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Power-Electronic Systems for the Grid Integration of Renewable Energy Sources Zbigniew Leonowicz, PhD Based on: J.M. Carrasco, J.T Bialasiewicz, et al:Power-Electronic.

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Presentation on theme: "Power-Electronic Systems for the Grid Integration of Renewable Energy Sources Zbigniew Leonowicz, PhD Based on: J.M. Carrasco, J.T Bialasiewicz, et al:Power-Electronic."— Presentation transcript:

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

2 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

3 Introduction 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 leads to new management structures

4 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

5 Outline (detailed) 1.Current technology and future trends in variable-speed wind turbines 2.Power-conditioning systems used in grid- connected photovoltaic (PV) 3.Research and development trends in energy-storage systems

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

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


9 Variable-speed turbine with DFIG Converter feeds the rotor winding Stator winding connected directly to the grid Small converter Low price

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

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

12 ENERCON multipole synchronous generator reduced losses lower costs increased reliability

13 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

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

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

16 Integrated gated control thyristor (IGCT) - ABB

17 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 ON-state voltage dropIGCTs have a lower ON-state voltage drop- losses 2x lower

18 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.

19 Requirements

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

21 IEC-61400-21 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 50 th harmonic

22 Other Standards High-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 constantswitching frequency of the inverter is not constant Can be not multiple of 50 HzCan be not multiple of 50 Hz

23 Transmission Technology for the Future Offshore installation.

24 HVAC Disadvantages: High distributed capacitance of cables Limited length

25 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.

26 HVDC LCC-based Line-commutated converters Many disadvantages Harmonics

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

28 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.

29 Comparison

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

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

32 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

33 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

34 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.

35 Variable-speed wind turbine with hydrogen storage system

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

37 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.

38 IEC 61000-3-2

39 Islanding PV Generator Converter AC-DC Local Loads Grid

40 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

41 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

42 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


44 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 triple-junction manufacturing technology in 2009.

45 Design of PV-Converters IGBT technology mustInverters 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


47 Converter topologies Central inverters Module-oriented or module-integrated inverters String invertersString inverters

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

49 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 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.


51 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.

52 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.

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

54 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

55 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%

56 Cost Cost effectiveness using similar circuits as in single-phase power-factor-correction (PFC) circuits variable-speed drives (VSDs)

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

58 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

59 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

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

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

62 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

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

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

65 Multi-string Inverters Flexible Every string can be controlled individually.

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

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

68 Power Decoupling Capacitors

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

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

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

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

73 AC Modules 1.100-W single-transistor flyback-type HF- link inverter 100 W, out 230 V, in 48 V, 96%, pf=0,955

74 AC modules 2.105-W combined flyback and buck–boost inverter 105 W, out 85V, in 35V, THD <5%

75 AC modules 3.Modified Shimizu Inverter (160W, 230, 28V, 87%)

76 AC modules 4.160-W buck–boost inverter in 100V out 160V

77 AC modules 5.150-W flyback dc–dc converter with a line-frequency dc–ac unfolding inverter in 44V, out 120V

78 AC modules 6.100-W flyback dc–dc converter with a PWM dc–ac inverter 30V – 210 V

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

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

81 String Inverters Single-stage Dual-stage

82 String Inverter a transformerless half-bridge diode- clamped three-level inverter

83 String Inverter two-level VSI, interfacing two PV strings

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

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

86 Evaluation and Discussion component ratings relative cost lifetime efficiency

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

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

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

90 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 high-power systems, like the ac module, the string, and the multistring inverters

91 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

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

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

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

95 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”

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


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

99 Flywheel-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

100 Applications of flywheels

101 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

102 Hydrogen-storage systems Storable transportable, highly versatile efficient clean energy carrier fuel cells to produce electricity

103 Hydrogen technology Storage –compressed or liquefied gas –by using metal hydrides or carbon nanotubes Technologies

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

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

106 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

107 Battery Storage Several types of batteries Discharge rate limited by chemistry

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

109 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

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

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