1

The Roles of Power Electronics in 

Renewable Energy Deployment

Eduard Muljadi

National Renewable 

Energy Laboratory

Panel presented at the 

2014 IEEE Power and 

Energy Society (PES) 

Conference on 

Innovative Smart Grid 

Technologies (ISGT)

Washington, DC

February 19-22, 2013

NREL/PR-5D00-61378

Source: NREL
Outline

• Introduction

• Renewable energy resources

• Grid integration

• Power electronics control

• Environment

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Introduction

• Applications 

 mW or MW level

 Isolated or grid connected

• Variability of the source 

 Temporal (seconds, hour, day, week, season) 

 Spatial (continental, local, plant)

• Large area coverage – Diversity (resource and 

electrical characteristics)

• Operation 

 Normal/abnormal

 Balanced/unbalanced

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4

 

 PV Plant (5~50 MW) 

 

 

 Rooftop PV 

 (1~30 kW)

 Mobile 9-kW PV System 

 Bechler Meadows Ranger Station 

 Yellowstone National Park 

Introduction

Photo from SunPower Corp., NREL 23816

Photo by Dennis Schroeder, NREL 22192

Photo from DOE/FEMP, 

NREL 27638
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California Region

Spring Peaking Summer Peaking Peak Shift

24-Hour Wind Resource

24-Hour Solar Resource

Wind Resource – Midwest Wind Resource – West Coast

Low Output

8 a.m. – 14 p.m.

High Output

8 a.m. – 12 p.m.
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Offshore Wind Resource

 

 

U.S. Wind Resource

 

 

Solar Resource

 

 

Resources
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Synchronous Generator (left) vs.

Wound Rotor Induction Generator (right)

What will happen if there is a sudden 

disturbance in the transmission 

network? To power AC-DC-AC power 

converter to operate WTG in 

variable speed
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Generator Sgen 4,000-W (Siemens) 

1,300 to 2,235 MVA 

Wind Turbine 

Generator

1-6 MW

Conventional Power Plant vs. Renewable Energy Power Plant

Plant Diversity Photo by David Hicks, NREL 18454

Photo fromhttp://www.energy.siemens.com/hq/en/ U.S. NRC image of a modern steam turbine generator. Photo from Wikipedia
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Type 1 Wind Turbine Generator (1% Slip) 

Type 3 Wind Turbine Generator ( + 30% Slip) Type 4 Wind Turbine Generator (Full Conversion)

(100% Slip)

Type 2 Wind Turbine Generator (10% Slip) 
Examples of 

Renewable Energy Resources 

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Photo by David Hicks, NREL 19881

Photo by David Hicks, NREL 18557

Photo by Bill Timmerman, NREL 08989

Photo by Greg Glatzmaier, NREL 19807
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Photo by Dennis Schroeder, NREL 27806
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Reference: Li, Y.; Yu, Y. H. (2012). 

Synthesis of Numerical Methods for 

Modeling Wave Energy Converter-Point 

Absorbers: Preprint. 36 pp.; NREL Report 

No. JA-5000-52115.
Grid Integration

• Interconnection

• Operation

• Standards – Grid codes 

• Ancillary services (inertial response, 

frequency and governor response, reserves) 

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Transmission to 

other utilities

115kV and higher 

Power plant Utility Scale Wind Farms (above 10 MW)

Small Wind Farm (less 

than 10 MW)

26-115 kV

Single Large Wind 

Turbine

Small Wind Turbine

Subtransmission

Customers

Distribution

Customers

Distribution Customers

4-35 kV 120-480 V

Step-up

transformer

Step-up

transformer

Step-down

transformer

Step-down

transformer

Step-down

transformer

Distributed Interconnections per 

IEEE-1547 Wind Farm Interconnections

Interconnection

Credit: NREL’s TGIG presentation archive.
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Base 

Generators Adjustable

Generators

(Conventional

Gen., RE)

Variable

Generators

(RE)

Reserves

 Storage

Base 

Loads

Adjustable

Loads

Variable

Loads

Power Losses

Real Power

Freq UP

Freq Down

Line

Capacitance Switched

Capacitors Synchronous

Generators/

Condensers

 Static 

Compensation

Line

Inductance

Inductive

Load

Induction

Motors/Generators

Reactive Power

Voltage UP

Voltage Down

Real and Reactive Power Balance

(To Keep Frequency and Voltage Constant)
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Voltage Ride Through

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-1 0 1 2 3 4

Time (sec)

Voltage (p.u.)

661

661A

LVRT-WECC Prop.

HVRT-WECC Prop.

Fault Ride-Through Capability

• Wind power plants should be able to stay online under transient faults/disturbances.

• The voltage should tolerate 0 p.u. for 15 ms

(9 cycles).

• Wind power plants should be able to regulate 

the power factor between 0.95 leading/lagging.

• Wind power plants should have a SCADA 

system to allow remote access and monitoring.

 

Voltage vs. Maximum Clearing 

Time as Described in IEEE 1547

Adaptable to 

Regional Code
Power Electronics Control

• Generator level

• Plant level

• Transmission level

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Maximize Energy Capture

Load Control 

Gearbox Preservation

Power Quality

Real and Reactive Power Control

Short/Long-Term DC 

Distributed Storage

Provide Active Damping to 

Power System Network

At the 

Generator Level
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Maximize Energy Capture

Power Quality

Real and Reactive Power Control

Short/Long-Term DC 

Distributed Storage

Provide Active Damping to 

Power System Network

At the 

Generator Level
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•Many (hundreds) of wind turbines (1 MW to 5 MW each) 

•Prime mover: Wind (wind turbine) – Renewable (free, natural, pollution free)

•Controllability: Curtailment

•Predictability: Wind variability based on wind forecasting, influenced more by 

nature (wind) than humans, based on maximizing energy production 

(unscheduled operation).

•Located at wind resource; may be far from the load center.

•Generator: Four different types (fixed speed, variable slip, variable speed, 

full converter) – Non-synchronous generation

•Types 3 and 4: Variable speed with flux-oriented controller (FOC) via power 

converter. Rotor does not have to rotate synchronously.

At the Plant Level
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A

B

C

+

RPMC RPMA

A

B

C

+

RPMC RPMA

5 10 15 20 25

0

1 106 ×

2 106 ×

Max Cp

8 m/s

9.2 m/s

10.8 m/s

12 m/s pitch=0 deg

12 m/s pitch=4.75 deg

Power vs RPM for different wind speeds

RPM (low speed shaft)

Power (W)

1.5 106 ⋅

11 21

A

B

C

D

D’

∆P

5 10 15 20 25

0

1 106 ×

2 106 ×

Max Cp

8 m/s

9.2 m/s

10.8 m/s

12 m/s pitch=0 deg

12 m/s pitch=4.75 deg

Power vs RPM for different wind speeds

RPM (low speed shaft)

Power (W)

1.5 106 ⋅

11 21

A

B

C

D

D’

∆P

Trated limit

Type 1 & 2

Type 3 & 4

At the Plant Level

Inertial Response from a Wind 

Turbine
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(Load 

Center)

Wind Power 

Generator

Other

Generators

VAR

Compensation

to Help

Regulate 

Voltage

Storage

∆δ, ∆V VR

∆P, ∆Q

∆δ, ∆V 

Wind

At the 

Transmission Level

Variability
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Thermal Limit

(Thin Wire)

Stability Limit

(High Impedance,

Long Distance,

Weak Grid)

Wind Power 

Generator

Wind Power 

Generator

Storage

Storage

(Load Center)

FACTS

At the 

Transmission Level

Transmission Constraints
Environment

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Google Map of a wind power plant, Tehachapi, CA

Photo by Stephen Drouilhet, NREL 05626

Reference: McNiff, B. (2002). Wind Turbine Lightning Protection Project: 

1999-2001. 100 pp.; NREL Report No. SR-500-31115. 

Reference: NREL Software Aids Offshore Wind Turbine Designs (Fact

Sheet). Innovation Impact: Wind, NREL (National Renewable Energy

Laboratory). (2013). 1 pg.; NREL ReportNo. FS-6A42-60377.
Conclusions

• Cost reduction in the past 20 years

• Many and diverse opportunities for Power Electronics

 Generation, transmission, and distribution

• Know the limitations

 Thermal, magnetic, electric (voltage, current) etc.

• Know the applications

 Environment: Ocean, land-based, isolated, clusters

 Opportunities to work in parallel (PV – Wind – CSP) 

• Leverage existing and future technologies

 Other industries (drives, transportation, ship building)

 Modern technologies (smart control, wireless, condition monitoring, 

cyber physical and security, synchrophasor, market driven)

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The information contained in this presentation is subject to a government license.