Generators

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

GENERATORS - OVERVIEW

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.

GENERATORS - TYPES OF DAMAGE

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.

1.https://strathprints.strath.ac.uk/53399/1/Carroll_etal_TEC2015_reliability_comparison_of_wind_turbines.pdf