Cell capacitor sizing in modular multilevel converters and hybrid topologies

Abstract

This paper presents a method to calculate the minimal size of cell capacitors in multilevel VSCs which meets a maximum voltage deviation criterion under ideal conditions. This method is applied to the Modular Multilevel Converter (MMC), the Alternate Arm Converter (AAC) and the hybrid multilevel converter with ac-side cascaded H-bridge cells (AC-CHB). The results show that the newer VSC topologies exhibits smaller energy deviation in their stacks, leading to an overall smaller volume of cell capacitors for the converter station but often accompanied by some compromises such as higher power losses or degraded DC current waveform quality.

Performance comparison of phase shifted PWM and sorting method for modular multilevel converters

“…In order to evaluate the modulation methods an MMC model with low number of SMs (4 SMs per arm) and a model with high number of modules (40 SMs per arm) has been built in simulation software named PLECS. The SM capacitance for the two cases has been chosen based on [11]:…”

Augmented inertial response of Multi-Level Converters using internal energy storage

“…This action allows the converter to store energy. The energy content of an MMC is small and varies according to the rating of the converter, on average it is 30-40 kJ/MVA [12], [13].…”

Comparative Analysis of an MV Neutral Point Clamped AC-CHB Converter With DC Fault Ride-Through Capability

“…The hybrid MMC contains a combination of FB and HB SMs and has been shown to provide an improved efficiency compared to the FB-MMC whilst still providing DC fault ride-through [6]. However, all MMC topologies require a high level of energy storage, in the form of SM capacitors, to minimize SM voltage ripple [12]; this results in a large penalty in terms of volume and cost. The AAC uses director switches to alternate the conduction period of each arm resulting in a reduced number of SMs [8] and lower energy storage requirement compared to the MMC [12], whilst also being able to provide DC fault ride-through.…”

“…However, all MMC topologies require a high level of energy storage, in the form of SM capacitors, to minimize SM voltage ripple [12]; this results in a large penalty in terms of volume and cost. The AAC uses director switches to alternate the conduction period of each arm resulting in a reduced number of SMs [8] and lower energy storage requirement compared to the MMC [12], whilst also being able to provide DC fault ride-through. The 2L-AC-CHB consists of a slow switching two-level (2L) main bridge followed by a stack of cascaded H-bridge SMs on the AC side which provide the required harmonic elimination.…”