Power-Electronic Systems for the Grid Integration

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Power-Electronic Systems for the Grid Integration of Renewable Energy Sources:

Power-Electronic Systems for the Grid Integration of Renewable Energy Sources Zbigniew Leonowicz, PhD Based on: J.M. Carrasco , J.T Bialasiewicz , et a l: Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey , IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006 .


Outline N ew trends in power electronics for the integration of wind and photovoltaic R eview of the appropriate storage-system technology Future trends in renewable energy systems based on reliability and maturity


Introduction Increasing number of renewable energy sources and distributed generators N ew strategies for the operation and management of the electricity grid I mprove the power-supply reliability and quality L iberalization of the grids leads to new management structures

Power-electronics technology:

Power-electronics technology P lays an important role in distributed generation I ntegration of renewable energy sources into the electrical grid F ast evolution , due to : development of fast semiconductor switches introduction of real-time controllers

Outline (detailed):

Outline (detailed) Current technology and future trends in variable-speed wind turbines Power-conditioning systems used in grid-connected photovoltaic (PV) R esearch and development trends in energy-storage systems

Wind turbine technology:

Wind turbine technology W ind-turbine market has been growing at over 30% a year Important role in electricity generation Germany and Spain

New technologies - wind turbines:

Variable-speed technology – 5% increased efficiency Easy control of active and reactive power flows R otor acts as a flywheel (storing energy ) No flicker problems Higher cost (power electronics cost 7%) New technologies - wind turbines


DFIG http://www.windsimulators.co.uk/images/DFIG.gif

Variable-speed turbine with DFIG:

Variable-speed turbine with DFIG C onverter feeds the rotor winding S tator winding connected directly to the grid Small converter Low price

Simplified semi-variable speed turbine:

Simplified semi-variable speed turbine Rotor resistance of the squirrel cage generator - varied instantly using fast power electronics

Variable-Speed Concept Utilizing Full-Power Converter:

Variable-Speed Concept Utilizing Full-Power Converter Decoupled from grid


ENERCON multipole synchronous generator reduced losses lower costs increased reliability http://www.wwindea.org/technology/ch01/imgs/1_2_3_2_img1.jpg

Full converter:

Full converter driver controlling the torque generator, using a vector control strategy Energy Transfer Control of the active and reactive powers total-harmonic-distortion control Energy storage CONTROL of Vdc

Rectifier and chopper:

Rectifier and chopper step-up chopper is used to adapt the rectifier voltage to the dc-link voltage of the inverter.

Semiconductor-Device Technology:

Semiconductor-Device Technology Power semiconductor devices with better electrical characteristics and lower prices I nsulated G ate B ipolar T ransistor (IGBT) is main component for power electronics

Integrated gated control thyristor (IGCT) - ABB:

I ntegrated gated control thyristor (IGCT) - ABB

Comparison between IGCT and IGBT:

C omparison between IGCT and IGBT IGBTs have higher switching frequency than IGCTs IGCTs are made like disk devices – high electromagnetic emission , cooling problems IGBTs are built like modular devices - lifetime of the device 10 x IGCT IGCTs have a lower ON-state voltage drop - losses 2x lower

Grid-Connection Standards for Wind Farms:

Grid-Connection Standards for Wind Farms Voltage Fault Ride-Through Capability of Wind Turbines turbines should stay connected and contribute to the grid in case of a disturbance such as a voltage dip. Wind farms should generate like conventional power plants, supplying active and reactive powers for frequency and voltage recovery, immediately after the fault occurred .



Power-Quality Requirements for Grid-Connected Wind Turbines:

Power-Quality Requirements for Grid-Connected Wind Turbines - flicker + interharmonics Draft IEC-61400-21 standard for “power-quality requirements for Grid Connected Wind Turbines ”

IEC Standard IEC-61400-21:

IEC Standard IEC-61400-21 Flicker analysis Switching operations. Voltage and current transients Harmonic analysis (FFT) - r ectangular windows of eight cycles of fundamental frequency . THD up to 50 th harmonic

Other Standards:

Other Standards H igh -frequency (HF) harmonics and interharmonics IEC 61000-4-7 and IEC 61000-3-6 methods for summing harmonics and interharmonics in the IEC 61000-3-6 To obtain a correct magnitude of the frequency components, define window width, according to the IEC 61000-4-7 switching frequency of the inverter is not constant Can be not multiple of 50 Hz

Transmission Technology for the Future:

Transmission Technology for the Future Offshore installation.


HVAC Disadvantages: High distributed capacitance of cables Limited length


HVDC More economic > 100 km and power 200-900 MW 1) Sending and receiving end frequencies are independent. 2) Transmission distance using dc is not affected by cable charging current . 3) Offshore installation is isolated from mainland disturbances 4) Power flow is fully defined and controllable. 5) Cable power losses are low. 6) Power-transmission capability per cable is higher.

HVDC LCC-based:

HVDC LCC-based Line-commutated converters Many disadvantages Harmonics

HVDC VSC based:

HVDC VSC based HVDC Light – HVDC Plus Several advantages- flexible power control, no reactive power compensation, …

High-Power Medium-Voltage Converter Topologies:

High-Power Medium-Voltage Converter Topologies Multilevel-converter 1) multilevel configurations with diode clamps 2) multilevel configurations with bidirectional switch interconnection 3) multilevel configurations with flying capacitors 4) multilevel configurations with multiple three-phase inverters 5) multilevel configurations with cascaded single-phase H-bridge inverters .


Comparison http://hermes.eee.nott.ac.uk/teaching/h5cpe2/

Multilevel back-to-back converter for direct connection to the grid:

Multilevel back-to-back converter for direct connection to the grid

Low-speed permanent-magnet generators:

Low-speed permanent-magnet generators power-electronic building block (PEBB)

Direct-Drive Technology for Wind Turbines:

Direct-Drive Technology for Wind Turbines R educed size L ower installation and maintenance cost F lexible control method Q uick response to wind fluctuations and load variation Axial flux machines

Future Energy-Storage Technologies in Wind Farms:

Future Energy-Storage Technologies in Wind Farms Zinc bromine battery High energy density relative to lead-acid batteries • 100% depth of discharge capability • High cycle life of >2000 cycles at • No shelf life • Scalable capacities from 10kWh to over 500kWh systems • The ability to store energy from any electricity generating source

Hydrogen as a vehicle fuel:

Hydrogen as a vehicle fuel Electrical energy c an be produced and delivered to the grid from hydrogen by a fuel cell or a hydrogen combustion generator. The fuel cell produces power through a chemical reaction and energy is released from the hydrogen when it reacts with the oxygen in the air.

Variable-speed wind turbine with hydrogen storage system:

Variable-speed wind turbine with hydrogen storage system

PV Photovoltaic Technology:

PV Photovoltaic Technology PV systems as an alternative energy resource C omplementary E nergy -resource in hybrid systems Necessary : high reliability reasonable cost user-friendly design

PV-module connections:

PV-module connections T he standards EN61000-3-2, IEEE1547, U.S. National Electrical Code (NEC) 690 IEC61727 power quality, detection of islanding operation, grounding structure and the features of the present and future PV modules.

IEC 61000-3-2:

IEC 61000-3-2


Islanding PV Generator Converter AC-DC Local Loads Grid

Market Considerations PV:

Market Considerations PV Solar-electric-energy grow th consistently 20%–25% per annum over the past 20 years 1) an increasing efficiency of solar cells 2) manufacturing-technology improvements 3) economies of scale

PV growth:

PV growth 2001, 350 MW of solar equipment was sold 2003, 574 MW of PV was installed. In 2004 increased to 927 MW S ignificant financial incentives in Japan , Germany, Italy and France triggered a huge growth in demand In 2008, Spain installed 45% of all photovoltaics, 2500 MW in 2008 to an drop to 375 MW in 2009


Perspectives World solar photovoltaic (PV) installations were 2.826 gigawatts peak (GWp) in 2007, and 5.95 gigawatts in 2008 The three leading countries (Germany, Japan and the US) represent nearly 89% of the total worldwide PV installed capacity. 2012 are and 12.3GW - 18.8GW expected


Efficiency M arket leader in solar panel efficiency (measured by energy conversion ratio) is SunPower , ( San Jose USA) - 23.4% market average of 12-18 %. E fficienc y of 42% achieved at the University of Delaware in conjunction with DuPont ( concentration ) in 2007. The highest efficiency achieved without concentration is by Sharp Corporation at 35.8% using a proprietary triple-junction manufacturing technology in 2009 .

Design of PV-Converters:

Design of PV-Converters IGBT technology Inverters must be able to detect an islanding situation and take appropriate measures in order to protect persons and equipment PV cells - connected to the grid PV cells - isolated power supplies

Converter topologies:

Converter topologies Central inverters M odule -oriented or module-integrated inverters String inverters

Multistring converter:

Multistring converter Integration of PV strings of different technologies and orientations

Review of PV Converters:

Review of PV Converters S. B. Kjaer, J. K. Pedersen, F.Blaabjerg „ A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules”, IEEE TRANSACTIONS ON INDUSTRY A PPLICATIONS, VOL. 41, NO. 5, SEPTEMBER/OCTOBER 2005 Demands Defined by the Grid - standards (slide 37) EN standard (applied in Europe) allows higher current harmonics the corresponding IEEE and IEC standards.


Islanding Islanding is the continued operation of the inverter when the grid has been removed on purpose, by accident, or by damage D etection schemes - active and passive. The passive methods - monitor grid parameters. The active schemes introduce a disturbance into the grid and monitor the effect.

Grounding & ground faults:

Grounding & ground faults The NEC 690 standard - system grounded and monitored for ground faults Other Electricity Boards only demand equipment ground of the PV modules in the case of absent galvanic isolation Equipment ground is the case when frames and other metallic parts are connected to ground.

Power injected into grid:

Power injected into grid Decoupling is necessary p –instantaneous P - average

Demands Defined by the Photovoltaic Module:

Demands Defined by the Photovoltaic Module Voltage in the range from 23 to 38 V at a power generation of approximate 160 W, and their open-circuit voltage is below 45 V. New technolgies - voltage range around 0.5 - 1.0 V at several hundred amperes per square meter cell

Maximum Power Point Tracker:

Maximum Power Point Tracker EX.: ripple voltage should be below 8.5% of the MPP voltage in order to reach a utilization ratio of 98%


Cost C ost effective ness using similar circuits as in single-phase power-factor-correction (PFC) circuits variable-speed drives (VSDs)

High efficiency:

High efficiency wide range of input voltage and input power very wide ranges as functions of solar irradiation and ambient temperature.

Meteorological data:

Meteorological data . (a) Irradiation distribution for a reference year. (b) Solar energy distribution for a reference year. Total time of irradiation equals 4686 h per year. Total potential energy is equal to 1150 kWh=(m 2 y ear ) 130 W/m 2


Reliability long operational lifetime most PV module manufacturer offer a warranty of 25 years on 80% of initial efficiency The main limiting components inside the inverters are the electrolytic capacitors used for power decoupling between the PV module and the single-phase grid

Topologies of PV inverters:

Topologies of PV inverters Centralized Inverters String Inverters Multi-string Inverters AC modules & AC cell technology

Centralized Inverters:

Centralized Inverters PV modules as series connections (a string ) series connections then connected in parallel, through string diodes Disadvantages !

String Inverters:

String Inverters Reduced version of the centralized inverter single string of PV modules is connected to the inverter no losses on string diodes separate MPPTs increases the overall efficiency

AC module:

AC module inverter and PV module as one electrical device No mismatch losses between PV modules Optimal adjustment of MPPT high voltage-amplification necessary

Future topologies:

Future topologies Multi-String Inverters AC Modules AC Cells …

Multi-string Inverters:

Multi-string Inverters Flexible E very string can be controlled individually.

AC cell:

AC cell One large PV cell connected to a dc–ac inverter Very low voltage New converter concepts

Classification of Inverter Topologies:

Classification of Inverter Topologies Single-stage inverter Dual stage inverter Multi-string inverter

Power Decoupling:

Power Decoupling Capacitors

Transformers and Types of Interconnections:

Transformers and Types of Interconnections Component to avoid (line transformers= high size, weight, price) High-frequency transformers Grounding,

Types of Grid Interfaces:

Types of Grid Interfaces I nverters operating in current-source mode Line-commutated CSI switching at twice the line frequency

Voltage-Source Inverters:

Voltage-Source Inverters standard full-bridge three-level VSI


VSI H alf -bridge diode - clamped three-level VSI

AC Modules:

AC M odules 100-W single-transistor flyback -type HF-link inverter 100 W, out 230 V, in 48 V, 96%, pf=0,955

AC modules:

AC modules 105-W combined flyback and buck–boost inverter 105 W, out 85V, in 35V, THD <5%

AC modules:

AC modules Modified Shimizu Inverter (160W, 230, 28V, 87%)

AC modules:

AC modules 160-W buck–boost inverter in 100V out 160V

AC modules:

AC modules 150-W flyback dc–dc converter with a line-frequency dc–ac unfolding inverter in 44V, out 120V

AC modules:

AC modules 100-W flyback dc–dc converter with a PWM dc–ac inverter 30V – 210 V

AC modules:

AC modules 110-W series-resonant dc–dc converter with an HF inverter toward the grid 30-230V , 87%

AC modules:

AC modules dual-stage topology Mastervolt Soladin 120 in 24-40V, out 230V, 91%, pf=0,99

String Inverters:

String Inverters Single-stage Dual-stage

String Inverter:

String Inverter a transformerless half-bridge d iode - clamped three-level inverter

String Inverter:

String Inverter two-level VSI, interfacing two PV strings

SMA Sunny Boy 5000TL:

SMA Sunny Boy 5000TL three PV strings, each of 2200 W at 125 - 750 V, with own MPPT

PowerLynx Powerlink PV 4.5 kW:

PowerLynx Powerlink PV 4.5 kW three PV strings, each 200 - 500 V , 1500 W

Evaluation and Discussion:

Evaluation and Discussion component ratings relative cost l ifetime efficiency


Results Dual-stage CSI = large electrolytic decoupling capacitor VSI = small decoupling electrolytic capacitor.

Results - Efficiency:

Results - Efficiency Low efficiency=87% C=68 m F 160V High efficiency=93% C=2,2 mF 45V

Discussion - String Inverters:

Discussion - String Inverters The dual-grounded multilevel inverters p.82 – good solution but quite large capacitors 2x640 m F 810V -> half-period loading bipolar PWM switching toward the grid p.83 & 84 (no grounding possible , large ground currents ) – 2x1200 m F 375 V current-fed fullbridge dc–dc converters with embedded HF transformers, for each PV string – p.85 – 3x 310 m F 400V

Resume – PV Inverters:

Resume – PV Inverters Large centralized single-stage inverters should be avoided P referable location for the capacitor is in the dc link where the voltage is high and a large fluctuation can be allowed without compromising the utilization factor HFTs should be applied for voltage amplification in the AC module and AC cell concepts Line-frequency CSI are suitable for low power, e.g., for ac module applications. H igh -frequency VSI is also suitable for both low- and high-power systems, like the ac module, the string, and the multistring inverters

Converter topologies (general):

Converter topologies (general) PV inverters with dc/dc converter (with or without isolation) PV inverters without dc/dc converter (with or without isolation) Isolation is acquired using a transformer that can be placed on either the grid or low frequency (LF) side or on the HF side

HF dc/dc converter:

HF dc/dc converter full-bridge single-inductor push–pull double-inductor push–pull

Another classification:

Another classification number of cascade power processing stages -single-stage -- dual-stage -----multi-stage There is no any standard PV inverter topology


Future very efficient PV cells roofing PV systems PV modules in high building structures

Future trends:

Future trends PV systems without transformers - minimize the cost of the total system cost reduction per inverter watt - make PV-generated power more attractive AC modules implement MPPT for PV modules improving the total system efficiency „ plug and play systems ”


Research MPPT control THD improvements reduction of current or voltage ripple standards are becoming more and more strict



Energy Storage Systems:

Energy Storage Systems Improvement of Quality Support the Grid during Interruption Flywheels – spinni ng mass energy (commercial application with active filters)


F lywheel-energy-storage low-speed flywheels ( < 6000 r/min) with steel rotors and conventional bearings modern high-speed flywheel systems ( to 60 000 r/min) advanced composite wheels ultralow friction bearing assemblies, such as magnetic bearings

Applications of flywheels:

A pplications of flywheels


Research Experimental alternatives for wind farms =flywheel connected to the dc link Control strategy = regulate the dc voltage against the input power surges/sags or sudden changes in the load demand Similar approach applied to PV systems, wave energy D-static synchronous compensator (STATCOM) Frequency control using distributed flywheels

Hydrogen-storage systems:

H ydrogen-storage systems S torable transportable, highly versatile e fficient clean energy carrier fuel cells to produce electricity

Hydrogen technology:

Hydrogen technology Storage compressed or liquefied gas by using metal hydrides or carbon nanotube s Technologies

Compressed-Air Energy Storage -CAES:

Compressed-Air Energy Storage - CAES Energy storage in compressed air Gas turbines


Supercapacitors 350 to 2700 F at of 2 V. modules 200 - to 400 V long life cycle s uitable for short discharge applications < 100 kW.

Superconducting Magnetic Energy Storage (SMES):

Superconducting Magnetic Energy Storage (SMES) energy in a magnetic field without resistive losses ability to release large quantities of power during a fraction of a cycle

Battery Storage:

Battery Storage S everal types of batteries Discharge rate limited by chemistry

Pumped-Hydroelectric Storage (PHS):

Pumped-Hydroelectric Storage (PHS) variable-speed drive s 30 - 350 MW, efficiencies around 75%.


Conclusions power-electronic technology plays a very important role in the integration of renewable energy sources optimize the energy conversion and transmission control reactive power minimize harmonic distortion to achieve at a low cost a high efficiency over a wide power range


Conclusions Achieve a high reliability tolerance to the failure of a subsystem component. common and future trends for renewable energy systems have been described. W ind energy is the most advanced technology Regulations favor the increasing number of wind farms. T he trend of the PV energy leads to consider that it will be an interesting alternative in the near future

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