Protective relay
[1]: 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, overvoltage, reverse power flow, over-frequency, and under-frequency.
[4] However, due to their very long life span, tens of thousands of these "silent sentinels"[5] 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.
Induction relays can respond to the product of two quantities in two field coils, which could for example represent the power in a circuit.
"[10]: 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.
For AC circuits, the principle is extended with a polarizing winding connected to a reference voltage source.
In a large installation of electromechanical relays, it would be difficult to determine which device originated the signal that tripped the circuit.
"Moving coil" meters use a loop of wire turns in a stationary magnet, similar to a galvanometer but with a contact lever instead of a pointer.
Induction relays require alternating current; if two or more coils are used, they must be at the same frequency otherwise no net operating force is produced.
[11] These electromagnetic relays use the induction principle discovered by Galileo Ferraris in the late 19th century.
In order to operate, the magnetic system in the relays produces torque that acts on a metal disc to make contact, according to the following basic current/torque equation:[14]
Once the upper and lower electromagnets are energised they produce eddy currents that are induced onto the metal disc and flow through the flux paths.
In an overcurrent condition, a value of current will be reached that overcomes the control spring pressure on the spindle and the braking magnet, causing the metal disc to rotate towards the fixed contact.
A relatively large standby current is required to maintain the tube filament temperature; inconvenient high voltages are required for the circuits, and vacuum tube amplifiers had difficulty with incorrect operation due to noise disturbances.
Static relays eliminated or reduced contact bounce, and could provide fast operation, long life and low maintenance.
The world's first commercially available digital protective relay was introduced to the power industry in 1984 by Schweitzer Engineering Laboratories (SEL) based in Pullman, Washington.
[3] In spite of the developments of complex algorithms for implementing protection functions the microprocessor based-relays marketed in the 1980s did not incorporate them.
[25] 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.
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 (SCADA) interface, monitoring of contact inputs, metering, waveform analysis, and other useful features.
This is particularly so in long-distance high voltage or multi-terminal circuits or in lines that are series or shunt compensated[24]: 3 They also offer benefits in self-testing and communication to supervisory control systems.
Each type, however, shares a similar architecture, thus enabling designers to build an entire system solution that is based on a relatively small number of flexible components.
[32] The various protective functions available on a given relay are denoted by standard ANSI device numbers.
The time interval between the instant pick-up value and the closing contacts of the relay is very low.
[36]: 42 Secondly if the source impedance varies and becomes weaker with less generation during light loads then this leads to slower clearance time hence negating the purpose of the IDMT relay.
It is evident from the relay characteristic equations that a larger TMS will result in a slower clearance time for a given PMS (Ir) value.
Time grading with other protection systems is therefore not required, allowing for tripping without additional delay.
Current transformers in a differential scheme must be chosen to have near-identical response to high overcurrents.
A synchronism checking relay provides a contact closure when the frequency and phase of two sources are similar to within some tolerance margin.
