Electromechanical protective relays operate by either, or. Unlike switching type electromechanical with fixed and usually ill-defined operating voltage thresholds and operating times, p. Electromechanical protective relays operate by either, or. Unlike switching type electromechanical with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays have well-established, selectable, and adjustable time and current (or other operating parameter) operating characteristics. Protection relays may use arrays of, shaded-pole, magnets, operating and restraint coils, solenoid-type operators, telephone-relay contacts, and phase-shifting networks. Protective relays can also be classified by the type of measurement they make. A protective relay may respond to the magnitude of a quantity such as voltage or current. Induction relays can respond to t. In, a protective relay is a device designed to trip a when a is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current,, reverse flow, over-frequency, and under-frequency. Microprocessor-based digital protection relays now emulate the original devices, as well as providing types of protection and supervision impractical with electromechanical relays. provide only rudimentary indication of the location and origin of a fault. In many cases a single microprocessor relay provides functions that would take two or more electromechanical devices. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays. However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world. Important transmission lines and generators have cubicles dedicated to protection,. Electromechanical relays can be classified into several different types as follows: "Armature"-type relays have a pivoted lever supported on a hinge or knife-edge pivot, which carries a moving contact. These relays may work on either alternating or direct current, but for alternating current, a shading coil on the pole is used to maintain contact force throughout the alternating current cycle. Because the air gap between the fixed coil and the moving armature becomes much smaller when the relay has operated, the current required to maintain the relay closed is much smaller than the current to first operate it. The "returning ratio" or "differential" is the measure of how much the current must be reduced to reset the relay. A variant application of the attraction principle is the plunger-type or solenoid operator. A is another example of the attraction principle. "Moving coil" meters use a loop of wire turns in a stationary magnet, similar to a but with a contact lever instead of a pointer. These can be made with very high sensitivity. Another type of moving coil suspends the coil from two conductive l. The various protective functions available on a given relay are denoted by standard. For example, a relay including function 51 would be a timed overcurrent protective relay. An overcurrent relay is a type of protective relay which operates when the load current exceeds a pickup value. It is of two types: instantaneous over current (IOC) relay and definite time overcurrent (DTOC) relay. The is 50 for an IOC relay or a DTOC relay. In a typical application, the over current relay is connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize to trip a circuit breaker. The DTOC relay has been used extensively in the United Kingdom but its inherent issue of operating slower for faults closer to the source led to the development of the IDMT relay. A definite time over-current (DTOC) relay is a relay that operates after a definite period of time once the current exceeds the pickup value. Hence, this relay has current setting range as well as time setting range. An instantaneous over-current relay is an overcurrent relay which has no intentional time delay for operation. The contacts of the relay are closed instantly when the current inside the relay rises beyond the operational value. The time interval between the instant pick-up value and the closing contacts of the relay is very low. It has low operating time and starts operating instantly when the value of current is more than the relay setting. This relay operates only when the impedance between the source and the relay is less than that provided in the section. An inverse-time over-current (ITOC) relay is an overcurrent relay which operates only when the magnitude of their operating current is inversely proportional to the magnitude of the energize quantities. Th. Relays can also be classified by their type of power source. • Self-powered relays operate on energy derived from the protected circuit, such as through the current transformers used to measure line current. Self-powered relays are advantageous in terms of cost and reliability as they do not require a separate power supply.• Auxiliary-powered relays rely on a battery or external AC supply. Some relays can use either AC or DC. The auxiliary supply must be highly reliable during a system fault to ensure the relay can operate.• Dual-powered relays are powered by the protected circuit and through an auxiliary power source which acts as a backup.
