Abstract
The series-connection of modules in multilevel converters are prone to energy imbalances in the dc capacitor due to the differences between the power absorbed and consumed. In renewable energy applications where the primary source is directly connected to each module, energy imbalances can be even worse if the primary sources are affected by unpredictable weather conditions. Therefore, control strategies are required to compensate such energy imbalances, while maintaining the correct converter operation. Focusing our attention on a cascaded transformer multilevel inverter called Isolated Multi-Modular Converter, this paper introduces the combination of two control strategies aimed at providing a wide range of operation under imbalanced energy states. A general analytical model, including the regulation capability and differences with an existing strategy are presented to demonstrate the performance of the control proposed. The effectiveness of the proposal is validated through experimental results based on a three-phase multilevel prototype.
Highlights
Cascaded multilevel converters based on H-bridge (CHB) configurations are among the most suitable multilevel topologies for large-scale photovoltaic (PV) power plants [1]–[4]
The error between the voltage reference given by a maximum power point tracking (MPPT) algorithm or any other reference and the voltage measurement is processed through a PI controller
EXPERIMENTAL RESULTS In an attempt to clarify and validate the Isolated Multi-Modular Converter (IMMC) working under energy imbalances, a 4.8 kW prototype with two modules per arm is tested with both control strategies
Summary
Cascaded multilevel converters based on H-bridge (CHB) configurations are among the most suitable multilevel topologies for large-scale photovoltaic (PV) power plants [1]–[4]. Cascaded transformer multilevel inverters (CTMI) are some of these configurations [7]–[13] They consist of several H-bridge modules connected in parallel to the same dc source, while the ac side is connected to low-frequency transformers. In order to control power imbalances between modules of the same phase, a capacitor voltage balancing is proposed in [22] This strategy phase shifts the modulated voltage according to the dc voltage level. Power imbalances in the IMMC have been controlled based on the amplitude voltage compensation [17] This control strategy was previously proposed for a cascaded H-Bridge multilevel converter in [22], [23] and for a cascaded T-Type configuration in [24].
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