Power Conversion
Systems for
Microgrids
Burak Ozpineci
Yan Xu
Tom King
Tom Rizy
Oak Ridge National Laboratory
Utility-Scale Power Electronics Needs
Devices
Device Issues
● Reliability
● Low cost
Modules
Converters
● Higher efficiency
Module Issues
Converter Issues
●Wide bandgap
materials
● Thermal
Management
● Modularity
● Higher switching
frequency
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●High temperature
operation
Managed
by UT-Battelle
for the U.S. Department of Energy
● Availability
● High Temperature
Packaging
● Low cost
● Low maintenance
● Low cost
● Higher Efficiency
Presentation_name
System
System Issues
● Stability
● Reliability
Distributed Energy and Storage (DE/DS)
Power Conversion Technologies
DC
DC
DC
AC
AC
DC
DC
DC
AC
DC
For this discussion the inverter might or might not include the dc/dc
converter and/or the rectifier
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Objective of the Session
• Define specifications for inverters for microgrids
• Identify the technical and cost challenges for deploying
inverter technology within microgrids
• Focus on three elements of the inverter (lowering the
cost and advancing state-of-the-art)
– Hardware
– Control / Operations - Functionality
– Topologies
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for the U.S. Department of Energy
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Inverter Specifications for Microgrids
• Power levels (20kW, 50kW, 100kW, 250kW, higher?)
• Voltage levels (480V - 13.8kV)
– Today – Low voltage inverter with step-up transformer
– Future – Medium voltage rated/ distribution system grade inverter?
• Efficiency
– Today – 97%
– Future – 99% (67% reduction in losses)
• Switching frequency
– Today – 10-20kHz
– Future – 50kHz
• Thermal management
• Cost
• Difference between a microgrid inverter and grid-connected inverter
– Functionality and control
– Inverter topologies for medium voltage inverters
– Other differences?
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Inverter Hardware Components
• DC/DC converters
–
–
–
–
Power switches (semiconductor switches and their packaging)
Passive components (inductors, capacitors, high frequency transformers)
Controllers and gate drivers
Communication
• Inverters/Rectifiers (typically voltage source converters)
–
–
–
–
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Power switches (semiconductor switches and their packaging)
Coupling inductor/transformer
Controllers and gate drivers
Communication
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for the U.S. Department of Energy
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Microgrid Inverter Control Functions
• Today – Fundamental functions
– On-grid: MPPT, droop control during high frequency, local voltage
regulation to a pre-set level
– Islanding: P-f droop control, Q-V droop control
– Transition: mode change based on local voltage and current sensors
• Future – Advanced functions with communication and
microgrid central controller
– On-grid: following the P, Q (or V) references dispatched from
microgrid central controller or distribution operation center
– Islanding: frequency secondary control, optimized Q dispatch, source
and load dynamics
– Transition: situation awareness, mode switch and power re-dispatch
based on local and microgrid information
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Inverter Controllable Independent Variables
Active power
related variables
Active
power
Active
current
Power
factor
Frequency
Reactive power
Yes
Yes
Yes
Yes
Reactive current
Yes
Yes
Yes
Yes
Power factor
Yes
Yes
NA
Yes
Local voltage
Yes
Yes
Yes
Yes
Reactive power
related variables
9
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for the U.S. Department of Energy
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Inverter and Microgrid Topologies
• Inverter
– More efficient inverter topologies
needed to reduce the operational costs
– New inverter topologies
• Multilevel inverters
• Matrix converters
• High frequency link converters
• Microgrid
– AC microgrids
– DC microgrids
– DC-AC hybrid microgrids
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Factors Affecting Inverter Cost
• Capital costs
– Components
•
•
•
•
•
Power device cost
Number of devices – new topologies?
Current/voltage sensors – sensor elimination?
Communication – autonomy?
Control
– Standardization of devices and converters
– Scalability and modularity
– Reliability
• Operational costs
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– Added functionality (volt/var support, frequency regulation,
etc)
– Efficiency
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“Sunshot”
Dollar-a-Watt PV Program
• What are the current
number for microgrid
converters?
• What should be the target
numbers for future
microgrid converters?
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for the U.S. Department of Energy
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Challenges/Opportunities
• Multiple inverter operation (grid-connected/stand alone)
– Collaborating inverters
– Plug and Play
– Hierarchy - autonomous and coordinated
• Reliability
– Warranties up to 20-30 years
– Reliable active and passive components
– Operating under harsh conditions
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Challenges/ Opportunities
• Standardization
– Microgrid specific standard inverter or
– Modified grid-connected inverters
• Scalability/Modularity
– Medium voltage inverter
– Determine cost benefit of medium voltage inverters
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Challenges/ Opportunities
• Compatibility/interoperability – Plug ‘n Play operation
for cost reduction
– Inverter
– Communications
– Control
• Efficiency
– High efficiencies at high switching frequencies or elevated
temperatures
• EMI
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Key R&D areas where DOE can make
an impact
• Medium voltage microgrid inverter
• Advanced inverter controls for microgrids
• Robust operation during fault conditions
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