Question

Briefly describe following with respect to protective relaying:

(a) Principle of operation and characteristics of induction type relays. (6)

(b) Static and digital relays. (5)

(c) Protection of alternators, motors, transformer and busbar. (5)

Answer 1

Part (a)

An induction relay works on the same principle as an induction motor. It has a laminated electromagnet (with a shading ring or split phase) producing a rotating or travelling magnetic field, and a moving disc or cup in which eddy currents are induced.
The interaction between the flux and the eddy currents produces a torque on the disc proportional to the product of the fluxes and the sine of the phase angle between them, i.e. proportional to the current (or power) being measured.
The disc is restrained by a spring and a permanent magnet (damping magnet) provides braking torque proportional to speed.
When the current exceeds the relay setting, the operating torque exceeds the restraining torque and the disc rotates, closing the contacts after a time determined by the disc travel.
Characteristics: the relay has an inverse time-current characteristic - the operating time decreases as the current increases. It can be made inverse, very inverse, or definite minimum time (IDMT). The time setting and current (plug) setting can be adjusted. The damping magnet gives a smooth, accurate time delay.

Part (b)

Static relays: use solid-state electronic circuits (transistors, diodes, operational amplifiers, comparators) instead of moving parts to measure the current/voltage and provide the time-current characteristic. They have no moving disc; the operating time is produced by electronic timing circuits. They are faster, more accurate, have no mechanical wear, and can provide complex characteristics. They require a d.c. auxiliary supply.
Digital (numerical) relays: use a microprocessor or digital signal processor. The analogue current/voltage signals are converted to digital form by an A/D converter and processed by software. They can provide multiple protection functions (overcurrent, earth fault, differential, distance) in one unit, with programmable settings, self-monitoring, fault recording, and communication with a monitoring system. They are highly accurate, flexible, and can be tested and set remotely.

Part (c)

Alternators: overcurrent, earth fault, differential (for large machines), reverse power (to prevent motoring), over/under voltage, over/under frequency, field failure, and stator earth fault protection.
Motors: overload (thermal), short-circuit (overcurrent), earth fault, single-phasing (phase failure), under-voltage, and locked-rotor protection. Large motors may have differential protection.
Transformer: overcurrent, earth fault, differential (main protection for internal faults), overloading (thermal), Buchholz relay (gas and oil surge) for oil-filled transformers, and over-fluxing protection.
Busbar: differential (biased) protection which compares the currents entering and leaving the busbar; on an internal fault the currents are unbalanced and the relay trips all breakers connected to the busbar, isolating the fault quickly to prevent extensive damage.

✓ Verified Model Answer (Text Solution)