WTT Electrical Systems & Power Generation 2 — Questions and Answers
Question 1: What is the primary function of a crowbar circuit in a wind turbine's power converter?
- To disconnect the generator from the grid during faults
- To protect the DC link capacitors by shorting the rotor circuit during voltage transients (Correct answer)
- To regulate reactive power output during normal operation
- To synchronize the generator frequency with the grid
Correct answer: To protect the DC link capacitors by shorting the rotor circuit during voltage transients
A crowbar circuit protects the rotor-side converter by shorting the rotor windings during grid voltage transients, preventing DC link overvoltage.
Question 2: Which type of grounding system is most commonly used in wind turbine nacelle electrical panels in the US?
- Ungrounded delta
- High-resistance grounded
- Solidly grounded wye (Correct answer)
- Corner-grounded delta
Correct answer: Solidly grounded wye
Solidly grounded wye systems are standard in US wind turbine panels because they limit fault voltages and are required by NEC for most applications.
Question 3: A wind turbine's power factor is measured at 0.85 lagging. What corrective action would bring it closer to unity?
- Add series inductance to the output circuit
- Inject capacitive reactive power via the inverter or capacitor banks (Correct answer)
- Reduce the active power output of the generator
- Increase the rotor excitation current to draw more reactive power
Correct answer: Inject capacitive reactive power via the inverter or capacitor banks
Injecting capacitive reactive power (leading) offsets the inductive (lagging) component, improving power factor toward unity.
Question 4: What does the term 'slip' refer to in an induction generator used in a wind turbine?
- The mechanical play between the gearbox and shaft coupling
- The difference between rotor speed and synchronous speed expressed as a percentage (Correct answer)
- The voltage drop across the stator windings at rated load
- The angular offset between rotor magnets and stator poles
Correct answer: The difference between rotor speed and synchronous speed expressed as a percentage
Slip is the percentage difference between synchronous speed and actual rotor speed; in an induction generator operating above synchronous speed, slip is negative.
Question 5: During a Low Voltage Ride-Through (LVRT) event, what is the wind turbine required to do according to US grid codes?
- Trip offline immediately to protect internal components
- Remain connected and support the grid with reactive current injection (Correct answer)
- Switch to islanded operation using local battery storage
- Reduce active power to zero and wait for voltage recovery
Correct answer: Remain connected and support the grid with reactive current injection
US grid codes (FERC Order 661-A) require wind turbines to stay connected during voltage dips and inject reactive current to support grid voltage recovery.
Question 6: What is the purpose of a dV/dt filter installed on the output of a wind turbine's PWM inverter?
- To reduce harmonic currents injected into the grid
- To limit the rate of voltage rise that could damage generator winding insulation (Correct answer)
- To improve the power factor at the point of common coupling
- To synchronize switching frequency with grid frequency
Correct answer: To limit the rate of voltage rise that could damage generator winding insulation
PWM inverters produce fast voltage pulses; dV/dt filters slow these transitions to protect motor/generator windings from insulation stress caused by voltage reflections.
Question 7: A 690 V wind turbine generator output is stepped up to 34.5 kV for collection. What transformer winding configuration is most common for this application?
- Delta-Delta
- Wye-Wye
- Delta primary, Wye secondary (Correct answer)
- Wye primary (grounded), Delta secondary
Correct answer: Delta primary, Wye secondary
A Delta-Wye (delta primary, grounded wye secondary) configuration is standard for wind turbine pad-mount transformers, providing grounding on the MV collection side.
WTT Electrical Systems & Power Generation 2
What is the primary function of a crowbar circuit in a wind turbine's power converter?