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Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Overview of Distributed Power.

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Presentation on theme: "Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Overview of Distributed Power."— Presentation transcript:

1 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre

2 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

3 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Distributed power generation Relatively small generating units and storage technologies Provide electric capacity and/or energy at or near consumer sites to meet specific customer needs 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

4 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Renewable energy systems Source: Billman, Advances in Solar Energy submission, 1/8/99

5 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre World energy consumption The growth of energy demand in 2007 remained high despite high energy prices China has surpassed the EU

6 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre World energy production The relative market share of oil is decreasing respect coal and gas

7 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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).

8 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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.

9 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

10 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Wind energy Bigger and more efficient ! 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!

11 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

12 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

13 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

14 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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)

15 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

16 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Smart micro-grids (SMG) 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. 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.

17 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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. Hydrogen distribution network Photovoltaic systems highly integrated in the buildings

18 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Information Technology Networking problems...

19 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Information Technology Networking possible solutions... Automated Demand Response Color-based indication of grid status from Dr. Peter Palensky’s contribution to IEEE – IECON 2008 Panel Discussion Session On Industrial Electronics for Renewable Energy

20 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Wind systems

21 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  Limited speed range (-30% to +20%, typical)  Small-scale power converter (Less power losses, price)  Complete control of active P ref and reactive power Q ref  Need for slip-rings  Need for gear Doubly-fed induction generator - wounded rotor  Producers: Vestas, Gamesa, NEG Micon, GE Wind, Nordex, REpower Systems, DEWind  Power range: 0.85 MW to 4.2 MW Wind turbine systems

22 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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 Induction generator - Squirrel cage rotor  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 Wind turbine systems

23 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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 Synchronous generator - External magnetized inverter or diode-bridge + chopper  Producers: Enercon, Largey,  Power range: 0.6 MW to 4.5 MW Wind turbine systems

24 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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 Synchronous generator - Permanent magnets inverter or diode-bridge + chopper  Producers: Largey, Mitsubishi, Pfleiderer Wind Energy  Power range: 0.6 MW to 4.5 MW Wind turbine systems

25 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre SG Example 1 20 kW mini-WT multipolar permanent magnet synchronous generator with axial flux produced by JONICA IMPIANTI (JIMP)

26 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre SG Example 2 from “WindBlatt 02/03” WT Enercon 300 kW multipolar synchronous generator installed in Antartica

27 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre SG Example 3 Multibrid WT 5 MW multipolar synchronous generator (Multi) with ibrid gear (brid) for offshore applications Prokon Nord synchronous generator with permanet magnets surface mounted and radial flux 3 kV NPC converter from Alstom or ABB

28 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Trends Power electronics is now in wind turbines - Direct-driven genertaor market share is growing no gear-box

29 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Basic power conversion and control: Wind turbine systems control

30 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Basic demands: Electrical: Mechanical: Interconnection (conversion, synchronization) Overload protection Active and reactive power control Power limitation (pitch) Maximum energy capture Speed limitation/control Reduce acoustical noise Control loops with different bandwidth Wind turbine systems control

31 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre - Controllers (internal) - Modulation - Overall system control Wind turbine systems control

32 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Control of permanent magnet synchronous generator system Wind turbine systems control

33 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Control of synchronous generator system - Control of active and reactive power Wind turbine systems control

34 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Control of doubly-fed induction generator system Wind turbine systems control

35 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Detailed example Operating range

36 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Control of doubly-fed induction generator system (generator-side) - Complete control of active and reactive power Wind turbine systems control

37 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Detailed example Basic power flow

38 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Photovoltaic systems

39 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

40 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Both technologies are on the market! Efficiency 93-95% On low frequency (LF) side On high frequency (HF) side PV inverters with boost converter and isolation Boosting inverter with HF trafo based on FB boost converter [2]Boosting inverter with LF trafo based on boost converter

41 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Transformerless PV inverters with boost Efficiency >95% Leakage current problem Safety issue Efficiency > 96% Extra diode to bypass boost when Vpv > Vg Boost with rectified sinus reference Time sharing configuration FB inverter + boost Typical configuration

42 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre Frequency analysis of voltage to earth Vpe for FB with UP and BP PWM switching Spectrum of voltage to earthSpectrum of leakage current Based on I Cp and V Cp 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 V AB, V PE and I PE for FB-UP

43 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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  V PE 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! High efficiency topologies derived from H- bridge FB with Bipolar PWM Switching

44 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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  V PE 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! High efficiency topologies derived from H-bridge FB with Unipolar PWM Switching

45 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre  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  V PE 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! High efficiency topologies derived from H-bridge FB with Hybrid PWM Switching

46 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  V PE 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

47 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  V PE 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

48 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  V PE 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)

49 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  V PE 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)

50 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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.

51 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  The switching ripple in the current equals 1x switching frequency  high filtering needed  Voltage across filter is unipolar  low core losses  V PE 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)

52 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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  V PE 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)

53 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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.

54 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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”

55 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

56 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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 dP/dV = 0, MPP

57 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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!

58 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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 V DC is controlled in the dc-ac inverter the change of the duty- cycle will change voltage at the output of the PV panels, V PV 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

59 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

60 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

61 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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

62 Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre 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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