DC-BUS capacitor rating of the back-to-back NPC converters

1. Alcalá University Department of Electronics DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, SantiagoCóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco Huerta, Carlos Girón Department of Electronics. Alcalá University marta.alonso@depeca.uah.es emilio@depeca.uah.es Researching group in Control and Power Electronics Systems SAAEI 2006

2. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

3. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

4. Alcalá University Department of Electronics Introduction • Converter structures for variable-speed wind turbines  back to back VSC • Increase of wind turbine power multilevel converters as interface between generator and the utility grid • Multilevel topologyNPC DC-bus capacitors Critical Design Researching group in Control and Power Electronics Systems SAAEI 2006

5. Alcalá University Department of Electronics Introduction • Same factors of two-levels topology • Low frequency ripple due iNPunbalance • DC-bus capacitors • Efects of unbalance • Aims of this presentation • Over-voltages in power electronics devices and DC-bus capacitors • Decrease of quality of currents • Even an inappropiate operation • Unbalance in the DC-bus capacitors due to iNP on a back-to-back converter • Relating the ripples due to iNP e iDC • Obtaining analytical equations for each ripple of a back-to-back converter Researching group in Control and Power Electronics Systems SAAEI 2006

6. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

7. Alcalá University Department of Electronics Relation between DC-bus capacitors in two-levels and three-levels converters • Two-levels converter • Working as PWM rectifier • ΔuDC Maximum allowed ripple • TSW IGBTs switching period • Working as active filter (These expressions are valid for VSC connected to the grid with passive an active load) CDC≥537.5μF Rectifier Very conservative! CDC≥4277.2μF Filter Researching group in Control and Power Electronics Systems SAAEI 2006

8. Alcalá University Department of Electronics Relation between DC-bus capacitors in two-levels and three-levels converters • Three-levels converter • CDC dependes on the low frequency ripple due to the NP current • fiNP = 3·fm • Dynamic circuit of NP connection: • Assumptions: • Each converter independently controlled • CP=CN=2CDC • Only fundamental harmonics are considered Researching group in Control and Power Electronics Systems SAAEI 2006

9. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

10. Alcalá University Department of Electronics INP Calculation • SVPWM: ma=1 and DPF=0 • SPWM: ma=1 and DPF=0 • THSPWM • Under the same condition the third order component is relatively smaller Researching group in Control and Power Electronics Systems SAAEI 2006

11. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

12. Alcalá University Department of Electronics Calculation of DC-bus capacitors • Harmonics components of the ripple of the DC-bus capacitors: Zero frequency Due to unbalances in the voltage of the two banks. (Depends on modulation technique.) If modulation signal has no offset Equalization resistances Conmutation frequency or high frequency ripple Two-levels converter Unbalance grid voltages or harmonics (2 ωbase) 3 ωmod or low frequency ripple due to iNP Three-levels converter The most restrictive! Researching group in Control and Power Electronics Systems SAAEI 2006

13. ΔvP= ΔvN Alcalá University Department of Electronics Calculation of DC-bus capacitors: NPC VSC • Analysing the VSC1 working as active rectifier iDC ripple are exactly the same in CP and CNNO unbalance iNP ripple is absorbed by CP and CN unbalance CDC due to iNP Variation of the voltages in the two banks of capacitors of a NPC in function of iNP If iNP triangular If iNP sinusoidal Worst case Researching group in Control and Power Electronics Systems SAAEI 2006

14. Alcalá University Department of Electronics Calculation of DC-bus capacitors: NPC VSC CDC due to iDC Total ripple  ripple iNP + ripple iDC Negligible Researching group in Control and Power Electronics Systems SAAEI 2006

15. Alcalá University Department of Electronics Calculation of DC-bus capacitors: back-to-back NPC VSC iNPR and iNPI have different frequency  capacitors for each frequency iNPR = iNPI with the same frequency  iNP = 0  The worst situation  iNP=2iNPI Researching group in Control and Power Electronics Systems

16. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

17. Alcalá University Department of Electronics Simulation and experimental results Simulation ModulationTHSPWM Inverter VSC2 Rectifier VSC1 Researching group in Control and Power Electronics Systems SAAEI 2006

18. Alcalá University Department of Electronics Simulation and experimental results Experimental Researching group in Control and Power Electronics Systems SAAEI 2006

19. Alcalá University Department of Electronics Contents • Introduction • Relation between DC-bus capacitors in converters of two-levels and three-levels • INP calculation • Calculation of DC-bus capacitors: • NPC VSC • Back to back NPC VSC • Simulation and experimental results • Conclusions Researching group in Control and Power Electronics Systems SAAEI 2006

20. Alcalá University Department of Electronics Conclusions • The analytical expressions to calculate DC-bus capacitors of a back-to-back three-level NPC converter are not only obtain but also verified through simulations and practical results. • If a NP voltage controller is added, the DC-bus capacitors decrease. • The highest value in CDC is obtained when grid voltage unbalances appearusing controllers is possible decrease this value too ACKNOWLEDGMENT This work has been financied by the Spanish administration (ENE2005-08721-C04-01) Researching group in Control and Power Electronics Systems SAAEI 2006

21. Alcalá University Department of Electronics DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, SantiagoCóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco Huerta, Carlos Girón Department of Electronics. Alcalá University marta.alonso@depeca.uah.es emilio@depeca.uah.es Researching group in Control and Power Electronics Systems SAAEI 2006