# Wind Energy Questions and Answers – Wind Energy Conversion Systems (WECS) – 2

This set of Wind Energy Questions and Answers for Campus interviews focuses on “Wind Energy Conversion Systems (WECS) – 2”.

1. How is the speed varied in limited variable speed WECS?
a) Variable rotor resistance
b) Fixed rotor resistance
c) Fixed capacitance
d) Variable rotor

Explanation: Limited variable speed WECS is an evolution of fixed-speed WECS. The variable speed is achieved with the help of a variable rotor resistance which is controlled by power electronics. Thus, the total resistance is adjustable.

2. Which generator is used in variable speed WECS?
a) Squirrel-cage induction
b) Doubly-fed induction
c) Induction squirrel
d) Induction generator

Explanation: Double-fed induction generator is used in a variable speed WECS. The name doubly-fed comes from the fact that the stator voltage is applied from the grid and the rotor voltage is impressed by the power converter.

3. Which of the following components are used as converters in power electronics converter?
a) BJT
b) Earphone
c) IGBT
d) Wi-Fi driver

Explanation: Two IGBTs are used as converters, namely the rotor side and the grid side converter. BJT is not used as converter. Both, earphone and Wi-Fi driver are not electrical components. Earphones are used to listen to music and Wi-Fi driver is used to connect to the Wi-Fi.

4. What is the function of rotor side converter?
a) To control the DC-link voltage
b) To ensure operation at large power factor
c) To control generator in terms of active and reactive power
d) To ensure operation at low power factor

Explanation: The function of rotor side converter is to control the generator. It controls the generator in terms of active and reactive power by ensuring minimum power loss during power conversion. The rotor side converter is made up of IGBT.

5. What is the function of grid side converter?
a) To ensure operation at low power factor
b) To control generator in terms of reactive power
c) To control generator in terms of active power
d) To control the DC-link voltage

Explanation: The function of grid side converter is to control the DC-link voltage. It also ensures operations at a large power factor. An IGBT is used to make the converter.
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6. What is sub-synchronous mode in doubly-fed induction generator (DFIG)?
a) Rotor runs below synchronous speed
b) Rotor runs above synchronous speed
c) Rotor runs at synchronous speed
d) Slip power is fed into AC power supply

Explanation: In a doubly-fed induction generator (DFIG), sub-synchronous mode is a mode where the rotor runs below the synchronous speed. This is conducive towards using wind power at lower wind speed.

7. What is super-synchronous mode in doubly-fed induction generator (DFIG)?
a) Rotor runs below synchronous speed
b) Rotor runs above synchronous speed
c) Rotor runs at synchronous speed
d) Rotor takes power from AC mains

Explanation: In a doubly-fed induction generator (DFIG), super-synchronous mode is a mode where the rotor runs above the synchronous speed. This is conducive towards feeding the slip power into AC mains supply with the help of an inverter-converter combination.

8. Which of the following generators are mostly used in WECS?
a) PMSG and SCIG
b) DFIG and SCIG
c) DFIG and PMSG
d) VAWT and HAWT

Explanation: Doubly-fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG) are the two most commonly used generators in WECS. SCIG is typically used in a fixed speed WECS which has a low efficiency. VAWT and HAWT are types of wind turbines.

9. Which of the following is a flowchart depicting WECS modelling?
a) Wind turbine aerodynamic model → gear train model → power grid → generator (PMSG/DFIG)
b) Wind turbine aerodynamic model → generator (PMSG/DFIG) → gear train model → power grid
c) Wind turbine aerodynamic model → generator (PMSG/DFIG) → power grid → gear train model
d) Wind turbine aerodynamic model → gear train model → generator (PMSG/DFIG) → power grid

Explanation: Wind turbine is connected to the electrical generator through a coupling device gear train. The output of the generator is connected to the power grid via a power controller to minimize power loss and ensure system protection.

10. _______ speed WECS is the most flexible in terms of the generator used.
a) Full variable
b) Limited variable
c) Half variable
d) Fixed

Explanation: Full variable speed WECS is the most flexible in terms of the generator used. Various generators like squirrel-cage induction generator (SCIG), wound-rotor synchronous generator (WRSG) or permanent-magnet synchronous generator (PMSG) can be used.

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