Distance relays are characterized by having two input quantities proportional to the voltage and current at a particular point in the power system, referred to as the relaying point. Ideal static distance relays have characteristics independent of actual magnitudes of voltage and current but dependent only on their ratio and phase angle between them. The versatile family of distance relays includes impedance relays, reactance relays and mho relays. The measurement of impedance, reactance or admittance is done by comparing input current and voltage. Hence distance relays have voltage and current as input quantities. In a static distance relay it is necessary that the two input quantities are similar i.e., voltage/voltage or current/current because they are not electrically separate as they are in case of electro—magnetic relays (in an impedance relay magnets are energized by voltage and currents). A practical static distance protection scheme includes a starting, measuring and timing elements made up of solid-state devices. The output unit is usually a moving coil relay. The starting element is usually an over current relay. The output is given to the measuring element. Phase comparators are employed in the measuring devices. The measuring device determines whether the fault is within the protected zone or not. A tripping signal is initiated in case the fault is within the protected zone. In case the fault is outside protected zone, the timer unit starts which initiates zone-wise protection.

A block diagram of a distance relay based on current comparison principle is given in fig 25. The line PT secondary is connected to auxiliary PT and the output of auxiliary PT is converted into current and this current is compared with the output of the auxiliary CT.

Static distance relays do not have any moving part so they operate much faster (operating time of the order of some milli-seconds) and without risk of incorrect tripping as compared to electro-magnetic relays. Static distance relays are accurate over a wider range of fault currents and line lengths and require much lower burden as compared to their counterparts in electro-magnetic relays. Static distance relays are compact in size and have better stability under power swing conditions. Static distance relays are extensively used for protection of medium and long transmission lines, parallel feeders and unit back-up protection as well as inter-connected and T-connected lines.

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