T 2 (3 T₂ (4) Machine 1 FLY Machine 2 Krossy 147 YA For the three-phase power system with single-line diagram, equipment ratings and per-unit reactances are given as follows: Machines 1 and 2: 120 MVA, 10.5 kV, X = X2 = 0.2, X₁ = 0.03, X = 0.03; Transformers 1 and 2: 120 MVA, 11D/115Y kV, X₁= X2 = X0 = 0.05. Select a base of 120 MVA, 115 kV for the transmission line. On that base, the series reactances of the line are X1 X2 = 0.3 and Xo = 2X1 per unit. With a nominal system voltage of 115 kV at bus 3, machine 2 is operating as a motor drawing 40 MVA at 0.8 power factor lagging. Compute the fault current for single-line-to-ground fault at bus 3

T 2 (3 T₂ (4) Machine 1 FLY Machine 2 Krossy 147 YA For the three-phase power system with single-line diagram, equipment ratings and per-unit reactances are given as follows: Machines 1 and 2: 120 MVA, 10.5 kV, X = X2 = 0.2, X₁ = 0.03, X = 0.03; Transformers 1 and 2: 120 MVA, 11D/115Y kV, X₁= X2 = X0 = 0.05. Select a base of 120 MVA, 115 kV for the transmission line. On that base, the series reactances of the line are X1 X2 = 0.3 and Xo = 2X1 per unit. With a nominal system voltage of 115 kV at bus 3, machine 2 is operating as a motor drawing 40 MVA at 0.8 power factor lagging. Compute the fault current for single-line-to-ground fault at bus 3

Power System Analysis and Design (MindTap Course List)

6th Edition

ISBN:9781305632134

Author:J. Duncan Glover, Thomas Overbye, Mulukutla S. Sarma

Publisher:J. Duncan Glover, Thomas Overbye, Mulukutla S. Sarma

Chapter3: Power Transformers

Section: Chapter Questions

Problem 3.37P: Three single-phase two-winding transformers, each rated 25MVA,54.2/5.42kV, are connected to form a...

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Three single-phase two-winding transformers, each rated 3kVA,220/110volts,60Hz, with a 0.10 per-unit leakage reactance, are connected as a three-phase extended autotransformer bank, as shown in Figure 3.36(c). The low-voltage winding has a 110 volt rating. (a) Draw the positive-sequence phasor diagram and show that the high-voltage winding has a 479.5 volt rating. (b) A three-phase load connected to the low-voltage terminals absorbs 6 kW at 110 volts and at 0.8 power factor lagging. Draw the per-unit impedance diagram and calculate the voltage and current at the high-voltage terminals. Assume positive-sequence operation.
For large power transformers rated more than 500 kVA, the winding resistances, which are small compared with the leakage reactances, can often be neglected. (a) True (b) False
Three single-phase two-winding transformers, each rated 25MVA,34.5/13.8kV, are connected to form a three-phase bank. Balanced positive-suence voltages are applied to the high-voltage terminals, and a balanced, resistive Y load connected to the low-voltage terminals absorbs 75 MW at 13.8 kV. If one of the single-phase transformers is removed (resulting in an open connection) and the balanced load is simultaneously reduced to 43.3 MW (57.7 of the original value), determine (a) the load voltages Va,Vb, and Vc; (b) load currents Ia,Ib, and Ic; and (c) the MVA supplied by each of the remaining two transformers. Are balanced voltages still applied to the load? Is the open transformer overloaded?
Three single-phase two-winding transformers, each rated 25MVA,54.2/5.42kV, are connected to form a three-phase Y- bank with a balanced Y-connected resistive load of 0.6 per phase on the low-voltage side. By choosing a base of 75 MVA (three phase) and 94 kV (line-to-line) for the high-voltage side of the transformer bank, specify the base quantities for the low-voltage side. Determine the per-unit resistance of the load on the base for the low-voltage side. Then determine the load resistance RL in ohms referred to the high-voltage side and the per-unit value of this load resistance on the chosen base.
Consider a single-phase electric system shown in Figure 3.33. Transformers are rated as follows: XY15MVA,13.8/138kV, leakage reactance 10 YZ15MVA,138/69kV, leakage reactance 8 With the base in circuit Y chosen as 15MVA,138kV determine the per-unit impedance of the 500 resistive load in circuit Z, referred to circuits Z, Y, and X. Neglecting magnetizing currents, transformer resistances, and line impedances, draw the impedance diagram in per unit.
Consider three ideal single-phase transformers (with a voltage gain of ) put together as three-phase bank as shown in Figure 3.35. Assuming positive-sequence voltages for Va,Vb, and Vc find Va,Vb, and VC. in terms of Va,Vb, and Vc, respectively. (a) Would such relationships hold for the line voltages as well? (b) Looking into the current relationships, express IaIb and Ic in terms of IaIb and Ic respectively. (C) Let S and S be the per-phase complex power output and input. respectively. Find S in terms of S.
The per-unit equivalent circuit of two transformers Ta and Tb connected in parallel, with the same nominal voltage ratio and the same reactan of 0.1 per unit on the same base, is shown in Figure 3.43. Transformer Tb has a voltage-magnitude step-up toward the load of 1.05 times that of Ta (that is, the tap on the secondary winding of Tb is set to 1.05). The load is represented by 0.8+j0.6 per unit at a voltage V2=1.0/0 per unit. Determine the complex power in per unit transmitted to the load through each transformer, comment on how the transformers share the real and reactive powers.
Consider the oneline diagram shown in Figure 3.40. The three-phase transformer bank is made up of three identical single-phase transformers, each specified by X1=0.24 (on the low-voltage side), negligible resistance and magnetizing current, and turns ratio =N2/N1=10. The transformer bank is delivering 100 MW at 0.8 p.f. lagging to a substation bus whose voltage is 230 kV. (a) Determine the primary current magnitude, primary voltage (line-to-line) magnitude, and the three-phase complex power supplied by the generator. Choose the line-to-neutral voltage at the bus, Va as the reference Account for the phase shift, and assume positive-sequence operation. (b) Find the phase shift between the primary and secondary voltages.
A 30 MVA, 11 kV generator has a reactance of 0.10p.u.on its own base. Determine the per-unit reactance when referred to base kVA of 50,000 kVA and base kV of 33 kV.
explain why Transformers are necessary in all stages of power distribution systems. Three-phase voltages may be stepped up or down by three-phase transformers
7. System Model and Per Unit T1 Line T2 BE 31.25miles 3 Impedance Load R1+jXL-0.1+j0.5 p.u. 30 MVA 30 MVA 13 kV X-0.1 p.u. 14 kV 13/220 kV X-0.2 p.u. 35 MVA * Transformer T2 is composed of "three single-phase" transformers, each rated 40/3 MVA, 132.8:14 kV with a leakage reactance of 0.1 per unit. (a) Fill in the appropriate base values. Gen. G Trans. Ti Line Trans. T: Load New MVA Base New KV Base (b) Calculate the impedance values. Gen G Trans. T1 Line Trans. T2 Load p.u. Impedance
Three single-phase transformers, each rated 10 MVA , 66.4/12.5 kV, 60 Hz , with an equivalent series reactance of 0.1 per unit divided equally between primary and secondary, are connected in a three-phase bank. The high-voltage windings are Y-connected and their terminals are directly connected to a 115-kV three-phase bus. The secondary terminals are all shorted together. Find the currents entering the high-voltage terminals and leaving the low-voltage terminals if the low-voltage windings are (a) Y-connected and (b) Δ- connected.