Current Transformers :
Distinction
between CT and conventional transformers;
Equivalent circuit and phasor diagrams; errors; ALF; Voltage
developed across CT secondary; CT ratio; Short time
withstand current; Bar and wound primary CTs Classification
based on application. (Metering, Protection and PS class.);
CT specification.
Potential
Transformers :
Errors;
voltage factor; connections (star, delta, open delta and ‘V’
connections); PT ratio; Insulation level ;
Classification based on application
( Metering, Protection class );
Protection of PT; PT specification.
Fault
Level Calculations :
Impedance
diagram; per unit and percentage impedance; sequence
components; impedance to be considered for different network
elements like generator, transformer, transmission line,
motor, cable, etc; default figures where data not available;
fault calculation procedures; step by step fault level
evaluation exercise for phase fault and ground faults.
Relay
Setting & Co-ordination :
Primary &
back up protection; need for co-ordination; functional
characteristics of protection scheme, including sensitivity,
selectivity (discrimination by time, current and both time
and current), and speed; Discrimination time (co-ordination
interval); Criteria for setting pick ups and time dial for
DMT and IDMT relays.
Tools
available to reduce fault clearance time like instantaneous
over-current; Instantaneous earth fault & Pilot wire
protection; overload vs over current protection.
HT
Motor Protection :
Major
protection elements; thermal over load; over current vs
overload; critical review of motor and relay thermal
characteristics; major short comings; locked rotor; speed
switch; negative sequence; unbalance vs negative sequence;
short-circuit; ground fault (zero sequence); use of residual
connected CT and CBCT; differential protection; under
voltage protection; under current protection; special
protection requirement for wound motor; rotor unbalance;
additional protection for synchronous motor; loss of field.
LT
Motor Protection :
Causes for LT
Motor damage, Bimetal overload relay, HRC fuses, cost
effective Motor Protection relay for contactor controlled
motor, co-ordination between fuse and relay, Application of
thermistors.
Generator
Protection :
Protection
elements; Stator over current; Stator earth fault (95% and
100% ); Differential High impedance and low impedance
schemes; Over voltage protection; Over fluxing protection;
Rotor earth fault; Field over heating; Loss of excitation;
Bearing vibration; Motoring (reverse power and low forward
power) protection; Unbalanced armature current protection;
Voltage controlled over current protection; Voltage
restrained over current protection; Under frequency
protection, withstand times for steam, hydro, DG and gas
units; Comparison of protection practices from different
references.
Busbar
Protection :
Protection
classification; High, medium and low impedance schemes; High
impedance – current based scheme, AC & DC schematics,
CT specification, work out for exciting current, KPV and
stabilizing resistor, need for non-linear resistor, location
of CTs to avoid blind spots; High impedance – Voltage
based scheme, performance for internal and external faults,
voltage setting calculations; Medium impedance scheme, one
cycle protection, relay operation and characteristics for
internal and external fault conditions; Low impedance
schemes, numerical bus bar protection, stabilized
differential algorithm & directional current comparison
algorithm; Isolator auxiliary contacts – need for
reliability.
