Renewable Energy Grid
Integration
Jian Sun
Professor and Director
CFES 2011 Annual Conference
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Grid Integration Issues
•
•
•
•
Cost, Reliability & Efficiency of Grid Interface
Grid Congestion, Weak Grids
Variability of Renewable Production
Bidirectional Power Flow in Distribution Networks,
Localized Voltage Stability Problems
• Possible Solutions
– Enhance Transmission Lines, Active Grid Control
– Energy Storage: Large Scale (e.g. Pumped Hydro) for
Balance of Power; Fast Acting Storage for Stability
– Load Management and Demand Response
CFES 2011 Annual Conference
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Power Electronics as an Enabler
Renewable
Generation
Energy
Storage
Load
Manag.
Power Grid
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AC Generation
Synchronous Generator
B()
v(t)
PWM VSC Converter

t
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Generator Control
Large Wind
Generator
Traditional
Generator
Excitation
Excitation
Control
Control
Limited Controllability at
Low Frequencies
Complex Control at High
Frequencies
Prime
Prime Mover
Mover
Control
Control
DC-Link
DC-Link
Control
Control
Turbine
Turbine Speed
Speed
Control
Control
0.01
0.1
Grid
Grid Q
Q&
&V
V
Control
Control
1
Grid
Grid
Synchronization
Synchronization
Current
Current
Control
Control
10
Frequency (Hertz)
100
CFES 2011 Annual Conference
Semiconductor
Semiconductor
Switching
Switching
1000
10000
5
Research Areas and Projects
• Overall Theme: Operation and Control of Power Grids with
Pervasive Power Electronics
– Fundamental Research
– Small-Scale System Demonstration
• Wind Turbines Integration with Weak Grids (GE)
• High-Voltage DC Transmission for Offshore Wind (Stanford)
• DC Micro Grid for Integration of Renewable Energy and
Energy Storage (Ultralife, NYSERDA)
• Smart Grid Voltage Stability and Control (NSF)
• Multi-Terminal HVDC for Offshore Wind Farms and Bulk
Power Transmission (ARPA-E, GE, CURENT ERC)
CFES 2011 Annual Conference
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Voltage Instability
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Impedance-Based Method
Im
+

Zs
Vs
+
Vl
Zl

Source
Load
Vl ( s )
Z l (s)
1


Vs ( s ) Z l ( s )  Z s ( s ) 1  Z s ( s )
Z l (s)
Gain
Margin
Re
1
c
Phase
Margin
• Partition System into a Source and a Load Subsystem
• Determine Source Subsystem Output Impedance (Zs) and Load
Subsystem Input Impedance (Zl)
• System is Stability if Zs/Zl Meets Nyquist Stability Criterion
CFES 2011 Annual Conference
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Distributed Generation Test-Bed
Passive Impedance Bank
3 208 V
Utility-Grid
~/
Solar
Invert.
Reactors
Invert.
Invert.
/
Fuel Cell
Rect.
Filter
Active Grid Simulator
Wind
3 208 V
Simulated Grid
Load
• A Simulated Distribution Grid with
– Programmable Voltage, Frequency Variation, Harmonics
– Programmable Grid Impedance
• Various Distributed Generation Sources & Loads
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Grid-Connected Inverter Modeling
ib
vb
ic
abc
PWM
Kd
abc
vqm
dq


vdm

Kd
Hi(s)
Hi(s)
vc
PLL
dq
HPLL(s)
id


abc
idr

dq
vd
Z p 0 s   sL 
iq

vq
Grid
va
Filter
ia
iqr
Vdc
 H i ( s  j 2 πf )
2
V
Z n 0 s   sL  dc  H i ( s  j 2 πf )
2

 1 V1 H PLL ( s  j 2πf1 ) Vdc I1
H i ( s )
Z p s   Z p 0 ( s )1 
 2 1  V1 H p ( s  j 2 πf1 ) V1

 1 V1 H PLL ( s  j 2 πf1 ) Vdc I1

Z n s   Z n 0 ( s )1 
H i ( s )
 2 1  V1 H p ( s  j 2 πf1 ) V1

CFES 2011 Annual Conference
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1
1
High Inductive Grid Impedance Causes
Resonance with Capacitive Inverter
Output Impedance
Magnitude (dB)
Harmonic Resonance
50
40
30
Zp
Zg
Zn
20
10
100 Hz
1 kHz
10 kHz
← Hz
Phase (deg.)
150
100
50
0
50
100
150
-
← =175°
100 Hz
CFES 2011 Annual Conference
1 kHz
10 kHz
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Mitigating Harmonic Resonance
• Increase Inverter Impedance to Avoid Intersection
with Grid Impedance
Inverter Currents with
• Reshape Inverter Output
Reduced PLL Bandwidth
Impedance to Ensure
Sufficient Stability Margin
• Current Compensator
• PLL Design
• Online Grid Impedance
Identification, Adaptive
Control
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HVDC for Offshore Wind Farms
690 V

ωmech
PM
1600 rpm


7 km
Direct-Drive Technology
Speed
Source



ωmech
PM
1600 rpm




7 km
33kV AC
Bus
690 V
Speed
Source

ωmech
PM
1600 rpm



HVDC Rectifier HVDC
(VSC or LCC)
690 V

7 km
690 V
Speed
Source

ωmech
PM
1600 rpm
1600 rpm


400*2.5MW Turbines
Speed
Source



ωmech
PM




7 km
300
300 MVA
MVA
STATCOM
STATCOM
AC Bus
Bus
AC
Filters
Filters
Speed
Source
Stability & Control of AC Collection Bus
690 V
7 km
7km Cable
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DC Micro Grid
Supervisory Control
Ultra
Caps
=
Hardware
Embedded Control
Switching
Protection
10-6
10-2
~
DC Bus
Loading/Unloading
Weather Change
1
=
Supervisory
Control & Com
Subsystem-Level
Autonomous Control
Function, Stability
Power Quality
10-4
=
102
CFES 2011 Annual Conference
Hourly, Daily &
Seasonal Variation
104
106
Second
108
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Summary
• Research Focus on Future Grid with Pervasive Power
Electronics
– Renewable Energy
– Energy Storage, Dynamic Load Management
– Active Grid Control
• AC, DC & Hybrid AC-DC Networks
– Stability & Compatibility Over a Wide Frequency Range
• Local Control with Predictable Global Effects
• Impedance-Based Analytical Approach
• Test-Bed as a Platform for System Demonstration
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