High Voltage Power Electronics Technologies for
Integrating Renewable Resources into the Grid
RenewElec Workshop
Carnegie Mellon University
October 22, 2010 – Pittsburgh, PA
Dr. Gregory F. Reed & Brandon M. Grainger
Power & Energy Initiative
University of Pittsburgh, Swanson School of Engineering
Electrical & Computer Engineering Department
Overview
• Background
• Technology and Infrastructure Challenges
• Power Electronic Technologies
• HVDC Systems
• FACTS Devices
• Summary
2
Background
3
Background
Challenges with Renewable Integration
• Integration of conventional generation resources
(coal, petroleum, and natural gas) and renewable sources
(solar and wind) present technological obstacles to the current
system and practices
• Focus of Work: Characterize common obstacles and present
solutions that derive from the interconnection of transmission
technologies for better renewable integration
 FACTS Compensation Devices for AC Infrastructure Expansion
 Conventional and Voltage-Source Converter Based HVDC
Transmission Technology
• Important Factor: Multiple hybrid configurations can be
considered for more economic and reliable grid interconnection
4
Texas / ERCOT Example
Texas as a Model: Trends to Observe
• Generation portfolio consists of traditional fossil generation
sources such as coal, petroleum, and natural gas. It also
boasts a strong supply of renewable generation, most notably,
wind power; and clean nuclear energy
 Stands as the U.S. leader
in wind generation capacity
with 7.892 GW installed
 CREZ Project will add 10 GW
more wind power……
 2,300 miles of new 345-kV
transmission with shunt and
series dynamic compensation
5
Renewable Resources
Geographic Intensity of Highest Penetration Potential
Wind Speed Across the US
Solar Intensity Across the US
6
Technological and Infrastructure Challenges
7
Challenges and Issues
Issues that are Turbine (Rotating Machine) Related
• Turbine Tripping
Loss in generators can lead to major cascading issues
• Subsynchronous Resonance (SSR)
Contributor to turbine shaft damage, SSR results from turbine
tensional vibration that is amplified by series capacitors.
•Reactive Power Consumption
Induction generators require substantial amounts of reactive
power during operation. This power is pulled from the grid and
can cause depressed voltage conditions and stability problems.
Transmission Infrastructure Issues
• Power System Dynamic Performance
• Moving New/Distant Resource Portfolios to Load Centers
• Operations in New Market and Regulatory Conditions
8
Challenges and Issues
Issues Related to Dispatch of Generation Resources
• Voltage Instability
Large differences between the output voltage of the generating
utility and grid operating voltage at the point of common coupling
can lead to instability on the grid.
Changes in wind speed can contribute to this issue
• Voltage Flicker
Wind and solar power generators are non-dispatchable (fuel
source is inherently variable by nature) often resulting in
fluctuations in output voltage.
9
Power Electronics Available for Improved
Integrated Generation Management (IGM)
10
Power Electronics for IGM
Inspiring Quote:
“Up until now we’ve just been connecting wind farms to the grid.
What we need to be doing is integrating them. Power electronics
will enable us to do this by controlling the power flows. It’s a
solution that’s starting to be used, but NOWHERE, near to the
extent that will be needed in the future.” (Wind Directions, 2008)
11
Power System Basics
Power Generation, Transmission and Distribution
Distribution
Generation
Mechanical-toElectrical Energy
Conversion
Transmission
Electrical Power Used
and Electrical-to-Mechanical
Energy Conversion
FACTS / HVDC – High Capacity Power Electronics
are applied here for improved operation, reliability, etc.
12
Power Electronics for IGM
Evaluation of AC & HVDC for Future Generation Options
• Many of today’s interconnections make use of high voltage AC
transmission to integrate many alternative energies to the
electric network.
But is it the most optimal, reliable, and secure option for future
infrastructure expansion in all cases?
• Renewable resources located further from load centers
There is a distance at which HVDC becomes economically more
attractive compared to AC.
Why? AC cable transmission suffers from excessive reactive
current drawn by cable charging capacitances. Reactive shunt
compensation required to absorb excessive reactive power and
avoid overvoltage conditions
13
HVDC
HVDC Transmission and HVDC BTB-Link
DC
Transmission
Lines
~ or ~
AC
Network
(A)
AC
Network
(B)
DC-Link
Converter
Station A
Converter
Station B
14
HVDC
Planning Considerations
• Planners should consider the HVDC backbone systems and
AC systems with FACTS compensation to achieve the needed
capacity and system security.
Two Types of HVDC Technologies
• Current-Source Converters (Thyristor Based)
• Voltage-Source Converters (Advanced Semiconductor Based)
15
HVDC
Summary Comparison of HVDC Technologies:
16
HVDC
Advantages of HVDC Systems:
More power can be transmitted more efficiently over long
distances by applying HVDC
HVDC lines can carry 2 to 5 times the capacity of an AC line
of similar voltage
Interconnection of two AC systems, where AC lines would
not be possible due to stability problems or both systems
having different nominal frequencies
HVDC transmission is necessary for underwater power
transfer if the cables are longer than 50km
Power flow can be controlled rapidly and accurately
17
FACTS
FACTS: Flexible AC Transmission Systems
Greater demands are being placed on the transmission
network and will continue. At the same time, its becoming
more difficult to acquire new rights of way for new
transmission infrastructure/lines.
FACTS open the door for new opportunities in controlling
power, enhancing the usable capacity of present and future
transmission; improving system performance, reliability and
security; and validating the use of power electronics to
enhance power systems operation and dynamic
performance.
18
FACTS
FACTS: Flexible AC Transmission Systems
 Function: Shunt and Series Compensation
 Static Var Compensator (SVC) and
Voltage Sourced Converter (VSC-based) STATCOM
Conventional
SVC
STATCOM
Mechanically
Switched
Thyristor
Controlled
Converter
Slow VARs
Fast VARs
Better, Faster VARs
19
FACTS
Advantage of FACTS Devices
Efficient Installations: 12 to 18 month timeframe
Increased System Capacity: Maximum operational
efficiency of existing transmission lines and other equipment
Enhanced System Reliability: Provide greater voltage
stability and power flow control, which improves system
reliability and security
Improved System Controllability: Intelligence built into the
grid, ability to instantaneously respond to disturbances &
redirect power flows
Investment: Less expensive than new transmission lines
20
Power Electronics Technologies
A View of the Smart Grid
Power Generation
Power Flow Control
System Reliability
Inter-area Control
Inter-tie Reliability
Inter-connected
ITC/RTO System
S/S
Wind Farm
Interconnections
BTB DC
HVDC / BTB
UPFC
Improved
Power Quality
Voltage Control
Power System Stability
S/S
SVC /
Load
STATCOM
Voltage
Support
Inter-connected
Power System
Load
STATCOM / SVC
Enhanced
Import Capability
STATCOM / SVC
Increased
Transmission Capacity
Load
S/S
HVDC / BTB
FSC / TCSC
21
Summary and Conclusions
•
Needs are developing in the electric power sector for improved integrated
generation management (IGM) with respect to the increase in green energy
resource penetration.
•
Many of the challenges faced for IGM and the new green resource portfolios
that are emerging are within the power transmission delivery sector. There is
a strong need for applying advanced transmission technologies to assure
safe, reliable, and efficient electricity delivery.
•
Future applications and development requirements for power electronics and
control technologies in a diversified generation environment, with respect to
power system dynamic performance, are needed.
•
In general, the case is made for employing more power electronics control
technologies throughout transmission and distribution systems for
strategically interconnecting green energy resources.
•
Combinations of FACTS and HVDC transmission technologies can provide
optimal solutions and enhanced investment for utilities and generation
providers alike – we need continued development and deployment !!
22