25-08-2017, 09:32 PM
POWER SYSTEM PROTECTION
POWER SYSTEM.pdf (Size: 128.58 KB / Downloads: 131)
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.
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.