Power Electronic Converters for Efficient Operation of the Modular HVDC Generator for Offshore Wind Power
Master thesis
Date
2020Metadata
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- Institutt for elkraftteknikk [2503]
Abstract
Offshore vindkraft spås å ha en eksponentiell vekst de neste tiårene da verden trengerkostnadseffektiv og fornybar energi. Den modulære HVDC (ModHVDC) generatoren eret nytt design for å oppnå 100 kV HVDC i ett omformingssteg uten transformatorer. Ved åsegmentere statoren til en permanentmagnetgenerator blir hvert statorsegment å regne somen trefase generator. Ved å koble segmentene til seriekoblede kraftomformere muliggjøresHVDC i ett omformingssteg. Dermed tar teknologien sikte på å utvide bruken av HVDC ioffshore vindkraft.
I oppgaven har teknologien vært konseptualisert i en 10MW vindturbin. Kraftomformeren(e)har vært fokusområdet i vindturbinsystemet. To forskjellige omformeres ytelse i applikasjonenhar vært sammenlignet, tap tilknyttet halvlederne i omformerne og kontrollmetoderav omformerne har vært studert. De neste tre avsnittene oppsummerer metode ogresultat tilhørende hvert punkt.
Ytelse ble studert ved å sammenligne en “three-level neutral point clamped” (3L-NPC)omformer og en “two-level voltage source converter” (2L-VSC). Ytelse ble målt ved åstudere tilstandsvariablene i systemet, kurvene til spenning og strøm og kraftkvaliteten itillegg til tap og effektivitet for begge omformere for en rekke forskjellige vindhastigheter.Dette ble gjennomført i simuleringsprogrammet Simulink hvor tilhørende oppsett og resultatergis i kapittel 5. Resultatene viste at systemet var stabilt for begge omformerne.3L-NPC-omformeren viste bedre kraftkvalitet, redusert variasjon i DC-link strøm, laveretap og høyere effektivitet sammenlignet med 2L-VSC-omformeren. Basert på resultateneble det konkludert med at 3L-NPC-omformeren utkonkurrerer sin motpart og det er anbefaltat førstnevnte omformer brukes i videre studier eller praktiske forsøk i istedenforsistnevnte.
Tap i 3.3, 4.5 og 6.5 kV industrielt tilgjengelige IGBT moduler ble beregnet i kapittel 4 vedbruk av analytiske beregningsmodeller. Formålet var å studere og sammenligne forskjellenemellom °a bruke flere 3.3 kV moduler, få 6.5 kV moduler eller et kompromiss ved åbruke 4.5 kV moduler. Resultatene viste at 3.3 kV modulen hadde de laveste tapene. Allemodulene ble evaluert i begge omformerne og resultatene viste at 3L-NPC-omformerenvar mer effektiv enn sin motpart i alle tilfeller, hvilket simuleringsresultat i kapittel 5støtter.
Kontroll av DC-busspenningene med tilhørende utfordringer og løsninger ble presentert ikapittel 3. En casestudie med åtte generator/omformer moduler med normalfordelte parametereble brukt som utgangspunkt for å studere kontrollmetodene. Resultatene viser atman enten må akseptere overbelastning av moduler eller senke effekten fra noen modulerfor å ha identiske DC-busspenninger. Overbelastning medførte at en modul ble belastetmed 0.048 pu (8 A) over nominell verdi, mens kraftreduksjonen var på 4.5 %, hvilket kantilsvare 1.3 GWh/år for en 10 MW offshore vindturbin. Offshore wind power is projected to have an exponential growth in the coming decadesas the world needs affordable low-carbon and renewable energy resources. The modularHigh Voltage Direct Current (ModHVDC) generator is a new design for generator andelectrical drive train, that proposes a transformer-less concept with a single conversionstep to achieve 100 kV HVDC potential. By segmenting the stator of a permanent magnetsynchronous generator, the machine forms multiple equivalent three-phase generators.Connecting these stator segments to series-connected power converters enables HVDC ina single conversion step. Thus, this technology aims at extending the use of HVDC inoffshore wind power grids.
This master thesis emphasizes the power electronic converters related to the ModHVDCmachine. Special technical challenges arise due to the stator segmentation and multiplepower converters. Adequate control methods are required for high performance and reliableoperation. Additionally, the 100 kV DC-potential necessitates dedicated convertersfor safe and efficient operation. Research for energy-efficient and high performing powerelectronic converters were the focus for this thesis, where power converter performance,semiconductor losses and DC-bus voltage control methods was studied. The intention wasto use the results for future lab-scale realization of the ModHVDC generator to increasethe technical readiness level of the technology.
A comparison between a three-level neutral point clamp converter (3L-NPC) and a conventionaltwo-level voltage source converter (2L-VSC) was carried out in terms of theirperformance. More precisely, performance was measured by studying the state variablesbehaviour, voltage and current waveform, power quality, losses and efficiency for bothconverters when the wind turbine was subjected to various wind speeds. This was conductedin Simulink, where simulation setup, results and a summary are presented in chapter5. The results showed that even though a stable operation was achieved with both converters,the 3L-NPC showed better power quality, reduced DC-link current ripple, lowerlosses and higher efficiency than the 2L-VSC. Based on the results, the 3L-NPC converterwas concluded to be a suitable converter for use and future research for the ModHVDCgenerator.
Semiconductor losses with a 3.3, 4.5 and 6.5 kV industrially available IGBT module werecalculated in chapter 4 by using analytical calculation loss models. The purpose ofthe study was to compare benefits of using multiple lower voltage rated modules or fewerhigher voltage rated modules with both converters. The results showed that the 3.3 kVIGBT module had the lowest losses. Additionally, the calculation supported the simulationresults as the 3L-NPC converter was more efficient than the 2L-VSC.
DC-bus voltage control methods were studied in chapter 3 and concerns balancing theDC-bus voltages of all converters. Both the challenge and potential solutions for controlstrategies were presented. Eighth generator/converter modules were assigned with normaldistributed parameters for simulating a natural voltage variation between modules in afull-scale application. The results show that the alternatives for having identical DC-busvoltages are either accepting overloading of some modules or lower the power output ofeach module to the module with lowest output power. For this specific case, the former ledto a current overloading of 0.048 pu (8 A), while the latter resulted in a power reductionof 4.5 %, which could accumulate to 1.3 GWh/year for a 10 MW offshore wind turbine.