Subject Title : Introduction to Electrical Systems
Subject Code : SEHH2270
Electrical Systems代考 Detemine the intemal generated voltage of the generator if it is supplying 90% rated current at 0.95 power factor lagging.
This question paper has a total of SIX pages (including this covering page).
Instructions to Candidates:
1. There are TWO sections in this paper.
Section A (60%) – Answer ALL questions in this section in the answer book provided.
Section B (40%) -Answer any TWQ out of THREE questions. Each question carries 20marks. If you answer more than two questions, only the first two attempted questions will be marked. Indicate in your answer book clearly which two questions you are attempting. Electrical Systems代考
2. Begin each question on a fresh page in the answer book provided.
3. Show all your calculations clearly and neatly. Reasonable steps should be shown.
4. Candidates are NOT allowed to retain this paper.
5. Some useful equations are provided on pages 5 and 6.
6. Electronic calculators, including programmable calculators, may be used provided that the calculators are battery powered, silent in operation, with neither printout nor graphic / word display facilities and do not use dot-matrix technology in the main display. All programmes stored in the calculator should have been cleared. Other electronic devices with graphic /word-display facilities (such as databank watches) are not permited.
Constants: Electrical Systems代考
Permitivity of free space (8) = 8.85 x 10-12 Permeability of free space (40) = 4π x10-7 H/m
Authorised Materials:
CALCULATOR YES NO
(All programmes stored should be cleared.) [√] [ ]
SPECIFICALLY PERMITTED ITEMS [ ] [√]
Section A (60%) Electrical Systems代考
Answer ALL questions in this section in the answer book provided.
Question A1
A 20 kVA, 440/ 220 V, 50 Hz single phase transfomer has the following test resuts:
Open-circuit test (measured from high-voltage side): Voc = 440 V, loc= 1.675 A, Poc= 240 w
Short-circuit test (measured from low-voltage side): Vsc= 13.4V, Isc= 90A, Psc= 195 w
Find the approximate equivalent circuit parameters of the transformer referred to the high-voltage side.
Question A2
Figure 1 shows a single-phase power system.
(a) If we choose Vas1 = 23 kV and Sbael = 100 MVA as the base voltage and base power in region 1, what are the base values of voltage and power in regions 2 and 3?
(b) Based on results in part (a), determine Zoase in regions 2 and 3.
Question A3
(a) Briefly describe ONE type of power plant that you will suggest to a power company to construct for handling the future increasing demands of electricity use.
(b) Name ONE type of circuit breaker and how it can protect the electrical power system.
Question A4 Electrical Systems代考
A 3-phase, Y-connected, 50 Hz, 4-pole, 415 v, suirercage induction motor has the fllowing test data:
Determine the equivalent circuit parameters of the motor.
Question A5
A stranded conductor consists of 4 identical round conductors each having a radius of 0.5 cm as shown in Figure 2 below. Determine the GMR of the stranded conductor.
Section B (40%)
Answer any TWO out of THREE questions. Each question carries 20 marks. If you answer more than two questions, only the first two attempted questions will be marked. Indicate in your answer book clearly which two questions you are attempting.
Question BI
A 20 MVA, 11 kV, 3-phase, Y-connected, non-salient pole synchronous generator has a synchronous reactance ofj5.2 Q per phase and negligible armature resistance.
(a) Detemine the intemal generated voltage of the generator if it is supplying 90% rated current at 0.95 power factor lagging.
(b) Using the result in part (a), determine the new armature current of the generator if the field current is increased by 20%.
(C) Based on resuts in parts (a) and (), compute the real and reactive powers of the generator.STATE whether the generator absorbs or supplies real and reactive powers.
Question B2 Electrical Systems代考
A 3-phase, Y-connected, 50 Hz, 6-pole, 380 V, squirel-cage induction motor has the fllowing equivalent circuit parameters:
R1=1Ω，R2=4.19， X1=Xz=j5.64Ω， XM= j30.3 Ω.
Assume the rotational loss of the motor is 23 W.
(b) Determine the output power of the motor.
(b) Determine the output power of the motor.
Question B3
A 500 V DC shunt motor takes 35 A and runs at 1200 rpm at full load. The armature resistance is 1.2 Q and the field resistance is 200 8. Assume magnetic linearity and flux is directly proportional to the field current.
(a) Find the internal generated voltage of the motor at full load.
(b) Determine what resistance should be inserted in series to the field resistance to reduce the speed to 1000 rpm at the same torque.
