Assessment of the Impact of Split Storage within Modular Multilevel Converter

Abstract

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.

Blackstart Capability of Modular Multilevel Converters From Partially-Rated Integrated Energy Storage

“…An interesting approach is investigated in [14]- [16], where the authors study MMCs with unbalanced distributed ES. This arrangement could facilitate standalone converter start-up, however it requires constant energy balancing during normal operation to counteract the asymmetric power contribution from different ES units and also affects the semiconductor ratings owing to the extra current flowing in the arms.…”

Cost-performance framework for the assessment of Modular Multilevel Converter in HVDC transmission applications

“…To identify the number of parallel capacitors required in the submodule bank of capacitors, , an expression of the rms current is required. The SM capacitor bank will be subjected to currents with frequencies at the line frequency and double the line frequency [33]. The rms value can be found by evaluating (8).…”

A Submodule With Integrated Supercapacitors for HVDC-MMC Providing Fast Frequency Response

“…In this context, modular multilevel systems have received a significant interest [6]- [8]. Instead of using a common DC bus, the ESS can be freely distributed among the submodules (SMs) [9] and controlled independently at a converter level. The first publications about modular structures with ESS focus on Cascaded H-Bridge Converter (CHB) and Modular Multilevel Converter (MMC) topologies but solely for the purpose of connecting an ESS to the grid at medium voltage level [10]- [14].…”