A New Scheme for Nearest Level Control with Average Switching Frequency Reduction for Modular Multilevel Converters

Abstract

This paper proposes a new NLC (Nearest Level Control) scheme for MMCs (Modular Multilevel Converters), which offers voltage ripple reductions in the DC capacitor of the SM (Sub-Module), the output voltage harmonics, and the switching losses. The feasibility of the proposed NLC was verified through computer simulations. Based on these simulation results, a hardware prototype of a 10kVA, DC-1000V MMC was manufactured in the lab. Experiments were conducted to verify the feasibility of the proposed NLC in an actual hardware environment. The experimental results were consistent with the results obtained from the computer simulations.

A Simple Staircase Modulation for A Cascaded H-bridge Multilevel Inverter

“…The nearest level to the reference voltage is selected. The output voltage V will be a staircase voltage with a minimum possible error with respect to the reference voltage V * [27][28][29][30][31][32]. The nearest output voltage level V is calculated by:…”

Offset Voltage Control Scheme for Modular Multilevel Converter Operated in Nearest Level Control

“…To implement the space vector modulation, the absolute values of maximum and minimum pole voltage should be identically set as Equation (18). In this way, the offset voltage is defined as Equation (19).…”

“…The pole voltage which is the sum of the phase voltage and the offset voltage can be decided by the switching status of SM on each phase of MMC. (17) To implement the space vector modulation, the absolute values of maximum and minimum pole voltage should be identically set as Equation (18). In this way, the offset voltage is defined as Equation (19).…”

“…This method does not support equal charging and discharging for each SM during one power cycle. So, the voltage balancing algorithm for the SM capacitors is required [17][18][19][20].…”

“…Total harmonic distortion (THD) is rather high when the operation level is low. In particular, the MMC in MVDC can be more influenced due to the small number of SMs [18,19]. This paper proposes a new offset voltage control scheme for the MMC operated in NLC to improve the THD of the output voltages and to generate linear phase voltages in the over-modulation region as well as the normal-modulation region.…”