-90
-45 -30 -15
-75 -60
Min forward angle
10
88 deg
85 deg
73 deg
100% of In
20% of In
1% of In
Figure 156: Phase angle 88 amplitude ( Directional mode = Forward)
4.2.2.9
Application
The directional earth-fault protection (F)DEFxPDEF is designed for protection and
clearance of earth faults and for earth-fault protection of different equipment
connected to the power systems, such as shunt capacitor banks or shunt reactors,
and for backup earth-fault protection of power transformers.
Many applications require several steps using different current start levels and time
delays. (F)DEFxPDEF consists of two different stages.
• Low (F)DEFLPDEF
• High DEFHPDEF
(F)DEFLPDEF contains several types of time delay characteristics. DEFHPDEF is
used for fast clearance of serious earth 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 earthing, the phase-to-earth 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 earthing method, where φ is the angle between the residual current
and the reference residual voltage (-Uo). 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 earth.
System neutral earthing is meant to protect personnel and equipment and to
reduce interference for example in telecommunication systems. The neutral earthing
sets challenges for protection systems, especially for earth-fault protection.
1MRS758755 C
Protection functions
REC615 & RER615
Technical Manual
317
