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