Trapezoidal logic control module PFSK164

Trapezoidal logic control module PFSK164

Trapezoidal logic control module PFSK164

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 125.

The 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 right figure of figure 125.

The load encroachment algorithm will increase the possibility to detect high fault resistances, especially for phase-to-earth faults at remote line end. For example, for a given setting of the load angle ArgLd for Phase selection with load encroachment, quadrilateral characteristic function (FDPSPDIS), the resistive blinder for the zone measurement can be expanded according to the figure 125 given higher fault resistance coverage without risk for unwanted operation due to load encroachment. This is valid in both directions.

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.