Comparative Evaluation of Transformer-less Configurations ...
Transcript of Comparative Evaluation of Transformer-less Configurations ...
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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