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Speed Variation

Operation at variable speed offers the possibility of increased ‘grid friendliness,’ load reduction and some minor energy benefits.  Among wind turbines over 1 MW, out of 95 distinct models from 29 different manufacturers, 9 were fixed speed, 14 had two-speed systems and 72 employed variable speed.  This shows that it is almost mandatory for MW scale turbines to have some degree of speed variation and that continuously variable speed is the predominant choice.

Variable speed operation is realised in many ways, each differing in significant details.  Direct drive systems have a natural capability for a very wide speed range, although even there some restriction on minimum speed may reduce the cost of power electronics.  The ‘conventional’ variable-speed concept, using a geared drive train, connects the generator to the network through a power electronic converter and enables systems that may have wide or narrow speed ranges.  The electrical energy is generated at variable frequency – a frequency related to the rotational sped of the rotor – and then converted, by the converter or inverter (both power electronic devices) to the frequency of the grid.  There are several possible configurations, based on both synchronous and induction generators.

The most popular system is currently the doubly fed induction generator (DFIG), also called the wound rotor induction generator (WRIG).  This provides almost all the benefits of full-range variable speed drives, but only a proportion, perhaps one third, of the power passes through the converter.  The power converter thus has approximately a third of the size, cost and losses of a conventional variable-speed drive.  In this concept, the stator of the electrical machine is connected directly to the network, and the rotor circuit is connected via the power converter.  This is a modern version of the classical Kramer or Scherbius system.  The DFIG has a more limited speed range than the conventional variable-speed drive (approximately 1.5-2:1, compared to 2.5:1 or more).  This speed range, however, is sufficient to provide the benefits listed above.  The conventional option of a power converter, with the same rating as the generator, is unlikely to compete with the DFIG until the cost of power electronic converters falls substantially and the efficiency improves.  There is evidence that this point may have been reached, with some manufacturers moving over to fully rated converters.  In this respect, the potential for improved efficiency in avoiding the DFIG route may come to outweigh cost differentials.  Also, some of the benefit of a DFIG system has been eroded by more stringent network requirements impacting on DFIG system cost.

The so-called 'squirrel-cage induction generator' may be used with a fully rated converter, as in some Siemens designs.  Other novel generator configurations have been proposed for wind turbine applications, including the Switched Reluctance machine (SR – also known as variable reluctance).  All rely on full-size power converters, and are therefore also at a disadvantage relative to the DFIG.  The DFIG configuration used at present requires slip-rings to transfer power to and from the rotor circuit.  There is an alternative method, which in effect transfers the rotor power magnetically, called the brushless doubly-fed induction generator (BDIG), which avoids the use of slip-rings.  However, at least one generator manufacturer has concluded that such machines are inherently larger and more expensive than the slip-ring option.  There is no commercial turbine using the BDIG.  As the experience of WRIG with slip rings is good in wind turbines, this remains the preferred option.  Slip-ring maintenance intervals of six months are achieved, and may be stretched to yearly.