Smart Grid and Renewable
Energy Grid Integration
Jian Sun, Professor and Director
Department of ECSE & Center for Future
Energy Systems
IEEE USA 5-4-2012
1
How Smart Can We
Make This Grid?
IEEE USA 5-4-2012
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Smart Grid Drivers
• Need to Use Renewable Energy
– Peak Oil; Energy Security
– GHG Emission; Climate Change
•
•
•
•
•
Electrification of Transportation Sector
Energy Storage
Demand Response; Efficient Utilization
Stronger Transmission Network
Intelligent, Bidirectional Distribution System
IEEE USA 5-4-2012
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Energy is a National Priority
Energy
Security
Renewable
Energy
Energy
1
Efficiency
2
Climate
Change
Nuclear
3
Energy
IEEE USA 5-4-2012
Green
Economy
4
Role of Power Electronics
Renewable
Generation
Energy
Storage
Load
Manag.
Smart Grid
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Production of AC
With Electric Machines
B()
v(t)
With Power Electronics

t
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Traditional vs. Wind Generators
Large Wind
Generator
Traditional
Generator
Prime
Prime Mover
Mover
Control
Control
Excitation
Excitation
Control
Control
Limited Controllability at Low
Frequencies
Complex Control & Dynamics
at High Frequencies
DC-Link
DC-Link
Control
Control
Turbine
Turbine Speed
Speed
Control
Control
0.01
0.1
Grid
Grid
Synchronization
Synchronization
Grid
Grid Q
Q&
&V
V
Control
Control
1
Current
Current
Control
Control
10
Frequency (Hertz)
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Semiconductor
Semiconductor
Switching
Switching
1000
10000
7
Grid Operation & Control
109
109
Fast, Autonomous Control
of Many Units
Number of Units (N)
108
107
106
108
107
106
Central Control
Manual Dispatch
105
105
104
104
103
103
102
102
102
0.1
0.1
1
1
10
10
100
Control
IEEE
USA Frequency
5-4-2012
102
1000
(F)
103
10000
104 Hz
100000.
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Impedance is a Key Parameter
Im
+

Zs
Vs
+
Vl
Gain
Margin
Zl

Source
Re
1
Load
Vl ( s )
Z l (s)
1


Vs ( s ) Z l ( s )  Z s ( s ) 1  Z s ( s )
Z l (s)
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
IEEE USA 5-4-2012
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Grid-Parallel Inverter Stability
Voltage-Source System
Current-Source System
• Grid-Connected Inverters are Controlled as Current Sources
• Different System Model and Stability Requirement
• Ratio of Grid Impedance to Inverter Output Impedance Must
Meet Nyquist Stability Criterion
IEEE USA 5-4-2012
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An Example – Solar Inverter
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Grid Impedance
• Line + Transformer + Generator Impedance
• Typically Inductive at Fundamental Frequency
– Focus of Traditional Power System Theory
– Weak Grid
• Resonance at Harmonic Frequencies
• Effects of Loads; Variability with Time
• Effects of Neighboring Renewable Sources
– Active Control; Different from Passive Impedance
IEEE USA 5-4-2012
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Inverter Output Impedance
• Depends on Physical Design and Control
– Filter Inductors and Capacitors (L, LC, LCL)
– Current & Voltage Control, Grid Synchronization
• Inverter Impedance Modeling
– Native Circuit & Control Models are Nonlinear
• Small-Signal Impedance has to be Used
– Time-Varying Operation; No DC Operation Point
• Traditional Linearization Methods cannot be Applied
IEEE USA 5-4-2012
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Small-Signal Modeling
• Phasor-Based Methods
– Not Compatible with Impedance-Based Analysis
– Limited to Line Fundamental Frequency
• DQ-Transformation Method
– Impedance in DQ-Coordinate System is Difficult
to Measure and Interpret
– Coupling between DQ Axes Requires Generalized
Nyquist Criterion
• Direct Harmonic Linearization
IEEE USA 5-4-2012
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Three-Phase Converter Modeling
• Decomposition Using Symmetric Components
– Positive-Sequence Impedance
– Negative-Sequence Impedance
– Zero-Sequence Impedance – Usually Open-Circuit
• Single-Phase Model for Each Sequence Component
• No Crossing Coupling between Positive and Negative
Sequence Subsystems
va
vb
vc
ia
ib
ic
Positive Sequence
+
vp
ip

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Negative Sequence
+
vp
in

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Smart Grid System Test-Bed
• Need a Controllable Grid to
– Emulate Different Grid Conditions
– Test Analysis Method and System Theory
– Demonstrate System Control Techniques
• A System Test-Bed has been Developed
– Grid Simulator
– Programmable Voltage, Frequency, Harmonic Contents,
and Impedance
– Single or Three-Phase Operation, 75 kW Power
– Standalone, Grid Parallel Mode, Micro Grid
IEEE USA 5-4-2012
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=
~
=
~
=
=
Utility
Grid
~
Simulated Grid with Programmable
Volt/Freq/Impedance
~
 Inverters (20)
Grid Simulator
Central
Inverters (3)
PV Simulators
~
=
=
~
G
G
M
M
4th Gen Wind Turbine Simulator
Electronic
Loads
IEEE USA 5-4-2012
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Single-Phase Solar Inverter
Lp = 0 mH
Grid Voltage (500V/div)
Lp = 12.8 mH
Grid Voltage (500V/div)
Grid Current (10A/div)
Grid Current (10A/div)
IEEE USA 5-4-2012
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Harmonic Resonance
5
Ih/I1 (%)
Lp = 0 mH
Lp = 12.8 mH
4
3
2
1
0
2
7
12
17
22
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32
37
h
19
Three-Phase Wind Inverter
ia (5 A/div.) ib (5 A/div.) ic (5 A/div.)
rd
PLL Bandwidth 100 Hz
PLL Bandwidth 10 Hz
Sequence
Gain Margin
Phase Margin
Sequence
Gain Margin
Phase Margin
Positive
1.04 dB
5°
Positive
>15 dB
25°
Negative
15 dB
42°
Negative
>15 dB
55°
IEEE USA 5-4-2012
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Nature of Harmonic Resonance
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Inverter Impedance Shaping
• Grid Synchronization Methods
• Current Control Loop
• Active Damping
• Online Grid Impedance Identification
• Adaptive Control
• Inverter Interactions in Wind Farms
IEEE USA 5-4-2012
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HVDC for Offshore Wind Farms
690 V

ωmech
PM
1600 rpm


Direct-Drive Technology
Speed
Source



ωmech
PM
1600 rpm



7 km

7 km
33kV AC
Bus
690 V
Speed
Source

ωmech
PM
1600 rpm




7 km
690 V
Speed
Source

ωmech
PM
1600 rpm
1600 rpm


400*2.5MW Turbines
Speed
Source



ωmech
PM




HVDC Rectifier HVDC
(VSC or LCC)
690 V
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
IEEE USA 5-4-2012
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Multi-Terminal HVDC
• DC Output from Individual Turbines
• Series and Parallel Connections
• Modular Voltage-Source Converter Design
IEEE USA 5-4-2012
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Hybrid AC-DC System Test-Bed
~
~
=
=
MT HVDC
Utility Grid
~
~
=
=
=
~
~
=
Real-Time
Simulator
IEEE USA 5-4-2012
=
~
AC DG
Test-Bed
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Summary
• Renewable Energy and Electric Transportation Will
Drive Smart Grid Development
• Energy Storage and Demand Management Required
• Ubiquitous Use of Power Electronics
– New Stability Problems at High Frequencies
– New Modeling and Analysis Tools Needed
– Fast, Autonomous Control are Essential
• New Impedance-Based System Analysis Methods
• Hardware-in-the-Loop System Test-Bed for
Validation and Demonstration
IEEE USA 5-4-2012
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