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7.1.2.3 Load encroachment

GUID-6785BF05-2775-4422-8077-A663D01C6C07 v6

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

54. 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 54. The load encroachment algorithm will increase

the possibility to detect high fault resistances, especially for phase-to-earth faults at

the remote line end. For example, for a given setting of the load angle ArgLd, the

resistive blinder for the zone measurement can be set according to figure 54

affording higher fault resistance coverage without risk for unwanted operation due

to load encroachment. Separate resistive blinder settings are available in forward

and reverse direction.

The use of the load encroachment feature is essential for long heavily loaded lines,

where there might be a conflict between the necessary emergency load transfer and

necessary sensitivity of the distance protection. The function can also preferably be

used on heavy loaded, medium long lines. For short lines, the major concern is to

get sufficient fault resistance coverage. Load encroachment is not a major

problem. .

ArgLd

RLdFw

IEC09000248-3-en.vsdx

RLdRv

[1]

IEC09000248 V3 EN-US

Figure 54: Load encroachment phenomena and shaped load encroachment

characteristic

[1]

[1] RLdRv=RLdRvFactor*RLdFw

Section 7 1MRK 505 393-UEN B

Impedance protection

112 Line differential protection RED650 2.2 IEC

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