1. A large power plant generates electricity at 12.0 kV. Its old transformer once converted the voltage to 335 kV. The secondary of this transformer is being replaced so that its output can be 750 kV for more efficient cross-country transmission on upgraded transmission lines. (a) What is the ratio of turns in the new secondary compared with the old secondary? (b) What is the ratio of new current output to old output (at 335 kV) for the same power? (c) If the upgraded transmission lines have the same resistance, what is the ratio of new line power loss to old?

1. A large power plant generates electricity at 12.0 kV. Its old transformer once converted the voltage to 335 kV. The secondary of this transformer is being replaced so that its output can be 750 kV for more efficient cross-country transmission on upgraded transmission lines. (a) What is the ratio of turns in the new secondary compared with the old secondary? (b) What is the ratio of new current output to old output (at 335 kV) for the same power? (c) If the upgraded transmission lines have the same resistance, what is the ratio of new line power loss to old?

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 3.33P: Consider the three single-phase two-winding transformers shown in Figure 3.37. The high-voltage...

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Consider the three single-phase two-winding transformers shown in Figure 3.37. The high-voltage windings are connected in Y. (a) For the low-voltage side, connect the windings in , place the polarity marks, and label the terminals a, b, and c in accordance with the American standard. (b) Relabel the terminals a, b, and c such that VAN is 90 out of phase with Va for positive sequence.
An infinite bus, which is a constant voltage source, is connected to the primary of the three-winding transformer of Problem 3.53. A 7.5-MVA,13.2-kV synchronous motor with a sub transient reactance of 0.2 per unit is connected to the transformer secondary. A5-MW,2.3-kV three-phase resistive load is connected to the tertiary Choosing a base of 66 kV and 15 MVA in the primary, draw the impedance diagram of the system showing per-unit impedances. Neglect transformer exciting current, phase shifts, and all resistances except the resistive load.
A 120 kVA, 1350/330 V single phase transformer has the following parameters: R1 = 0.34 Q, X1 = 0.59 0, R2 = 0.014 Q, X1 = 0.029 Q, Rc = 380 and Xm = 320 Q. The no load current is 4.7 A at a power factor of 0.36 lagging and the no load losses are 4.6 kW. For a full load and 0.9 power factor lagging with 300 V load terminal voltage, determine (a) equivalent circuit parameters referred to primary side, (b) percentage regulation and (c) full load efficiency. O:01 AM
3. Three 1100/110-V transformers connected in delta-delta supply a lighting load of 100 kW. One of the transformers is damaged and removed for repairs. Find: (a) What currents were flowing in each transformer when the three transformers were in service? (b) What current flows in each transformer when the third is removed? And (c) The output kVA of each transformer if the transformers connected in open A supply the full-load with normal heating? [(a) primary = 30.3 A; secondary = 303 A %3D (b) primary = 30.3 V3 A; secondary = 303 V3 A]
Q/A single-phase transformer with a capacity of 100 kVA and a voltage of 2000/400 and the equivalent circuit elements are as follows R1=0.10 x1=0.20 R2= 0.250 X2 = 0.75 RO = 500 XO = 150 It feeds a 90 kVA load at a voltage of 2000 volts and a power factor of 8.0 delayed. Calculate the initial voltage and current? Rogl JXql a Mmm T I', 2₁ Acti Go to
Q/A single-phase transformer with a capacity of 100 kVA and a voltage of 2000/400 and the equivalent circuit elements are as follows R1=0.10 x1=0.20 R2= 0.250 X2 = 0.75 RO = 500 XO = 150 It feeds a 90 kVA load at a voltage of 2000 volts and a power factor of 8.0 delayed. Calculate the initial voltage and current? Regl www JXcql 1₁-1'₂ d Z₁
4. The load cycle of a three-phase 100 kVA, 11/0.4 kV, 50HZ transformer for one day is given in the table below. The transformer has full-load copper losses of 2.5 kW and core losses at rated voltage and frequency of 1.8 kW. Time interval 6-8 8-12 12-14 14-17 17-19 19-22 22-6 (hour of the day) Load (%) 60 90 75 95 80 15 Power factor 0.95 0.8 0.9 0.85 0.88 0.6 (a) Sketch the load cycle of the transformer for the given period. (b) Calculate the energy efficiency of the transformer for the given period.
A 100VA single-phase transformer inside a refrigerator has 100 turns in its primary coil and 20 turns in its secondary. A 120v ac source is connected to the primary. a. Is this an example of a step-up or a step-down transformer? b. What is the secondary voltage? c. Indicate why a transformer is needed in a fridge – why two different voltages are needed in a fridge, and indicate a part of the fridge that is powered by the secondary voltage of this transformer. d. Ifa 202 load is connected to the secondary, what percentage of its rated will the transformer supply? e. If the primary is reduced to 90 turns, what will the secondary voltage be with the same 120v primary source? power
6. A generator delivers power through a transmission line at 23 kV. The resulting line loss across the 23-kV transmission line is 7.6 MW. To reduce the losses, a step-up transformer rated 23 KV/230 kV is used. What will be the new loss in MW across a 230-kV transmission line? (four decimal places)
In the circuit in Figure 2, the capacitor connected to the a-b terminals of the transformer anda circuit consisting of resistance is defined as a load circuit. a)Find the average power spent by the load circuit? b) How much of the average power calculated in a) by the resistance in the load circuit and how muchspent by the capacitor? Comment on the results obtained?
1. Three 1100/110-V transformers connected delta-delta supply a lighting load of 100 kW. One of the transformers is damaged and removed for repairs. Find (a) What currents were flowing in each transformer when the three transformers were in service ? (b) What current flows in each transformer when the third is removed ? and (c) The output kVA of each transformer if the transformers connected in open A supply the full-load with normal heating ? [(a) primary = 30.3 A; secondary 303 A (b) primary = 30.3 3A; secondary = 303/3A %3! (c) 33.33 kVA) (Elect. Machines-I, Gwalior Univ. Apr. 1977)
Given : 240 /120 V , 10 KVA transformer r1=0.130 , r2= 0.030 , x1= 0.20, x2= 0.050 Find: The full load copper los using a.primary terms and b.secondary terms