Abstract
The cross regulation effect in multi-output DC/DC converters offers a reliable support for the grid integration of multilevel inverters by balancing the capacitor voltages. The capacitor voltage balancing by single input dual output boost converter is often realised by conventional three-level switching scheme. The three-level operation benefits lower inductor ripple current, but it limits the maximum possible compensation voltages. In this study, the entire operating modes of the boost converter is presented and all the possible cases which contribute to the voltage balancing are employed for balancing the capacitor voltages in a three-level neutral point clamped inverter. A proportional-integral controller based duty ratio control and pulse delay control are used for DC link voltage regulation and capacitor voltage balancing. Since the classical state-space averaging technique is not suitable for SIDO converters, inductor current ripple averaging technique is utilised for controller design. The circuit simulation is performed in Matlab/Simulink. The digital controller is realised using the Virtex-5FPGA in Labview/CompactRIO module. Both simulation and experimental results are presented to validate the controller performance.
Highlights
The increased grid integration of renewable energy resources, especially from offshore platforms, demands power transmission at high voltage to limit the current and thereby, associated losses
If the Pulse delay control (PDC) begins after the boost control and especially the NP voltage is higher than the achievable compensation voltage for that particular duty ratio, high voltage oscillation occurs at the NP voltage and that reflects on the duty ratio as well
If the NP voltage is less than the possible compensation voltage at specific duty ratio, the PDC brings the capacitor voltages to equilibrium without any oscillation
Summary
The increased grid integration of renewable energy resources, especially from offshore platforms, demands power transmission at high voltage to limit the current and thereby, associated losses. PDC technique is based on the dynamic variation of pulse delay to compensate the capacitor voltages, which results different cases, including either two, three or four modes of operation of the SIDO boost converter. The authors extend the operation of the converter to eight different cases to compensate the neutral voltage imbalance where the delay varies from zero to the switching period, T. This paper discusses the control and implementation of the SIDO boost converter for DC voltage boosting and PDC based NP voltage balancing It operates as front-end of a three-level NPC inverter. The simulation and experimental results are provided to validate the DC link and NP voltage control techniques
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