Distance relay (resistance measurement relay):
Distance relay is a protection relay and the break time of relay is depended to resistance of the wire. Almost, the open circuit time of relay should be corresponded to the location of fault. So, this time should be subject to the certain direction of short circuit energy. The distance of fault from relay is corresponded to the impedance of the section of wire and the more distance, the impedance would be higher. There is a direct relation between resistance and the length of the wire, therefore, by using distance relay over the lines, transferring energy solve the protection and set the consecutive relay time problems.All Distance relays operate according to the measuring the relay distance from fault place. When the value of minimum fault current is large enough to comprise to load current, distance relay will be useful, because this type of relay is not sensitive to the value of current and just measures the impedance (distance of fault location from the relay). The internal impedance of relay is shown by Z0 sign. This impedance is equal to the impedance of the line section (the section which the relay should protect it).
Operation of Distance relay
Distance relays operate like overcurrent voltage against short circuit faults. These relays clear the faults when the line impedance is lower than set value. Otherwise, they don’t operate and clear the fault. The resistance of consumers are not settable, so, the values of the consumer resistance will be ignored and according to the Z=U/I equation, when there is an short circuit fault, because of increasing the current, the impedance will be decreased and the more it is close to relay, the relay will be operate sooner and clear the fault. In the grid which includes many distance relays, if a fault appear in the grid, all relays will be excited, but only the relay which is close to the location of fault will be operate and other relays will go to their usual state.For protecting transmission lines, distance relays used based on arranging the fixed settings. Usually, the operation zone of these relays change by changing network conditions and it cause the false operation of relay. By using procedures which have capability of data processing and pattern detecting, such as applying Microprocessor and smart algorithms, new relays can be usefull and they will have High accuracy.