Unbalanced voltage across the dc-link capacitors of classical diode clamped multilevel inverters (DCMI) generates lower order harmonics in the output voltage and increases the voltage stress on the switching devices which may result in permanent damage to the switching devices. This study proposes a space vector modulation (SVM) based voltage balancing strategy for a new 5-level multiple-pole multilevel diode-clamped inverter (M2DCI) topology to eliminate the voltage drift phenomena. The 5-level M2DCI topology which is derived from 3-level DCMI topology uses lesser number of clamping diodes compared with the conventional 5-level DCMI topology. An effective switching function model of a new multilevel inverter is derived and is used for dc-link capacitor voltage control. The method utilises the redundant vector property to balance the dc-link capacitor voltages without using any auxiliary hardware. The dependence of the capacitor voltage variation on the load power factor and modulation index has been extensively studied for 5-level M2DCI. The range of operation for the new topology with the proposed control strategy is also presented based on the simulation studies in the Matlab/Simulink ® and PSIM environment and verified using experimental results.
Low-voltage ride-thorough capability is among the challenges in the operation of medium-and large-scale gridconnected photovoltaic power plants (PVPPs). In addition, reactive power injection during voltage sags is required by power system operators in order to enhance the voltage of the point of common coupling. The performance of medium-and largescale grid-connected PVPPs during these events is studied. An algorithm for the calculation of current references, in the dqframe, during voltage sags is introduced, which considers the inverter current limitation, grid code requirements and the amount of extracted power from photovoltaic strings. The proposed algorithm uses the full current capacity of the inverter in injecting active or reactive powers to the grid during voltage sags, which leads in a better grid voltage enhancement. The performance of proposed control strategies is investigated on a 150-kVA PVPP connected to the 12.47-kV medium-voltage test-case system simulation model during different fault conditions. An experimental setup of the 3.3-kVA grid-connected three-level neutral-pointclamped inverter with a dc/dc converter illustrates and validates the performance of the controller in injecting required active/ reactive power and supporting the network voltage.
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