Fundamentals Of Power System Protection By Paithankar Pdf Free Download
Electromechanical protective relays at a generating plant. The relays are in round glass cases. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. In, a protective relay is a device designed to trip a when a fault is detected.: 4 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, over-voltage, 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. Electromechanical relays provide only rudimentary indications of involved phase and zone targets.
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, with many individual electromechanical devices, or one or two microprocessor relays.
The theory and application of these protective devices is an important part of the education of a who specializes in. The need to act quickly to protect circuits and equipment as well as the general public often requires protective relays to respond and trip a breaker within a few thousandths of a second. In some instances these clearance times are prescribed in legislation or operating rules. A maintenance or testing program is used to determine the performance and availability of protection systems. Based on the end application and applicable legislation, various standards such as ANSI C37.90, IEC255-4, IEC60255-3, and IAC govern the response time of the relay to the fault conditions that may occur. Contents. Operation principles Electromechanical protective relays operate by either, or.: 14 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 induction disks, shaded-pole,: 25 magnets, operating and restraint coils, solenoid-type operators, telephone-relay contactsand phase-shifting networks. Protective relays can also be classified by the type of measurement they make.: 92 A protective relay may respond to the magnitude of a quantity such as voltage or current. Induction relays can respond to the product of two quantities in two field coils, which could for example represent the power in a circuit. 'It is not practical to make a relay that develops a torque equal to the quotient of two a.c. This, however is not important; the only significant condition for a relay is its setting and the setting can be made to correspond to a ratio regardless of the component values over a wide range.' : 92 Several operating coils can be used to provide 'bias' to the relay, allowing the sensitivity of response in one circuit to be controlled by another. Various combinations of 'operate torque' and 'restraint torque' can be produced in the relay.
By use of a permanent magnet in the, a relay can be made to respond to current in one direction differently from in another. Such are used on direct-current circuits to detect, for example, reverse current into a generator. These relays can be made bistable, maintaining a contact closed with no coil current and requiring reverse current to reset. For AC circuits, the principle is extended with a polarizing winding connected to a reference voltage source. Lightweight contacts make for sensitive relays that operate quickly, but small contacts can't carry or break heavy currents. Often the measuring relay will trigger auxiliary telephone-type armature relays.
In a large installation of electromechanical relays, it would be difficult to determine which device originated the signal that tripped the circuit. This information is useful to operating personnel to determine the likely cause of the fault and to prevent its re-occurrence. Relays may be fitted with a 'target' or 'flag' unit, which is released when the relay operates, to display a distinctive colored signal when the relay has tripped.
Types according to construction Electromechanical Electromechanical relays can be classified into several different types as follows:. attracted armature. moving coil. induction.
motor operated. mechanical. thermal '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: 14 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 ligaments, allowing very long travel of the coil. Induction disc overcurrent relay. Main article: Digital protective relays were in their infancy during the late 1960s.
An experimental digital protection system was tested in the lab and in the field in the early 1970s. Unlike the relays mentioned above, digital protective relays have two main parts: hardware and software: 5. The world's first commercially available digital protective relay was introduced to the power industry in 1984. In spite of the developments of complex algorithms for implementing protection functions the microprocessor based-relays marketed in the 1980s did not incorporate them. A microprocessor-based digital protection relay can replace the functions of many discrete electromechanical instruments. These relays convert voltage and currents to digital form and process the resulting measurements using a microprocessor. The digital relay can emulate functions of many discrete electromechanical relays in one device, simplifying protection design and maintenance.
Fundamentals Of Power System Protection By Paithankar Pdf Free Download
Each digital relay can run self-test routines to confirm its readiness and alarm if a fault is detected. Digital relays can also provide functions such as communications interface, monitoring of contact inputs, metering, waveform analysis, and other useful features. Digital relays can, for example, store multiple sets of protection parameters, which allows the behavior of the relay to be changed during maintenance of attached equipment. Digital relays also can provide protection strategies impossible to implement with electromechanical relays. This is particularly so in long distance high voltage or multi-terminal circuits or in lines that are series or shunt compensated: 3 They also offer benefits in self-testing and communication to supervisory control systems. A dual powered protection relay powered by the current obtained from the line by a CT.
The striker is also shown. Self-powered relays operate on energy derived from the protected circuit, through the current transformers used to measure line current, for example. This eliminates the cost and reliability question of a separate 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.
Dual powered relays can be also auxiliary powered, so all batteries, chargers and other external elements are made redundant and used as a backup. References.