Renewable energy and grid integration challenges

A review on Power-Electronic Systems for Grid Integration of Renewable Energy Sources

We hand hold you to “Change For Good”

Compiled by B.Somasundaram

Grid Integration of Renewable Energy Sources – An Outline • New trends in power electronics for the integration of wind and photovoltaic • Review of the appropriate storage-system technology • Future trends in renewable energy systems based on reliability and maturity • Increasing number of renewable energy sources and distributed generators • New strategies for the operation and management of the electricity grid • Improve the power-supply reliability and quality • Liberalization of the grids We hand hold you to “Change For Good”


Power-electronics technology • Plays an important role in distributed generation • Integration of renewable energy sources into the electrical grid Fast evolution, due to: a. development of fast semiconductor switches b. introduction of real-time controllers c. High Speed Communication techniques We hand hold you to “Change For Good”


Renewable Energy - Wind Turbines • Wind-turbine market has been growing at over 30% a year • Important role in electricity generation • Rapid decrease in cost • Capacity tripled by advancement in integration of multiple wind turbines and low velocity wind harvesting techniques • Gear less technology up to 2 MW • Variable-speed technology – 5% increased efficiency • Easy control of active and reactive power flows • power electronics cost is only 7%



2006 5 MW 600’

2000 850 kW 265’


2003 1.8 MW 350’


Double Fed Induction Generator



Variable speed turbine with Double Fed Induction Generator • Converter feeds the rotor winding • Stator winding connected directly to the grid • Small converter • Low price



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



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 We hand hold you to “Change For Good”


Full converter

Energy storage

Energy Transfer Control of the active and reactive powers total-harmonicdistortion control

driver controlling the torque generator, using a vector control strategy Compiled by B.Somasundaram We hand hold you to “Change For Good”

Rectifier and chopper

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



Semiconductor-Device Technology • Power semiconductor devices with better electrical characteristics and lower prices • Insulated Gate Bipolar Transistor (IGBT) is main component for power electronics



Integrated gated control thyristor (IGCT) - ABB



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



Requirements



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



IEC Standard IEC-61400-21 1. Flicker analysis 2. Switching operations. Voltage and current transients 3. Harmonic analysis (FFT) - rectangular windows of eight cycles of fundamental frequency. THD up to 50th harmonic



Other Standards • High-frequency (HF) harmonics and • interharmonics IEC 61000-47 and IEC 61000-3-6 • • methods for summing harmonics and interharmonics in the IEC 61000-3-6

61000-4-7 switching frequency of the inverter is not constant Can be not multiple of 50 Hz

• To obtain a correct magnitude of the frequency components, define window width, according to the IEC We hand hold you to “Change For Good”


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

• Line-commutated converters • Many disadvantages • Harmonics



HVDC VSC based

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



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 threephase inverters 5) multilevel configurations with cascaded singlephase H-bridge inverters. We hand hold you to “Change For Good”


Comparison http://hermes.eee.nott.ac.uk/teaching/h5cpe 2/ 1 0.8 0.6

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



Low-speed permanent-magnet generators


building Compiledpower-electronic by B.Somasundaram block (PEBB)

Direct-Drive Technology for Wind Turbines

•Reduced size •Lower installation and maintenance cost •Flexible control method •Quick response to wind fluctuations and load

variation

•Axial flux machines We hand hold you to “Change For Good”


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 We hand hold you to “Change For Good”


Hydrogen as a vehicle fuel • Electrical energy can 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



PV Photovoltaic Technology • PV systems as an alternative energy resource • Complementary Energy-resource in hybrid systems Necessary: • high reliability • reasonable cost • user-friendly design



PV-module connections The 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



Islanding

PV Generator We hand hold you to “Change For Good”

Converter AC-DC

Local Loads Grid Compiled by B.Somasundaram

Market Considerations PV • Solar-electric-energy growth 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 • 2001, 350 MW of solar equipment was sold 2003, 574 MW of PV was installed. • In 2004 increased to 927 MW • Significant 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 • Market leader in solar panel efficiency (measured by energy conversion ratio) is SunPower, (San Jose USA) - 23.4% • market average of 12-18%. • Efficiency 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 triplejunction manufacturing technology in 2009.



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 • Central inverters • Module-oriented or module-integrated inverters • String inverters



Multistring converter • Integration of PV strings of different technologies and orientations



Review of PV Converters •

S. B. Kjaer, J. K. Pedersen, F.Blaabjerg „A Review of Single-Phase GridConnected Inverters for Photovoltaic Modules”, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, 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 • Detection schemes - active and passive. 1. The passive methods -monitor grid parameters. 2. The active schemes introduce a disturbance into the grid and monitor the effect.



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 • Decoupling is necessary • p –instantaneous • P - average



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 We hand hold you to “Change For Good”


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 • Cost effectiveness • using similar circuits as in single-phase power-factorcorrection (PFC) circuits • variable-speed drives (VSDs)



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



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=(m2 year) 130 W/m2



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 • Centralized Inverters • String Inverters • Multi-string Inverters • AC modules & AC cell technology



Centralized Inverters

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



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

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



Future topologies • Multi-String Inverters • AC Modules • AC Cells • …



Multi-string Inverters

• Flexible • Every string can be controlled individually.



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



Classification of Inverter Topologies • Single-stage inverter

• Dual stage inverter • Multi-string inverter



Power Decoupling • Capacitors



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



Types of Grid Interfaces • Inverters operating in current-source mode

Line-commutated CSI switching at twice the line frequency



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



VSI • Half-bridge diode-clamped three-level VSI



AC Modules 1. 100-W single-transistor flyback-type HF-link inverter • 100 W, out 230 V, in 48 V, 96%, pf=0,955



AC modules 2. 105-W combined flyback and buck–boost inverter •

105 W, out 85V, in 35V, THD half-period loading • bipolar PWM switching toward the grid p.83 & 84 (no grounding possible, large ground currents) – 2x1200 µF 375 V • current-fed fullbridge dc–dc converters with embedded HF transformers, for each PV string – p.85 – 3x 310 µF 400V



Resume – PV Inverters • Large centralized single-stage inverters should be avoided • Preferable 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. •

High-frequency VSI is also suitable for both low- and highpower systems, like the ac module, the string, and the multistring inverters



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 • full-bridge • single-inductor push–pull • double-inductor push–pull



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 • 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

