WTT Electrical Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a wind turbine's crowbar circuit?
- To limit rotor speed during high winds
- To protect the power converter from overvoltage during grid faults (Correct answer)
- To disconnect the generator from the grid automatically
- To regulate reactive power output
Correct answer: To protect the power converter from overvoltage during grid faults
A crowbar circuit clamps excessive voltage across the DC link capacitors during grid faults, protecting the power converter from damage.
Question 2: Which type of cable insulation is most commonly specified for nacelle wiring exposed to oil and UV radiation?
- PVC (polyvinyl chloride)
- XLPE (cross-linked polyethylene)
- EPR (ethylene propylene rubber) (Correct answer)
- Neoprene-jacketed multi-conductor
Correct answer: EPR (ethylene propylene rubber)
EPR insulation offers excellent resistance to ozone, UV, and petroleum-based lubricants found in nacelle environments.
Question 3: In a doubly-fed induction generator (DFIG), the rotor windings are connected to the grid through:
- A diode rectifier bridge only
- A back-to-back AC/DC/AC power converter (Correct answer)
- A step-down autotransformer
- A soft-starter relay bank
Correct answer: A back-to-back AC/DC/AC power converter
DFIG rotors use a back-to-back converter (rotor-side and grid-side) to inject variable-frequency current and enable variable-speed operation.
Question 4: What does a power factor of 0.85 lagging mean for a wind turbine operating at rated output?
- The turbine is absorbing reactive power from the grid (Correct answer)
- The turbine is supplying reactive power to the grid
- The turbine's real and reactive power outputs are equal
- The turbine's voltage lags the grid frequency by 0.85 Hz
Correct answer: The turbine is absorbing reactive power from the grid
A lagging power factor indicates the load (or generator) is consuming reactive power (inductive behavior), drawing VArs from the grid.
Question 5: A wind turbine technician measures 480 V AC between two phases but 0 V between each phase and ground. This indicates:
- A properly grounded delta system
- An open ground fault on one phase
- A high-resistance ground on the neutral
- An ungrounded (floating) delta system (Correct answer)
Correct answer: An ungrounded (floating) delta system
In an ungrounded delta system, phase-to-phase voltage is normal but there is no reference to ground, so phase-to-ground readings are undefined or zero.
Question 6: Which instrument is best suited for locating a high-resistance connection in a wind turbine's 690 V bus system?
- Megohmmeter (Megger)
- Clamp-on ammeter
- Infrared thermographic camera (Correct answer)
- Phase sequence tester
Correct answer: Infrared thermographic camera
Infrared thermography detects heat signatures from high-resistance connections under load without requiring circuit de-energization.
Question 7: During commissioning, a technician performs a polarity test on a 34.5 kV step-up transformer. What is the purpose of this test?
- To verify the transformer turns ratio is within tolerance
- To confirm correct winding connections before paralleling or connecting to the grid (Correct answer)
- To measure insulation resistance between primary and secondary
- To check the transformer's impedance percentage
Correct answer: To confirm correct winding connections before paralleling or connecting to the grid
Polarity testing ensures the H1/X1 terminal markings are correct so the transformer can be safely paralleled or connected to the grid with correct phase relationships.
WTT Electrical Systems 2
What is the primary purpose of a wind turbine's crowbar circuit?