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A PWM Multilevel Current-Source Inverter Used for Grid-Connected Wind Energy Conversion System
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Energy Procedia
Energy Procedia (2011)16 000–000 Energy 00 Procedia (2012) 461 – 466 www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
A PWM Multilevel Current-Source Inverter Used for GridConnected Wind Energy Conversion System Jianyu Baoa, Weibing Baob, Jie Gong a* b
a Ningbo Institute of Technology,Zhejiang University,Ningbo,315100,China Zhijiang College, Zhejiang University of Technology,Hangzhou,310024,China
Abstract This paper proposes a grid-connected wind energy conversion system (WECS) based on a PWM multilevel currentsource inverter (MCSI) topology. The topology used here is derived from the multilevel voltage-source inverter (MVSI) by dual conversion, this allows the wealth of existing knowledge relating to the operations, modulations and control strategies of multilevel VSI to be immediately applied to such multilevel CSI. The topology are supplied with two independent DC current-sources, 2n+1 current levels are obtained at the output. In the proposed control scheme, DC-link current controller is used to regulate and stabilize dc link current, output power factor controller is used to independently control the real power or reactive power, and unity power factor is easily achieved at grid side. Validation of models, control and steady-state and transient performances of a WECS based on 5-level CSI is carried out in the MATLAB/Simulink environment. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. [name organizer]
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of
Keywords: grid-connected; wind energy conversion system (WECS); multilevel current source inverter(MCSI); unity power factor
1. Introduction Multilevel voltage-source inverter (VSI) topology has been widely used in grid-connected applications for wind energy conversion system (WECS). However, when employing VSI as the power conversion circuit in WECS, due to its inherent buck characteristics, a boost converter used to step up the DC-link voltage should be inserted, thus adding to the cost and complexity of the whole conversion system. Current-source inverter (CSI), as the dual part of VSI, offers advantages over VSI in terms of inherent
* Corresponding author. Tel.: +0-086-574-8813-0236; fax: +0-086-574-8822-9505. E-mail address: [email protected].
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society . doi:10.1016/j.egypro.2012.01.075
462
Jianyu Bao et al. / Energy Procedia 16 (2012) 461 – 466 Author name / Energy Procedia 00 (2011) 000–000
boosting and short-circuit protection capabilities, direct control of the output current, longer lifetime of the storage unit [1]. The reason behind CSI not being as popular as VSI is that inductors used in CSI as storage elements have higher conduction losses and thus lower energy storage efficiency compared to DC-link capacitors of VSI. However, some of the disadvantages of traditional inductors can now be overcome owing to the development of superconducting magnetic energy storage (SMES) technology, and hence their use is becoming more attractive, especially for very large current applications. Recently, the interest of researchers for multilevel CSIs for various applications has noticeably increased. The published literatures about multilevel CSI are mainly focused on the topology constructions and the corresponding modulation strategy. Most of those proposed three-phase topologies are constructed by proper combinations of single-phase or three-phase CSI uint [2][5], the whole system is usually carrier phase-shifted (CPS) SPWM controlled or operated at one preset mode. Recently, there emerged a new three-phase MCSI topology [6] which is derived from an improved three-phase MVSI topology by applying dual transformation, and thus the wealth of existing knowledge relating to modulations of MVSI can be immediately mapped into such kind of three-phase MCSI topology. However, the closed-loop control strategy used in high power applications are not well investigated further as in the case of multilevel VSI, thus the practical use of such topology is degraded. For those multilevel CSIs used in grid-connected wind energy system, by nature, consist of n number of CSI units connected in parallel, which can be capable of producing 2n+1 levels of output current [7][8], but the modulation strategy is complex and should still be met with 3-logic rule of CSI. Therefore, it is meaningful to explore how a three-phase multilevel CSI tailored to high-power wind turbine grid-connected applications with proper closed-loop control. 2. System description The whole wind power conversion system shown in Fig. 1 consists of a wind turbine, a permanent magnet synchronous generator (PMSG), two thyristor-based rectifiers used to generate DC currentsources, and a PWM-based three-phase multilevel CSI. Grid is assumed to be stiff and can be simplified as a voltage-source in series with little source-impedance (Lo and Ro). Lo represents the sum of line impedance and leakage inductance of transformer, while Ro is referred to the transformer and line loss.
Fig. 1. A PMSG WECS based on thyristor rectifier and multilevel CSI
The aim of this paper is to investigate the three-phase multilevel current-source inverter topology tailored to high-power grid-connected wind turbine applications using PMSG designed for direct drive. Therefore, the simple control for thyristor-based rectifier to obtain constant DC-link current according to the variable wind speed is only considered, while the maximum power point tracking (MPPT) control for generator rectifier is not especially developed. On the grid side, with the proper control of multilevel CSI,
Jianyu Bao et al. / Energy Procedia 16 (2012) 461 – 466 Author name / Energy Procedia 00 (2011) 000–000
the active and reactive power can be separately regulated under the condition of variable wind speed. Apart from this, the unity power factor at the grid-side can be easily achieved by means of a Phase Locked Loop (PLL) control. 3. Control of grid-side multilevel CSI 3.1. PWM based multilevel CSI topology Fig. 2 gives the proposed 5-level CSI topology, which is derived from the improved three-phase flyingcapacitors 5-level VSI through dual transformation. Such multilevel inverter is powered by two independent current-sources, there are two complementary switch pairs: (Sx1, Sx4), (Sx2, Sx3) (x=a, b, c) in each phase, and the dc current source 2Idc is equally divided by sharing-inductors Ly (y=a, b, c), so the currents through every inductor are all Idc. According to the dual relationship, the existing knowledge relating to modulation of multilevel VSI’s can be immediately applied to such multilevel CSI. It is well established that the carrier disposition PD PWM strategy creates the lowest line-to-line harmonic distortion for a multilevel VSI [9]. If this 5-level CSI is Phase-Disposition (PD) PWM controlled, that is, when the current reference waveforms are compared against the upper triangular carrier, phase current outputs switch between +2Idc and +Idc, while when they are compared against the lower triangular carrier, phase current outputs switch between +Idc and 0. The mechanism of 5-level current generation is described in detail in [6].
Fig. 2. The new three-phase 5-level CSI topology
3.2. Grid-connected control scheme of AC-side current The block diagram of the proposed control scheme is shown in Fig. 4. The main purpose of such control strategy is to independently regulate active and reactive power injected into the grid for variable speed operation. Usually, the active power flowing into the grid is exacted from the rectifier with MPPT control, and the grid-connected multilevel CSI is expected to generate the reactive power according to the requirements of grid. Meanwhile, the grid-side currents output from the multilevel CSI should be as close as possible to a high-quality sinusoidal waveform.
463
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Jianyu Bao/etEnergy al. / Energy Procedia 16 (2012) 461 – 466 Author name Procedia 00 (2011) 000–000
To simplify the control structure, the multilevel CSI control is developed based on grid voltage oriented synchronous frame. Three-phase AC currents injected into the grid are projected on a d-q synchronous frame which is rotating at an angular speed ω, where, ω is adjusted to be synchronized with the grid frequency by means of a Phase Locked Loop (PLL) [10]. Therefore, three-phase AC currents are sinusoidal functions of the grid frequency, and d-axis, q-axis current components become time-invariant in steady-state, and thus the controller design is greatly simplified. In Fig. 3, the inputs to the PLL block is the three-phase sinusoidal voltages from the grid, and its output is the angular speed ω for abc-to-dq and dq-to-abc transformations. Three-phase voltages are transformed into d- and q-axis components based on [11][12], and the q-axis voltage vsq is regulated to zero using a PI controller. If q-axis voltage vsq can be regulated to zero, the active and reactive powers, Ps and Qs, will be independent of the voltage vsq. Therefore, active power, Ps and reactive power, Qs can be expressed in terms of currents isd and isq as:
Ps = 1.5vsd isd
(1)
Qs = 1.5vsd isq
(2)
From Equation (1) and (2), Ps is proportional to, and be controlled by, isd; similarly, Qs can be controlled by isq. Therefore, based on grid voltage oriented control, the grid voltage has only d-axis component vsd, while q-axis component vsq=0 if the d-axis is aligned to the voltage phasor. The active and reactive power can therefore be independently controlled by regulating the d-q reference currents.
θ (ω )
θ (ω )
Fig. 3. Control scheme of grid-side AC current
In Fig. 3, the outputs of two PI regulators are variable modulation indexes based on d-q frame, by using dq-to-abc transformations, the three-phase sinusoidal current reference signals for PD-PWM control are achieved. 4. Simulation results To verify the operations of grid-connected WECS, a simulation system based on a three-phase 5-level CSI is set up, and the simulation is carried out in MATLAB/Simulink environment. Some main parameters are as follows: LDCx (x=1, 2)=50mH, the sharing-inductors Lx(x=a, b, c) =20mH. R0=0.4Ω, L0=4mH, Cfx(x=a, b, c)=50μF; the 5-level CSI was operated with an output frequency of 50 Hz, and a PDPWM switching frequency of 1050 Hz (pulse ratio = 21). Fig. 4 presents the response of the grid-connected WECS, including outputs of the thyristor rectifier and 5-level CSI due to a step change for wind speed. At t = 0.3s, the DC-link current reference value is changed from 50A to 100A due to change of wind speed, as shown in Fig. 4(a). After being shortly regulated by DC-link current controller, the DC-link output current of each thyristor rectifier follows the
Jianyu Baoname et al. // Energy Energy Procedia Procedia 00 16 (2011) (2012) 000–000 461 – 466 Author
reference current very well in steady-state, as shown in Fig. 4(b) and (c). By supplying these two dc current-sources to the three-phase 5-level CSI, the PD-PWM controlled 5-level switching waveform of phase-a is obtained, as shown in Fig. 4(d), Fig. 4(e) shows the filtered current of phase-a, which is almost sinusoidal, and will be injected into the grid.
Fig. 4. Step response of thyristor rectifier and three-phase 5-level CSI
Fig. 5. Grid voltage and current waveforms at different power factor
Fig. 5 shows the performance of the grid-connected WECS based on three-phase 5-level CSI, where, different displacement power factors are generated at the grid-side by independently regulating the reference values of the d-axis and q-axis components, which are described as idref and iqref respectively in Fig. 3. By setting idref =0.8(pu) and iqref =0(pu), the in-phase waveforms of current and voltage can be seen in Fig. 5(a), and thus the unity power factor is achieved. If iqref is changed to 0.2(pu), then the leading displacement power factor is produced as shown in Fig. 5(b). Similarly, the lagging displacement power factor is produced when iqref is set to -0.2(pu), as shown in Fig. 5(c). In Fig. 5(d), idref is kept constant, and iqref is changed from 0.2(pu) to -0.2(pu), the transitions between leading and lagging reactive power injected into the grid can be seen in Fig. 5 (e) and (f).
465
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Jianyu Bao/etEnergy al. / Energy Procedia 16 (2012) 461 – 466 Author name Procedia 00 (2011) 000–000
5. Conclusion This paper presented a grid-connected WECS structure based on a three-phase PWM 5-level CSI. This three-phase 5-level CSI is PD-PWM controlled and provides relatively low line current harmonics at the grid, which is similar to that of line-line voltage for 5-level VSI. For direct-driven wind energy conversion system applications, the dc-link current can be adjusted according to the variation of wind speed through phase-control thyristor rectifier. At the grid side, control scheme of independent active and reactive power control was developed. Based on grid voltage oriented control, q-axis component of grid voltage is regulated to zero though PLL, therefore the active and reactive power can be independently controlled by regulating the d-q reference currents. According to the requirements of power balance control, the leading, unity, or lagging displacement power factor is produced. Meanwhile, the three-phase AC currents result from the multilevel CSI-based wind energy conversion system which will be injected into the grid are almost sinusoidal for all operating conditions. Acknowledgements This work was supported by Natural Science Foundation of Zhejiang Province, through grant number Y1111002. References [1] J. Dai, D. Xu, B. Wu. A Novel Control System for Current Source Converter Based Variable Speed PM Wind Power Generators. IEEE 38th Annual Power Electronics Specialists Conference, Orlando, Florida, USA, 2007, p. 1852-1857. [2] Xiong, Y., Li, Y.L., Yang, X., Zhang, Z.C. A New Three-Phase Five-Level Current-Source Inverter. IEEE Conference on APEC05, 2005, p.424-427. [3] Bao, J.Y., Bai, Z.H., Wang, Q.S., Zhang, Z.C. A New Three-Phase 5-Level Current-Source Inverter. Journal of Zhejiang University SCIENCE A, 2006, 7(12): 1973-1978. [4] Kwak, S.S., Toliyat, H.A. Multilevel Converter Topology Using Two Types of Current-Source Inverters. IEEE Trans. on Industry Applications, 2006, 42(6):1558-1564. [5] Bai, Z.H., Zhang, Z.C., Zhang,Y. A Generalized Three-Phase Multilevel Current Source Inverter with Carrier Phase-Shifted SPWM. IEEE Conference on PESC07, 2007, p.2055-2060. [6] Bao, J.Y., Bao,W.B., Wang, S.R., ZHANG, Z.C. Multilevel Current Source Inverter Topologies Based on the Duality Principle. IEEE Conference on APEC, 2010, p.1097-1100. [7] Pierluigi T, Andrew AR, Thomas AL. Wind Turbine Current-Source Converter Providing Reactive Power Control and Reduced Harmonics. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, 2007, 43( 4):1050-1060. [8] Miteshkumar P, Bin W, Navid RZ. DC Link Current Control of CascadedCurrent Source Converter Based Offshore Wind Farms. IEEE International Electric Machines & Derives Conference, 2011, p.807-812. [9] McGrath, BP, Holmes, DG. Multicarrier PWM Strategies for Multilevel Inverters, IEEE Trans. on Industrial Electronics, 2002, 49(4):858-867. [10] Chung SK. A phase tracking system for three phase utility interface inverters. IEEE Transactions on Power Electronics. 2000, 15(3): 431-438. [11] Kazerani M, Dash, PP. Dynamic Modeling and Performance Analysis of a Grid-Connected Current-Source Inverter-Based Photovoltaic System. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, 2011, 2(4): 443-450. [12] Kazerani M, Dash, PP. A Multilevel Current-Source Inverter Based Grid-Connected Photovoltaic System. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, 2011, 2(4): 443-450.
Energy Procedia
Energy Procedia (2011)16 000–000 Energy 00 Procedia (2012) 461 – 466 www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
A PWM Multilevel Current-Source Inverter Used for GridConnected Wind Energy Conversion System Jianyu Baoa, Weibing Baob, Jie Gong a* b
a Ningbo Institute of Technology,Zhejiang University,Ningbo,315100,China Zhijiang College, Zhejiang University of Technology,Hangzhou,310024,China
Abstract This paper proposes a grid-connected wind energy conversion system (WECS) based on a PWM multilevel currentsource inverter (MCSI) topology. The topology used here is derived from the multilevel voltage-source inverter (MVSI) by dual conversion, this allows the wealth of existing knowledge relating to the operations, modulations and control strategies of multilevel VSI to be immediately applied to such multilevel CSI. The topology are supplied with two independent DC current-sources, 2n+1 current levels are obtained at the output. In the proposed control scheme, DC-link current controller is used to regulate and stabilize dc link current, output power factor controller is used to independently control the real power or reactive power, and unity power factor is easily achieved at grid side. Validation of models, control and steady-state and transient performances of a WECS based on 5-level CSI is carried out in the MATLAB/Simulink environment. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. [name organizer]
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of
Keywords: grid-connected; wind energy conversion system (WECS); multilevel current source inverter(MCSI); unity power factor
1. Introduction Multilevel voltage-source inverter (VSI) topology has been widely used in grid-connected applications for wind energy conversion system (WECS). However, when employing VSI as the power conversion circuit in WECS, due to its inherent buck characteristics, a boost converter used to step up the DC-link voltage should be inserted, thus adding to the cost and complexity of the whole conversion system. Current-source inverter (CSI), as the dual part of VSI, offers advantages over VSI in terms of inherent
* Corresponding author. Tel.: +0-086-574-8813-0236; fax: +0-086-574-8822-9505. E-mail address: [email protected].
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society . doi:10.1016/j.egypro.2012.01.075
462
Jianyu Bao et al. / Energy Procedia 16 (2012) 461 – 466 Author name / Energy Procedia 00 (2011) 000–000
boosting and short-circuit protection capabilities, direct control of the output current, longer lifetime of the storage unit [1]. The reason behind CSI not being as popular as VSI is that inductors used in CSI as storage elements have higher conduction losses and thus lower energy storage efficiency compared to DC-link capacitors of VSI. However, some of the disadvantages of traditional inductors can now be overcome owing to the development of superconducting magnetic energy storage (SMES) technology, and hence their use is becoming more attractive, especially for very large current applications. Recently, the interest of researchers for multilevel CSIs for various applications has noticeably increased. The published literatures about multilevel CSI are mainly focused on the topology constructions and the corresponding modulation strategy. Most of those proposed three-phase topologies are constructed by proper combinations of single-phase or three-phase CSI uint [2][5], the whole system is usually carrier phase-shifted (CPS) SPWM controlled or operated at one preset mode. Recently, there emerged a new three-phase MCSI topology [6] which is derived from an improved three-phase MVSI topology by applying dual transformation, and thus the wealth of existing knowledge relating to modulations of MVSI can be immediately mapped into such kind of three-phase MCSI topology. However, the closed-loop control strategy used in high power applications are not well investigated further as in the case of multilevel VSI, thus the practical use of such topology is degraded. For those multilevel CSIs used in grid-connected wind energy system, by nature, consist of n number of CSI units connected in parallel, which can be capable of producing 2n+1 levels of output current [7][8], but the modulation strategy is complex and should still be met with 3-logic rule of CSI. Therefore, it is meaningful to explore how a three-phase multilevel CSI tailored to high-power wind turbine grid-connected applications with proper closed-loop control. 2. System description The whole wind power conversion system shown in Fig. 1 consists of a wind turbine, a permanent magnet synchronous generator (PMSG), two thyristor-based rectifiers used to generate DC currentsources, and a PWM-based three-phase multilevel CSI. Grid is assumed to be stiff and can be simplified as a voltage-source in series with little source-impedance (Lo and Ro). Lo represents the sum of line impedance and leakage inductance of transformer, while Ro is referred to the transformer and line loss.
Fig. 1. A PMSG WECS based on thyristor rectifier and multilevel CSI
The aim of this paper is to investigate the three-phase multilevel current-source inverter topology tailored to high-power grid-connected wind turbine applications using PMSG designed for direct drive. Therefore, the simple control for thyristor-based rectifier to obtain constant DC-link current according to the variable wind speed is only considered, while the maximum power point tracking (MPPT) control for generator rectifier is not especially developed. On the grid side, with the proper control of multilevel CSI,
Jianyu Bao et al. / Energy Procedia 16 (2012) 461 – 466 Author name / Energy Procedia 00 (2011) 000–000
the active and reactive power can be separately regulated under the condition of variable wind speed. Apart from this, the unity power factor at the grid-side can be easily achieved by means of a Phase Locked Loop (PLL) control. 3. Control of grid-side multilevel CSI 3.1. PWM based multilevel CSI topology Fig. 2 gives the proposed 5-level CSI topology, which is derived from the improved three-phase flyingcapacitors 5-level VSI through dual transformation. Such multilevel inverter is powered by two independent current-sources, there are two complementary switch pairs: (Sx1, Sx4), (Sx2, Sx3) (x=a, b, c) in each phase, and the dc current source 2Idc is equally divided by sharing-inductors Ly (y=a, b, c), so the currents through every inductor are all Idc. According to the dual relationship, the existing knowledge relating to modulation of multilevel VSI’s can be immediately applied to such multilevel CSI. It is well established that the carrier disposition PD PWM strategy creates the lowest line-to-line harmonic distortion for a multilevel VSI [9]. If this 5-level CSI is Phase-Disposition (PD) PWM controlled, that is, when the current reference waveforms are compared against the upper triangular carrier, phase current outputs switch between +2Idc and +Idc, while when they are compared against the lower triangular carrier, phase current outputs switch between +Idc and 0. The mechanism of 5-level current generation is described in detail in [6].
Fig. 2. The new three-phase 5-level CSI topology
3.2. Grid-connected control scheme of AC-side current The block diagram of the proposed control scheme is shown in Fig. 4. The main purpose of such control strategy is to independently regulate active and reactive power injected into the grid for variable speed operation. Usually, the active power flowing into the grid is exacted from the rectifier with MPPT control, and the grid-connected multilevel CSI is expected to generate the reactive power according to the requirements of grid. Meanwhile, the grid-side currents output from the multilevel CSI should be as close as possible to a high-quality sinusoidal waveform.
463
464
Jianyu Bao/etEnergy al. / Energy Procedia 16 (2012) 461 – 466 Author name Procedia 00 (2011) 000–000
To simplify the control structure, the multilevel CSI control is developed based on grid voltage oriented synchronous frame. Three-phase AC currents injected into the grid are projected on a d-q synchronous frame which is rotating at an angular speed ω, where, ω is adjusted to be synchronized with the grid frequency by means of a Phase Locked Loop (PLL) [10]. Therefore, three-phase AC currents are sinusoidal functions of the grid frequency, and d-axis, q-axis current components become time-invariant in steady-state, and thus the controller design is greatly simplified. In Fig. 3, the inputs to the PLL block is the three-phase sinusoidal voltages from the grid, and its output is the angular speed ω for abc-to-dq and dq-to-abc transformations. Three-phase voltages are transformed into d- and q-axis components based on [11][12], and the q-axis voltage vsq is regulated to zero using a PI controller. If q-axis voltage vsq can be regulated to zero, the active and reactive powers, Ps and Qs, will be independent of the voltage vsq. Therefore, active power, Ps and reactive power, Qs can be expressed in terms of currents isd and isq as:
Ps = 1.5vsd isd
(1)
Qs = 1.5vsd isq
(2)
From Equation (1) and (2), Ps is proportional to, and be controlled by, isd; similarly, Qs can be controlled by isq. Therefore, based on grid voltage oriented control, the grid voltage has only d-axis component vsd, while q-axis component vsq=0 if the d-axis is aligned to the voltage phasor. The active and reactive power can therefore be independently controlled by regulating the d-q reference currents.
θ (ω )
θ (ω )
Fig. 3. Control scheme of grid-side AC current
In Fig. 3, the outputs of two PI regulators are variable modulation indexes based on d-q frame, by using dq-to-abc transformations, the three-phase sinusoidal current reference signals for PD-PWM control are achieved. 4. Simulation results To verify the operations of grid-connected WECS, a simulation system based on a three-phase 5-level CSI is set up, and the simulation is carried out in MATLAB/Simulink environment. Some main parameters are as follows: LDCx (x=1, 2)=50mH, the sharing-inductors Lx(x=a, b, c) =20mH. R0=0.4Ω, L0=4mH, Cfx(x=a, b, c)=50μF; the 5-level CSI was operated with an output frequency of 50 Hz, and a PDPWM switching frequency of 1050 Hz (pulse ratio = 21). Fig. 4 presents the response of the grid-connected WECS, including outputs of the thyristor rectifier and 5-level CSI due to a step change for wind speed. At t = 0.3s, the DC-link current reference value is changed from 50A to 100A due to change of wind speed, as shown in Fig. 4(a). After being shortly regulated by DC-link current controller, the DC-link output current of each thyristor rectifier follows the
Jianyu Baoname et al. // Energy Energy Procedia Procedia 00 16 (2011) (2012) 000–000 461 – 466 Author
reference current very well in steady-state, as shown in Fig. 4(b) and (c). By supplying these two dc current-sources to the three-phase 5-level CSI, the PD-PWM controlled 5-level switching waveform of phase-a is obtained, as shown in Fig. 4(d), Fig. 4(e) shows the filtered current of phase-a, which is almost sinusoidal, and will be injected into the grid.
Fig. 4. Step response of thyristor rectifier and three-phase 5-level CSI
Fig. 5. Grid voltage and current waveforms at different power factor
Fig. 5 shows the performance of the grid-connected WECS based on three-phase 5-level CSI, where, different displacement power factors are generated at the grid-side by independently regulating the reference values of the d-axis and q-axis components, which are described as idref and iqref respectively in Fig. 3. By setting idref =0.8(pu) and iqref =0(pu), the in-phase waveforms of current and voltage can be seen in Fig. 5(a), and thus the unity power factor is achieved. If iqref is changed to 0.2(pu), then the leading displacement power factor is produced as shown in Fig. 5(b). Similarly, the lagging displacement power factor is produced when iqref is set to -0.2(pu), as shown in Fig. 5(c). In Fig. 5(d), idref is kept constant, and iqref is changed from 0.2(pu) to -0.2(pu), the transitions between leading and lagging reactive power injected into the grid can be seen in Fig. 5 (e) and (f).
465
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Jianyu Bao/etEnergy al. / Energy Procedia 16 (2012) 461 – 466 Author name Procedia 00 (2011) 000–000
5. Conclusion This paper presented a grid-connected WECS structure based on a three-phase PWM 5-level CSI. This three-phase 5-level CSI is PD-PWM controlled and provides relatively low line current harmonics at the grid, which is similar to that of line-line voltage for 5-level VSI. For direct-driven wind energy conversion system applications, the dc-link current can be adjusted according to the variation of wind speed through phase-control thyristor rectifier. At the grid side, control scheme of independent active and reactive power control was developed. Based on grid voltage oriented control, q-axis component of grid voltage is regulated to zero though PLL, therefore the active and reactive power can be independently controlled by regulating the d-q reference currents. According to the requirements of power balance control, the leading, unity, or lagging displacement power factor is produced. Meanwhile, the three-phase AC currents result from the multilevel CSI-based wind energy conversion system which will be injected into the grid are almost sinusoidal for all operating conditions. Acknowledgements This work was supported by Natural Science Foundation of Zhejiang Province, through grant number Y1111002. References [1] J. Dai, D. Xu, B. Wu. A Novel Control System for Current Source Converter Based Variable Speed PM Wind Power Generators. IEEE 38th Annual Power Electronics Specialists Conference, Orlando, Florida, USA, 2007, p. 1852-1857. [2] Xiong, Y., Li, Y.L., Yang, X., Zhang, Z.C. A New Three-Phase Five-Level Current-Source Inverter. IEEE Conference on APEC05, 2005, p.424-427. [3] Bao, J.Y., Bai, Z.H., Wang, Q.S., Zhang, Z.C. A New Three-Phase 5-Level Current-Source Inverter. Journal of Zhejiang University SCIENCE A, 2006, 7(12): 1973-1978. [4] Kwak, S.S., Toliyat, H.A. Multilevel Converter Topology Using Two Types of Current-Source Inverters. IEEE Trans. on Industry Applications, 2006, 42(6):1558-1564. [5] Bai, Z.H., Zhang, Z.C., Zhang,Y. A Generalized Three-Phase Multilevel Current Source Inverter with Carrier Phase-Shifted SPWM. IEEE Conference on PESC07, 2007, p.2055-2060. [6] Bao, J.Y., Bao,W.B., Wang, S.R., ZHANG, Z.C. Multilevel Current Source Inverter Topologies Based on the Duality Principle. IEEE Conference on APEC, 2010, p.1097-1100. [7] Pierluigi T, Andrew AR, Thomas AL. Wind Turbine Current-Source Converter Providing Reactive Power Control and Reduced Harmonics. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, 2007, 43( 4):1050-1060. [8] Miteshkumar P, Bin W, Navid RZ. DC Link Current Control of CascadedCurrent Source Converter Based Offshore Wind Farms. IEEE International Electric Machines & Derives Conference, 2011, p.807-812. [9] McGrath, BP, Holmes, DG. Multicarrier PWM Strategies for Multilevel Inverters, IEEE Trans. on Industrial Electronics, 2002, 49(4):858-867. [10] Chung SK. A phase tracking system for three phase utility interface inverters. IEEE Transactions on Power Electronics. 2000, 15(3): 431-438. [11] Kazerani M, Dash, PP. Dynamic Modeling and Performance Analysis of a Grid-Connected Current-Source Inverter-Based Photovoltaic System. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, 2011, 2(4): 443-450. [12] Kazerani M, Dash, PP. A Multilevel Current-Source Inverter Based Grid-Connected Photovoltaic System. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, 2011, 2(4): 443-450.