Research Focus: Improving wind turbine power conversion in variable speed wind turbines

Kenneth Eloghene Okedu, Kitami Institute of Technology, Department of Electrical and Electronic Engineering, Japan.

A new section where authors highlight their peer-reviewed research published in Renewable Energy Focus Journal begins with a novel solution that could potentially protect Variable Speed Wind Turbine Doubly Fed Induction Generator converters during grid fault scenarios.

Title of Peer Reviewed and Accepted Research Paper

  • The augmentation of Doubly Fed Induction Generator (DFIG) variable speed wind turbines with a new T-type Grid Side Converter (GSC) during transient conditions (Click here for full text access to the Paper, subscription or pay per view available).


  • Kenneth Eloghene Okedu, Kitami Institute of Technology, Department of Electrical and Electronic Engineering, Japan.

What are the key findings of your research (in brief)?

The proposed T-type Grid Side Converter (GSC) DFIG converter scheme in the research paper is much easier to achieve with less switching circuitry when compared to the conventional 3-level multilevel converters. Also, this scheme avoids the use of an external traditional expensive control strategy in achieving DFIG wind farm stability during grid disturbance.

The proposed converter topology for DFIG wind generators creates room for a sophisticated and robust converter controller implementation that would protect the fragile, vulnerable VSWT DFIG converters during grid fault scenarios.

Consequently, it would enable modern wind farms taking advantage of DFIG to remain grid connected - based on the grid codes or requirements to supply power to utilities during occurrences of “transients” - instead of having to disconnect from the grid. Also, instead of phasing out earlier wind turbines using the fixed speed technology, wind farms in the future could be composed of both fixed and variable speed wind turbines. This is because the variable speed wind turbines could help stabilise the fixed speed wind turbines and the entire wind farm during grid disturbances, at a lower cost, and without requiring expensive external reactive power compensators.

Can you give some more broad, technical details?

Amongst the various renewable energy sources, the use of wind energy is very common, considering the technological advancement of wind turbines and power electronics applications.

The common wind turbines either use Fixed Speed Wind Turbines (FSWT), made of the Squirrel Cage Induction Generator (SCIG) technology; or the Variable Speed Wind Turbine (VSWT), made of the Doubly Fed Induction Generator (DFIG)/Permanent Magnetic Synchronous Generator (PMSG).

The response of wind generators to grid disturbances is an important issue nowadays due to increased wind on the grid. Therefore, it is important that utilities and grid companies study the effects of various voltage sags on the corresponding wind turbine responses. Grid codes or requirements were formulated with the effective operation of grid-connected wind farms in mind. Such emerging grid codes demand that wind farms should have a good performance with respect to voltage control capability, and robust behaviour against frequency and voltage variations under fault condition.

The SCIG is used in general for FSWT generators due to its superior characteristics (i.e. brushless and rugged construction; low cost; maintenance free; and operational simplicity). However, it requires large reactive power to recover the air gap flux when a short circuit fault occurs in the power system. Therefore, reactive power compensation from power network or other devices is needed for a FSWT wind farm. The installation of compensation units in FSWT farms to overcome voltage sag during a grid fault (like static synchronous compensators (STATCOM); superconducting magnetic energy storage (SMES); and energy capacitor system (ECS) increases the system overall cost.

In general, the amount of the necessary dynamic reactive power compensation depends on the type of wind turbine generator system (WTGS), and is influenced by the relevant electrical and mechanical parameters of that system.

The doubly fed induction generator (DFIG) has very attractive characteristic as a wind generator because the power processed by the converter is only a fraction of the total power rating of the DFIG. This is typically 20%-30%, and therefore its size, cost, and losses are much smaller compared to a full size power converter used in other types of variable speed wind generators.

DFIG can operate at a wide range of speeds (depending on the wind speed or other specific operation requirements). Thus, it allows better capture of wind energy. The dynamic slip control and pitch control are the other salient features which help to augment the system stability. In addition, DFIG has better system stability during short-circuit faults in comparison with IG, because of its capability of independent control of active and reactive power output. This superior dynamic performance of the DFIG results from the frequency converter, which typically operates with sampling and switching frequencies of above 2 kHz. Therefore, it is paramount to use a VSWT system like a DFIG to stabilise a FSWT (IG) in a wind farm, because the DFIG system can also control reactive power in a similar manner to a STATCOM, SMES, or ECS, and thus the reactive power compensation can be implemented at a lower cost.

Why do these findings matter?

The results show improved performance of the DFIG wind generator during transient conditions considering the proposed scheme, thus saving the cost of traditional external expensive circuitry and more switching Insulated Gate Bipolar Transistors (IGBTs) used in the conventional 3-level multilevel converters for the DFIG Fault Ride Through (FRT) capability.

In addition, the simple extension of the conventional 2-level Grid Side Converter (GSC) of the DFIG wind generator to a 3-level topology by the bidirectional middle switch with two Insulated gate Bipolar Transistors (IGBTs) in common emitter connections, leads to low switching losses; low forward voltage drop; and only one more isolated gate drive supply per phase leg; when compared with the conventional 2-level converter.

Accessing the full text version of the Paper
'The augmentation of Doubly Fed Induction Generator (DFIG) variable speed wind turbines with a new T-type Grid Side Converter (GSC) during transient conditions (Click here for full text access to the Paper)'.

About the Author
Kenneth Eloghene Okedu was Massachusetts Institute of Technology (MIT) ETT Fellow, in the department of Electrical and Computer Engineering, 2013 at Cambridge, Boston, USA. He obtained his Ph.D. in the department of Electrical and Electronic Engineering, Kitami Institute of Technology, Hokkaido, Japan in 2012. He received his B.Sc. and M. Eng. degrees in Electrical and Electronic Engineering from the University of Port Harcourt, Nigeria in 2003 and 2007 respectively, where he was retained as a Faculty since 2005 till date.  He was also a visiting Faculty to The Petroleum Institute, (ADNOAC), Abu Dhabi. He is presently a visiting Faculty to Caledonian College of Engineering, Muscat, Oman (Glasgow Caledonian University). His research interests include renewable energy, stabilization of wind farm using doubly fed induction generator variable speed wind turbine, augmentation and integration of renewable energy into power systems, grid frequency dynamics considering wind energy penetration, FACTS devices and power electronics, renewable energy storage and energy management systems, hydrogen, fuel cells and power system stability.

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