Power transformer rating = 30 MVA
Voltage Ratio = 33/66 kV
Rated voltage (in kV) HV side = 66
Rated voltage (in kV) LV side = 33
Vector Group = YNyn0
CT ratio (HV) side = 400/1 A
CT ratio (LV) side = 600/1 A
Minimum Tap = – 10%
Maximum Tap = +10%
% Impedance = 10
Full Load current HV side = MVA/ voltage rating * √3
=30*1000000/ 66000*√3
= 262.743 A
CTR= 400/1 A
Current on CT secondary = 262.74/400
= 0.656 A
Ratio Compensation = 1/0.656 = 1.524
Full Load current LV side = MVA/ voltage rating * √3
=30*1000000/ 33000*√3
= 524.87 A
CTR= 600/1 A
Current on CT secondary = 524.87/600
= 0.874 A
Ratio Compensation = 1/0.874 = 1.144
Initial setting = 0.2 x In (full tap changer range -10 % to +10 % )
Slope 1 = 30 % (full tap change range that is 10+10 =20 +10% extra bias)
Irest 1: = 0.67 (Starting point of slope 1 )
half of the rated CT secondary ,for extra stabilization additional margin
Slope 2 = 2 times slope1 + 10 % safety margin
= 70 %
Irest 2 = 2.5 (starting value of slope 2)
Differential High set = 7.5
Transformer short ckt. current = FLA/ % Z
= 262.7/0.1= 2620 A
CTR = 400/1 A 2620/400= 6.55 this setting must be greater than the transformer inrush current
51 Relay setting (IDMT) Calculation
CT ratio: 400/1A
Full load amp of xmer : 262.7 A
110 % of full load amps
1.1 x 262.7 = 288.97 A
PLUG SETTING : 110 % OF FLA / CTR
288.97/400 = 0.72
PLUG SETTING = 0.72
TMS = 0.09
CURVE : Normal Inverse
relay operating time = 0.14/ (psm) 0.02-1 * TMS
Relay operating time = 0.09 * 14/ (6.55)0.02-1
Relay operating time =279 ms
50 Relay setting (Instantaneous O/C)
600 % FLA = 5*262.7= 1576.2= 1576.2/400 = 3.94
CT sec = 1A
Setting = 3.94
Selected tripping characteristic: DMT
Delay = 0.10 sec
51N Relay setting ( IDMT)
CTR : 400/1A
TMS: 0.08
SETTING: 0.2 X In (20 % of FLA )=52.54 A
relay operating time: 0.14/ (psm) 0.02-1 * TMS
Relay operating time : 137 ms
Curve: NI (Normal Inverse)
50N Relay Setting (Instantaneous E/F )
200 % OF FLA= 525.4= 525.4/400= 1.31
Setting: 1.3 x In
Delay : 0.06 sec
Selected Tripping Characteristics : DMT
Under voltage protection
PT ratio = 66000 kV/3/110 V /3
Phase to phase Voltage = 110V
Phase to neutral voltage = 63.5 V
77% of 110 V = 84.7 V
Delay = 3 sec (to Avoid unwanted tripping)
Over voltage protection
110% OF 110 V= 121 V
Delay = 0 sec
Over fluxing protection
V/F = 110/50 = 2.2 =1 P.U
For 1.10 P.U V/F = 2.2 *1.1*50= 121 Volts Phase to phase
Ph-E= 121/√3 = 69.9 V
V/F pick up > = 1.10
Delay = 10.00 s
V/F pick up >>= 1.20
Delay= 1.00 s
HV REF protection
Vs= IF X T(Rct +2RL)
FLA = 262.7 Amps
Max fault current = 2620 A
CTR = 400/1 A
Rct= 3 ohm
RL = 0. 3 ohm
Vs = 2620/400(3+2*0.3)
Vs= 23.58 V
RS = VS/IS
IS = 0.1 A= 23.58/0.1
RS= 235 .8 OHM
REF SETTING= 0.1 X In
Delay = 0 sec
LV REF Protection
Vs= IF X T(Rct +2RL)
FLA = 524.8 Amps
Max fault current = 5248 A
CTR = 600/1 A
Rct= 3 ohm
RL = 0. 3 ohm
Vs = 5248/600(3+2*0.3)
Vs= 31.48 V
RS = VS/IS
IS = 0.1 A= 31.48/0.1
RS= 314.8 OHM
REF SETTING= 0.1 X In
Delay = 0 sec
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