Distance relays play a critical role in the power system, as they effectively carry and respond to the voltage and current (i.e., impedance) between the relay location and the fault location.
The impedance is constant every mile, allowing these relays to cope with the unpredictable power system environment.
As power systems become more complex, fault currents no longer remain constant, but fluctuate with changes in generation and system configuration.
In contrast to the traditional directional overcurrent relay, which is unable to cope with many unexpected situations, the setting of the distance relay is more stable to the change of factors outside the protection line.
There are three main types of distance relays, each with its own unique application and operating characteristics.
1.Remote relays
The non-directional characteristics of the remote relay are centered on the origin of the R-X diagram and are mainly used for fault detection.
2.Impedance relays
The impedance relay is also circular and has characteristics at the origin of the r-x diagram. The same applies to fault detection.
3.Admittance relays
Admittance relays are the most common distance relays. In the experimental protocol, trip relays are often made;In the step size scheme, it acts as a backup relay. It passes through the origin of the R-X diagram and is directional. In the electromechanical design, it is circular;In solid-state designs, the shape can correspond to the transmission line impedance.
4.Reactance relays
The linear nature of the reactance relay only reacts to the reactance of the protection line. Non-directional, often used as a supplementary trip relay for admittance relays, especially efficient in short-circuit fault arc resistance and line length of the same order of magnitude.
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