Interlinking Converter Control Techniques to Improve Power Quality in Hybrid AC-DC Microgrids

Abstract

Hybrid AC-DC microgrids (HMGs) have been studied recently in order to simultaneously exploit AC and DC microgrids (MGs). This dissertation develops control techniques for interlinking converter (IC) which is utilized to link MGs in HMGs. With the proposed control techniques, IC simultaneously manages various AC and DC voltage quality issues along with power flow among MGs. In islanded HMGs, where utility grid is not considered, power quality is seriously degraded with highly distorted bus voltage when nonlinear loads are applied. In order to obtain sinusoidal bus voltage, we propose an IC control scheme which maintains bus voltage by providing harmonic currents caused by the nonlinear load. Moreover, the active power between DC and AC distributed generations (DGs) is also accurately balanced by IC. In the proposed algorithm, the desired IC current is determined by only considering the IC terminal voltage instead of the load current. Consequently, power converters in HMGs are operated autonomously with low system cost because communication links among them are not needed. When AC MG connects to the utility grid, HMGs become grid-connected HMGs. In grid-connected HMGs, we have considered different conditions such as: the abnormal grid voltage, the utilized nonlinear load and the wide variation of DC load power. A versatile IC control strategy is proposed in order to enhance HMGs power quality under such severe conditions. Without implementing any additional hardware compensator, IC with the proposed control scheme can mitigate various issues such as distortion, sag/swell and imbalance in AC voltage and DC bus voltage restoration despite of wide load power variation. Both the desired power and the current among MGs are provided by IC since IC current references are generated by only detecting buses voltages. Therefore, AC bus voltage is balanced and sinusoidal with proper magnitude while DC bus voltage is constantly kept at the nominal voltage. In grid-connected HMGs, DC bus voltage is highly ripple when single-phase inverter with variable frequencies (SPI-VF) is operated in DC MG. In order to eliminate the ripple voltage, we present an IC control strategy which can adaptively deal with ripple voltage at different frequencies by controlling IC ripple current. In the proposed approach, instead of SPI current/voltage, only DC bus voltage is required to generate the IC ripple current reference. Besides that, power among MGs is also balanced by controlling IC fundamental current. As a consequence, AC MG can support DC MG in case DC load power changes widely. And DC bus voltage is well maintained at the nominal voltage without ripple. Simulation and experimental results are carried out to validate performance of the proposed IC control techniques.