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Relay Slectivity & Setting of20/25 MVA Trafo in details:-
1st For18.75 MVA Generator:-
TDPS Make,1500 R.P.M,15 MW,11 KV,987 A,0.8 Power Factor Alternator Total MVA =15/0.8 =18.75 MVA.
18.75 MVA Alternator Sub-Transient Reactence ( Xd") =25 %.
Take Base MVA as100 MVA.
Now Xd" =25 X100 /18.75 =133 % ---------------------------------------- (A)
2nd for20/25 MVA11 KV/33 KV GT:-
20 /25 MVA GT, % Z Voltage =10 .
Condition-1 ( Only20/25 MVA Trafo, nothing Else)
Max Short Ckt Current in33 KV Side = Tr. MVA X %Z / Root3 X KV =25000 X10 /1.732 X33 =4.373 KA.
Condition2 for20/25 MVA Trafo:-
Take base MVA as100 MVA.
Now % Z = %Z X Base MVA / Trafo MVA =10 X100 /25 =40 % ----------------------- ( B)
Taking18.75 MVA TG &25 MVA Trafo in Series Feeding in33 KV load BUS:-
Take Base MVA as100 MVA.
Adding18.75 MVA TG % Reactance +25 MVA Trafo % Z( Equation A + B ) =133 +40 =173.3 %.
Now Total Fault MVA in33 KV Bus, Including18.75 MVA TG &25 MVA Trafo =100 X100 /173 =57.8 MVA.
Max Fault Current =57800 / Root3 X33 =1011 Ampere.
Now Taking18.75 MVA TG +20/25 MVA Trafo +2 X7.2 MVA SMS +1 X6.5 MVA Ferro:-
33 KV Bus total Fault MVA as is70 MVA,
33 KV Bus total Fault Current =70000 / Root3 X33 =1226 Ampere.
20 /25 MVA Trafo relay Setting:-11KV /33 KV Delta / STAR.33 KV Star Neutral is solidly grounded,33 KV side CT Ratio500/1 &11 KV side CT ratio1200/1 :-
Note:- As we are having18.75 MVA TG feeding in11/33 KV GT, so Take20 MVA as reference for better Co-ordination Setting in33 KV BUS.
20 MVA Trafo33 KV side FLC350 Ampere.
O/C Setting:-(3 O/C Relay):-
Now Plug Setting Multiplier =1226 /500 X0.70 =3.5 .
So in Normally Inverse Curve relay operating time @ 3.5 times P.S.M =0.14 / PSM To the power0.02 -1 =5.51 Seconds.
Assumed operating Curren of Downstream O/C say650 mili seconds.
Allowed a grade margin of300 mili seconds Trip time required should be0.95 Seconds.
Now T.M.S ( Time Multi plier Setting ) =5.51 X0.170 =0.95 Seconds.
NOW PSM =0.7 & TMS =0.170 as per short CKT analysis.
E/F Setting:-
PSM =1226 /500 X0.2 =12.26.
In Normally Inverse Curve relay operating time @12.26 times P.S.M =0.14 / PSM To the power0.02 -1 =2.72 Seconds.
Assumed operating Curren of Downstream O/C say650 mili seconds.
Allowed a grade margin of300 mili seconds Trip time required should be0.95 Seconds.
Now T.M.S ( Time Multi plier Setting ) =2.72 X0.35 =0.95 Seconds.
NOW PSM =0.20 & TMS =0.35 as per short CKT analysis.
Instantaneous O/C Relay in Defenite Time Charecteristic on33 KV side in case of Short CKT Fault:-
As33 KV1250 Amps VCB breaker STC is 25 KA/1 sec
Current Setting = 1226/500 =2.5 X CT Ratio.
Time Delay ( DT) =0.05 –0.1-0.11 seconds According to Trafo Charging Current.
Differential Setting of20/25MVA Transformer :
CT CONFIGURATION:-
W1 Phase I/P =1 Amp.
W1 Phase CT Ratio =500/1
W2 Phase I/P =1 Amp.
W2 Phase CT Ratio =1200/1
Ig-1 Phase I/P =1 Amp.
Ig-1 Phase CT Ratio =500/1
Ig-2 Phase I/P =1 Amp.
Ig-2 Phase CT Ratio =1200/1
Setting Considering20 MVA on33 KV as well as11 KV side coordinating with18.75 MVA TG
W1 ICT Multiplier =1.42 X
W2 ICT Multiplier =1.15 X
87 BD Element = Enable
87 BD initial Setting =0.2 X In
87 BD1st Bias slope =0.4 X
87 BD1st Bias slope limit =4 X In
87 BD2nd Bias Slope type = Line
87 BD2nd Bias Slope =2 X
87 BD Delay =170-200 mili sec considering internal fault &/or Charging current.
87 BD Inrush Action = Inhibit.
REF Relay on Star side & Sensitive Instantaneous E/F relay on Delta side :-
33 KV Star side having Neutral solidly Grounded to earth, so there must be a Restricted E/F relay on Star Side.
As we know Delta winding (11 KV Side) provides Zero sequence isolation between Star winding & the source, so a residually connected E/F relay will not operate in case of E/F fault on Star side. Therefore a sensitive Instantaneous E/F (High Impedance mode) relay on Delta side will give protection in case of Star side fault, without requiring any time Co-ordination.
I HOPE ITS3 RELAY TYPES FOR HEALTHY PROTECTION., ONE SUCH IS NUMERIC RELAY OTHERS I COULD NOT ABLE TO REMEMBER .,
