WTT Electrical Systems & Power Generation 3 — Questions and Answers
Question 1: Which standard governs the testing and certification of electrical equipment used in potentially explosive atmospheres in wind turbine nacelles?
- IEC 61400-1
- ATEX / IECEx (IEC 60079 series) (Correct answer)
- NEC Article 430
- IEEE 519
Correct answer: ATEX / IECEx (IEC 60079 series)
ATEX (EU) and IECEx (international), based on the IEC 60079 series, govern equipment used in explosive atmospheres such as areas near hydraulic oil mist or battery rooms.
Question 2: What is the function of a varistor (MOV) installed across the terminals of a wind turbine's electrical panel?
- To regulate output voltage under varying load conditions
- To absorb transient overvoltages and protect downstream components (Correct answer)
- To provide galvanic isolation between control and power circuits
- To limit inrush current during transformer energization
Correct answer: To absorb transient overvoltages and protect downstream components
Metal oxide varistors clamp transient overvoltages (lightning, switching surges) by conducting heavily once a threshold voltage is exceeded, protecting connected equipment.
Question 3: A technician measures 480 V line-to-line on a three-phase system. What is the approximate line-to-neutral voltage?
- 240 V
- 277 V (Correct answer)
- 208 V
- 339 V
Correct answer: 277 V
Line-to-neutral voltage equals line-to-line voltage divided by √3 (1.732), so 480 ÷ 1.732 ≈ 277 V.
Question 4: What is the primary reason wind turbines use IGBT-based converters rather than SCR (thyristor) converters for variable-speed operation?
- IGBTs are less expensive and easier to source globally
- IGBTs can be turned both ON and OFF by gate signal, enabling PWM for sinusoidal output (Correct answer)
- SCRs require liquid cooling not available in nacelle environments
- IGBTs operate at lower voltages suitable for generator output
Correct answer: IGBTs can be turned both ON and OFF by gate signal, enabling PWM for sinusoidal output
IGBTs are fully controllable (turn-on and turn-off), allowing PWM control for low-harmonic sinusoidal output; SCRs are only turn-on controllable and require natural or forced commutation.
Question 5: In a Type 4 (full-converter) wind turbine, what component decouples the generator frequency from the grid frequency?
- The step-up transformer
- The full-scale power converter (back-to-back converter) (Correct answer)
- The gear box ratio selection
- The soft-starter relay
Correct answer: The full-scale power converter (back-to-back converter)
The back-to-back full-scale converter rectifies generator AC to DC, then inverts to grid-frequency AC, completely decoupling generator and grid frequencies.
Question 6: When megger testing (insulation resistance testing) cables in a wind turbine, what minimum insulation resistance is generally considered acceptable for 600 V rated cables?
- 100 kΩ
- 1 MΩ (Correct answer)
- 100 MΩ
- 1 GΩ
Correct answer: 1 MΩ
IEEE 43 and general industry practice set 1 MΩ as the minimum acceptable insulation resistance for low-voltage rotating machine and cable insulation systems.
Question 7: What protection relay function detects when a wind turbine continues to energize a de-energized section of grid (islanding)?
- ANSI 27 — undervoltage relay
- ANSI 81 — frequency relay (over/under frequency) (Correct answer)
- ANSI 50/51 — overcurrent relay
- ANSI 87 — differential relay
Correct answer: ANSI 81 — frequency relay (over/under frequency)
Anti-islanding protection uses frequency relays (ANSI 81) and voltage relays to detect the abnormal frequency or voltage that occurs when a turbine feeds an isolated grid section.
WTT Electrical Systems & Power Generation 3
Which standard governs the testing and certification of electrical equipment used in potentially explosive atmospheres in wind turbine nacelles?