In a 25 MVA 11/33 GT how many types of relay we use for healthy protection.11 KV CT 1200/1 & 33 KV side CT 500/1.1250 A Siemens VCB?

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 .,