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

Aims, pre-requisites, teaching methods 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 Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Course contents Control of DPGS Exercises (computer simulations) 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) Marco Liserre liserre@ieee.org

Course contents Exercises (laboratory) Expected knowledge 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] Marco Liserre liserre@ieee.org

Bibliography Control of PWM converters for renewable energy systems 1. N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics: Converters, Applications, and Design” Wiley, 2002, ISBN-10: 0471226939 2. 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. Marco Liserre liserre@ieee.org

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

Power Electronics Scenario GTOs are already obsolete. IGBT and IGCT will compete High voltage high power silicon carbide power devices will play important roles Device 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) Converter IGCT will be more important for high power Probably Silicon carbide will oust the Silicon device Marco Liserre liserre@ieee.org

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 DSP and FPGA control are very common Phase control so common now will tend to be obsolete gradually in future predictive control frequency shaping Marco Liserre liserre@ieee.org

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 High power GTO converters in utility system use phase-shift PWM (multi-stepping) with multi-phase transformer for voltage and phase angle control Asia & Pacific and China are growing: it is not only important to have technology know-how for European market but also for other markets, hence to be able to build systems that can operate in different voltage and frequency conditions The systems should be universal production Marco Liserre liserre@ieee.org

The Importance Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

The PWM grid converter frequency behavior voltage harmonic order 1 h n PWM carrier and sideband harmonics 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 . . . . Marco Liserre liserre@ieee.org

The Need Marco Liserre liserre@ieee.org

A glance to the distributed power generation Current Power System Future Power System Less central power plants and more Distributed Power Generation Systems Marco Liserre liserre@ieee.org

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! Marco Liserre liserre@ieee.org

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) Marco Liserre liserre@ieee.org

A glance to the renewable energy systems Photovoltaic systems require high-efficient and multi-functional grid converters Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

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 . . Marco Liserre liserre@ieee.org

A glance to the power quality Series and parallel active filters enhance grid power quality compensating voltage sag, harmonic, reactive power, etc . Marco Liserre liserre@ieee.org

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 . . Marco Liserre liserre@ieee.org

The opportunities and challenges Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Passivity-based control of a cascade converter Marco Liserre liserre@ieee.org

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) Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Main topics Grid monitoring: detection and synchronization Current control: harmonic rejection and stability Micro-grid management and grid support: power control strategies Marco Liserre liserre@ieee.org

Grid monitoring: detection and synchronization Marco Liserre liserre@ieee.org

Detection of grid conditions islanding detection Test to verify the detection of the islanding condition in a short time Description A higher penetration of distributed power generation systems (DPGS), involving both conventional and renewable technologies, is changing the power system face. There is a clear evolution towards active grids that could include a significant amount of storage systems, could work in island mode and could be connected through flexible transmission systems. This complex scenario will put different requirements on the DPGS units depending on their size and on their level of integration with the power system. Anyway the monitoring of the grid condition will be always a crucial feature of the DPGS units of every level. The detection of a possible island condition, nevertheless which will be the standards, recommendation or utilities codes, will be always important in a power system with a significant amount of DPGS. Test to verify immunity of the method (no false trip) to frequency variation Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Current control: harmonic rejection and stability Marco Liserre liserre@ieee.org

Current Control and LCL-filter IEEE Std 1547-2003 "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems", 2003. IEC Standard 61000-3-6, “Electromagnetic Compatibility, Assessment of Emission Limits for Distorting Loads in MV and HV Power Systems”, 1996. harmonic limit 5th 5-6 % 7th 3-4 % 11th 1.5-3 % 13th 1-2.5 % Marco Liserre liserre@ieee.org

Harmonic rejection Using Multiple Synchronous Reference Frames (MSRFs) Using selective filters based on resonant controllers Marco Liserre liserre@ieee.org

Hybrid solution Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Rejection of grid voltage background distortion no harmonic control harmonic control Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

High current non-linearity resonant controller repetitive controller a a THD= 8.1% THD= 4.9% 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]. Marco Liserre liserre@ieee.org

Grid converters connected through an LCL-filter magnitude (Db) L1+L2 frequency (Hz) ripple attenuation Marco Liserre liserre@ieee.org

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. Frequency [Hz] -50 50 D(z)G(z) D(z)Gd(z) Magnitude [dB] 10 2 3 i L i L1 i g 2 c C e f v v c R d Marco Liserre liserre@ieee.org

Active damping The aim is to shape the harmonic spectrum around the resonance frequency Gf GAD z-1GAD Gf Marco Liserre liserre@ieee.org

Genetic algorithm active damping optimal position of the poles optial position of the poles final result of GA The Genetic Algorithm (GA) simulates Darwin’s theory on natural selection and Mendel’s work in genetics on inheritance: the stronger individuals are likely to survive in a competing environment. In short the GA finds the optimum solution combining a set of randomly chosen solutions. In the following the term “individual” indicates the possible solution, the terms “gene” indicates one of the parameters of the solution and the “fitness value” indicates the degree of goodness of the individual. Other methods exist: ant colony, particle swarm, etc They can optimize either the parameters or the controller form GA optimize this controller Marco Liserre liserre@ieee.org

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 Marco Liserre liserre@ieee.org

Micro-grid management and grid support: power control strategies Marco Liserre liserre@ieee.org

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) Marco Liserre liserre@ieee.org

Droop control for grid forming or supporting The droop control is not only used in island application when it is needed to a have a wireless load sharing but also in order to support the grid In this case grid-feeding and grid-forming schemes can be modified accordingly including droop control grid feeding grid forming Marco Liserre liserre@ieee.org

P&Q Control Strategies Under Grid Fault Instantaneous Active Reactive Control (IARC) Distorted and unbalanced current Instantaneous power perfectly controlled Overcurrent trip risk Positive- Negative-Sequence Compensation (PNSC) Active or reactive power without oscillations Balanced Positive Sequence (BPS) q is a phase-shift operator in the time-domain which obtains the quadrature-phase waveform (90-degrees lag) of the original in-phase waveform. Bothe active and reactive power with oscillations Marco Liserre liserre@ieee.org

Conclusions Key drivers Technology for different country voltages/frequencies and codes/standards Avoid bulky transformers, reduce part count, increase efficiency Stability of new power systems based on DPGS Major challenges are: Synchronization with the grid Stability of current and voltage loops Proper harmonic control Detecting the grid disconnection without communication Managing soft re-connection to the grid Micro-grid control and optimum management of energy storage Marco Liserre liserre@ieee.org