Load Encroachment - ABB REL650 Applications Manual

1MRK 506 334-UUS A
6.1.2.7
Application manual
×
28707 L
=
Rarc
1.4
I
EQUATION1456 V1 EN
where:
L represents the length of the arc (in meters). This equation applies for the distance protection zone 1.
Consider approximately three times arc foot spacing for the zone 2 and wind speed of approximately
30 m/h
I is the actual fault current in A.
In practice, the setting of fault resistance for both phase to ground RFPGx and phase to
phase RFPPx (where x is 1-5 depending on selected zone) should be as high as possible
without interfering with the load impedance in order to obtain reliable fault detection.
In combined quadrilateral and mho characteristic, these settings are used specifically by
quadrilateral mode of operation. Dynamic mho characteristic self-expansion feature takes
care of the fault resistance.

Load encroachment

Five zone distance protection, quadrilateral and mho characteristic (ZQMPDIS, 21)
exhibits two important characteristics of the distance protection. These characteristics can
be separately configured by the setting CharPGZx (where x = 1-5) for phase to ground
loops and by CharPPZx (where x = 1-5) for phase to phase loops. This selectable feature
gives an additional flexibility for line protection.
In some cases the load impedance might enter the zone characteristic without any fault on
the protected line. The phenomenon is called load encroachment and it might occur when
an external fault is cleared and high emergency load is transferred on the protected line.
The effect of load encroachment is illustrated to the left in figure
entrance of the load impedance inside the characteristic is of course not allowed and the
way to handle this with conventional distance protection is to consider this with the
settings, that is, to have a security margin between the distance zone and the minimum
load impedance. This has the drawback that it will reduce the sensitivity of the protection,
that is, the ability to detect resistive faults.
The IED has a built in function which shapes the characteristic according to the left of
figure 61 and figure 62.
Section 6
Impedance protection
(Equation 48)
61 and figure 62. The
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