Q1. A// Choose the correct answer: 1. One of the advantage of using a per unit system in power system calculations is: a) To increase the accuracy of the results b) no computation are required to refer an impedance from one side of a transfo c) The per unit impedances of machines of the same type and widely different rating usually lie within a wide range. 2. The load is modelled in fault calculation of a power system by: a) A constant impedance b) An EMF in series with an impedance c) A constant impedance for a static load and an EMF in series with an impedance for rotating machines 3. The most famous fault in a power system is a) Symmetrical three phase fault b) Single line to ground fault c) Line to line fault 4. The type of unsymmetrical faults at which the sequence currents are equal( i.e 19 = 1 = 1) is: %3D a) Double line to ground fault b) Line to line fault c) Single line to ground fault 5. The ohmic reactance of a 0.1 pu, 30 kVA, 240/480 V single phase transformer referred to the low voltage side is: a) 0.768 2 b) 6.25 2 c) 0.192 2

Q1. A// Choose the correct answer: 1. One of the advantage of using a per unit system in power system calculations is: a) To increase the accuracy of the results b) no computation are required to refer an impedance from one side of a transfo c) The per unit impedances of machines of the same type and widely different rating usually lie within a wide range. 2. The load is modelled in fault calculation of a power system by: a) A constant impedance b) An EMF in series with an impedance c) A constant impedance for a static load and an EMF in series with an impedance for rotating machines 3. The most famous fault in a power system is a) Symmetrical three phase fault b) Single line to ground fault c) Line to line fault 4. The type of unsymmetrical faults at which the sequence currents are equal( i.e 19 = 1 = 1) is: %3D a) Double line to ground fault b) Line to line fault c) Single line to ground fault 5. The ohmic reactance of a 0.1 pu, 30 kVA, 240/480 V single phase transformer referred to the low voltage side is: a) 0.768 2 b) 6.25 2 c) 0.192 2

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

Chapter7: Symmetrical Faults

Section: Chapter Questions

Problem 7.6P

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Consider the adopted per-unit system for the transformers. Specify true or false for each of the following statements: (a) For the entire power system of concern, the value of Sbase is not the same. (b) The ratio of the voltage bases on either side of a transformer is selected to be the same as the ratio of the transformer voltage ratings. (c) Per-unit impedance remains unchanged when referred from one side of a transformer to the other.
With the same transformer banks as in Problem 3.47, Figure 3.41 shows the oneline diagram of a generator, a step-up transformer bank, a transmission line, a stepown transformer bank, and an impedan load. The generator terminal voltage is 15 kV (line-to-line). (a) Draw the per-phase equivalent circuit, aounting for phase shifts for positive-sequence operation. (b) By choosing the line-to-neutral generator terminal voltage as the reference, determine the magnitudes of the generator current, transmiss ion-line current, load current, and line-to-line load voltage. Also, find the three-phase complex power delivered to the load.
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.
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.
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TOPIC: Transmission Lines, Power Systems, and Powerplants:INSTRUCTIONS:- Answer in this format: Given, Illustration, Required Conversion, Solution, Final Answer.- Step-by-step solution, do not skip even simple calculations to avoid confusion.- If answered in written form, make sure it is readable.PROBLEM:The per-unit impedance of a system is 0.7 pu. The base KVA is 300 kVA, and the base voltage is 11 kV. What is the per-unit impedance referred to as the 400-KVA and 38-KV base values? 0.0782 0.0735 0.0674 0.0691
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