Control of PWM converters for renewable energy systems

1. Control of PWM converters for renewable energy systems Marco Liserre liserre@ieee.org

2. Control of PWM converters for renewable energy systems Aims, pre-requisites, teaching methods Grid-connected PWM converters are gaining increasing importance in view of a growing contribution of Distributed Power Generation Systems (DPGS) to the total power flow in the European electric utility. This is also owed to an increasing inflow from Renewable Energy Sources (RES). The course reviews some of the most important aspects related to the advanced control of grid-connected PWM converters with attention paid to DPGS based on RES. Pre-requisites Basic power converters. Control theory Lectures, supported by projector and blackboard, personalized feedback and coaching to improve every aspect of the student's work. Slides and exercises will be available at http://www.tf.uni-kiel.de/etech/LEA/?a=links the day before the lecture. Slides will be available in printed version the day of the lecture for students. Teaching methods

3. Control of PWM converters for renewable energy systems Course contents Power Converters for Distributed Power Generation Systems • Grid-connected PWM voltage source converters: opportunities and challenges • Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) • Grid requirements to connect DPGS based on RES • Review of modulation and basic control system, harmonic compensation • Grid filter design and stability of the current control loop • Grid converter operation (dc and ac control loops) • Grid synchronization • Grid Converter control and future functions • Modulation and control for cascaded multilevel converters • Non-linear control Control of the grid-connected power converter

4. Control of PWM converters for renewable energy systems Course contents Control of DPGS • Anti-islanding techniques for small DPGS • Control of Grid Converters Under Grid Faults (Low Voltage Ride Through- LVRT) • Micro-grid operation Droop control • HVDC, STATCOM, Active filter • Modulation, PI control and P+res control, Harmonic control • LCL-filter: stability issues • Synchronization of the converter • Cascaded control of grid converter • Anti-islanding • LVRT Exercises (computer simulations)

5. Control of PWM converters for renewable energy systems Course contents Exercises (laboratory) • LCL-filter stability problems • STATCOM operation of the grid converter to support the grid voltage Expected knowledge Knowledge of the main issues related to power conditioning in DPGS based on renewable energy systems, function of the grid converter Examination method Oral based on a presentation of a research described in a scientific paper. A general knowledge of the course contents is expected Course assistant Dipl. Ing. Jörg Dannehl [jda@tf.uni-kiel.de]

6. Control of PWM converters for renewable energy systems Bibliography 1. N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics: Converters, Applications, and Design” Wiley, 2002, ISBN-10: 04712269392. B. Bose, “Modern Power Electronics and A.C. Drives”, Prentice Hall, 2001, ISBN 013016743.3. D.G. Holmes and T. Lipo, Pulse Width Modulation for Power Converters : Principles and Practice, 2003, ISBN 0471208140.4. M. P. Kazmierkowski, R. Krishnan, F. Blaabjerg, “Control in Power Electronics”, Academic Press, 2002, ISBN 0-12-40277205.5. J. Machowski, J. Bialek, J. Bumby, “Power System Dynamics: Stability and Control ” Wiley, 2008, ISBN-10: 0470725583. 6. T. Ackermann, “Wind Power in Power Systems”. John Wiley & Sons, Ltd., 2005.7. F. Blaabjerg, R. Teodorescu, M. Liserre, A. V. Timbus, “Overview of Control and Grid Synchronization for Distributed Power Generation Systems”, IEEE Transactions on Industrial Electronics, October 2006, vol. 53, no. 5, pp. 1398-1408. 8. R. Teodorescu, F. Blaabjerg, M. Liserre and P. Chiang Loh, “Proportional-Resonant Controllers and Filters for Grid-Connected Voltage-Source Converters”, IEE proceedings on Electric Power Applications, September 2006, vol. 153, no. 5, pp. 750-762. 9. M. Liserre, R. Teodorescu, F. Blaabjerg, “Stability of Photovoltaic and Wind Turbine Grid- Connected Inverters for a Large Set of Grid Impedance Values”, IEEE Transactions on Power Electronics, January 2006, vol. 21, no.1, pp. 263-272. 10. P. Rodriguez, A. Timbus, R. Teodorescu, M. Liserre and F. Blaabjerg, “Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults”, IEEE Transactions on Industrial Electronics, October 2007, vol. 54, no. 5, pp. 2583-2592.

7. Grid-connected PWM voltage source converters: opportunities and challenges Marco Liserre liserre@ieee.org

8. Device • GTOs are already obsolete. IGBT and IGCT will compete • High voltage high power silicon carbide power devices will play important roles Converter • Multi-level converters (particularly diode-clamped and cascaded H-bridges) will be the most important • Multi-MW induction and synchronous motor drives now routinely use multi-level PWM converters (instead of traditional cycloconverters) Power Electronics Scenario

9. Power Electronics Scenario Optimal design and control • Intelligent control and optimal design are indispensable tools • Controllers based on PWM will be the dominant technology (average-based or on-off) • The choice in high power system will be between the frequency-domain approach or time-domain approach (predictive) and efficiency will be the driver • Diagnosis and fault-tolerant control will be a standard feature for high power converters predictive control frequency shaping

10. Power Electronics Scenario Utility applications • Power electronics is revolutionizing the field of power engineering • Voltage-fed multi-terminal HVDC will be very important • FACTS and STATCOM will be very important for P and Q control • Renewable energy systems (wind and photovoltaic) are becoming very important consumption Grid-connected PWM voltage source converters will be the intelligent interface for loads, generation systems, storage systems and flexible transmission production

11. The Importance

12. The PWM grid converter, a kind of new synchronous machine ? • The synchronous machine has a central role in the centralized power system • The “synchronous converter” major player in the future power system • Interfacing power production, consumption, storage and transportation within the future power system based on smart grids • Based on semiconductor technology and signal processing

13. PWM carrier and sideband harmonics 1 n voltage h harmonic order The PWM grid converter frequency behavior • The PWM grid converter is equivalent to multiple synchronous machines • The grid converter can control the active and reactive power flow in a vast frequency range 1 h n . . . .

14. The Need

15. A glance to the distributed power generation Current Power System Future Power System • Less central power plants and more Distributed Power Generation Systems

16. A glance to the renewable energy systems • Wind systems require optimized grid converter at high power • 3.6-6 MW prototypes running • 2 MW WT are still the "best seller" on the market!

17. A glance to the renewable energy systems • Doubly-fed is the most adopted soltion in wind systems • Full power converter can be used either with asynchronous generator or synchronous generator (multipole permanent magnet gearless solution is the most promising)

18. A glance to the renewable energy systems • Photovoltaic systems require high-efficient and multi-functional grid converters

19. A glance to the transmission system • Right Of Way (ROW) restrictions • Need of connection: distance between production and consumers, economics of scale, wider choice of generating plants, reduction in reserve capability, etc • Increase of power carrying capability vs transient stability

20. A glance to the transmission system • separate control loops active and reactive power • active power control • one station controls the active power • other station controls the DC-link voltage • reactive power control • reactive power or AC side voltage HVDC based on PWM grid converter offers . .

21. A glance to the power quality Series and parallel active filters enhance grid power quality compensating voltage sag, harmonic, reactive power, etc .

22. A glance to the load demand • Active rectifier is adopted as active front-end for medium and high power systems like multi-drive systems and single drives working frequently in regenerative operation like cranes, elevators . .

23. The opportunities and challenges

24. The increase in the power leads to the use of more voltage levels: • Single-cell converter • Multi-cell converter • Design and Control challenges and opportunities: • Lower switching frequency • More powerful computational device • Solutions: • Non-linear analysis • Optimization with deterministic and stochastic techniques

25. Single-cell converter • Wind turbine systems: high power -> 5 MW converter • Photovoltaic systems: many dc-links for a transformerless solution • predictive control to achieve the best control performance with minimum commutation • advanced grid filter design to deal with a low switching frequency

26. Multi-cell converter • Many converters forming cells connected in series to share the power • Both for wind and photovoltaic solutions • Passivity-based control to manage the power transfer from each cell independently • Reliability study to optimize each component and the choice of the cell structure

27. Passivity-based control of a cascade converter

28. Dynamical test dc voltage reference step on one bus dc load steps on the two buses leading to different loads Measured DC voltages [50 V/div] and grid current [4 A/div] (2330 mF)

29. Modified Phase Shifted Carrier PWM • The different dc voltages can be managed using a proper modulation original modified Shifting angles =0º, 120º and 240º Shifting angles =0º, 36º and 191º • The original PSC-PWM angles can be obtained as a particular solution • Asymmetrical PWM angles can be obtained dividing the obtained results by 2

30. Main topics • Grid monitoring: detection and synchronization • Current control: harmonic rejection and stability • Micro-grid management and grid support: power control strategies

31. Grid monitoring: detection and synchronization

32. islanding detection Test to verify the detection of the islanding condition in a short time Detection of grid conditions Test to verify immunity of the method (no false trip) to frequency variation

33. Synchronization • Synchronization will be crucial for all the grid connected inverters to adapt their behavior in any grid condition • Single PLL based on a second order integrator acting as a sinusoidal follower is the building block of a class of advanced synchronization methods

34. Synchronization • Detection of the positive and negative sequences will be important during grid-faults • Three-phase system synchronization needs a vectorial approach and a dual PLL

35. Current control: harmonic rejection and stability

36. Current Control and LCL-filter • IEC Standard 61000-3-6, “Electromagnetic Compatibility, Assessment of Emission Limits for Distorting Loads in MV and HV Power Systems”, 1996. • IEEE Std 1547-2003 "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems", 2003.

37. Harmonic rejection • Using Multiple Synchronous Reference Frames (MSRFs) • Using selective filters based on resonant controllers

38. Hybrid solution

39. Repetitive current control The resonant controller can track a sinusoidal signal, a repetitive controller can track a periodic signal The control action should be limited

40. Rejection of grid voltage background distortion no harmonic control harmonic control

41. Rejection of harmonics caused by non-linearities The frequency behaviour of the non-linear inductance can be studied splitting the model in a linear part and a non-linear part in accordance with the Volterra theory. The Volterra-series expansion of the flux is

42. Volterra-series expansion inductor model input current at ω1= 50 Hz input current at ω2= 150 Hz input current at (ω1 + ω2 ) flux spectrum of the non-linear inductance When two sinusoids of different frequencies are applied simultaneously intermodulation components are generated They increase the frequency components in the response of the system and the complexity of the analysis

43. High current non-linearity repetitive controller resonant controller a a THD= 4.9% THD= 8.1% b b Grid current with non-linear inductor and repetitive controller: a) (1) grid current [10A/div]; (2) grid voltage [400V/div]; (A) grid voltage spectrum [10V/div]; (B) grid current spectrum [0.5A/div]; (C) a period of the grid voltage; (D) a period of the grid current; b) a period of the grid current (simulation results) [10A/div]. Grid current with non-linear inductor and resonant controller: a) (1) grid current [10A/div]; (2) grid voltage [400V/div]; (A) grid voltage spectrum [10V/div]; (B) grid current spectrum [0.5A/div]; (C) a period of the grid voltage; (D) a period of the grid current; b) a period of the grid current (simulation results) [10A/div].

44. Grid converters connected through an LCL-filter L1 LCL magnitude (Db) L1+L2 frequency (Hz) ripple attenuation

45. Magnitude [dB] 50 D(z)G(z) 0 D(z)Gd(z) -50 2 3 10 10 Frequency [Hz] Passive damping • As the damping resistor increases, both stability is enforced and the losses grow but at the same time the LCL-filter effectiveness is reduced. i i L L1 i g c 2 C v e v f c R d

46. Activedamping • The aim is to shape the harmonic spectrum around the resonance frequency Gf GAD z-1GADGf

47. optimal position of the poles optial position of the poles final result of GA Genetic algorithm active damping GA optimize this controller

48. Comparison with non-linear optimisation method • Comparison with the non-linear Levenberg-Marquardt optimisation method already used for passive damping design 0.92 1.12 • The non-linear least-square method finds a point characterized by 1.12 while the absolute minimum is 0.92

49. Micro-grid management and grid support: power control strategies

50. Introduction • The grid converter can operate as grid-feeding or grid-forming device • Main control tasks • manage the dc-link voltage (if there is not a dc/dc converter in charge of it) • inject ac power (active/reactive) • A third option is the operation as grid-supporting device (voltage, frequency, power quality)