10. switched inductor quasi-z-source inverter for pmsg based wind energy conversion system

S.Sathish kumar and S.Chinnaiya 41

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 4, Issue. 1, June-2013.

Switched Inductor Quasi-z-source Inverter for PMSG based Wind Energy Conversion System

S.Sathish Kumar and S. Chinnaiya

Abstract: For standalone wind energy conversion system Permanent magnet synchronous generator is best suited because of no need of separate excitation, maintenance free and available in moderate ratings up to 50 Kilowatts. The main objective of this paper is to improve the wind energy conversion system by implementing Switch inductor quasi-z-source inverter and Battery storage system. Switch inductor quasi-z-source inverter has high efficiency and the input current is continuous, suppress inrush current and avoid shoot through fault in inverter when compared to Z-source inverter based wind energy conversion system. An effective control technique for the inverter, based on the pulse width modulation (PWM) method, has been developed to make the line voltages at the point of common coupling (PCC) balanced when the load is unbalanced. By proper control of battery current through dc–dc converter has been done to reduce the electrical torque pulsation of the PMSG under an unbalanced load condition Keywords-Wind Energy Conversion System (WECS), Battery Storage system (BSS), Permanent-magnet Synchronous generator (PMSG), Switched Inductor quasi-Z-Source inverter (SL- q-ZSI)

I. INTRODUCTION The electricity requirements of the world including India are increasing at alarming rate and the power demand has been running ahead of supply. It is also now widely recognized that the fossil fuels and other conventional resources, presently being used for generation of electrical energy, may not be either sufficient or suitable to keep pace with ever increasing demand of the electrical energy of the world. The recent severe energy crisis has forced the world to develop new and alternative methods of power generation, which could not be adopted so far due to various reasons.Thus this project Elucidates about Wind energy sources, and how we can generate power from Wind energy sources.The Indian wind energy sector has an installed capacity of 14158.00 MW (as on March 31, 2011). In terms of wind power installed capacity according to World wind energy Association, India is ranked 5th in the World. Today India plays a major role in the global wind energy production. Indian Wind Energy Association has estimated that with the current level of technology, the ‘on-shore’ potential for utilization of wind energy for electricity generation is of the order of 65,000 MW. Thus this project will consider the factor such as power handling performance based on system efficiency of power deliver to the output.

S.Sathish kumar persuing his Master degree in Power Electronics & Drives, K.S.R.C .E, Tamilnadu. Email: [email protected]

Some energy sources can be connected directly to the distribution network, but in the case of variable speed wind turbine (VSWT) systems it is necessary to use a power converter that interfaces the source and the grid.. Different types of generators can be used in wind energy conversion systems (WECS), but permanent magnet synchronous generators (PMSG) dominate the market. The main advantage of PMSG is the possibility of multipole design that offers slow speed operation and the possibility of gearless WECS construction. Another advantage of the PMSG is maintenance free operation since there are no brushes. The main drawback of variable speed wind turbine based PMSG is the dependence of its output voltage on the rotation speed. This drawback can be easily overcome with the help of an appropriate interface converter. The schematic of the standalone system using PMSG-based wind turbine is shown. In this paper, using battery as the storage devices, a small-scale standalone power supply system based on wind energy is considered. The main objectives are

a) To achieve effective control coordination among the PMSG, battery to maintain the dc-link voltage constant.

b) To maintain constant and balanced voltages at the ac bus (or load bus) as three phase dynamic loads need a balanced three-phase supply for their proper operation.

c) To reduce the effect of unbalanced load current on the torque pulsation of PMSG.

II. STATE OF THE ART TOPOLOGIES AND ISSUES One of the simplest interface converter topologies consists of uncontrolled rectifier and load side converter. This topology is the cheapest solution due to low number of controlled switches and simple control system which is necessary only for the load inverter. The main disadvantage of this solution is the absence of DC link voltage regulation possibility that leads to lack of extracted power at low speeds.

This topology offers a possibility to extract power at low wind speeds. Energy storage system is used to store the excessive power generated by PMSG at high wind speed and deliver the power to load through the inverter when wind speed is less than cut-in speed. Implementation of the uncontrolled rectifier allows the proposed system efficient and economic. The topology of wind energy system [10] does not discuss


International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 3, Issue. 1, May-2013.

about Energy storage system to store excessive power generated by PMSG. Implementation of Z-source inverters (ZSI) has become popular in recent years in applications [3].The ZSI has inherent voltage boost and buck capability using the shoot through switching states in each phase leg of the PWM inverter. This enables the proposed wind generation system to achieve the demanded variable-speed operation. The utilization of quasi-Z-source (qZS) network in the interface converter (Fig. 2) in addition to the benefits of the Z-source based converter offers continuous input current of the qZS-network as well as reduced DC voltage of the capacitor C2 [2].

Fig.1. PMSG based WECS with Z-Source inverter

Fig.2. PMSG based WECS with Quasi-Z-Source inverter

This paper proposes a possibility for further improvement of the quasi-Z-source based back-to-back interface converter by the introduction of the switched inductor quasi-Z-source network [2] (Fig. 2). In contrast to converters presented in Fig. 1 the new topology offers the increased voltage boost capability of the load-side inverter with energy storage system.

III. MODELLING OF PMSG

Permanent Magnet Synchronous Generator converts the mechanical power from aerodynamic system to ac electrical power, which is then converted to dc power through uncontrolled diode bridge rectifier connected with dc link at its dc port. The power is transferred to the load through another IGBT pulse width modulation (PWM) inverter. The electrical model of the PMSG has been developed. It is typically implemented in the dq rotating reference frame.

The equivalent circuits of the PMSG in direct and quadrature axes are shown in Figure. 3. The stator voltage equations in the d-q reference frame, and , are given as follows

= - - +

= - - -

Where, and respectively represent the inductance and resistance of the PMSG winding, ϕ represents the magnet flux, is the electrical rotational speed of generator, and , are the direct and quadrature components of the machine currents respectively.

Fig.3.Equivalent circuits of PMSG: (a) d-axis (b) q-axis. The electromagnetic torque, Te, is given as.

= p ( - ) +z )

where , are the two axes machine inductances; p is the number of pole pairs.

In surface mounted PMSGs, = = . Hence, the electromagnetic torque can be rewritten as follows.

= p . The relation between angular frequency of the stator voltage (electrical angular velocity) , and mechanical angular velocity of the generator rotor is given below as

.

IV. OPERATION PRINCIPLE OF SWITCHED INDUCTOR QUASI-Z-SOURCE INVERTER

As stated above, this paper proposes the switching inductor (SL) technique to be implemented in the traditional qZSI to improve its voltage boost properties. The proposed SL qZS network consists of three inductors (L1…L3), four diodes (D1…D4) and two capacitors (C1 and C2). Coupled with the Load side PWM inverter, the SL qZS network forms the SL quasi-Z-source inverter (SL qZSI).

Similarly to the traditional qZSI , the SL qZSI has two main types of operational states at the DC side: non-shoot-through states and shoot-through states



Let us assume that the operating period T of the SL qZSI consists of a shoot-through state and an active state

+ where and are the duty cycles of an active and shoot-through states, correspondingly.

In order to simplify the analysis of the circuit, it was assumed that the diodes, capacitors and inductors of the SL-qZS network are lossless. Fig. 4 shows the equivalent circuits of the SL qZSI operating in the CCM for the shoot-through (a) and active (b) states. At the steady state the average voltage of the inductors over one operating period is zero:

Based on that fact and defining the shoot-through duty cycle as and the non-shoot-through duty cycle as , the inductors voltages over one operating period could be represented as:

Equations for average capacitor voltages , and peak inverter input voltage are derived from the steady state analysis:

The voltage conversion ratio of the inverter can be expressed by the following

where is the modulation index. The modulation index is connected with the shoot-through duty cycle by the following relation

a)

b)

Fig.4. Equivalent circuits of SL qZSI: a) during shoot state and b) during active state

V. PWM CONTROL FOR THE PROPOSED SL-QZSI Three basic PWM control methods, simple, maximum , and constant boost control ], work with the proposed SLqZSI. The maximum boost control method serves as an example for the analysis, simulation, and experiment in this paper. Maximum boost control turns all traditional zero states into shoot-through states to obtain the largest possible duty cycle. When using the maximum boost control method [8], the average duty cycle of the shoot-through state (D), as defined as ,

Where M is the modulation index.

VI. PROPOSED WIND ENERGY SYSTEM This section unveils the operation principle of the new proposed interface converter (Fig. 5) in residential PMSG based wind turbines with power rating up to 15 kW



Fig.5. Proposed PMSG based WECS with Battery storage system

Operation modes of the interface converter Generally, PMSG based Variable Speed Wind Turbines have three distinct modes of operation: silent mode, variable speed operation mode and constant speed mode. A turbine is silent in two cases: If wind speed is below a cut-in level or above the cutoff speed then there will be no generation of power by generator. At that battery storage system supplies power to load. Turbines operate at variable speed in the wind velocity range from cut-in to rated wind speed. Rated wind speed differs by turbine types, but often has the value of 12 meters per second. Constant speed mode takes place above the rated wind speed and output power of the turbine remains constant at this mode. In this paper we considered PMSG with 5 pole pairs as a power source in this research. Its line voltage is 250 V at 100 rad/sec of the rotor speed, but it can operate up to 220rad/sec. This speed is considered as the maximum power operational point for the turbine and the generator. Generator power reaches 3250 W at this point with the output voltage of 583 V. Cut-in speed for a turbine is 62rad/sec of rotor speed and it can produce 80 W, but the generator voltage is only 48 V at this point. So this is the lowest input voltage for an interface converter. The necessary voltage boost is obtained in two steps. The rectifier stabilizes the DC link voltage to a 300 V level when the generator voltage is below 152 V. The transferred power of the converter lies between 40 W and 330 W at this mode. The controlled rectifier works as diode rectifier when the generator voltage is above 152 V. In this mode the DC link voltage is changed proportionally to the generator voltage, at the range from 250 V in rated generator speed conditions up to 600 V at the maximal speed. The SL qZS network with appropriate inverter control is stabilizing the

peak value of the inverter side DC link voltage to 490 V despite the voltage variations on the generator side DC link. The inverter input voltage regulation is obtained by changing the shoot-through duty cycle . The peak value of the inverter side DC link voltage is so high due to lower modulation index M if compared with voltage source inverters. Modulation index is limited by shoot-through states implemented in SL qZSI.The variation range of the voltage conversion ratio G lies between 1.44 and 2.4 that corresponds to the generator side DC link voltage range from 250 V up to 150 V. Since the modulation index M and the boostfactor B are the functions of the appropriate values of shoot-through can be found for minimum and maximum G values. The variation range of the shoot-through duty cycle DS is from 0.17 in maximum speed conditions to 0.27 in the cut-in speed conditions. Analysis of simulation and experimental results A series of simulations and experiments were performed to verify the proper operation of the proposed SL qZS Inverter with Battery storage system. Tests were performed at three characteristic operation points of the VSWT system to demonstrate the converters operation modes in the entire generator voltage and power range. These points are: cut-in speed (low voltage and minimum power), rated speed (corresponds to 7.5 m/s to 15m/s wind speed) and maximum generator speed, power conditions. General parameters of experimental setup based on 6 kW PMSG and proposed interface converter are summarized in Table 1. The simple boost control technique was implemented for the SL qZSI during simulations and tests. The switching frequency of proposed inverter is 10 kHz.



Table: 1. Key Parameters of the Proposed Converter

Input dc Voltage 48

Output line to line voltage is 120

q-Z-source network

L1=L2=L3 1Mh

C1=C2 1000µF

Switching frequency 10kHz

Three phase output filter

Lf 1Mh

Cf 20µF

Table: 2. Key Parameters of the Proposed Converter

Number of poles 10

Rated speed 153 rad/sec

Stator Inductance (Ls) 8.4 mH

Rated Torque 40 Nm

Rated Power 6 KW

it is seen that battery either delivers or absorbs the power according to the difference between the PMSG output power and load demand. The initialization of SOC of the battery is carried out near about 0.8 so that the battery will be charged due to the power mismatch and the SOC will hit the 0.8 limit. When SOC of battery becomes more than 0.8, surplus power should go to load.

Fig.6. DC link voltage

Fig.7. Simulation waveforms of load current

Fig.8. Simulation Waveforms for load Voltage

Table .3.Output values of the proposed system

Three phase output current 12 Amps

Three phase output voltage 440 Volts

THD 2.05%

Fig.9. Spectrum of load side current in Proposed WECS



The simulated waveforms of output current and output voltage for the proposed Wind energy conversion system is shown in the figure 6and figure 7 with variable wind speed and constant wind speed at t= 12 m/s. It is clear the output voltage and output current remain constant for variable speed condition and constant speed.

VII. CONCLUSION The interface of switch inductor quasi-z-source inverter with pmsg based Wind energy conversion system has several advantages has reduced passive components (i.e) uses only three inductor and two capacitor and four diodes compared to the SL-ZSI has several drawbacks: 1) it adds six diodes and two inductors, compared with the classical ZSI, which increases size, cost, and loss; 2) it does not share a dc ground point between the source and converter; 3) the input current is discontinuous. Voltage stress SL-q ZSI compared to traditional ZSI is very low. Overall system reliability his improved because of elimination of shoot-through fault. Inclusion of Permanent Magnet Synchronous Generator in Wind energy conversion system also has high efficiency and good power factor. So total system has good performance and reliability.

VIII. REFERENCES

[1]. The World Wind Energy Association,www.wwindea.org. [2]. C. N. Bhende, S. Mishra, and Siva Ganesh Malla, “Permanent Magnet

Synchronous Generator-Based Standalone Wind Energy Supply System” IEEE Trans.Sustainable Energy, vol. 2, NO. 4,pp.361-372, Oct. 2011.

[3]. Minh-Khai Nguyen, Young-Cheol Lim, and Geum-Bae Cho “Switched-Inductor Quasi-Z-Source Inverter” IEEE Trans.Power Electronics,vol. 26, NO. 11, Nov 2011.

[4]. Seyed Mohammad Dehghan and Mustafa Mohamadian, “A New Variable-Speed Wind Energy Conversion System Using Permanent-Magnet Synchronous Generator and Z-Source Inverter”IEEE Trans. Energy Conversion, vol. 24, NO. 3, Sept. 2009.

[5]. Bhim Singh and Gaurav Kumar Kasal “Solid State Voltage and Frequency Controller for a Stand Alone Wind Power Generating System” IEEE Trans. Power Electronics, vol. 23, NO. 3, MAY 2008.

[6]. Bhim Singh and Shailendra Sharma, Member, “Design and Implementation of Four-Leg Voltage-Source-Converter-Based VFC for Autonomous Wind Energy Conversion System” IEEE Trans. Industrial Electronics, vol. 59, NO. 12, Dec. 2012.

[7]. Hernan De Battista and Ricardo J. Mantz “Dynamical Variable Structure Controller for Power Regulation of Wind Energy Conversion Systems” IEEE Trans. Energy Conversion, vol. 19, NO. 4, Dec. 2004.

[8]. Md. Enamul Haque, Michael Negnevitsky, and Kashem M. Muttaqi, Senior Member, IEEE (2010) “A Novel Control Strategy for a Variable-Speed Wind Turbine With a Permanent-Magnet Synchronous Generator” IEEE Trans. Industry Applications, vol. 46, NO. 1, Feb.2010.

[9]. Li Wang, and Guang-Zhe Zheng “Analysis of a Micro turbine Generator System Connected to a Distribution System Through Power-Electronics Converters” IEEE Trans. Sustainable Energy, vol. 2, NO. 2, APRIL 2011

[10]. Kaigui Xie, Zefu Jiang, and Wenyuan Li, Fellow, IEEE (2012) “Effect of Wind Speed on Wind Turbine Power Converter Reliability” IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 1, MARCH 2012.pp.96-104.

[11]. Shuhui Li, Senior Member, IEEE, Timothy A. Haskew, Senior Member, IEEE, Richard P. Swatloski, and William Gathings (2012) “Optimal and Direct-Current Vector Control of Direct-Driven PMSG Wind Turbines” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 27, NO. 5, MAY 2012.pp.2325-2337.

[12]. Shinzo Takata (2010) “Steady-State Analysis of a Permanent-Magnet-Assisted Salient-Pole Synchronous Generator” IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 25, NO. 2, JUNE 2010.pp.388-393

[13]. L. Bisenieks and D. Vinnikovs “Switched inductor qZ Source back to Back converter for VWST with PMSG”ISSN 1392-1215.

[14]. Miao Zhu, Kun Yu, and Fang Lin Luo, “Switched Inductor Z-Source Inverter” IEEE Trans. Power Electronics, vol. 25, NO. 8, Aug. 2010.

[15]. Muhammed Rashid .Phd.,Editor in chief, Professor of University of florida, “Power Electronics Handbook” 2ndEdition.

[16]. Gary L.Johnson “Wind Energy system” 2nd Edition.

S.SathishKumar received B.E degree in Electrical & Electronics in Sree Sastha Institute of Engineering &Technology. He is pursuing Master of Engineering in Power Electronics & drives at K. S. R. College of Engineering. He presented a paper in an International conference at Maria college of Engineering. His research interests include Electrical machines and drives and Wind Energy conversion system.

S.Chinnaiya received B.E degree in Electrical and Electronics Engineering in Mahendra Engineering College in 2004. He received M.E degree in Power Electronics and Drives in Bannari Amman Institute of Technology in 2006. He is pursuing PhD under Anna University, Chennai. His area of interest includes Power Electronics Converter, Digital Controller based Drives.

10. switched inductor quasi-z-source inverter for pmsg based wind energy conversion system

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