DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco Huerta, Carlos Girón Department of Electronics. Alcalá University SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

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 SAAEI 2006 Alcalá UniversityDepartment of Electronics Critical Design DC-bus capacitors Researching group in Control and Power Electronics Systems

Introduction DC-bus capacitors Efects of unbalance Aims of this presentation SAAEI 2006 Alcalá UniversityDepartment of Electronics Same factors of two-levels topology Low frequency ripple due i NP  unbalance 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 i NP on a back-to-back converter Relating the ripples due to i NP e i DC Obtaining analytical equations for each ripple of a back-to-back converter Researching group in Control and Power Electronics Systems

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

Relation between DC-bus capacitors in two-levels and three-levels converters Two-levels converter –Working as PWM rectifier  Δu DC  Maximum allowed ripple T SW  IGBTs switching period –Working as active filter  SAAEI 2006 Alcalá UniversityDepartment of Electronics (These expressions are valid for VSC connected to the grid with passive an active load) Δu DC 10%u DC SnSn 100kVA T SW 400μs U dc 610V C DC ≥537.5μF C DC ≥4277.2μF Very conservative! Rectifier Filter Researching group in Control and Power Electronics Systems

Relation between DC-bus capacitors in two-levels and three-levels converters SAAEI 2006 Alcalá UniversityDepartment of Electronics Three-levels converter –C DC dependes on the low frequency ripple due to the NP current –f iNP = 3·f m –Dynamic circuit of NP connection: Assumptions: –Each converter independently controlled –C P =C N =2C DC –Only fundamental harmonics are considered Researching group in Control and Power Electronics Systems

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

I NP Calculation SVPWM: m a =1 and DPF=0 SPWM: m a =1 and DPF=0 SAAEI 2006 Alcalá UniversityDepartment of Electronics THSPWM –Under the same condition the third order component is relatively smaller Researching group in Control and Power Electronics Systems

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

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

Calculation of DC-bus capacitors: NPC VSC Analysing the VSC1 working as active rectifier SAAEI 2006 Alcalá UniversityDepartment of Electronics i DC ripple are exactly the same in C P and C N  NO unbalance i NP ripple is absorbed by C P and C N  unbalance Δv P = Δv N Variation of the voltages in the two banks of capacitors of a NPC in function of i NP If i NP triangular If i NP sinusoidal Worst case C DC due to i NP Researching group in Control and Power Electronics Systems

Calculation of DC-bus capacitors: NPC VSC SAAEI 2006 Alcalá UniversityDepartment of Electronics C DC due to i DC Total ripple  ripple i NP + ripple i DC Negligible Researching group in Control and Power Electronics Systems

Calculation of DC-bus capacitors: back-to-back NPC VSC Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems i NPR and i NPI have different frequency  capacitors for each frequency i NPR = i NPI with the same frequency  i NP = 0  The worst situation   i NP =2i NPI 

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

Simulation and experimental results Fsw2.5kHz C p =C N 1000μF RiRi 0.075Ω LILI 0.75mH f base 25 Hz U base 400V RiRi 0.075Ω LILI 0.75mH f base 50 Hz U base 400V SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems Inverter VSC2 Rectifier VSC1 Modulation  THSPWM Simulation

Simulation and experimental results SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems Experimental

Contents Introduction Relation between DC-bus capacitors in converters of two-levels and three-levels I NP calculation Calculation of DC-bus capacitors: –NPC VSC –Back to back NPC VSC Simulation and experimental results Conclusions SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems

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 C DC is obtained when grid voltage unbalances appear  using controllers is possible decrease this value too SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems ACKNOWLEDGMENT This work has been financied by the Spanish administration (ENE C04-01)

DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco Huerta, Carlos Girón Department of Electronics. Alcalá University SAAEI 2006 Alcalá UniversityDepartment of Electronics Researching group in Control and Power Electronics Systems