1. Control of PWM converters for renewable energy systems
Marco Liserre
Marco Liserre

2. 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 the day before the lecture. Slides will be available in printed version the day of the lecture for students.
Teaching methods
Marco Liserre

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
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4. 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)
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5. 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
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6. 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: B. Bose, “Modern Power Electronics and A.C. Drives”, Prentice Hall, 2001, ISBN D.G. Holmes and T. Lipo, Pulse Width Modulation for Power Converters : Principles and Practice, 2003, ISBN M. P. Kazmierkowski, R. Krishnan, F. Blaabjerg, “Control in Power Electronics”, Academic Press, 2002, ISBN J. Machowski, J. Bialek, J. Bumby, “Power System Dynamics: Stability and Control ” Wiley, 2008, ISBN-10: T. Ackermann, “Wind Power in Power Systems”. John Wiley & Sons, Ltd., 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 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 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 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
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7. Grid-connected PWM voltage source converters: opportunities and challenges
Marco Liserre
Marco Liserre

8. 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
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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
DSP and FPGA control are very common
Phase control so common now will tend to be obsolete gradually in future
predictive
control
frequency shaping
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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
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
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11. The Importance
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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
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13. 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
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14. The Need
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15. A glance to the distributed power generation
Current Power System
Future Power System
Less central power plants and more Distributed Power Generation Systems
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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!
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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)
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18. A glance to the renewable energy systems
Photovoltaic systems require high-efficient and multi-functional grid converters
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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
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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 . .
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21. A glance to the power quality
Series and parallel active filters enhance grid power quality compensating voltage sag, harmonic, reactive power, etc .
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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 . .
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23. The opportunities and challenges
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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
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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
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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
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27. Passivity-based control of a cascade converter
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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)
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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
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30. Main topics Grid monitoring: detection and synchronization
Current control: harmonic rejection and stability
Micro-grid management and grid support: power control strategies
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31. Grid monitoring: detection and synchronization
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32. 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
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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
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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
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35. Current control: harmonic rejection and stability
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36. Current Control and LCL-filter
IEEE Std "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems", 2003.
IEC Standard , “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 %
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37. Harmonic rejection Using Multiple Synchronous Reference Frames (MSRFs)
Using selective filters based on resonant controllers
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38. Hybrid solution
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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
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40. Rejection of grid voltage background distortion
no harmonic control
harmonic control
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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
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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
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43. 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].
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44. Grid converters connected through an LCL-filter
magnitude (Db)
L1+L2
frequency (Hz)
ripple attenuation
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45. 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
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46. Active damping
The aim is to shape the harmonic spectrum around the resonance frequency Gf
GAD
z-1GAD Gf
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47. 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
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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
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49. Micro-grid management and grid support: power control strategies
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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)
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51. 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
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52. 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
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53. 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
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