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1CFES 2011 Annual Conference
Renewable Energy Grid Integration
Jian SunProfessor and Director
2CFES 2011 Annual Conference
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
3CFES 2011 Annual Conference
Power Electronics as an Enabler
Power Grid
Renewable GenerationRenewable Generation
Energy StorageEnergy Storage
Load Manag.Load
Manag.
4CFES 2011 Annual Conference
Synchronous Generator PWM VSC Converter
t
AC Generation
B()
v(t)
5CFES 2011 Annual Conference
Generator Control
0.01 0.1 1 10 100 1000 10000Frequency (Hertz)
Semiconductor Switching
Semiconductor Switching
Current ControlCurrent Control
DC-Link Control
DC-Link Control
Turbine Speed Control
Turbine Speed Control
Grid Q & V Control
Grid Q & V Control
Prime Mover Control
Prime Mover Control
Excitation Control
Excitation Control
Trad
ition
alG
ener
ator
Larg
e W
ind
Gen
erat
or Grid Synchronization
Grid Synchronization
Limited Controllability at Low FrequenciesComplex Control at High Frequencies
6CFES 2011 Annual Conference
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)
7CFES 2011 Annual Conference
Voltage Instability
8CFES 2011 Annual Conference
Impedance-Based Method
c
1
Phase Margin
Gain Margin
Im
Re
• 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
+
Zs
ZlVs Vl
+
Source Load
)()(1
1)()(
)()()(
sZsZsZsZ
sZsVsV
l
ssl
l
s
l
9CFES 2011 Annual Conference
Distributed Generation Test-Bed
Rect.
Fuel Cell
Solar
Wind
Invert.
Invert.
Invert.
Load
Reactors
Filter / ~ /
3 208 VSimulated Grid
3 208 VUtility-Grid
Passive Impedance Bank
• A Simulated Distribution Grid with– Programmable Voltage, Frequency Variation, Harmonics– Programmable Grid Impedance
• Various Distributed Generation Sources & Loads
Active Grid Simulator
10CFES 2011 Annual Conference
Grid-Connected Inverter Modeling
HPLL(s) vq vd
PLLPWM
iqrHi(s)
Kd
Kd
idr
Hi(s)
id
iq
abcdq
abcdq
iaib
ic
abcdq
vqm vdm
va
vb
vc Filte
r
Grid
1
i1
1dc
11
1PLL10
1
i1
1dc
11
1PLL10
)()π2(1
)π2(211)(
)()π2(1
)π2(211)(
sHV
VfjsHVfjsHVsZsZ
sHV
VfjsHVfjsHVsZsZ
pnn
ppp
I
I
)π2(2
)π2(2dc
0
dc0
fjsHVsLsZ
fjsHVsLsZ
in
ip
11CFES 2011 Annual Conference
100 Hz 1 kHz 10 kHz
1020304050
100 Hz 1 kHz 10 kHz
-150-100-50
050
100150
Harmonic Resonance
Phas
e (d
eg.)
Mag
nitu
de (d
B
)← =175°
pZ gZ
← Hz
nZ
High Inductive Grid Impedance Causes Resonance with Capacitive Inverter Output Impedance
12CFES 2011 Annual Conference
Mitigating Harmonic Resonance• Increase Inverter Impedance to Avoid Intersection
with Grid Impedance• Reshape Inverter Output
Impedance to Ensure Sufficient Stability Margin
• Current Compensator• PLL Design• Online Grid Impedance
Identification, Adaptive Control
Inverter Currents with Reduced PLL Bandwidth
13CFES 2011 Annual Conference
HVDC for Offshore Wind Farms
SpeedSource
1600 rpm
ωmech
690 V
7 km
PM
SpeedSource
1600 rpm
ωmech
690 V
7 km
PM
SpeedSource
1600 rpm
ωmech
690 V
7 km
PM
SpeedSource
1600 rpm
ωmech
690 V
7 km
PM
SpeedSource
1600 rpm
ωmech
690 V
7 km
PM
400*2.5MW Turbines
33kV AC Bus
7km Cable
Direct-Drive Technology HVDC Rectifier(VSC or LCC)
HVDC
300 MVA STATCOM300 MVA
STATCOM
AC
Bus
Fi
lters
AC
Bus
Fi
lters
Stability & Control of AC Collection Bus
14CFES 2011 Annual Conference
DC Micro Grid
Second
10-6 10-4 10-2 1 102 104 106 108
Hourly, Daily & Seasonal Variation
Loading/Unloading Weather Change
Function, Stability Power Quality
SwitchingProtection
Hardware Embedded Control
Subsystem-Level Autonomous Control
Supervisory Control & Com
Ultra Caps
=~
Supervisory Control
DC Bus= =
15CFES 2011 Annual Conference
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
