Abstract
When a distance relay protects a transmission line located on a dual circuit tower, a coupling effect will occur between the two circuits. Transposition of the circuits can reduce the mutual impedances, but this does not cater to the zero-sequence mutual coupling impedance during earth faults. As a result, the impedance measured by a distance relay under phase-to-earth fault conditions in these circumstances will not represent the correct impedance to the fault point unless these effects are taken into account. On multi-circuit lines, primarily if they operate in parallel, a zero-sequence mutual coupling should be considered when calculating settings for distance protection function. A 220 kV parallel line sharing the same tower was analysed using DigSilent Power Factory in the simulations. Phase-to-earth faults in different configurations were analysed on this system, and the reach of the protection relay was then estimated for operation. The results confirm how a protection relay can overreach and underreach in a distance protection scheme due to the influence of mutual coupling.
Highlights
Mutual coupling of overhead lines occurs when two or more circuits are in close proximity in the same corridor
Distance relays must be compensated against the coupling effect between parallel lines since the current flowing in one phase of one line induces voltage onto adjacent phases of the other parallel line
The short-circuit sweep time-distance diagrams are shown in Figure 14, from which it can be seen that the reach of the relay changes depending on the parallel line configuration
Summary
Mutual coupling of overhead lines occurs when two or more circuits are in close proximity in the same corridor. Environmental conditions and planning restrictions are forcing utilities to install more and more parallel lines This configuration imposes a unique problem for the associated line protection relays [1]. The voltage drop along the transmission line is equal to the product of the fault current Ie and the impedance fault Za. Distance relays must be compensated against the coupling effect between parallel lines since the current flowing in one phase of one line induces voltage onto adjacent phases of the other parallel line. Distance relays must be compensated against the coupling effect between parallel lines since the current flowing in one phase of one line induces voltage onto adjacent phases of the other parallel line This will otherwise affect the measured impedance under fault conditions.
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