Research

Direct power control - a new control method for wind power generations
Wind power is one of the most promising and fastest growing renewable energy resources. The wind power would generate over 1,866 TWh of electricity by 2020. This would mean that wind power would meet about 8.0% of global electricity demand in 2020 [1]. In the global wind market, the doubly-fed induction generator (DFIG) has become one of the most popular configurations in wind energy generation systems. The DFIG has several advantages including the maximum power capture over a wide speed range and decoupled active and reactive power control. It also allows the use of a partially rated converter which reduces the system cost.
However, previous control methods are mainly based on vector control (VC). In VC approach, a modulator such as space vector modulation (SVM) is needed to generate the gates-driving pulses. Besides, it requires appropriate decoupling, introduces rotary transformation and needs much tuning work to ensure the system stability.
To overcome the large amount of tuning work and reduce the control complexity in the VC, direct power control (DPC) were proposed for DFIGs in wind energy systems [2]. The actual output powers P and Q are first estimated, then compared with the references P_ref and Q_ref. The errors are sent to two fixed band hysteresis comparators to produce digitized signals dP and dQ. Next, the voltage vector is selected from a pre-defined switching table according to the digitized signals and the position of the generator flux [3]. DPC is characterized by fast dynamic response and simple structure, so it has attracted much attention in both academic and industry communities.


 

Fig. 1 Configuration of the DFIG based wind turbine system.
Fig. 1 shows a cascaded brushless DFIG based wind power generation systems and its electric control center. The cascaded brushless DFIG is constructed by two identical wound-rotor induction machines, which is driven by a wind turbine. Fig. 2 presents the dynamic response of the cascaded brushless DFIG with sinusoidal active power reference under variable speed from 0.6 p.u. (450 rpm) to 1.3 p.u. (975 rpm). It can be seen that during the dynamic process, the active and reactive powers can track their references very well, showing excellent dynamic performance and tracking ability. In addition, the stator and rotor currents vary with powers in a safe manner.









Fig. 2 Experimental results: (a) active power and reactive power, (b) rotor current, stator current and wind speed.

To sum up, this work proposes a new control strategy of DFIG based on DPC for wind energy applications. The active and reactive powers are directly controlled by applying a proper voltage vector according to a switching table, thus the control system is simpler and tuning effort is not needed. Experimental results prove the effectiveness of the proposed strategy, which allows quick dynamic responses, good tracking behavior, and is capable of improving the power quality. These merits show that the proposed control strategy are very suitable for wind energy applications.


Author: 
of the DFIG based wind turbine system.
Jiefeng Hu, associate professor, department of automation science and electrical engineering, Beihang University, E-mail: jhu@buaa.edu.cn

REFERENCES
[1] Wind Energy Council, “Global wind energy outlook 2012,” November. 2012.
[2] J. Hu, J. Zhu, Y. Zhang, G. Platt, Q. Ma and D. G. Dorrell, “Predictive direct virtual torque and power control of doubly fed induction generators for fast and smooth grid synchronization and flexible power regulation,” IEEE Trans. Power Electronics, vol. 28, no. 7, pp. 3182–3194, July 2013.
[3] J. Hu, J. Zhu, and D. G. Dorrell, “A new control method of cascaded doubly fed induction generators using direct power control,” IEEE Trans. Energy Conversion, vol. 29, no. 3, pp. 771–779, September 2014.

The outcome of this work has been publish in IEEE Transactions on Power Electronics and IEEE Transactions on Energy Conversion. The total citation up to now is 29. These journals are the topmost Q1 zone SCI journals.