Overview on Reliability of Modular Multilevel Cascade Converters

Multi-level converters have been used extensively in modern industry which calls for energy conversion with high-power and high-or medium-voltage.Because of its modularity and scalability,the multi-level converter with modular structure can be extended to different voltage levels and has a variety of forms in practical applications.It has attracted much attention from academia in the past decade,however,as a result of the numerous vulnerable power electronics sub-modules,significant challenges remain with regards to reliability.After summarizing the current research status of modular multilevel cascade converters,the main issues of reliability are reviewed in the paper.Firstly,the failure cases are thoroughly surveyed and classified,and the main failure causes are analyzed.Secondly,the reliability evaluation methods are reviewed and applied to the modular multilevel cascade converters.Thirdly,some promising measures to improve the reliability are presented and discussed,including parameter selection,redundancy design,fault-tolerant control and so on.Then,a complete reliability-oriented design procedure for the modular multilevel cascade converters is proposed.Finally,the challenges and opportunities to improve the reliability are concluded.

A distributed VSG control method for a battery energy storage system with a cascaded H-bridge in a grid-connected mode

With the high penetration of renewable energy,new challenges,such as power fluctuation suppression and inertial support capability,have arisen in the power sector.Battery energy storage systems play an essential role in renewable energy integration.In this paper,a distributed virtual synchronous generator(VSG)control method for a battery energy storage system(BESS)with a cascaded H-bridge converter in a grid-connected mode is proposed.The VSG is developed without communication dependence,and state-of-charge(SOC)balancing control is achieved using the distributed average algorithm.Owing to the low varying speed of SOC,the bandwidth of the distributed communication networks is extremely slow,which decreases the cost.Therefore,the proposed method can simultaneously provide inertial support and accurate SOC balancing.The stability is also proved using root locus analysis.Finally,simulations under different conditions are carried out to verify the effectiveness of the proposed method.

Stability-improvement Method of Cascaded DC-DC Converters with Additional Voltage-error Mutual Feedback Control

The interaction between the source and load converters in cascaded DC-DC converters may cause instability.Thus,improving the stability of cascaded DC-DC converters is important.To solve the above-mentioned problem,a flowchart to improve the control method is established by calculating the eigenvalue sensitivity of a time-domain model of cascaded DC-DC converters.Further,an additional voltage-error mutual feedback control method is firstly proposed based on the flowchart provided in this study to improve the stability of cascaded DC-DC converters.Subsequently,the influence of the proposed mutual feedback control on the stability of cascaded DC-DC converters is analyzed.Finally,the effectiveness of the proposed control method is verified by simulation and experiment.

Inter-cluster Voltage Balancing Control of Modular Multilevel Cascaded Converter Under Unbalanced Grid Voltage

This paper presents a novel inter-cluster direct current(DC)capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter(MMCC)-based static synchronous compensator(STATCOM)under unbalanced grid voltage.The negative-sequence component of grid voltage at the point of common connection(PCC)causes unbalanced active power flow in the phase limbs of converter.This leads to the imbalance of DC voltages of the sub-module capacitors across the MMCC phases,and consequently,the malfunction of converter.The proposed solution is to inject both negative-sequence current(NSC)and zero-sequence voltage(ZSV)into the phase limbs of MMCC.A quantification factor Qf is used to achieve the sharing of inter-cluster active pow-er between the NSC and ZSV injection methods.Accurate determination of the quantification factor has been presented.In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced,it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion.The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STATCOM with 3-level bridge cells as sub-modules.The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.

Finite-time robust control for transformerless H-bridge cascaded STATCOM with star configuration

This article presents a finite-time robust control(FTRC)of a transformerless STATCOM based on a cascaded multilevel H-bridge converter(CMHC)with star configuration.The FTRC is first proposed for the current loop control of a CMHC-based transformerless STATCOM by using the finite time robust control theory.Taking the parameters,perturbations and external disturbances into account and using coordinate transformation method,the nonlinear dynamic model of the CMHC-based transformerless STATCOM is transformed into a standard nonlinear port-controlled dissipative Hamiltonian(PCDH)structure.Based on the PCDH structure,an FTRC is designed for the CMHC-based transformerless STATCOM to improve the transient stability and oscillation damping of power system.Finally,the simulation results demonstrate that the FTRC has better dynamic performance and strong robustness in comparison with the passivity-based control of the CMHC-based transformerless STATCOM.