Challenges in Integrating Renewable Sources into the Electric Power Grid:

A Power Electronics Perspective

Bingsen Wangbingsen@asu.edu

Assistant Professor, Electrical EngineeringArizona State University

April 7, 2009

1

Topics

• Overview of renewable energy• Motivation of grid integration• Functional role of power electronics• The challenges• Summary

4/7/2009 2

Renewable vs. Sustainable Energy

• Renewable energy – is regenerative or virtually inexhaustible.

Focused on supply side.• Sustainable energy

– can be replenished at a rate higher than consumption and the waste stream can be naturally absorbed at a rate higher than injection. Focused on supply and use sides.

4/7/2009 3

Renewable Energy Sources

• Renewable energy sources include– Solar– Wind– Biomass– Hydro– Geothermal– Tidal

• Wind and solar enjoy very fast growth rate

4/7/2009 4

Renewable Portfolio Standards

Source: K.S. Cory and B.G. Swezey, “Renewable Portfolio Standards in the States: Balancing Goals and Implementation Strategies,” NREL/TP-670-41409, 2007.

4/7/2009 5

World PV Installation

4/7/2009 6

• Accumulated PV installed capacity reached 10.5 GW by the end of 2007.

Source: Renewables 2007 Global Status Report, www.ren21.net.

Data from “World Wind Energy Report 2008”

Wind Installations Worldwide

4/7/2009 7

7.5 9.713.7

18.024.3

31.239.3

47.7

59.0

74.2

93.8

121.2

29%

42%

32%

35%

28%26%

21%

24%26%

27%

29%

0%

10%

20%

30%

40%

0

20

40

60

80

100

120

140

Ann

ual G

row

th

Capa

city

(GW

)

Year

Wind Power Capacity in U.S.

• Installed wind capacity in U.S. reached 25 GW by the end of 2008.

Source: American Wind Energy Association

4/7/2009 8

Renewable Energy Conversion

• Converted to electricity – and delivered using the power system

infrastructure– or consumed locally

• Converted to fuel – and delivered through conventional

infrastructure

4/7/2009 9

Why Grid Integration

• Necessity– Geographic locations of sources may not

match the locations of load centers (particularly true for wind resources)

• For performance / economics– Integration provides ways to manage

resource output variability– Economically advantageous– Regulation

4/7/2009 10

Integration Involves

• Technical aspects– Physical layer– Information layer

• Market and economics• Policy and regulation• ...

4/7/2009 11

Power Electronics for Electric Power Applications

4/7/2009 12

Interconnection Levels

4/7/2009 13

Power Electronics

Power Electronics

• Electronics for processing power rather than just for processing signal information.

• Power flows and signal flows co-exist in a power electronics system.

4/7/2009 14

Power Processor

Signal Processor

Application of Power Electronics is Evolving

• Power devices drive the power flow path

• Signal processors drive the information flow path

4/7/2009 15

Mercury Arc Valve

Press pack IGBT

IGCT

State of the Art in High Power Conversion

• In electric drives, power throughput of matured single power converter has reached 10 MW.

• These high power converters switch at approximate frequency of 1 kHz.

• They operate at medium voltage 3.3 – 6.9 kV.• Modular scaling up to 30-100 MW which covers

majority of renewable interface.

4/7/2009 16

Role of Power Electronics in Integrating Renewable Energy Sources

4/7/2009 17

Functional Role Change

• Power electronics has been traditionally used to interface the power system with loads (such as motors).

• Power electronics will be broadly used to interface sources and with the power system.

4/7/2009 18

Source LoadPEPE

Higher PE Penetration

• Higher penetration of power electronics can be expected as use of renewables grows. Power electronics will be used for:– Grid interface– Energy storage– Power flow management

4/7/2009 19

Power Electronics

Wind Generation System - Yesterday

• In “Danish concept” wind generation systems, no power electronics is utilized.

4/7/2009 20

Gear Box

Squirrel Cage Induction Generator

Reactive Power Compensation

Wind Generation System - Today

• In wind generation systems equipped with DFIG, power converter handles about 30% power flow.

4/7/2009 21

Gear Box

Wound Rotor Induction Generator

Power Converter of 30% Rated Power

Wind Generation System - Tomorrow

• In the expected large wind generation systems (> 5 MW) that utilize PM generators, power converter handles full power.

4/7/2009 22

Permanent Magnet Synchronous Generator

Power Converter of 100% Rated Power

Energy Storage

• Energy storage is one of the options to deal with variability of renewable sources.– Pumped hydro– Compressed air– Supercapacitor– Flywheel– PHEV

• Majority of the energy storage technologies involve power electronics

4/7/2009 23

FACTS Devices and Integration

• FACTS devices will become more important at transmission level because of increased power flow traffic resulting from variability of renewable sources.

• Even at distribution levels, power flow control will become necessary as the radial distribution system is shifted to a meshed system.

4/7/2009 24

Series FACTS Device

• Power flow controller

4/7/2009 25

VinjV1 V2

X1+-

Vinj

V1

+V2

+

X2

Shunt FACTS Device

• Reactive power compensation

4/7/2009 26

V1 V2

X1

IinjV1

+V2

+

X2

Series and Shunt FACTS Device

• Unified power flow controller

4/7/2009 27

V1

X1

IinjV1

+V2

+

X2+-

Vinj

Adding Value with Power Electronics

• Voltage support by reactive power generated by power electronics

• Ramp up and down quickly• Improve power quality• Control of a large number of sources

centrally• Low voltage ride-through

4/7/2009 28

Evolution of Standards

• Larger grid support requirement is expected from renewable sources as reach higher penetration levels.

• One of the evolution examples is the low voltage ride through for wind power generation systems.

4/7/2009 29

Low Voltage Ride Through

4/7/2009 30

0.9

0.15

0.625 3.0

Wind Plant Required to Remain On-line

Wind Plant Not Required to Remain On-line

0

Vol

tage

at P

CC

(pu)

Time (sec)

Communications and Control

• Power electronics needs augmented communication capability to use smart grid features.

• Control in microgrids will rely heavily on power electronics.

4/7/2009 31

Technical Challenges

• New design perspective from downstream to upstream in power flow chain

• Interactions among much larger number of distributed resources with power electronic converters

• Integrated design in high power areas• Standardized design to improve inter-

operability

4/7/2009 32

Non-Technical Challenges

• Assessing the economic value of power electronics solutions– Cost reduction challenging today due to non-

standardized designs– Inclusion of lifecycle operating costs– Quantification of indirect benefits such as

GHG reduction• Less operational experience as compared

to conventional power generation sources4/7/2009 33

Myths That Need to be Dispelled!

• System “polluter” – harmonics?

• Complex failure modes?

4/7/2009 34

Summary

• Integration challenges must be addressed given anticipated growth penetration of renewable sources.

• Renewable energy integration will be enabled by power electronics.

• The new operating regime and design space present challenges for the needed use of power electronics.

4/7/2009 35