WTT Electrical Systems 3 — Questions and Answers
Question 1: What is the function of the slip rings and brushes in a wound-rotor induction generator?
- To transfer mechanical torque from the rotor to the nacelle frame
- To carry electrical current between the stationary and rotating rotor circuits (Correct answer)
- To provide a path for grounding the rotor shaft
- To sense rotor position for the pitch control system
Correct answer: To carry electrical current between the stationary and rotating rotor circuits
Slip rings and brushes create the electrical connection that allows current to flow between the stationary converter and the rotating rotor windings in a DFIG.
Question 2: A wind turbine's 24 VDC control system shows a ground fault alarm. The FIRST step a technician should take is:
- Replace the control panel fuses immediately
- Isolate circuit sections systematically to locate the fault before further energizing (Correct answer)
- Reset the alarm and continue monitoring
- Replace the entire cable harness in the nacelle
Correct answer: Isolate circuit sections systematically to locate the fault before further energizing
Systematic isolation narrows the fault location safely and prevents damage or misdiagnosis before the fault source is confirmed.
Question 3: What standard specifies electrical safety requirements for working on or near wind turbines in the United States?
- IEC 61400-1
- NFPA 70E (Correct answer)
- IEEE 519
- ANSI/UL 508A
Correct answer: NFPA 70E
NFPA 70E establishes arc flash and shock protection requirements for electrical workers, including wind turbine technicians in the US.
Question 4: In a three-phase system, if phase A current is 120 A, phase B is 118 A, and phase C is 119 A, the system is best described as:
- Severely unbalanced and requiring immediate shutdown
- Balanced within acceptable limits for generator operation (Correct answer)
- Open-phased on one winding
- Operating in single-phase mode
Correct answer: Balanced within acceptable limits for generator operation
A current imbalance of less than 2% (here about 1.7%) is within typical acceptable limits for three-phase generator operation.
Question 5: The purpose of a load break switch in a wind turbine's medium-voltage collection system is to:
- Interrupt fault current up to the system's short-circuit rating
- Disconnect circuits under normal load current without interrupting fault current (Correct answer)
- Provide surge protection for the step-up transformer
- Measure energy production for revenue metering
Correct answer: Disconnect circuits under normal load current without interrupting fault current
A load break switch can open and close under normal load conditions but is not rated to interrupt short-circuit fault currents, unlike a circuit breaker.
Question 6: What is the typical frequency of the AC voltage produced by a wind turbine generator before conversion for US grid connection?
- 60 Hz, matching the grid at all times
- Variable frequency, depending on rotor speed (Correct answer)
- 50 Hz, to match European standards
- 120 Hz, due to the high pole count
Correct answer: Variable frequency, depending on rotor speed
Variable-speed wind turbines produce variable-frequency AC that is converted to DC and then inverted to fixed 60 Hz for grid connection.
Question 7: Which protection relay function is used to detect a loss-of-grid (islanding) condition in a wind turbine?
- Overcurrent relay (ANSI 51)
- Under/over frequency relay (ANSI 81) (Correct answer)
- Differential relay (ANSI 87)
- Overvoltage relay (ANSI 59)
Correct answer: Under/over frequency relay (ANSI 81)
Under/over frequency relays (ANSI 81U/81O) detect frequency deviations caused by islanding and trip the turbine off the grid to prevent unsafe energization of a de-energized line.
WTT Electrical Systems 3
What is the function of the slip rings and brushes in a wound-rotor induction generator?