Coordinated Capacitor Voltage Balancing Method for Cascaded H-bridge Inverter with Supercapacitor and DC-DC Stage

Cascaded H-bridge inverter(CHBI) with supercapacitors(SCs) and dc-dc stage shows significant promise for medium to high voltage energy storage applications. This paper investigates the voltage balance of capacitors within the CHBI, including both the dc-link capacitors and SCs. Balance control over the dc-link capacitor voltages is realized by the dcdc stage in each submodule(SM), while a hybrid modulation strategy(HMS) is implemented in the H-bridge to balance the SC voltages among the SMs. Meanwhile, the dc-link voltage fluctuations are analyzed under the HMS. A virtual voltage variable is introduced to coordinate the balancing of dc-link capacitor voltages and SC voltages. Compared to the balancing method that solely considers the SC voltages, the presented method reduces the dc-link voltage fluctuations without affecting the voltage balance of SCs. Finally, both simulation and experimental results verify the effectiveness of the presented method.

Model Predictive Control for Cascaded H-Bridge PV Inverter with Capacitor Voltage Balance

We designed an improved direct-current capacitor voltage balancing control model predictive control(MPC)for single-phase cascaded H-bridge multilevel photovoltaic(PV)inverters.Compared with conventional voltage balanc-ing control methods,the method proposed could make the PV strings of each submodule operate at their maximum power point by independent capacitor voltage control.Besides,the predicted and reference value of the grid-connected current was obtained according to the maximum power output of the maximum power point tracking.A cost function was con-structed to achieve the high-precision grid-connected control of the CHB inverter.Finally,the effectiveness of the proposed control method was verified through a semi-physical simulation platform with three submodules.

FCS-MPC for four-switch three-phase AC-DC converters with DC-link capacitor voltage balancing

Due to the decrease in the number of switches for the four-switch three-phase alternating current-direct current(FSTP AC-DC)converter,it can easily lead to DC-link capacitor voltage imbalance and the system stability reduction.In order to solve these problems,a finite control set model predictive control(FCS-MPC)for FSTP AC-DC converters with DC-link capacitor voltage balancing is proposed.In this strategy,in order to facilitate calculation,theαβcoordinate system model is established and all voltage vectors are evaluated by establishing a cost function.During the whole process,phase locked loop(PLL)and complex modulation strategy are not required.In the new established cost function,the additional objective term of suppressing capacitor voltage fluctuation is to eliminate effectively the capacitor voltages oscillations and deviations and improve the system reliability.The simulation results show that the proposed strategy can keep the capacitor voltage balancing and has good dynamic and static performance.

Limits of Uneven Battery Power in Modular Multilevel Converter-based Battery Energy Storage Systems

For modular multilevel converter-based battery energy storage systems (MMC-BESS), uneven power among batteries of SMs will be deduced by battery aging, battery fault, etc., which will degrade performance and even lead to system failure. For maintaining the balance of capacitor voltage, this paper analyzes the limits of the uneven power of batteries, which are presented as the current limits in this paper. According to analysis, an analytical method is proposed that only the dc and fundamental frequency components of the arm current are used to calculate current limits. With the current limits it is able to evaluate the reasonability of power distribution among batteries. Meanwhile, increase of fundamental frequency component will enhance the current limits, and the dc component determines the size relationship between the absolute values of the upper and lower current limits. Finally, simulation model and experiment prototype are built for verifying the theoretical analysis and current limits calculation method, and satisfactory results are provided.

Capacitor Voltage Balancing Strategy Based on Sub-module Capacitor Voltage Estimation for Modular Multilevel Converters

The modular multilevel converter(MMC)is expected to be used extensively in high-voltage direct current(HVDC)transmission networks because of its superior characteristics over the line-commutated converter(LCC).A key issue of concern is balancing sub-module capacitor voltages in the MMCs,which is critical for the correct operation of these converters.The majority of voltage balancing techniques proposed thus far require that the measurement of the capacitor voltages use a reliable measuring system.This can increase the capital cost of the converters.This paper presents a voltage balancing strategy based on capacitor voltage estimation using the adaptive linear neuron(ADALINE)algorithm.The proposed estimation unit requires only three voltage sensors per phase for the arm reactors and the output phase voltages.Measurements of sub-module capacitor voltages and associated communication links with the central controller are not needed.The proposed strategy can be applied to MMC systems that contain a large number of sub-modules.The method uses PSCAD/EMTDC,with particular focus on dynamic performance under a variety of operating conditions.

Overview on Submodule Topologies,Modeling,Modulation,Control Schemes,Fault Diagnosis,and Tolerant Control Strategies of Modular Multilevel Converters

In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.

Modulation and Voltage Balancing Control of Dual Five-level ANPC Inverter for Ship Electric Propulsion Systems

Open-end winding motors are used extensively in ship electric propulsion systems,in which medium-voltage high-power inverters are a critical component.To increase the system voltage and power density,a dual five-level active neutral-point clamped(ANPC)inverter is proposed herein to drive medium-voltage open-end winding motors for ship electric propulsion.Each phase of this inverter comprises two five-level ANPC bridges and all the phases are powered by a common direct-current link.A hybrid modulation method is proposed to control this inverter.The series-connected switches in all the five-level ANPC bridges are operated at the fundamental frequency,and the other switches are controlled with a phase-shifted pulse-width modulation(PWM),which can achieve a natural balance between the neutral-point voltage and flying capacitor voltages in a carrier period.A closed-loop capacitor voltage balancing method based on adjusting the duty ratios of the PWM signals is proposed.The neutral-point voltage and flying capacitor voltages can be controlled independently and balanced without affecting the output phase voltage.Simulation and experimental results are presented to demonstrate the validity of this method.