Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit

EE311 ELECTRONIC AND ELECTRICAL PRINCIPLES 3

Date: xx May 2018

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 transmission power loss. 2 marks

(b) Transmission grids are highly interconnected networks. Identify and briefly explain the advantages that an interconnected grid provides to their operators and users. 2 marks

(c) A low voltage (415V) cable feeds a community of six properties, each of which have a peak load of 9kW and 0.95 lagging power factor. The community have a diversity factor of 1.5. Determine the maximum demand drawn by the community, and briefly explain how smart grid controls might change this value. 2 marks

(d) A 220kV, 50Hz, three phase transmission line is 50 km long. The characteristic parameters of the transmission line are: r=0.08Ω/km, x=1Ω/km. The voltage at the receiving end of the transmission line is 210kV. By using the short line model, determine the sending end voltage if the line is supplying a load of 100 MVA at a power factor of 0.85 lagging? 2 marks

(e) A full-loaded transformer has a copper loss of 400W and iron loss of 300W. Determine the power efficiency when the transformer is half loaded with 12kVA at a power factor of 0.8 lagging. 2 marks

Attempt ONE further optional question from this part.

Q2. (a) Consider a three phase synchronous motor. Describe the main physical parts of the motor and the basic principle by which it operates, and then sketch the motor’s per phase equivalent circuit, correctly labelling the relevant voltages, currents and impedances. 5 marks

(b) Starting from the equivalent circuit of the motor, prove that the per phase power from the synchronous machine is:

                   5 marks

(c) This three phase synchronous motor is rated as follows: 240kW, star connected, 4 pole. The motor delivers 100kW at a rotor angle of 21.5° when connected to a supply having a line voltage of 3.3kV. Assuming the armature resistance is negligible and the initial back emf equals the magnitude of the supply voltage, determine its synchronous reactance. 4 marks

(d) If the power delivered by the motor is now increased to 160kW:

(i) Calculate the new rotor angle.

(ii) Calculate the new supply current (as a phasor).

(iii) Show the resulting voltages and currents on a phasor diagram. 6 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: ZA = ZB = ZC = (11 = j4). Perform the following tasks:

(i) Derive the values of voltage and current measured by the voltmeter and ammeter shown in Figure Q3a; 2 marks

(ii) Calculate the value of real power measured by the wattmeter W1 considering its specific connection as shown in Figure Q3a. 3 marks

Assume that the impedances of the measuring instruments are negligible, i.e. zero for current measurement and infinite for voltage measurement.

(b) A small part of a three-phase network supplying a number of loads is shown in Figure Q3b. The grid infeed fault level is 750MVA.

(i) Using a common base of 100MVA, determine the resulting per unit values for the network components shown in Figure Q3b. Show these on a sketch of the single line diagram. 5 marks

(ii) Calculate the steady state fault level at busbar E in the network shown in Figure Q3b. Based on your result, choose appropriate breaking capacity of a circuit breaker for this busbar. Select from the following values: 150MVA, 200MVA, 250MVA. 8 marks

(iii) Would the circuit breaker selected in (ii) remain adequate if an additional distributed generator, with the same parameters as generator G in Figure Q3b, was connected to busbar E? Support your answer with quick calculation. 2 marks

PART B

Attempt the following compulsory question

Q4. (a) Explain why the characteristic impedance of a transmission line is a constant whereas the input impedance of a transmission line is not a constant. 2 marks

(b) State the mathematical expression used to determine the “voltage standing wave ratio” of a terminated lossless transmission line. What range of values do you expect? 2 marks

(c) Estimate the channel capacity C of a wireless transmission that uses a 54 MHz bandwidth and the signal to noise ratio is 3162. 2 marks

(d) Draw a PI diagram indicating how a laser power output P depends on a bias current I. 2 marks

(e) A plane wave in air is incident (normal to the surface) upon a lossless, nonmagnetic medium with εr = 4. What is the percentage of power transmitted into the medium? 2 marks

Attempt ONE further optional question from this part.

Q5. Answer TWO of the following topics:

(a) Material has conductivity  = 5 M/m, permittivity  = 10-9 F/m and permeability  =o = 4π × 10-7 H/m. Find the distance x at which the radio wave will be 90% attenuated if the transmitting frequency is:

(i) 50 kHz,

(ii) 50 MHz. 10 marks

(b) Based on wave attenuation and reflection measurements conducted at a frequency of 1 MHz on an unknown material, it was determined that it is a good conductor having intrinsic impedance