1. The document calculates the stabilizing resistance needed for a 40MVA transformer with a 66kV system voltage and 400:1 CT ratio to have a stability voltage of 18.023V and stabilizing resistor of 106.02 ohms dissipating 3.06W. 2. For a 40MVA transformer with an 11kV system voltage and 2000:1 CT ratio, the calculated stability voltage is 73.4V and stabilizing resistor is 734 ohms dissipating 7.34W. 3. For an earthing transformer with a 0.415kV system voltage and 750:1 CT ratio, the stability voltage is 78.5V and stabilizing resistor is 262 oh

1. 1 STABILIZING RESISTANCE FOR 40MVA TRANSFORMER REF FEEDER B11,... (87NH, 66KV side)
System voltage = 66 KV
CT ratio = 400/1 A
Transformer rated current (IL) = 349.91 A
% impedance (at 40MVA base) = 18 (From transformer test certificate)
Considering negative tolerance of 7.5% = 16.65
Through fault current = 350/0.1665 A
= 2101.557
Stability voltage, Vs = 1.5*If * (Rct + 2Rl)
2Rl = 1.187 Ohms (Site Value)
Total loop resistance (Rct + 2Rl) = 2.287 Ohms
Vs = 1.5*2101.55/400*2.287
= 18.023 Volts
Adopted setting, Ip.u. = 0.17 In
Stabilising resistor (Rs) = Vs/Ip.u.
= 106.02 ohms
Short term Power, P = Vs * Vs /R
= 3.06 W
2 STABILIZING RESISTANCE FOR 40MVA TRANSFORMER REF FEEDER A18,.. (87NL, 11KV side)
System voltage = 11 KV
CT ratio = 2000/1 A
Transformer rated current (IL) = 2099.5 A
% impedance (at 40MVA base) = 18 (From transformer test certificate)
Considering negative tolerance of 7.5% = 16.65
Through fault current = 2100/0.1665 A
= 12609
Stability voltage, Vs = If * (Rct + 2Rl)
2Rl = 2.6347 Ohms (Site Value)
Total loop resistance (Rct + 2Rl) = 11.6347 Ohms
Vs = 12609/2000*(11.6347)
= 73.4 Volts
Adopted setting, Ip.u. = 0.1 In
Stabilising resistor (Rs) = Vs/Ip.u.
= 734 ohms
Short term Power, P = Vs * Vs /R
= 7.34 W
3 STABILIZING RESISTANCE FOR FEEDER A18,...EARTHING TRANSFORMER REF (87NLE, 415V side)
System voltage = 0.415 KV
CT ratio = 750/1 A
Transformer rated current = 438.2 A
Through fault current = 8115.60 A
Stability voltage, Vs = If * (Rct + 2Rl)
2Rl = 1.25 Ohms (Site Value)
Total loop resistance; (Rct + 2Rl) = 7.25 Ohms
Vs = 8115.60/750*(7.25)
= 78.5 Volts
Adopted setting, Ip.u. = 0.30 In
Stabilising resistor (Rs) = Vs/Ip.u.
= 262 ohms
Short term Power, P = Vs * Vs /R
= 23.54 W