SecondGen
December 21, 2019, 10:57 AM
There are two identical LTC transformers (12/16/20 MVA, 115 kV to 13.8kV, 9%Z, ±10% in 5/8% steps) which are operating in parallel on differing taps, one step removed from each other. Calculate the resulting circulating current from a one-step tap discrepancy.
Step 1: Find the voltage driving the circulating current
Each transformer is one tap apart so the difference in voltage is: 5/8% (0.00625 pu)
V = Difference x (Vsec / 1.73205)
V = 0.00625 X (13,800 / 1.73205)
V = 0.00625 X 7967.438
V = 49.796
V = 49.8V
Step 2: Find the transformer base impedance
Z(base) = kV^2 / MVA
Z(base) = 13.8^2 / 12
Z(base) = 190.44 / 12
Z(base) = 15.87 ohms
Step 3: Find the transformer loop impedance
Z(loop) = 2 x Z(transformer)
Z(loop) = 2[9% x Z(base)]
Z(loop) = 2[0.09 x 15.87]
Z(loop) = 2[1.4283]
Z(loop) = 2.86 ohms (reactive)
Z(loop) = j2.86 ohms
Step 4: Calculate the resulting circulating current from a one-step tap discrepancy
I(circ) = Voltage / Z(loop)
I(circ) = 49.8 / j2.86
I(circ) = -j17.413 Amps
https://beckwithelectric.com/wp-content/uploads/docs/app-notes/Beckwith-App-Note-11.pdf
Step 1: Find the voltage driving the circulating current
Each transformer is one tap apart so the difference in voltage is: 5/8% (0.00625 pu)
V = Difference x (Vsec / 1.73205)
V = 0.00625 X (13,800 / 1.73205)
V = 0.00625 X 7967.438
V = 49.796
V = 49.8V
Step 2: Find the transformer base impedance
Z(base) = kV^2 / MVA
Z(base) = 13.8^2 / 12
Z(base) = 190.44 / 12
Z(base) = 15.87 ohms
Step 3: Find the transformer loop impedance
Z(loop) = 2 x Z(transformer)
Z(loop) = 2[9% x Z(base)]
Z(loop) = 2[0.09 x 15.87]
Z(loop) = 2[1.4283]
Z(loop) = 2.86 ohms (reactive)
Z(loop) = j2.86 ohms
Step 4: Calculate the resulting circulating current from a one-step tap discrepancy
I(circ) = Voltage / Z(loop)
I(circ) = 49.8 / j2.86
I(circ) = -j17.413 Amps
https://beckwithelectric.com/wp-content/uploads/docs/app-notes/Beckwith-App-Note-11.pdf