Аннотация
The impedances of overhead transmission lines and cables are needed for distance protection, fault locators
and power system calculation. Inaccurate line impedances can cause malfunctions of distance protection,
wrong displayed fault locations and faulty calculation results. The required values in symmetrical components
are the positive sequence impedance, the zero sequence impedance and the zero sequence coupling
impedance (only for double circuit lines). These parameters can be determined by measurement or by
calculation. The calculation of line impedances is based on ideal geometrical assumptions like homogeneous
ground resistivity and continuous line geometry. Varying ground resistivity, buried conductors and a missing
earth conductor between two towers are not considered in the idealized impedance calculation, although
they have a strong impact on the real line impedance. Therefore, the calculation method does normally
not provide capable and accurate results which allow reliable protection settings and fault location. A high
accuracy of line impedance values can be received by measuring line impedances instead of calculating
them. Accurately measured zero sequence impedances allow reliable and correct operation of distance
protection and fault locators.
Measurements have shown that in a significant number of cases the k factor of the measured lines is deviating
more than 20% from its actual value. This can result in zone under- or overreach and consequently, the
selectivity is lost.
This paper presents an advanced method for measuring the transmission line impedance. Based on these
measurements, the relevant parameters for distance relays are calculated like the positive sequence and
zero sequence impedance values, as well as the grounding (K) factor.
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