Smart Grid and Renewable Energy Grid Integration. Jian Sun, Professor and Director Department of ECSE & Center for Future Energy Systems

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1 Smart Grid and Renewable Energy Grid Integration Jian Sun, Professor and Director Department of ECSE & Center for Future Energy Systems 1

2 How Smart Can We Make This Grid? 2

3 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 3

4 Energy is a National Priority Energy Security Energy Efficiency 1 Climate Change Renewable Energy 2 Nuclear 3 Energy Green Economy 4

5 Role of Power Electronics Renewable Generation Energy Storage Load Manag. Smart Grid 5

6 Production of AC With Electric Machines With Power Electronics B( ) v(t) t 6

7 Traditional vs. Wind Generators Traditional Generator Prime Mover Control Excitation Control Limited Controllability at Low Frequencies Complex Control & Dynamics at High Frequencies Large Wind Generator Turbine Speed Control DC-Link Control Grid Q & V Control Grid Synchronization Current Semiconductor Control Switching Frequency (Hertz) 7

8 Grid Operation & Control Fast, Autonomous Control of Many Units Number of Units (N) Central Control Manual Dispatch Hz Control Frequency (F) 8

9 Impedance is a Key Parameter Im + V s Z s Source + V l Z l Load Gain Margin 1 Re Vl ( s) V ( s) Z s l Zl ( s) ( s) Z s ( s) 1 1 Z s ( s) Z ( s) l c Phase Margin Partition System into a Source and a Load Subsystem Determine Source Subsystem Output Impedance (Z s ) and Load Subsystem Input Impedance (Z l ) System is Stability if Z s /Z l Meets Nyquist Stability Criterion 9

10 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 10

11 An Example Solar Inverter 11

12 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 12

13 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 13

14 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 14

15 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 v a v b v c i a i b i c Positive Sequence + v p i p + Negative Sequence + v p i n 15

16 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 16

17 Central Inverters (3) PV Simulators 17 Simulated Grid with Programmable Volt/Freq/Impedance Inverters (20) Grid Simulator Utility Grid = ~ = ~ = ~ = ~ ~ = 4 th = G M ~ th Gen Wind Turbine Simulator Electronic Loads

18 Single-Phase Solar Inverter L p = 0 mh Grid Voltage (500V/div) L p = 12.8 mh Grid Voltage (500V/div) Grid Current (10A/div) Grid Current (10A/div) 18

19 Harmonic Resonance I h /I 1 (%) L p = 0 mh L p = 12.8 mh h 19

20 Three-Phase Wind Inverter i a (5 A/div.) i b (5 A/div.) i c (5 A/div.) rd PLL Bandwidth 100 Hz PLL Bandwidth 10 Hz Sequence Gain Margin Phase Margin Positive 1.04 db 5 Negative 15 db 42 Sequence Gain Margin Phase Margin Positive >15 db 25 Negative >15 db 55 20

21 Nature of Harmonic Resonance 21

22 Inverter Impedance Shaping Grid Synchronization Methods Current Control Loop Active Damping Online Grid Impedance Identification Adaptive Control Inverter Interactions in Wind Farms 22

23 HVDC for Offshore Wind Farms 690 V Stability & Control of AC Collection Bus Speed Source ω mech PM 1600 rpm Direct-Drive Technology Speed Source ω mech PM 1600 rpm Speed Source ω mech PM 1600 rpm 690 V 690 V 7 km 7 km 7 km 33kV AC Bus HVDC Rectifier (VSC or LCC) 300 MVA STATCOM HVDC Speed Source ω mech PM 1600 rpm Speed Source 400*2.5MW Turbines ω mech PM 1600 rpm 690 V 690 V 7 km 7 km 7km Cable AC Bus Filters 23

24 Multi-Terminal HVDC DC Output from Individual Turbines Series and Parallel Connections Modular Voltage-Source Converter Design 24

25 Hybrid AC-DC System Test-Bed Utility Grid ~ = ~ = MT HVDC ~ = ~ = = ~ Real-Time Simulator ~ = = ~ AC DG Test-Bed 25

26 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 26