DC-BUS capacitor rating of the back-to-back NPC converters Alcalá University Department of Electronics 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 cobreces@depeca.uah.es emilio@depeca.uah.es Researching group in Control and Power Electronics Systems EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

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 EPE-PEMC 2006

Introduction DC-bus capacitors Efects of unbalance 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 EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

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) Calculos sin control La expresion para el convertidor trabajando en dos niveles es en el caso de emplearlo para compensar desequilibrios en una carga de secuencia negativa a potencia nominal CDC≥537.5μF ΔuDC 10%uDC Sn 100kVA TSW 400μs Udc 610V Rectifier Very conservative! Filter CDC≥4277.2μF Researching group in Control and Power Electronics Systems EPE-PEMC 2006

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 EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

INP Calculation SVPWM: ma=1 and DPF=0 SPWM: ma=1 and DPF=0 THSPWM 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 EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

Calculation of DC-bus capacitors 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 ω mod or low frequency ripple due to iNP Three-levels converter The most restrictive! Researching group in Control and Power Electronics Systems EPE-PEMC 2006

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

Calculation of DC-bus capacitors: NPC VSC 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 EPE-PEMC 2006

Calculation of DC-bus capacitors: back-to-back NPC VSC 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 have different frequency  iNP = 0  The worst situation  iNP=2iNPI La corriente iNP que circula por los condensadores y que produce el desbalanceo tiene este valor, donde inpr es la corriente debida al rectificador e inpi la debida al inversor. Si estas dos corriente tienen diferentes frecuencias el cálculo de los condensadores se hace para cada una de ellas. Researching group in Control and Power Electronics Systems EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

Simulation and experimental results Alcalá University Department of Electronics Simulation ModulationTHSPWM Fsw 2.5kHz Cp =CN 1000μF Ri 0.075Ω LI 0.75mH fbase 25 Hz Ubase 400V 50 Hz Inverter VSC2 Objetivo: compara los resultados teóricos y prácticos Rectifier VSC1 Researching group in Control and Power Electronics Systems EPE-PEMC 2006

Simulation and experimental results Alcalá University Department of Electronics Experimental Objetivo: compara los resultados teóricos y prácticos Researching group in Control and Power Electronics Systems EPE-PEMC 2006

Contents Introduction 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 EPE-PEMC 2006

Conclusions Conclusions Acknowledgment Alcalá University Department of Electronics Conclusions Conclusions Acknowledgment Researching group in Control and Power Electronics Systems EPE-PEMC 2006