Comparative Evaluation of Transformer-less

Configurations for Wind Energy Conversion

Systems

Akinola A. Ajayi-Obe

Supervisor: Associate Prof. M.A. KhanAdvanced Machines and Energy Systems (AMES) Research Group

14th November 2017

By

Presentation Outline

• Brief Overview

• Review of Wind Turbine Transformers

• Why Transformer-less?

• Existing Transformer-Less Configurations For Wind Energy Conversion System.

• Comparative Evaluation and Discussion

• Conclusion

Brief Overview

• More power is extracted from the wind per turbine.• Turbine power capacity has increased from 50 kW to 7.5 MW• Projected to increase to 10 MW – 20 MW per turbine by year 2018• Average size of installed onshore and offshore turbines are about

1.5 MW and 3.6 MW.

Source: V. Yaramasu, et al, “High-Power Wind Energy Conversion Systems: State-of-the Art and Emerging Technologies,” in Proc. Of the IEEE, vol. 103, no. 5, pp. 740–780, May 2015.

Failure Rate in Respective Rated Power Group Versus Operational Year

• About 625 installed turbines was used in the survey• Operational time frame between 1997-2004.

Source: J. Ribrant, and L.M. Bertling, “Survey of Failures in Wind Power Systems with Focus on Swedish Wind Power Plants during 1997-2005”, IEEE, vol. 22, no. 1, pp. 167-173, March 2007.

Percentage Distribution of the Number of Failures for Swedish Wind Power Plants Between 2000-2004

Percentage of Downtime per Component in Swedish Wind Power Plants Between 2000-2004

Source: J. Ribrant, and L.M. Bertling, “Survey of Failures in Wind Power Systems with Focus on Swedish Wind Power Plants during 1997-2005”, IEEE, vol. 22, no. 1, pp. 167-173, March 2007.

Breakdown of the Failure Rates and Downtimes of Electrical Systems Components

Percentage of the Component Costs of a 2 MW WECS

Source: M. D. Reder, et al, "Wind Turbine Failures - Tackling current Problems in Failure Data Analysis", Journal of Physics: Conference Series, vol. 753, pp 27, 2016, ISSN 1742-6588.J. M. Perez, et al, “Wind Turbine Reliability Analysis”, Renew. And Sustainable Energy Reviews, No. 23, pp. 463-472, April 2013.

Why do wind turbine transformer fail?

➢ Utilize conventional power distribution transformer▪ Most economical option.

▪ Conventional transformer are designed for steady-state operation.

▪ Variable cycling in wind turbine is preeminent; i.e. leads to thermal cycling

and deteriorate the insulation of the transformer.

▪ Core losses of the transformer is amplified.

➢ Environmental factor▪ Marine air causes corrosion and condensation in the transformer.

➢ Grid code compliance▪ Imposes more electrical and thermal stress in the transformer.

Review of Wind Turbine Transformers

Vacuum Cast Coil Transformer

• Non-flammable, environmentally friendly based on solid insulation.

• No-load losses are significantly high.• Considered the heaviest transformer

due to the size of its core and coils.

Liquid-Immersed Transformer

• Mainly uses mineral oil as dielectric material.

• Flammable nature of mineral oil at high temperature may lead to fire disaster.

• Dielectric material allows better heat dissipation.

Bio-SLIM Transformer

• Made up of bio-degradable synthetic ester and cellulose paper.

• They possess high thermal conductivity which regulates the transformer temperature.

• Developed to create compact transformer.

Vacuum Cast Coil Liquid-Immersed Bio-SLIM

Size High Moderate Moderate

No-Load Losses High Moderate Low

Full-Load Losses Moderate Moderate Moderate

Reliability Low Moderate High

Cost Moderate Moderate High

Comparison of Different Wind Turbine Transformers

Why Transformer-less?

ZsVs

PCC BUS 1 BUS 2Yg ∆Yg∆Yg

Grid Substation Transformer WT Transformer WECS

150 kV/6.6-33 kV

ZsVs

PCC BUS 1 BUS 2Yg∆Yg

Grid Substation Transformer MV-WECS

150 kV/6.6-33 kV

Merits

• Eliminates the associated cost and drawback of wind turbine transformer from wind power plants.

• Simplifies the development of wind power plants.

• Reduced current transmission and minimized cable losses.

Demerits

• Injection of DC components into the grid.• Severe voltage sags are experienced by

the WECS.• Extensive use of complex grid-side

multilevel converter topology in the WECS.

6.6-33/0.69 kV

Transformer-Less Configuration for High

Power Wind Energy Conversion System

Generator-Converter

ConfigurationThree-Stage Power

Converter

Configuration

Classification of Transformer-Less Configuration for High Power Wind

Energy Conversion System.

Generator-Converter Configuration

Cdc

Cdc

MVCollection

Point

Cdc

Phase A Phase B Phase C

Cdc

Cdc

Cdc

Cdc

Cdc

Cdc

Stator Coil

H-Bridge Inverter

Single-switch Power Factor

Correction (PFC)

Single-Phase Rectifier

Phase A

Phase B Phase C

Stator Coil

Active Rectifier

H-Bridge InverterGround

Series-connected Modular Permanent Magnet Generator and MMC Topology

Star-connected Modular Permanent Magnet Generator and MMC Topology

Source: X. Yuan, et al, “A Transformer-less High Power Converter for Large Permanent Magnet Wind Generator Systems,” IEEE Trans., Vol. 3, No. 3, pp. 318-329, July 2012.C.H. Ng, et al, “A Multilevel Modular Converter for a Large, Light Weight Wind Turbine Generator,” IEEE Trans., Vol. 23, No. 3, pp. 1062-1074, May 2008.

Stator Coil Active

Rectifier

H-Bridge Inverter

Modular Permanent Magnet Generator and MMC Topology

• Air-cored, slot-less, multi-coil winding, creating a Modular PMSG.

• Stator windings of the generator is arranged into separate isolated coils.

• A pair of the stator coil winding provides the input voltage for each MMC

• Active rectifier controls the current extracted from each coil so that a unity power factor is obtained.

• Coil voltage depends on the PMSG speed while current is set by the control loop of the rectifier to stabilize the dc-link voltage.

• H-bridge inverter modules are connected in series on the AC side to form MMC.

• Provides a high degree of fault-tolerance.

• Few power semiconductor devices are required in the inverter.

Three-Stage Power Converter Configuration

Sa3

Sa2

Sa1

Sa4

Sa5

Sa6

Sb4

Sb3

Sb2

Sb1

Sb5

Sb6

Sc4

Sc3

Sc2

Sc1

Sc5

Sc6

C2

C1

C3

D1

D2

D3

D4

S1

S2

S3

Lin

Cin

Diode Rectifier

4-Level DC-DC

Converter

4-Level Diode-Clamped

Converter

Source: V. Yaramasu, et al, “A New Power Conversion System for Megawatt PMSG Wind Turbines Using Four-Level Converters and a Simple Control Scheme based on Two-Step Model Predictive Strategy – Part I: Modeling and Theoretical Analysis,” IEEE Journal, Vol. 2, No. 1, pp. 2-13, March 2014.

S1

S2

S3

Cin

S4

Cdc

Cdc

MVCollection

Point

Cdc Cdc

Cdc

Cdc

Cdc

Cdc

Cdc

Phase A Phase B Phase C

Diode Rectifier

High Frequency H-Bridge Inverter

High Frequency Single-Phase

Rectifier

H-Bridge Inverter

Sa2

Sa1

Sa4

Sa5

Sb4

Sb2

Sb1

Sb5

Sc4

Sc2

Sc1

Sc5

C2

C1

Cp1

Chopper Circuit

S1 S2 S3

Cin

S4 S5 S6

Cp2

Cn1 Cn2

Cp_r2Cp_r1

Lp_r1 Lp_r2

Cn_r1

Ln_r1

Cn_r2

Ln_r2

3-Level Diode-Clamped

Converter

Modular Switched Capacitor Based

Resonant Converter2-Level

Voltage Source Converter Series

Connected IGBTs

DD-PMSG Based WECS using High Frequency Link Multilevel Cascaded Medium-Voltage Converter

DD-PMSG Based WECS using Two-level VSC, Modular Switched-Capacitor Based Resonant Converter with Three-level Diode Clamped Multilevel Converter

Source: M. R. Islam, et al, “A High-Frequency Link Multilevel Cascaded Medium-Voltage Converter for Direct Grid Integration of Renewable Energy Systems,” IEEE Trans., Vol. 29, No. 8, pp. 4167-4182, August 2014.

M. Sztykiel, “High Voltage Power Converter for Large Wind Turbine,” Doctor of Philosophy Thesis, Department of Energy Technology, Aalborg University, June 2014.

PARAMETERS OF EXISTING TRANSFORMER-LESS WECS POWER ELECTRONICS CONVERTER

TOPOLOGIES

Parameters Generator-Converter MMC

High Frequency-Link MMC

Diode Rectifier+4L-DC-DC+ 4L-DCC

2L VSC+ Modular SCR+ 3L-DCC

Power Rating 2 MW 4.76 MW 5 MW 10 MW

Grid Voltage 11 kV 11 kV 6.6 kV 20 kV

No. of Modules per phase

24 4 1 1

Switching Frequency 1.2 kHz 1.5 kHz 900 Hz 1.05 kHz

Grid Frequency 50 Hz 50 Hz 50 Hz 50 HzIGBT Voltage Rating 1.7 kV 4.5 kV 6.5 kV 3.3 kV

No. of series-connected IGBT

1 1 1 7

Phase Current 148.5 A 250 A 620 A 408 A

Comparative Evaluation and Discussion

The conduction and switching loss of the IGBT/Diode module of the grid-side converter topologies are calculated using the following equations;

𝐸𝑐𝑜𝑛𝐼 = 𝑣0𝐼 + 𝑟0𝐼 ∙ 𝑖𝑝ℎ 𝑡 𝐵𝑐𝑜𝑛𝐼 ∙ 𝑖𝑝ℎ 𝑡 ∙ 𝑡𝑠 (1)

𝐸𝑐𝑜𝑛𝐷 = 𝑣0𝐷 + 𝑟0𝐷 ∙ 𝑖𝑝ℎ 𝑡 𝐵𝑐𝑜𝑛𝐷 ∙ 𝑖𝑝ℎ 𝑡 ∙ 𝑡𝑠 (2)

𝐸𝑜𝑛𝐼 = 𝐴𝑜𝑛𝐼 ∙ 𝑖𝑝ℎ 𝑡 𝐵𝑜𝑛𝐼 ∙𝑣𝑐𝑜𝑚

𝑣𝑑𝑐(3)

𝐸𝑜𝑓𝑓𝐼 = 𝐴𝑜𝑓𝑓𝐼 ∙ 𝑖𝑝ℎ 𝑡 𝐵𝑜𝑓𝑓𝐼 ∙𝑣𝑐𝑜𝑚

𝑣𝑑𝑐(4)

𝐸𝑜𝑓𝑓𝐷 = 𝐴𝑜𝑓𝑓𝐷 ∙ 𝑖𝑝ℎ 𝑡 𝐵𝑜𝑓𝑓𝐷 ∙𝑣𝑐𝑜𝑚

𝑣𝑑𝑐(5)

∴, 𝑃𝑎𝑣𝐼 = 𝑓𝑠𝑤 ∙ 𝐸𝑐𝑜𝑛𝐼 + 𝐸𝑜𝑛𝐼 + 𝐸𝑜𝑓𝑓𝐼 (6)

∴, 𝑃𝑎𝑣𝐷 = 𝑓𝑠𝑤 ∙ 𝐸𝑐𝑜𝑛𝐷 + 𝐸𝑜𝑓𝑓𝐷 (7)

FITTING PARAMETERS OF IGBT/DIODE MODULES

Parameter 1.7 kV/600 A 3.3 kV/800A 4.5 kV/600 A 6.5 kV/600 A

𝑣𝐶𝐸 900 V 1800 V 2250 V 3600 V

𝑣0𝐼 0.7 1 1 1

𝑟0𝐼 0.0023 0.09367 0.01861 0.09857

𝑣0𝐷 0.5 0.8 0.5 0.5

𝑟0𝐷 0.00133 0.00907 0.023196 0.08699

𝐴𝑜𝑛𝐼 0.00057942 0.000959466 0.006213403 0.010908105

𝐵𝑜𝑛𝐼 0.9351 1.115444805 0.950072933 1.001643596

𝐴𝑜𝑓𝑓𝐼 0.00066378 0.003771589 0.06854911 0.00437628

𝐵𝑜𝑓𝑓𝐼 0.88671 0.841860719 0.511257394 1.044655002

𝐵𝑐𝑜𝑛𝐼 0.79806 0.687596711 0.664827161 0.591830287

𝐵𝑐𝑜𝑛𝐷 0.52041 0.660487133 0.725344373 0.573661926

𝐴𝑜𝑓𝑓𝐷 0.008839 0.05906231 0.0196761 0.039192228

𝐵𝑜𝑓𝑓𝐷 0.43627 0.4227119 0.47047145 0.5742542

Generator-Converter MMC HF-Link MMC 3L-DCC 4L-DCC0

50

100

150

200

250

Pow

er L

oss

(Wat

t)

On-state Energy of the IGBT

On-State Energy of the Diode

Turn-On Energy of the IGBT

Turn-Off Energy of the IGBT

Turn-Off Energy of the Diode

Conduction and Switching losses of the IGBT and Diode Modules of the Grid-Side Converter Topology for Transformer-less WECS

Generator-Converter MMC HF-Link MMC 3L-DCC 4L-DCC0

0.5

1

1.5

2

2.5x 10

5

Avera

ge P

ow

er

Loss (

kW

)

Average Power Loss of IGBT

Average Power Loss of Diode

Average Power losses of the IGBT and Diode Modules of the Grid-Side Converter Topology for Transformer-less WECS.

Conclusion

Collection Point Medium Voltage

level

(kV)

Minimum DC-Link Voltage of the Grid-Side

Converter

(kV)

Required Number of Voltage levels of

the Grid-Side Converter

6.6 11 Five Level

11 17 Seven Level

22 34 Nine Level

33 51 Eleven Level

Medium Voltage Range of the Collection Point, the Minimum DC-link Voltage and the respective Voltage levels of the Grid-side Multilevel Power Converter:

Conclusion

Modular Multilevel Converter (MMC) topology shows better efficiency than the other grid-side converter topologies, due to the low voltage rated IGBT/diode modules.

Diode Clamped Converter (DCC) topology efficiency can be improved by using 4.5 kV rated IGBT device and operating at higher order multilevel voltage level (five-level and above).

Although, the MMC topology has been efficiency, the complexity of the generator-converter configuration in terms of the special stator winding arrangement is a major drawback.

The three stage power converter configuration is a more feasible approach for developing transformer-less connection for high-power wind energy conversion systems (WECS).

The application of conventional multilevel topologies to the grid-side converter of transformer-less WECS will be very complex. Due to the excessive use of clamping devices.

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