AXIS Explores
Generators
Overview
Generators consist of a rotating rotor with magnetic poles, surrounded by a static stator which contains the wiring coils.
Generators for WTGs are either Double Fed Induction Generators (DFIG) or Permanent Magnet Generators (PMG).
DFIG
- Geared WTGs use DFIG
- DFIG is the most common type of WTG for onshore wind
PMG
- Generally used by Direct Drive WTGs
- Traditionally more expensive although prices are dropping
Comparison of generators
DFIG
Pros:
- Cheaper and simpler gearbox helps absorb loads
Cons:
- Rotors hard to repair
- Current can pass through bearings
PMG
Pros:
- More efficient, lower maintenance
- No windings in rotor to fail
Cons:
- Complicated control system, including a fully rated converter
Most generator failures require repairs as opposed to replacement which reduces OpEx costs.
For DFIG WTGs, main causes of failure are:
- Slip ring failures
- Bearing failures caused by either gearbox/generator misalignment, inadequate lubrication, or electrical discharge from the converter
- Faults in the rotor or stator winding, e.g. insulation issues
For PMG WTGs, main causes of failure are:
- Converter failures
- Cooling system problems resulting in excessive heat or fire
- Inadequate lubrication
A study in 20151 found that generator failure rates for PMG WTGs are nearly three times that of DFIG WTGs.
This was driven by converter failures being six times higher in PMG vs DFIG; Failure rates of the actual generator in PMGs were significantly lower than in DFIGs.
