1. Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES)
Marco Liserre
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2. Outline
Introduction to distributed power generation and renewable energy systems
World energy scenario (including renewable energy)
Outlook on wind and photovoltaic energy
Integrating renewable energy sources with the future power system
Wind systems
Photovoltaic systems
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3. Distributed power generation
Provide electric capacity and/or energy at or near consumer sites to meet specific customer needs
Relatively small generating units and storage technologies
Either be interconnected with the electric grid or isolated from the grid in "stand- alone"
The location value is important to the economics and operation
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4. Renewable energy systems
Source: Billman, Advances in Solar Energy submission, 1/8/99
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5. World energy consumption
The growth of energy demand in 2007 remained high despite high energy prices
China has surpassed the EU
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6. World energy production
The relative market share of oil is decreasing respect coal and gas
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7. Renewable Energy scenario
In 2007 the world renewable energy production share has been calculated as 19 %.
However 16 % is due to hydraulic energy production, hence wind and photovoltaic (the most promising renewable sources) energy production is still very modest.
The goal of the European Community is to reach 20 % in 2020, however the EU-27 energy is only 17% of world energy.
USA with 22% of energy share may adopt similar goals under the pressure of public opinion concerned by environmental problems (in California the goal is 20 % in 2010).
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8. Renewable Energy scenario
However the policies of Asia and Pacific countries, with 35% of energy share, will be probably more important in the future energy scenario.
In fact countries like China and India require continuously more energy (China energy share increases 1 point every year from 2000).
The need for more energy of the emerging countries and the environmental concerns of USA and EU will drive the increase of the renewable energy production: the importance of renewable energy sources in the future energy scenario is not anymore under discussion !
The needed technology is available and it benefits of continuous improvement due to academic and industrial research activity
Knowledge transfer to industry on the basis of international conferences and workshops and educational programs.
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9. Renewable Energy scenario
Wind energy – highest development
Solar energy – next highest development
Wave energy – largely unexplored
Tidal energy – largely unexplored
Small hydro (<10MW), 47GW used, 180 GW untapped (70% in developing countries). Oldest technology (not covered)
Biomass 18GW used (2000), largely unexplored. Used in CHP
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10. Wind energy Bigger and more efficient !
3.6-6 MW prototypes running (Vestas, GE, Siemens Wind, Enercon)
Danish Vestas and Siemens Wind stand for over 40% of the worldwide market
2 MW WT are still the "best seller" on the market!
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11. Wind energy
Wind energy can benefit of huge investments in research and education.
Some of the most relevant goals of the research can be briefly summarized as:
to increase the power production of each wind turbine (over 5 MW),
to increase the penetration of small wind turbine systems (under 50 kW)
to create wind plants (preferably off-shore) that can behave similarly to standard oil & gas power plants respect to the grid (due to wind forecast and proper control strategies).
Educational investments are mainly done by universities to prepare a future category of engineers for the wind industry but also by leader wind companies that want to form highly specialized engineers through specific PhD programs
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12. Photovoltaic energy
The cost of PV electricity will reach the break-even point soon in many countries
Optimistic ! Silicon shortage has slowed the price reduction
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13. Photovoltaic energy
Despite the silicium shortage in the last years the PV industry is growing at more than 30%
PV Module technology is also developing fast toward higher efficiency and lower cost of 4-5 €/Wp, expected 3€/Wp in 5 years. From experience 7%/year fall
String technology is dominating. Multi-string for residential applications
Mini-central three-phase inverters 8-15 kW are emerging for modular configuration in medium and high power systems (commercial roof-tops)
Central inverters are available for plants up to MW range (1MW – SMA)
Reliability is increased now 5 years but extended 20 years (not free!)
Increase functionality available (built-in logger, communication, grid support, etc)
Cost is still high ( €/kWp) and high efforts are done in order to reduce it to €/kWp in the next 5 years by:
mass production
better topologies with fewer components
design-to-cost
PV electricity cost is expected to reach the break-even cost around 2015 where mass PV penetration is expected
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14. Photovoltaic energy
The most relevant goals of photovoltaic energy are 40% cost reduction of photovoltaic panels and of the power converter stage in 5 years and the increase of the efficiency of both and the reliability of the latter considerably.
These goals are driving the research towards several directions such as:
maximum power extraction algorithms,
advanced anti-islanding algorithms for higher safety levels
higher efficiency of the power converter (98 % efficiency is the goal for transformerless topologies)
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15. Power system evolution
Active distribution grids with a significant amount of medium-scale and small-scale generators (ranging from hundreds of kW to tens of MW), involving both conventional and renewable technologies, together with storage systems and flexible high-voltage transportation systems connecting those grids with lower cost and ROW (Right Of Way) restrictions.
The importance of storage in the overall scenario is crucial
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16. Smart micro-grids (SMG)
Within active grids, generators and loads can both play a role as operators in electricity markets
Distribution grids have to be equipped with protection systems and real-time control systems leading to smart micro-grids (SMG) usually operated in connection to distribution grids but with the capability of automatically switching to a stand-alone operation if faults occur in the main distribution grid, and then re-connected to the grid.
The safe operation in any condition (grid-connected or stand-alone) relies also on good simulation tools to predict the behavior of the overall system considering the specific operation of the renewable energy sources.
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17. Information Technology Networking
The operation of a SMG can result in higher availability and quality compared with strictly hierarchical management of power generation and distribution. The security of the system can be improved by the ability of feeding final users, reacting to demand variations in a short time by redispatching energy thanks to smart systems. This allows to reduce risks and consequences of black-outs, avoiding the increase of the global production.
Photovoltaic systems highly integrated in the buildings
Hydrogen distribution network
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18. Information Technology Networking
problems . . .
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19. Information Technology Networking
possible solutions . . .
Color-based indication of grid status
Automated Demand Response
from Dr. Peter Palensky’s contribution to IEEE – IECON 2008 Panel Discussion Session On Industrial Electronics for Renewable Energy
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20. Wind systems
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21. Wind turbine systems Doubly-fed induction generator - wounded rotor
Limited speed range (-30% to +20%, typical)
Small-scale power converter (Less power losses, price)
Complete control of active Pref and reactive power Qref
Need for slip-rings
Need for gear
Producers: Vestas, Gamesa, NEG Micon, GE Wind, Nordex, REpower Systems, DEWind
Power range: 0.85 MW to 4.2 MW
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22. Wind turbine systems Induction generator - Squirrel cage rotor
Full speed range
No brushes on the generator
Complete control of active and reactive power
Proven technology
Full-scale power converter
Need for a gear
Mainly for low power stand-alone
Producers: Verteco (converter rated for 50% power), Neg Micon, Siemens
Power range: 0.66 MW to 3.6 MW
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23. diode-bridge + chopper
Wind turbine systems
Synchronous generator - External magnetized
Full speed range
Possible to avoid gear (multi-pole generator)
Complete control of active and reactive power
Small converter for field
Need of slip-rings
Full scale power converter
Multi-pole generator may be big and heavy
inverter or
diode-bridge + chopper
Producers: Enercon, Largey,
Power range: 0.6 MW to 4.5 MW
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24. diode-bridge + chopper
Wind turbine systems
Synchronous generator - Permanent magnets
Full speed range
Possible to avoid gear (multi-pole generator)
Complete control of active and reactive power
Brushless (reduced maintenance)
No power converter for field (higher efficiency)
Full scale power converter
Multi-pole generator big and heavy
Permanent magnets needed
inverter or
diode-bridge + chopper
Producers: Largey, Mitsubishi, Pfleiderer Wind Energy
Power range: 0.6 MW to 4.5 MW
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25. SG Example 1
20 kW mini-WT multipolar permanent magnet synchronous generator with axial flux produced by JONICA IMPIANTI (JIMP)
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26. SG Example 2
from “WindBlatt 02/03” WT Enercon 300 kW multipolar synchronous generator installed in Antartica
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27. 3 kV NPC converter from Alstom or ABB
SG Example 3
Multibrid WT 5 MW multipolar synchronous generator (Multi) with ibrid gear (brid) for offshore applications
Prokon Nord
3 kV NPC converter from Alstom or ABB
synchronous generator with permanet magnets surface mounted and radial flux
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28. Trends 2002 no gear-box Power electronics is now in wind turbines
Direct-driven genertaor market share is growing
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29. Wind turbine systems control
Basic power conversion and control:
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30. Wind turbine systems control
Basic demands:
Electrical:
Interconnection (conversion, synchronization)
Overload protection
Active and reactive power control
Mechanical:
Power limitation (pitch)
Maximum energy capture
Speed limitation/control
Reduce acoustical noise
Control loops with different bandwidth
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31. Wind turbine systems control
Controllers (internal)
Modulation
Overall system control
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32. Wind turbine systems control
Control of permanent magnet synchronous generator system
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33. Wind turbine systems control
Control of synchronous generator system
- Control of active and reactive power
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34. Wind turbine systems control
Control of doubly-fed induction generator system
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35. Detailed example
Operating range
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36. Wind turbine systems control
Control of doubly-fed induction generator system (generator-side)
- Complete control of active and reactive power
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37. Detailed example
Basic power flow
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38. Photovoltaic systems
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39. PV Inverter Topologies
PV dc voltage typical low for string inverters  boost needed for low power
For high power (>100 kW) central PV inverters w/o boost, typical three-phase FB topologies with LV-MV trafo
Galvanic isolation necessary in some countries
LF/HF transformer (cost-volume issue)
A large variety of topologies
The optimal topology is not matured yet as for drives
Transformerless topologies having higher efficiency are emerging and the grid regulations are changing in order to allow them
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40. PV inverters with boost converter and isolation
On low frequency (LF) side
On high frequency (HF) side
Boosting inverter with LF trafo based on boost converter
Boosting inverter with HF trafo based on FB boost converter [2]
Both technologies are on the market! Efficiency 93-95%
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41. Transformerless PV inverters with boost
FB inverter + boost
Typical configuration
Time sharing configuration
Efficiency > 96%
Extra diode to bypass boost when Vpv > Vg
Boost with rectified sinus reference
Efficiency >95%
Leakage current problem
Safety issue
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42. Frequency analysis of voltage to earth Vpe for FB with UP and BP PWM switching
VAB, VPE and IPE for FB-UP
Spectrum of voltage to earth
Spectrum of leakage current
Based on ICp and VCp and different frequencies the leakage capacitance was calculated at: Cp=13.6nF (7.06nF/kWp). Cp is useful in high-frequency analysis and in damping resonances
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43. High efficiency topologies derived from H-bridge FB with Bipolar PWM Switching
S1 + S4 and S2 + S3 are switched complementary at high frequency (PWM)
No 0 output voltage possible
The switching ripple in the current equals 1x switching frequency  large filtering needed
Voltage across filter is bipolar  high core losses
No common mode voltage  VPE free for high frequency low leakage current
Max efficiency 96.5% due to reactive power exchange L1(2)<-> Cpv during freewheeling and due to the fact that 2 switched are simultaneously switched every switching
This topology is not suited to transformerless PV inverter due to low efficiency!
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44. High efficiency topologies derived from H-bridge FB with Unipolar PWM Switching
Leg A and Leg B are switched with high frequency with mirrored sinusoidal reference
Two 0 output voltage states possible: S1 and S2 = ON and S3 and S4 = ON
The switching ripple in the current equals 2x switching frequency  lower filtering needed
Voltage across filter is unipolar  low core losses
VPE has switching frequency components  high leakage current and EMI
Max efficiency 98% due to no reactive power exchange L1(2)<-> Cpv during freewheeling
This topology is not suited to transformerless PV inverter due to high leakage!
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45. High efficiency topologies derived from H-bridge FB with Hybrid PWM Switching
Leg A is switched with grid low frequency and Leg B is switched with high PWM frequency
Two 0 output voltage states possible: S1 and S2 = ON and S3 and S4 = ON
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE has square wave variation at grid frequency  high leakage current and EMI
High efficiency 98% due to no reactive power exchange L1(2)<-> Cpv during freewheeling and due to lower frequency switching in one leg.
This topology is not suited to transformerless PV inverter due to high leakage!
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46. High efficiency topologies derived from H-bridge H5 (SMA)– ηmax=98%
Extra switch in the dc link to decouple the PV generator from grid during zero voltage
Two 0 output voltage states possible: S5 = OFF, S1 = ON and S5 = OFF, S3 = ON
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE is sinusoidal with grid frequency component  low leakage current and EMI
High max. efficiency 98% due to no reactive power exchange as reported by Photon Magazine for SMA SunnyBoy 4000/5000 TL single-phase
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47. High efficiency topologies derived from H-bridge HERIC (Sunways)-ηmax=98%
Two 0 output voltage states possible: S+ and D- = ON and S- and D+ = ON
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE is sinusoidal has grid frequency component  low leakage current and EMI
High efficiency 98% due to no reactive power exchange as reported by Photon Magazine for Sunways AT series 2.7 – 5 kW single-phase
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48. High efficiency topologies derived from H-bridge FB – DC Bypass (Ingeteam)-ηmax=96.5%
Two extra switches switching with high frequency and 2 diodes bypassing the dc bus. The 4 switches in FB switch at low fsw
Two 0 output voltage states possible by “natural clamping# of D+ and D-
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE is sinusoidal and has grid frequency component  low leakage current and EMI
High max efficiency 96.5% due to no reactive power exchange as reported by Photon Magazine for Ingeteam Ingecon Sun TL series (2.5/3.3/6 kW, single-phase)
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49. High efficiency topologies derived from H-bridge REFU ηmax=98% -
Three-level output. Requires double PV voltage input in comparison with FB but it include time-shared boost
Zero voltage is achieved by shortcircuiting the grid using the biderectional switch
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE without high frequency component  low leakage current and EMI . No L in neutral!
High max efficiency 98% due to no reactive power exchange, as reported by Photon Magazine for Refu Solar RefuSol (11/15 kW, three-phase)
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50. High efficiency topologies derived from H-bridge Summary
Actually both HERIC, H5, REFU and FB-DCBP topologies are converting the 2 level FB (or HB) inverter in a 3 level one.
This increases the efficiency as both the switches and the output inductor are subject to half of the input voltage stress.
The zero voltage state is achieved by shorting the grid using higher or lower switches of the bridge (H5) or by using additional ac bypass (HERIC or REFU) or dc bypass (FB-DCBP).
H5 and HERIC are isolating the PV panels from the grid during zero voltage while REFU and FB-DCBP is clamping the neutral to the mid-point of the dc link.
Both REFU and HERIC use ac by-pass but REFU uses 2 switches in anti- parallel and HERIC uses 2 switches in series (back to back). Thus the conduction losses in the ac-bypass are lower for the REFU topology.
REFU and H5 have slightly higher efficiencies as they have only one switch switching with high-frequency while HERIC and FB_DCBP have two.
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51. High efficiency topologies derived from NPC
Half Bridge Neutral Point Clamped (HB-NPC)-ηmax=98% -
Three-level output. Requires double PV voltage input in comparison with FB. Typically needs boost.
Two 0 output voltage states possible: S2 and D+ = ON and S3 and D- = ON. For zero voltage during Vg>0, Ig<0, S1 and S3 switch in opsition and S2 and S4 for Vg<0, Ig>0
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE is equal –Vpv/2 without high frequency component  low leakage current and EMI . No L in N!
High max efficiency 98% due to no reactive power exchange, as reported by Danfoss Solar TripleLynx series (10/12.5/15 kW)
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52. High efficiency topologies derived from NPC
Conergy NPC -ηmax=96% -
Only 4 switches needed with 2 of them (S+ and S-) rated only Vpv/4
Three-level output. Requires double PV voltage input in comparison with FB. Typically needs boost.
Two 0 output voltage states possible using the bidirectional clamping switch (S+ and S-)
The switching ripple in the current equals 1x switching frequency  high filtering needed
Voltage across filter is unipolar  low core losses
VPE is equal –Vpv/2 without high frequency component  low leakage current and EMI . No L in N!
High max efficiency 96.1% due to no reactive power exchange, as reported by Conergy IPG series (2-5 kW single-phase)
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53. High efficiency topologies derived from NPC
Summary
The classical NPC and its “variant” Conergy-NPC are both three-level topologies featuring the advantages of unipolar voltage across the filter, high efficiency due to disconnection of PV panels during zero-voltage state and practical no leakage due to grounded DC link mid-point.
Due to higher complexity in comparison with FB-derived topology, these structures are typically used in three-phase PV inverters with ratings over 10 kW (mini-central).
These topologies are also very attractive for high power in the range of hundreds of kW) central inverters) where the advantages of multi-level inverters are even more important.
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54. PV Inverter Topologies -Conclusions
The “race” for higher efficiency PV inverters has resulted in a large variety of “novel” transformerless topologies derived from H-Bridge with higher efficiency and lower CM/EMI (H5, HERIC)
Equivalent high-efficiency can be achieved with 3-level topologies (ex NPC)
Today more than 70% of the PV inverters sold on the market are transformerless achieving 98% max conversion efficiency and 97.7% “european” (weighted) efficiency
Further improvements in the efficiency can be achieved by using SiC MosFets. ISE Fraunhofer-Freiburg reported recently 98.5% efficiency (25% reduction in switching + conduction losses)
For 3-phase systems the trend is to use 3 independent controlled single-phase inverters like 3xH5 or 3xHERIC but 3FB-SC and 3NPC (not proprietary) are also present on the market. 3NPC achieve higher efficiency 98%
The general trend in PV topologies is “More Switches for Lower Losses”
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55. Control Structure Overview
Basic functions – common for all grid-connected inverters
Grid current control
THD limits imposed by standards
Stability in case of grid impedance variations
Ride-through grid voltage disturbances (not required yet!)
DC voltage control
Adaptation to grid voltage variations
Ride-through grid voltage disturbances (optional yet)
Grid synchronization
Required for grid connection or re-
connection after trip.
PV specific functions – common for PV inverters
Maximum Power Point Tracking – MPPT
Very high MPPT efficiency in steady state (typical > 99%)
Fast tracking during rapid irradiation changes (dynamical MPPT efficiency)
Stable operation at very low irradiation levels
Anti-Islanding – AI as required by standards (VDE0126, IEEE1574, etc)
Grid Monitoring
Operation at unity power factor as required by standards
Fast Voltage/frequency detection
Plant Monitoring
Diagnostic of PV panel array
Partial shading detection
Ancillary Support – (future?)
Voltage Control
Frequency control
Fault Ride-through
Q compensation
DVR
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56. Introduction to Maximum Power Point Tracking - MPPT
The MPP is affected by temperature and irradiance.
The task of MPPT is to track this MPP regardless of weather or load conditions so that the PV system draws maximum power from the solar array.
The MPPT is a nonlinear and time-varying system that has to be solved.
All algorithms are based on the fact that, looking at the power characteristic, at the left of the MPP the dP/dV > 0, at the right dP/dV < 0 and at MPP dP/dV = 0
The task of MPPT in a PV energy conversion system is to tune continuously the system so that it draws maximum power from the solar array regardless of weather or load conditions.
Since the solar array has non ideal voltage-current characteristics and the conditions such as irradiance and ambient temperature that affect the output of the solar array are unpredictable, the tracker should deal with a nonlinear and time-varying system.
dP/dV = 0, MPP
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57. MPPT Comparison Most common methods:
Perturb&Observe – PO
Incremental Conductance – IC
Constant Voltage
Preliminary results indicate that IC method compares favorably with PO and CV methods
Still PO is preferred due to implementation simplicity
Combined PO+CV is best!
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58. Typical control structure for dual-stage PV inverter
The MPPT is implemented in the dc-dc boost converter.
The output of the MPPT is the duty-cycle function. As the dc-link voltage VDC is controlled in the dc-ac inverter the change of the duty-cycle will change voltage at the output of the PV panels, VPV as:
The dc-ac inverter is a typical current controlled voltage source inverter (VSI) with PWM and dc-voltage controller.
The power feedforward requires communication between the two stages and improves the dynamics of MPPT
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59. Typical control structure for single-stage PV inverter
In these topologies -which are becoming more and more popular in countries with low grid voltage (120V) like Japan and thus the voltage from the PV array is high enough- the MPPT is implemented in the dc-ac inverter
Also in topologies with boost trafo on ac side (SMA)
The output of the MPPT is the dc-voltage reference. The output of the dc-voltage controller is the grid current reference amplitude. The power feedforward improves the dynamic response as MPPT runs at a slow sampling frequencies (typ. 1 Hz).
A PLL is used to synchronize the current reference with the grid voltage
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60. Practical PV inverter control implementation
Dual-stage full-bridge PWM inverter with LCL filter and grid trafo
The current controller Gc can be of PI or PR (Proportional Resonant) type
Other non-linear controllers like hysteresis or predictive control can be used for current control
The dc voltage controller can be P type due to the integration effect of the typical large capacitor
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61. PV Inverter Control Structures - Conclusions
The most typical control structure is the current controlled voltage source inverter with PWM
Typically boost dc-dc converter is required
The MPPT is a necessary feature in order to extract the maximum power from a panel array at any conditions of irradiation and temperature.
PO and INC are the most used ones. PO+CV is also possible
According to the topology (dual- or single-stage) the MPPT is implemented in the dc-dc converter or in the dc-ac inverter
PR current controller better than PI control for sinusoidal references
PLL is typically required for synchronization
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62. Acknowledgment
Part of the material is or was included in the present and/or past editions of the
“Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice”
Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero,
Place: Aalborg University, Denmark
The course is held twice (May and November) every year
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