Question

Briefly describe following with respect to protective relaying:

(a) Principle, working 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 induction motor principle. A laminated electromagnet (with a shading ring or split phase) produces a rotating/travelling magnetic field, and a moving disc or cup has eddy currents induced in it.
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 provides damping torque proportional to speed.
When the current exceeds the setting, the operating torque exceeds the restraining torque and the disc rotates, closing the contacts after a time determined by the disc travel.
Characteristics: inverse time-current characteristic - operating time decreases as current increases. Can be inverse, very inverse, or definite minimum time (IDMT). Time and current (plug) settings are adjustable. The damping magnet gives a smooth, accurate time delay.

Part (b)

Static relays: use solid-state electronic circuits (transistors, op-amps, comparators) instead of moving parts. No moving disc; the time-current characteristic is produced by electronic timing circuits. Faster, more accurate, no mechanical wear, can provide complex characteristics. Require a d.c. auxiliary supply.
Digital (numerical) relays: use a microprocessor/DSP. Analogue signals are converted to digital by an A/D converter and processed by software. Provide multiple protection functions in one unit, with programmable settings, self-monitoring, fault recording, and communication. Highly accurate, flexible, and can be set and tested remotely.

Part (c)

Alternators: overcurrent, earth fault, differential (large machines), reverse power, over/under voltage, over/under frequency, field failure, stator earth fault.
Motors: overload (thermal), short-circuit, earth fault, single-phasing, under-voltage, locked-rotor; differential for large motors.
Transformer: overcurrent, earth fault, differential (main protection for internal faults), thermal overload, Buchholz relay (gas/oil surge), over-fluxing.
Busbar: biased differential protection comparing 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.

✓ Verified Model Answer (Text Solution)