Abstract
Doubly fed induction generator (DFIG) is a widely spread technology in modern wind turbines (WT) due to its capability to operate with variable speed, partial scale power converter, and ability to control active and reactive power independently. The main drawback of DFIG is its complicated protection systems. In the chapter, several strategies for DFIG protection are reviewed, and the authors provide a conclusion about their advantages. Penetration of renewable energy sources (in particular, wind power) have a huge impact on power systems; thus, wind turbines should be considered as conventional generation units in terms of frequency and voltage regulation. Modern grid codes require WT stay connected during grid fault and be capable to provide appropriate grid support. Therefore, it is important to implement a DFIG protection system that could meet grid code requirements.
TopBackground
In the last decade the penetration of the wind power into the power system has large-scale, and tripping several wind turbines (WTs) during voltage dips can further risk the stability of power system, and as worst case a system collapse (Hansen, Michalke, Sorensen, Lund, & Iov, 2007). So modern grid code require from wind turbine (WT) to act like conventional power generation unit and provide the capability of active and reactive power control, and more important, have an ability to ride through low voltage case.
Doubly fed induction generator (DFIG) has many advantages in terms of independent control of active and reactive power control in the range wider in comparison with the permanent magnet synchronous generator (PMSG). The main drawback of WTs with doubly fed induction generator is their vulnerability to grid disturbances: even small voltage sag can cause WT disconnection and even worse consequences. Protection circuit should be implemented properly to provide WTs ability to stay connected during the fault and support grid with reactive power injection.
This problem of DFIG was raised since DFIG found their application in WT industry. Earlier protection schemes provided a capability for WTs stay connected, but they were unable to support grid with reactive power injection.
To reduce excessive currents in the rotor during the fault, some new fault-control solutions for the rotor side converter (RSC) of DFIG have been developed. Stator transient impact on the DFIG fault ride through (FRT) capability is considered by means of introducing differential terms of stator flux into rotor voltage feed forward compensation terms (Hu, Sun, He, Zhao, 2006), and by the feed-forward of the faulty stator voltage (Liang, Qiao, Harley, 2010). Moreover, the RSC can be protected by counteracting zero and negative sequences of the stator flux (Xiang, Ran, Tavner, Yang, 2006) or by the feed-forward of the measured stator currents as a reference for the rotor currents controllers (Lima et al., 2008). Nevertheless, these techniques are not suitable for severe voltage sags. To keep DFIG connected to the grid during fault and post-fault condition without additional circuits in (Rathi, Mohan, 2005) proposed a method, where the controller is designed using H∞ technique and μ-analysis. However, the method is complicated and need a lot of computational resources and time. Authors in (Zhou, He, Sun, 2009; Santos-Martin, Rodriguez-Amenedo, Arnaltes, 2009; Shang, Hu, 2012), pay attention on improving performance during unsymmetrical fault case.
Nevertheless, the optimal strategy is not defined, because protection circuits developed during a long time, with different technologies and cost of power electronics. Different papers focused on one aspect of DFIG behavior during fault but do not consider others.
Key Terms in this Chapter
LVRT: Low voltage ride through is a capability of generation unit to withstand low voltage conditions.
Crowbar: A shunt resistance, connected into rotor circuit, and protecting RSC from overcurrent.
GSC: Grid side converter is an invertor, connected from the grid side. It has the general task of maintaining constant voltage on the DC side.
DFIG: Doubly fed induction generator is a wound rotor induction generator in which rotor windings powered by variable frequency, provided by power converter via slip rings.
RSC: Rotor side converter is an invertor, connected to the DFIG rotor. The main task is to regulate active and reactive output from DFIG.
SDRB: Series dynamic braking resistors is a protection device that can be connected to the rotor or stator side and damps excessive power.
Power Converter: A power electronics device, consisted of two inverters in back-to-back configuration for rectifying one frequency f 1 and then inverting DC into frequency f 2 . During this frequency conversion process power transferred as well.