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EE311 ELECTRONIC AND ELECTRICAL PRINCIPLES 3

Date: xx May 2019

PART A

Attempt the following compulsory question

Q1. (a) Quantify the relative advantage of a three-phase system over an equivalent three single-phase circuits in terms of voltage regulation, i.e. voltage drop along the transmission system. 2 marks

(b) The simplified load profile shown in Figure Q1b represents the demand at a substation over a 24 hour period. Calculate the corresponding load factor. 2 marks

(c) Describe the typical architecture for a rural power distribution network, and explain two changes that could be introduced to improve the network’s reliability of supply. 2 marks

(d) A 275kV, 50Hz, three phase transmission line is 30 km long. The characteristic parameters of the transmission line are: r=0.1 Ω/km, x=1.2 Ω/km. The voltage at the sending end of the transmission line is 275 kV. The power flow at the sending end of the line is measured to be 100 MW with a power factor of 0.9, lagging. Using the short line model, determine the voltage which would be measured at the receiving end. 2 marks

(e) A 33kV/11kV transformer has a primary winding impedance of (5+ j15) Ω and a secondary winding impedance of (0.5 + j1) Ω. What is the equivalent winding impedance when all transformer parameters are referred to the primary side? 2 marks

Attempt ONE further optional question from this part.

Q2. (a) For the three phase distribution system and loads shown in Figure Q2a:

(i) Determine suitable sizes for the two cables such that the load voltages remain within statutory limits. Show all your working, and use the table of cable sizes and parameters provided in Table Q2a. 5 marks

(ii) Suppose the network designer was aware that an additional load may be connected beside Load 2. Without calculation, describe how the selected cable sizes might be affected. 2 marks

(b) The tests shown in Table Q2b have been carried out on the 400V (line-line) 50Hz star-connected three-phase induction motor shown in Figure Q2b. Calculate the parameters of the IEEE machine model and draw the equivalent circuit (labelled with parameter values). 5 marks

(c) A 690V star-connected three-phase 4-pole induction motor rated at 100 kW mechanical output has the following parameters:

R1 = 0.25 Ω                                 X1 = 0.7 Ω

R2’ = 0.3 Ω                                  X2’ = 0.7 Ω

Mechanical losses = 6 kW              Xm = 50 Ω

(i) When driving a mechanical load while connected to a 690V line-line supply, the motor is found to run at 1363 rpm. Calculate the input current, power factor, and mechanical output power of the motor in these conditions. 6 marks

(ii) Explain which circuit element should be connected in parallel with the motor to improve its power factor. 2 marks

Q3. (a) In the diagram presented in Figure Q3a a three-phase star connected load is supplied from a balanced 415V three-phase supply. The phase impedances of the load are: