# 讲解 ENGI4467/ENGI44T10 Electrical Energy Conversion 4 2024讲解 Prolog

ENGI4467/ENGI44T10-WE01

Electrical Energy Conversion 4

2024

Question 1

(a)  In your own words, explain the principle of operation of reluctance machines. You should

explain the method of torque production and describe the main design features of switched and synchronous reluctance machines. Support your answer with relevant sketches and equations. Provide qualitative comments about reluctance machine reliability when compared to conventional machines. Give examples of potential applications. Marks will be awarded for the quality of the technical discussion as well as its content. (max. 500 words) [40%]

A fixed speed 4-pole, three-phase synchronous generator is connected to a 50 Hz supply with a line voltage of 11 kV. The generator produces 6 MW (real power) at a power factor of 0.97 lagging and has a synchronous reactance of 8 Ω/phase. The excitation curve is shown in Figure Q1.

(i)       Calculate the phase reactive power and state the phase apparent power in complex notation.

(ii)       Calculate the complex current.

(iii)      Calculate the internal (open circuit) voltage (Eo ) in complex notation, the magnitude

of Eo , and the load angle (degrees).

(iv)      Find the approximate rotor excitation current, Ix . [30%]

Figure Q1

(c)  The excitation current is adjusted such that Eo   is now around 6750 V. The load angle is fixed.

(i)        Explain what has happened to the generator’s real, reactive power and power factor. Support your explanation with appropriate calculations.

(ii)       Sketch the approximate phasor diagram for this condition. [30%]

Question 2

(a) In your own words, explain the main reasons for operating wind turbines at variable speed. Give examples of how this was achieved as wind turbines developed. You should support your answer with equations and your own diagrams/sketches. [20%]

(b) The need to operate wind turbines at variable speed has driven a change in the types of electrical machines used in multi-megawatt turbines. In your own words, describe the main differences between the various generator topologies found in variable speed wind turbines. Your answer should discuss electrical design and operational characteristics of the generators  when  implement within multi-megawatt wind turbines as well as the major reliability considerations. [50%]

A 3 m diameter direct drive tidal turbine has a cut-in flow rate of 1 m/s and reaches its rated power (and maximum rotational speed) at a flow rate of 4 m/s and a tip speed ratio of 1.6. The turbine will use a permanent magnet synchronous generator that will be connected to the 50 Hz electrical grid via a back-to-back power converter that controls the generator speed. The designers have said that the machine-side converter’s frequency is limited to a possible operating range of 3-15 Hz. The density of seawater is 1020 kg/m3 .

(c) Calculate the number of generator poles required to ensure the turbine captures maximum power at the rated flow rate. [20%]

(d) Calculate the approximate power range of the tidal turbine if operated under maximum power point tracking. [10%]