(i) Causes and consequences of dangerous currents: Faults, overloads and switching overcurrents. 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.