Page 321 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

long distance transmission line in case of light loading or no load operation of transmission system the receiving
at voltage often increases beyond the sending end voltage, leading to phenomena known as Ferranti effect. A
long transmission line can be considered to compost a high amount of capacitance and inductance distributed
across the entire length of the line.
This voltage can be controlled by placing shunt reactors at the receiving ends of the lines. Shunt reactor is
an inductive current element connected between line and neutral to compensate the capacitive current from
transmission lines.
Protective relays

Protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the
defective element from the rest of the system.
Primary winding of a current transformer(C.T.) which is connected in series with the line to be protected. Secondary
winding of C.T. and the relay operating coil.Tripping circuit which may be either AC or DC. It consists of a source
of supply, the trip coil of the circuit breaker and the relay stationary contacts.

Requirements of protective relaying (Fig 6)
• Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble and
disconnect the faulty part without disturbing the rest of the system.

• Speed: The relay system should disconnect the faulty section as fast as possible.
• Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity.
• Reliability: It is the ability of the relay system to operate under the pre-determined conditions.
• Simplicity. The relaying system should be simple so that it can be easily maintained.
• Economy. The most important factor in the choice of a particular protection scheme is the economic aspect.

Basic relays (Fig 7)
Relays are classified in to two
1 Electromagnetic attraction
2 Electromagnetic induction.

Fig 6 Fig 7

Pick-up current

It is the minimum current in the relay coil at which the relay starts to operate. So long as the current in the relay
is less than the pick-up value, the relay does not operate and the breaker controlled by it remains in the closed
position.

308

CITS : Power - Electrician & Wireman - Lesson 106-116