Power electronics applications in wind energy conversion system: A

Power Electronics Applications in Wind Energy Conversion System: A Review Rishabh Dev Shukla, Prof. R. K. Tripathi, Member, IEEE, and Sandeep Gupta

Abstract—This paper gives a review on the power electronic applications for wind energy conversion systems. Different types of wind energy conversion system (WECS) with different generators and power electronic converters are described, and different technical features are compared. The electrical topologies of WECS with different wind turbines are summarized and the possible uses of power electronic converters with wind farms are given. In conclusion, the possible methods of using the power electronic technology for improving wind turbine performance in power systems to meet the main grid connection requirements are discussed. Index Terms — DFIG, FACTs, HVDC, IGCT SCIG, voltage source inverter, WECS.

F

I. INTRODUCTION

ROM the past two decades, Wind energy is a very important renewable energy source. In present time global wind power potential is of the order of 11,000 GW. It is about 5% of the global installed power generation capacity. At the end of 2009, worldwide wind energy generating capacity is of 159.2 gigawatts (GW) [1]. Energy production was 340 TWh, which is about 2% of worldwide electricity usage and is growing rapidly, having doubled in the past three years. In past times, the technology used in WECS was based on squirrel-cage induction generators directly connected to grid. Recently, the technology moves towards variable speed. The controlling of the wind turbine becomes very important as the power level increases. Power electronics is an essential part of the Variable-speed WECS. It gives the ease for integrating the variable speed wind system units to achieve high efficiency and performance when connected to the Grid. Also, for Fixed speed WECS where the system is directly connected to the grid, power electronics switches (such as thyristor) are used as softstarters. Power electronics converters are used for matching the characteristics of wind energy generator to grid connection requirements, such as frequency, voltage, control of active and reactive power and harmonics, etc. Rishabh Dev Shukla, Research Scholar in Department of Electrical Engineering of Motilal Nehru National Institute of Technology, Allahabad (U.P.)-211004 INDIA (e-mail: [email protected]). Dr. R.K. Tripathi, Professor in Department of Electrical Engineering of Motilal Nehru National Institute of Technology, Allahabad (U.P.)-211004 INDIA (e-mail: [email protected]). Sandeep Gupta, Research Scholar in Department of Electrical Engineering of Motilal Nehru National Institute of Technology, Allahabad (U.P.)-211004 INDIA (e-mail: [email protected]).

II. WIND ENERGY CONVERSION SYSTEM The main elements of a wind turbine system are shown in Fig. 1. It consists of a turbine, a gearbox, a generator, a power electronic system, and a transformer for grid connection. Wind turbines graph power from wind with the help of turbine blades and convert it to mechanical power. It is very important to control and limit the converted mechanical power during higher wind speeds. It done either by stall control, active stall, or pitch control. [3], [4]. According to the rotation speed, wind energy conversion system (WECS) can be broadly classified into two categories; fixed speed, and variable speed. For variable speed WECSs, based on the rating of power converter related to the generator capacity, they can be further divided into wind generator systems with a partial-scale and a full-scale power electronic converter [5]. Wind power Rotor

Gearbox

Power Electronics Converter

Generator

Supply Grid

Power Transformer

Fig. 1. Main elements of a Wind Energy Conversion System.

The fixed speed WECSs have been used with a multiplestage gearbox and a SCIG directly connected to the grid through a transformer as illustrated in Fig 2. Induction generators with cage rotor can be used in the fixed speed wind turbines due to the damping effect. The SCIG operates only in a narrow range around the synchronous speed, the wind turbine outfitted with this type of generator is often called the fixed-speed wind energy conversion system [6], [7]. Since the SCIG always draws reactive power from the grid, during the 1980s this concept was extended with a capacitor bank for reactive power compensation. Smoother grid connection was also achieved by incorporating a soft-starter. Moreover, a pole-changeable SCIG has been used, which leads two rotation speeds. The variable speed WECSs have been used with a configuration is known as the DFIG concept, which corresponds to a variable speed wind turbine with a Wound rotor induction generator and power converter on the rotor circuit, as shown in Fig. 3.

978-1-4244-8542-0/10/$26.00 ©2010 IEEE

2

Fig. 2 Scheme of a fixed speed concept with SCIG system

The stator is directly connected to the grid, while the rotor is connected through a power electronic converter. The power converter controls the rotor frequency and thus the rotor speed. This concept supports a wide speed range operation, depending on the size of the frequency converter. Typically, the variable speed range is +30% around the synchronous speed [3], [7]–[10]. The rating of the power electronic converter is only 25–30% of the generator capacity, which makes this concept attractive and popular from an economic point of view.

grid commutated converters are mainly thyristor converters with high power capacity of 6 or 12 or even more pulses. A thyristor converter consumes inductive reactive power and it is not able to control the reactive power. Thyristor converters are mainly used for very high voltage and power applications, such as conventional HVDC systems. Self-commutated converter systems usually adopt pulse width-modulated (PWM) control methods; the semiconductors with turn-OFF ability, such as IGBTs, are mainly used. This type of converter may transfer both active power and reactive power [11], [12] in both directions (ac–dc or dc–ac). This means that the reactive power demand can be delivered by a PWM converter. The high-frequency switching of a PWM converter may produce harmonics and interharmonics (in order some kHz) and are relatively easier to be removed by small-size filters. Fig. 4. shows a typical power electronic converter consisting of IGBTs. Other types of power electronics converters also exist, including the multilevel converters, as shown in Fig. 5, and the matrix converter, as shown in Fig. 6. Particularly, the multilevel converters are more fascinating in such applications due to the voltage level of the converters and the decrease of the harmonics, and accordingly, the size of the output filters. The matrix converter is technically more complicated.

Fig. 3 Scheme of a variable speed concept with DFIG system

Synchronous generators, excited by an externally applied dc or by permanent magnets (PMs) are also used in WECSs. Synchronous machines powered by wind turbines may not be directly connected to the ac grid because of the requirement for significant damping in the drive train. The use of a synchronous generator leads to the requirement for a full rated power electronic conversion system to decouple the generator from the network. III.

ADVANCE POWER ELECTRONICS

Power electronics technology has changed rapidly during the last three decade. Their applications have been increasing, due to the advancements of the semiconductor devices and the microprocessor technology. The power electronic device technology is still undergoing in progress, including some important self-commutated devices, such as insulated gate bipolar transistor (IGBT), MOSFET, integrated gate commutated thyristor (IGCT), MOS-gate thyristors, and silicon carbide FETs. The breakdown voltage and/or current carrying capability of these devices are also continuously increasing. Significant study is going on to change the material from silicon to silicon carbide. This may severely increase the power density of the power converters. A converter, depending on the topology and application, may allow both directions of power flow. Basically there are two different types of power Electronics converter systems: grid commutated and self-commutated converter systems. The

Fig. 4. Circuit diagram of a VSC with IGBT.

Various possible technical solutions of WECSs are related to power electronics, since they can improve dynamic and steady-state performances, help to control the wind turbine generator, and decouple the generator from the electrical grid [5], [13]. Some major power electronic applications are described in this section A. SOFT-STARTER FOR FIXED-SPEED WECS Directly connecting a wind turbine to the grid is widely used in early WECSs. The scheme consists of an SCIG, connected using a transformer to the grid and operating at an almost fixed speed. The power can be limited aerodynamically either by stall control, active stall, or pitch control. The basic configurations of the fixed-speed concepts are shown in Fig.7

(a) Three-level neutral point clamped VSC.

(b) Three-level flying capacitor VSC. Fig. 5. Multilevel converters

Fig. 6. Circuit configuration of a matrix converter.

Connecting the induction generators to power system produces transients that are short duration with very high inrush currents, thus causing disturbances to both the grid and high torque spikes in the drive train of wind turbines with a directly connected induction generator. Such a transient disturbs the grid and limits the acceptable number of wind turbines. The high starting currents of induction generators are usually limited by a thyristor soft-starter. The current limiter or soft-starter, based on thyristor technology, typically limits the rms value of the inrush current to a level below two times of the generator rated current. The soft-starter has a limited thermal capacity and it is short circuited by a contactor, which carries the fullload current, when the connection to the grid has been completed. In addition to reduce the impact on the grid, the soft-starter also effectively dampens the torque peaks associated with the peak currents, and hence reduces the loads on the gearbox. B. POWER ELECTRONICS FOR VARIABLE-SPEED WECS WECS has many advantages. The wind turbine can increase or decrease its speed with the variation of the wind speed. This gives less wear and tear on the tower, gearbox, and other components in the drive train. Also, WECS can increase the production of the energy and reduce the fluctuation of the power fed into the grid. In WECS, the

Fig. 7. Fixed speed WECS with a power electronics soft starter

generator is normally connected to the grid through a power electronic converter system. For synchronous generators and induction generators without rotor windings, a full rated power electronic system is used between the stator of the generator and the grid, where the total power generated must be fed through the power electronic converter system [15]. For induction generators with rotor windings, the stator of the generator is connected to the grid directly, and the rotor is connected to a power-electronic-controlled resistor or connected to the network through slip rings and a power electronic converter. In wounded rotor induction generator with rotor resistance control also known as Dynamic Slip Control, the rotor windings are connected with variable resistors. The equivalent resistance in the circuit can be adjusted by an electronic control system, as shown in Fig. 8. As the resistance of the rotor windings increases, the slip increases. In this way, the generator speed can be varied in a limited range. Both cage induction generators and rotor resistance-controlled wounded induction generators need to operate at a speed greater than the synchronous speed to generate electricity.

978-1-4244-8542-0/10/$26.00 ©2010 IEEE

4

Fig. 8. WRIG with a rotor resistance converter (Dynamic Slip Control)

Both of them draw reactive power that might be given from the grid or from installed compensation equipment, such as capacitor banks or additional power electronic equipment. Economically, capacitor banks are normally used. Modern WECSs have FACT devices, such as thyristor switched capacitors (TSCs), static Var compensator (SVC), STATCOM etc, for more improved dynamic responses. There is more advance concept of Doubly Fed Induction Generator. The stator of a doubly fed induction generator (DFIG) is connected to the grid directly, while the rotor of the generator is connected to the grid by electronic converters through slip rings, as shown in Fig. 9. The generator can deliver energy to the grid at both speeds greater than synchronous speed and less than synchronous speed. The slip is varied with the power flowing through the power electronic circuit. The advantage is that only a part of the power generated is fed through the power electronic converter. Hence, the nominal power of the power electronic converter system can be less than the nominal power of the wind turbine. In general, the nominal power of the converter may be about 30% of the wind turbine power, enabling a rotor speed variation in the range of about ±30% of the nominal speed. By controlling the active power of the converter, it is possible to vary the rotational speed of the generator, and thus the speed of the rotor of the wind turbine. Self-commutated converter systems, such as IGBT-based switching converters, are generally used for this type of system. As shown in Fig. 9, the DFIG generally uses a back-toback converter, which consists of two bidirectional converters using a common dc link, one connected to the rotor and the other one to the grid. In Variable-speed WECSs, the power electronic converters control both the active and reactive power delivered to the grid. This gives potential for optimizing the grid integration with respect to steady-state operation conditions, power quality, voltage, and angular stability. The DFIG system also provides the high-quality power to the grid.

Fig. 9. Variable speed WECS with DFIG

The acoustical noise from the wind turbines can effectively be reduced since the system can operate at a lower speed when the wind becomes quiet. The dynamic response and controllability are excellent in comparison with traditional induction generator systems. The DFIG solution needs neither a soft-starter nor a reactive power compensator.

(a) Induction generator with gearbox

(b) Synchronous generator with gearbox.

(c) Multipole synchronous generator without gearbox. Fig. 10. WECSs with full-scale power converters.

5

C. WECS WITH CONVERTERS

FULL

RATED

POWER

ELECTRONIC

Cage induction generators and synchronous generators may be incorporated into power systems with full rated power electronic converters. A full scale power converter, between the generator and grid, gives the added technical performance. Generally, a back-to-back voltage source converter (VSC) is used in order to achieve full control of the active and reactive power, though with synchronous generators, diode rectifiers may be used [12], [16], [17], but in this case, the whole system is not a fully controlled. Since the generator is decoupled from the grid, the generator can operate at a wide variable frequency range for optimal operation while the generated active power will be sent to the grid through the grid-side converter that can be used for controlling the active and reactive power independently and the dynamic response may be improved. Fig. 10 shows three possible solutions with full-scale power converters. All three solutions have almost the same controllable characteristics since the generator is decoupled from the grid by a dc link. The grid-side converter enables the system to control active and reactive power very fast. IV.

CONCLUSIONS

This paper has reviewed the power electronic applications for wind energy conversion systems. Various WECS with different generators and power electronic converters are described. The electrical topologies of wind farms with different wind turbines are briefed. It has been shown that the wind farms consisting of different turbines may need different configurations for the best use of the technical merits. The wind turbine size is still increasing. Both onshore and offshore wind farms are quickly developing in a worldwide scale. While the wind turbine market continues to be dominated by conventional gear-driven wind turbine systems, the direct drive or one-stage gear, so-called multibrid-type wind system, is attracting attention. Variable-speed operation has many advantages. The DFIG dominates the present market for variablespeed gear-driven wind turbine systems, largely due to the fact that only the power generated in the generator rotor has to be fed through a power electronic converter system (25%–30%).On the other hand, variable-speed wind turbines with a full-scale power converter may be more effective and less complicated to deal with grid-related problems, including the possibility for active grid support, and the potential to operate wind turbines and wind farms as power plants. Therefore, variable speed wind turbine concepts with a full-scale power converter would become more attractive. Power electronics technology for WECSs have been actively researched, mainly the VSCs, including multiconverter configurations, are used. Compared with geared-drive WECSs, the main advantages of direct-drive WECSs are increased overall efficiency, reliability, and availability due to omitting the gearbox.

PM machines are more attractive and superior with higher efficiency and energy give up, higher reliability, and power-to-weight ratio compared with electricity-excited machines [5].With synchronous generators, diode rectifiers may be used as the machine side converters [12]. With the increasing levels of wind turbine penetration in current power systems, grid connection issues have posed a number of new challenges to WECS design and development. The grid connection demands is becoming a major issue in the wind energy conversion technology. Nowadays, grid connection requirements are becoming more strict. One of the requirements is that in case of a major grid disturbance, WECSs not only have to remain connected to the power system, but should also play an assisting role. In future, the percentage of wind energy on many grids is likely to be a significant part, thus advancement in WECSs as key grid company. Power electronic technologies, as the interfaces for wind turbines, for some energy storage systems, and as flexible ac transmission systems (FACTs) devices, such as STATCOM, will play a significant role in developing new state-of-the-art solutions for the future success of various new power generation and control concepts. V. REFERENCES [1] World Wind Energy Report 2009 (world wind energy Association) [2] Lecture Notes by Prof. Shireesh B. Kedare, Adjunct Assistant Professor, IITB. [3] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004 [4] Z.Chen and F. Blaabjerg, “Wind turbines—A cost effective power source,” Przeglad Elektrotechniczny, no. 5, pp. 464–469, 2004. [5] H. Li* Z. Chen, “Overview of different wind generator systems and their comparisons” IET Renewable Power Generation on 24th January 2007. [6] HANSEN AD, HANSEN LH: ‘Wind turbine concept market penetration over 10 years (1995– 2004)’, Wind Energy, 2007, 10, (1), pp. 81–97 [7] HANSEN LH, HELLE L, BLAABJERG F, ET AL.: ‘Conceptual survey of generators and power electronics for wind turbines’ Riso National Laboratory Technical Report Riso-R-1205(EN) Roskilde, Denmark, December 2001. [8] POLINDER H, MORREN J: ‘Developments in wind turbine generator systems’. Electrimacs 2005, Hammamet, Tunisia [9] DUBOIS MR, POLINDER H, FERREIRA JA: ‘Comparison of generator topologies for direct-drive wind turbines’. Proc. Nordic Countries Power and Industrial Electronics Conf. (NORPIE), Aalborg, Denmark, June 2000, pp. 22–26 [10] CARLSON O, GRAUERS A, SVENSSON J, ET AL.: ‘A comparison of electrical systems for variable speed operation of wind turbines’. European wind energy conf., 1994, pp. 500–505 [11] M. P. Kazmierkowski, R. Krishnan, and F. Blaabjerg, Control in Power Electronics—Selected Problems. London, U.K.: Academic, 2002. Economically, capacitor banks are normally used. Modern WECSs have FACT devices, such as [12] Z. Chen and E. Spooner, “Voltage source inverters for high-power, variable-voltage dc power sources,” Proc. Inst. Electr. Eng. Generation, Transmiss. Distrib., vol. 148, no. 5, pp. 439–447, Sep. 2001. [13] J. Zhang, Z. Chen, and M. Cheng, “Design and comparison of a novel stator interior permanent magnet generator for direct-drive wind turbines,” IET Proc. Renewable Power Generation, vol. 1, no. 4, pp. 203–210, Dec. 2007. [14] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electr on., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.

6 [15] Z.Chen, S.G´omez-Arnalte, andM.McCormick, “Afuzzy logic controlled power electronic system for variable speed wind energy conversion systems,”in Proc. IEEE PEVD 2000, London, U.K., Sep., pp. 114–119. [16] Z. Chen and E. Spooner, “A modular, permanent-magnet generator for variable speed wind turbines,” in Proc. IEE EMD 1995, pp. 453–457. [17] Z. Chen and E. Spooner, “Grid power quality with variable-speed wind turbines,” IEEE Trans. Energy Convers., vol. 16, no. 2, pp. 148–154, Jun. 2001. [18] Muller S, Deicke M, De Doncker RW. “Doubly fed induction generator systems for wind turbines”. IEEE Industry applications magazine 2002;8(3):26–33. [19] H. De Battista, R. J. Mantz, and C. F. Christiansen, “Dynamical sliding mode power control of wind driven induction generators,” IEEE Trans. Energy Convers., vol. 15, no. 4, pp. 451–457, Dec. 2000. [20] Z. Chen, F.Blaabjerg, andY.Hu, “Voltage recovery of dynamic slip control wind turbines with a STATCOM,” in Proc. IEEE IPEC 2005, Singapore, Apr., pp. 1093–1100. [21] Z. Saad-Saoud and N. Jenkins, “The application of advanced static VAr compensators to wind farms,” in Proc. Inst. Electr. Eng. Colloq. Power Electron. Renewable Energy, 1997, pp. 6/1–6/5. [22] J. Twidell, “Wind turbines: Technology fundamentals,” Renewable Energy World, vol. 6, no. 3, pp. 102–111, May/Jun. 2003.

[23] A. Kehrli and M. Ross, “Understanding grid integration issues at wind farms and solutions using voltage source converter FACTS technology,” in Proc. IEEE PES General Meeting, 2003, pp. 1822–1828. [24] P. Mattavelli, G. C. Verghese, and A. M. Stankovi´c, “Phasor dynamics of thyristor-controlled series capacitor systems,” IEEE Trans. Power Syst., vol. 3, no. 12, pp. 1259–1267, Aug. 1997. [25] Z. Chen, F. Blaabjerg, and Y. Hu, “Stability improvement of wind turbine systems by STATCOM,” in Proc. IEEE IECON 2006, pp. 4213– 4218. [26] C.A. Canizares and Z.T. Faur, “Analysis of SVC and TCSC Controllers in Voltage Collapse,” IEEE Trans. on Power Systems, Vol 14, No. 1, pp.1-8. Feb. 1999. [28] IEEE Power Engineering Society, FACTS Overview. IEEE Special Publication 95TP108, 1995. [29] IEEE Power Engineering Society, FACTS Applications. IEEE Special Publication 96TP116-0, 1996. [30] Z. Chen andY.Hu, “Dynamics performance improvement of a power electronic interfaced wind power conversion system,” in Proc. IEEE IPEMC 2004, Xi’an, China, Aug., pp. 1641–1646.