(i) Causes and consequences of dangerous currents: Faults, overloads and
switching over currents. Introduction to protection, trip circuit of a circuit breaker.
Functional characteristics of a relay, zone of protection, primary and backup protection.
(ii) CTs & PTs: Current transformer construction, measurement and protective CTs.
Type of potential transformers. Steady state ratio and phase angle errors in CTs and PTs.
Transient errors in CT and CVT (Capacitive Voltage Transformer).
Units: II-Overcurrent Protection:
HRC fuse and thermal relay. Overcurrent (OC) relays -
instantaneous, definite time, inverse time and inverse definite minimum time overcurrent
relays, time and current gradings. Induction disc type relay. Directional overcurrent relay,
30◦, 60◦ and 90◦ connections. Earth fault relay. Brief description of overcurrent protective
schemes for a feeder, parallel feeders and ring mains.
Units: III-Generator Protection:
Stator protection - differential and percentage differential
protection, protection against stator inter-turn faults, stator overheating protection. Rotor
protectionprotection against excitation and prime mover failure, field earth fault and
unbalanced stator currents (negative sequence current protection).
Units: IV
(i) Transformer Protection: Percentage differential protection, magnetizing
inrush current, percentage differential relay with harmonic restraint. Buchholz relay.
Differential protection of generator transfer unit.
(ii) Busbar Protection: Differential protection of busbars, high impedance relay scheme,
frame leakage protection.
Units: V
(i) Transmission Line Protection: Introduction to distance protection.
Construction, operating principle and characteristics of an electromagnetic impedance
relay. Effect of arc resistance. Induction cup type reactance and mho relays. Comparison
between impedance, reactance and mho relays. Three stepped distance protection of
transmission line.
(ii) Induction Motor Protection: Introduction to various faults and abnormal operating
conditions, unbalance supply voltage and single phasing. Introduction to protection of
induction motors- HRC fuse and overcurrent, percentage differential, earth fault and negative
sequence voltage relays.