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Tampere University of Technology
Electrical Engineering

Wind Power Systems DEE-54107 15.12. 2016

Enrigue Acha Programmable calculator allowed 5 guestions/& 6 p

Ouestion 1:
A) Explain briefly what a wind rose is and sketch a typical twelve-segment wind rose.

(1p)

B) Discuss briefly the impact that doubling the size of the turbine”s rotor diameter has
in power extraction from the wind by the turbine. (1p)

C) Draw the output power-wind speed characteristic of a wind turbine and label all its
relevant parts. (1p)

D) State what the tip speed ratio is. (1p)

E) Describe briefly how the power take-up may be controlled in a fixed-speed, large
wind turbine. (1p)

F) Describe briefly the role of the gear-box in fixed-speed, wind turbines which are
connected to the electric power grid. (1p)

Ouestion 2: JN p
A) When investigating suitable locations for a wind farm, tests show that Location 1
has 10% more wind resource than Location 2. Estimate the potential increase in
power that could be obtained by choosing Location 1 over Location 2. (1p)

B) Explain the term array loss in connection with a wind farm. (2p)

€) With the power coefficient, C,, expressed in terms of the interference factor, a, i.e.,

C, =4a(1-a)

show analytically or otherwise that C, carries a maximum value of 59%. (3p)

Ouestion 3:

A) State the main differences, in terms of connection to the supply, between a sguirrel-
cage induction generator with and with no po jätimies and a doubly-fed
induction generator when used in a wind turbine application. State what are the
advantages of the doubly-fed induction generator over the sguirrel-cage induction
generator with power electronics and the sguirrel-cage induction generator with no
power electronics? (4p)

B) The power input of a three-phase induction machine is 60 kW. The stator losses
total 1.5 kW. Find the total mechanical power developed by the machine when it
is made to operate as a generator with a slip of -1%. Assume rotationa! losses of

1.5%. (2p)
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Tampere University of Technology
Electrical Engineering

Wind Power Systems DEE-54107 15.12. 2016

Enrigue Acha Programmable calculator allowed 5 guestions/ & 6 p

Ouestion 4:
A) Sketch the per-phase eguivalent circuit of a three-phase, sguirrel-cage induction
generator, labelling carefully all its resistance and inductive reactance components.

(p)

B) For the per-phase eguivalent circuit of a three-phase, sguirrel-cage induction
generator, derive an expression for the eguivalent impedance, as seen from its
stator terminal. Make sure that you include the stator and rotor circuits as well as
the magnetizing branch. (2p)

C) Derive expressions for the active power and the reactive power at the generator”s
terminal, as a function of its eguivalent impedance as seen from its terminal and
the terminal voltage. (2p)

Ouestion 5: Describe in detail the role that HVDC transmission plays in evacuating the power
output of off-shore wind farms. In particular, state which are the transmission distances
where HVDC transmission may be the only practical option for the connection of off-
rss "shore-wind-farms. Use-as many-eguations and diagrams that you-may find-appropriate==
to aid your description. (6p)

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