Power-Electronic Systems for the Grid Integration

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Power-Electronic Systems for

the Grid Integrationof Renewable Energy Sources

Zbigniew Leonowicz, PhD

BasedBased on: J.M.on: J.M. CarrascoCarrasco, J.T, J.T BialasiewiczBialasiewicz,, et aet al:l:Power Power--ElectronicElectronic

Systems for the Grid IntegrationSystems for the Grid Integration of Renewable Energy Sources: Aof Renewable Energy Sources: ASurveySurvey,, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,

VOL. 53, NO. 4, AUGUST 2006VOL. 53, NO. 4, AUGUST 2006..

utline

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

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

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 semiconductorswitches

b. introduction of real-time controllers

utline (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

ind turbine technology

Wind-turbine market has been growing at

over 30% a year

Important role in electricity generation

Germany and Spain

² Variable-speed technology ² 5% increased

efficiency

² Easy control of active and reactive powerflows

² Rotor acts as a flywheel (storing energy)

² No flicker problems

² Higher cost (power electronics cost 7%)

ew technologies - wind turbines

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

Variable-speed turbine with DFIG

Converter feeds the rotor winding

Stator winding connected directly to the

converter

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

Decoupled from grid

ENERCON

multipolemultipole

synchronoussynchronous

generator generator

reduced

losses

lower costs

increased

reliabilityhttp://www.wwindea.org/technology/ch01/imgs/1_2_3_2_img1.jpg

Full converter

driver controlling the torque

generator, using a vector controlstrategy

Energy Transfer

Control of the activeand reactive powers

total-harmonic-distortion control

Energy storage

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) ismain 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 ² highelectromagnetic emission, cooling

problems

IGBTs are built like modular devices -lifetime of the device 10 x IGCT

IGCTs have a lower ONON--state voltage dropstate voltage drop-

losses 2x lower

Grid-Connection Standards for Wind

FarmsVoltage Fault RideVoltage Fault Ride--Through Capability ofThrough Capability of

Wind TurbinesWind Turbines

a. turbines should stay connected andcontribute 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 Grid-

Connected Wind Turbines - flicker + interharmonicsinterharmonics

Draft IEC-61400-21 standard for ´power-

quality requirements for Grid ConnectedWind Turbinesµ

IEC Standard IECIEC--6140061400--2121

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 andinterharmonics IEC

61000-4-7 and IEC61000-3-6

methods for summingharmonics andinterharmonics in the

IEC 61000-3-6 To obtain a correct

magnitude of thefrequency components,define window width,

according to the IEC61000-4-7

switchingswitching frequencyfrequency ofof

the inverter is notthe inverter is notconstantconstant

CanCan be notbe not multiplemultiple ofof50 Hz50 Hz

Transmission Technology for the

Future Offshore installation.

Disadvantages:

High distributed capacitance of cables

Limited length

MoreMore economiceconomic > 100 km and> 100 km and powerpower 200200--900 MW900 MW

1) Sending and receiving end frequencies areindependent.

2) Transmission distance using dc is not affected bycable charging current.

3) Offshore installation is isolated from mainlanddisturbances

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 ² 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 bidirectionalswitch interconnection

3) multilevel configurations with flyingcapacitors

4) multilevel configurations with multiplethree-phase inverters

5) multilevel configurations with cascaded

single-phaseH

-bridge inverters.

Comparison

0 0.005 0.01 0.015 0.02

Time /s

A m p l i t u d e

0 0.005 0.01 0.015 0.02

Time /s

A m p l i t u d e

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

Multilevel back-to-back converter for

direct connection to the grid

Low-speed permanent-magnet

generators

power-electronic buildingblock (PEBB)

Direct-Drive Technology for Wind

TurbinesReduced size

Lower installation and maintenance cost

Flexible control methodQuick response to wind fluctuations and load

variation

Axial flux machines

Future Energy-Storage Technologies

in Wind FarmsZincZinc brominebromine batterybattery

High energy density relativeto lead-acid batteries

100% depth of dischargecapability

High cycle life of >2000cycles at No shelf life Scalable capacities from

10kWh to over 500kWhsystems The ability to store energyfrom any electricitygenerating source

Hydrogen as a vehicle fuel

Electrical energy can be produced and

delivered to the grid from hydrogen by a

fuel cell or a hydrogen combustiongenerator.

The fuel cell produces power through a

chemical reaction and energy is releasedfrom 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 hybridsystems

NecessaryNecessary::

high reliability

reasonable cost

user-friendly design

PV-module connections

TThe standardshe standards

EN61000-3-2, IEEE1547,

U.S. National Electrical Code (NEC) 690 IEC61727

power quality, detection of islandingoperation, grounding

structure and the features of the presentand future PV modules.

IEC 61000-3-2

Islanding

PV Generator Converter AC-DC Local Loads Grid

Market Considerations PV

Solar-electric-energy growth consistently

20%²25% per annum over the past 20 years

1) an increasing efficiency of solar cells1) an increasing efficiency of solar cells

2) manufacturing2) manufacturing--technology improvementstechnology improvements

3) economies3) economies ofof scalescale

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% ofthe 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) isSunPower, (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 withoutconcentration is by Sharp Corporation at35.8% using a proprietary triple-junctionmanufacturing technology in 2009.

Design of PV-Converters

IGBT technology

Inverters mustmust be able to detect an

islanding situation and take appropriatemeasures 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

StringString invertersinverters

Multistring converter

Integration of PV strings of different

technologies and orientations

Review of PV Converters

S. B. Kjaer, J. K. Pedersen, F.Blaabjerg ÅA Review of Single-Phase Grid-

Connected Inverters for Photovoltaic Modulesµ, 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 demandequipment ground of the PV modules in

the case of absent galvanic isolation

Equipment ground is the case when framesand other metallic parts are connected to

ground.

Power injected into grid

Decoupling is necessary

p ²instantaneous

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

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 effectiveness

using similar circuits as in single-phasepower-factor-correction (PFC) circuits

variable-speed drives (VSDs)

High efficiency

wide range of input voltage and input

very wide ranges as functionsof 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 initialefficiency

The main limiting components inside the

inverters are the electrolytic capacitorsused 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 thenconnected in parallel, through

string diodes

Disadvantages !

String Inverters

Reduced version of the

centralized inverter

single string of PV modules isconnected 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 ofMPPT

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 dc²ac

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

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 buck²boost

inverter

105 W, out 85V, in 35V, THD <5%

AC modules

3. Modified Shimizu Inverter (160W, 230,

28V, 87%)

AC modules

4. 160-W buck²boost inverter

in 100V out 160V

AC modules

5. 150-W flyback dc²dc converter with a

line-frequency dc²ac unfolding inverter

in 44V, out 120V

AC modules

6. 100-W flyback dc²dc converter with a

PWM dc²ac inverter

30V ² 210 V

AC modules

110-W series-resonant dc²dc converter

with an HF inverter toward the grid

30-230V , 87%

AC modules

dual-stage topology Mastervolt Soladin 120

in 24-40V, out 230V, 91%, pf=0,99

String Inverters

Single-stage

Dual-stage

String Inverter

a transformerless half-bridge diode-

clamped three-level inverter

String Inverter

two-level VSI, interfacing two PV strings

SMA Sunny Boy 5000TL

three PV strings, each of 2200 W at 125-

750 V, with own MPPT

PowerLynx Powerlink PV 4.5 kW

three PV strings, each 200-500 V, 1500 W

Evaluation and Discussion

component ratings

relative cost

lifetime

efficiency

Results

Dual-stage CSI = large electrolytic

decoupling capacitor

VSI = small decoupling electrolytic

capacitor.

Results - Efficiency

Low efficiency=87%

C=68 QF 160V

High efficiency=93%

C=2,2 mF 45V

Discussion - String Inverters

The dual-grounded multilevel invertersp.82 ² good solution but quite largecapacitors 2x640QF 810V -> half-periodloading

bipolar PWM switching toward the gridp.83 & 84 (no grounding possible, large

ground currents) ² 2x1200 QF 375 V current-fed fullbridge dc²dc converters

with embedded HF transformers, for eachPV string ² p.85 ² 3x 310 QF 400V

Resume ² PV Inverters

Large centralized single-stage inverters should beavoided

Preferable location for the capacitor is in the dc

link where the voltage is high and a largefluctuation can be allowed without compromisingthe utilization factor

HFTs should be applied for voltage amplification inthe 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, thestring, and the multistring inverters

Converter topologies (general)

PV inverters with dc/dc converter (with or

without isolation)

PV inverters without dc/dc converter (withor without isolation)

Isolation is acquired using a transformerthat 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 push²pull

double-inductor push²pull

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 systemefficiency

Å plug and play systemsµ

Research

MPPT control

THD improvements

reduction of current or voltage ripple

standards are becoming more and morestrict

STORAGE

Energy Storage Systems

Improvement of Q uality

Support the Grid during Interruption

Flywheels ² spinning mass energy

(commercial application with active

filters)

Flywheel-energy-storage

low-speed flywheels (< 6000 r/min) with

steel rotors and conventional bearings

modern high-speed flywheel systems (to60 000 r/min) advanced composite wheels

ultralow friction bearing assemblies, such

as magnetic bearings

Applications of flywheels

Research

Experimental alternatives for wind farms

=flywheel connected to the dc link

Control strategy = regulate the dc voltageagainst 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

Storable

transportable,

highly versatile efficient

clean energy carrier

fuel cells to produce electricity

Hydrogen technology

Storage

² compressed or liquefied gas

² by using metal hydrides or carbon nanotubes Technologies

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 suitable for short discharge applications

<100 kW.

Superconducting Magnetic Energy

Storage (SMES) energy in a magnetic field without

resistive losses

ability to release large quantities of powerduring a fraction of a cycle

Battery Storage

Several types of batteries

Discharge rate limited by chemistry

Pumped-Hydroelectric Storage (PHS)

variable-speed drives

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 subsystemcomponent.

common and future trends for renewableenergy 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 inth f t

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