With the massive integration of renewable energies, power converters will become increasingly important in the electrical grid of the future. One of the expected consequences is the reduction of system inertia. A promising alternative for this problem is the integration of energy storage elements (ESEs) into modular multilevel converters (MMCs). This paper presents a novel design methodology for a 3-phase MMC with partial integration of energy storage (MMC-PIES). The arms of the proposed converter contain standard submodules without energy storage (SMs) and submodules with an integrated ESE (ES-SMs). ES-SMs with half-bridge (HB) and full-bridge (FB) topologies are explored, while an HB configuration is kept for standard SMs. The methodology includes a minimisation of the amplitude of the circulating current that needs to be injected and a method to calculate the specifications of the converter. Like the results found in the literature, the proposed approach shows that using FB cells demands less ES-SMs. However, it is identified that the capacitances and ESEs must be remarkably bulkier, rendering this topology less attractive. The results also show that the partial integration with HB ES-SMs is a viable solution, causing only a small impact on the SM capacitance and on the ESEs rating.
The introduction of Renewable Energy Sources (RES) and the electricity market liberalization have pointed out the need to upgrade the current electricity transmission network. High Voltage Direct Current (HVDC) transmission system is potentially the key. Recent advances in power electronics have allowed the introduction of power converters in power system applications as the Modular Multilevel Converter (MMC). In the same perspective, grid scale Energy Storage Systems (ESSs) have broadly appeared. The dual purposing to combine both systems can provide a completely new solution. Even though each submodule (SM) within modular converter can have its own ESS, decreasing the number of energy storage submodules (ES-SMs) may be required to reduce the cost. This paper deals with the opportunities to introduce split storage into an MMC. The analysis is focused on the internal energy exchange to maintain the proper function of the converter by using circulating current. Analysis shows that, SM capacitor voltage ripple or semiconductors load are greatly influenced by additional circulating current injection. This study helps to design embedded ESSs in the converter to provide new function like ancillary services for power system operation. Finally, a simulation of an MMC with ESS confirms analytical calculations.
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