Load Encroachment - ABB REL650 Technical Manual

Section 6
Impedance protection
6.1.7.8
108

( I 3 0 I K
L 1
IEC15000057 V2 EN-US
Figure 40: Simplified offset mho characteristic and voltage vector for phase
L1-to-earth fault
Operation occurs if 90 °≤β≤270 °.

Load encroachment

In some cases the measured load impedance might enter the set zone characteristic
without any fault on the protected line. This phenomenon is called load
encroachment and it might occur when an external fault is cleared and high
emergency load is transferred onto the protected line. The effect of load
encroachment is illustrated on the left in figure 41. A load impedance within the
characteristic would cause an unwanted trip. The traditional way of avoiding this
situation is to set the distance zone resistive reach with a security margin to the
minimum load impedance. The drawback with this approach is that the sensitivity
of the protection to detect resistive faults is reduced.
The IED has a built in feature which shapes the characteristic according to the
characteristic shown in figure 41. The load encroachment algorithm will increase
I • jX
L1

 
( I 3 0 I K ) 1 Z
L 1 N set
U
 
) 1 Z
N set
1MRK 506 382-UEN A
  

U U ( I 3 0 I K
comp 1 L 1 L 1

1 L
   

U U ( ( I 3 0 I K
comp 2 L 1 L 1
IEC15000057-2-en.vsdx
GUID-6785BF05-2775-4422-8077-A663D01C6C07 v6
Line distance protection REL650 2.2 IEC
Technical manual

) 1 Z
N set

) 1 ) Z
N set
I • R
L1