A Simple Operation Approach for Modular Multilevel Converter Under Grid Voltage Swell in Medium-Voltage Microgrids

Modular solid-state transformers (SSTs) are a promising technology in converting power from a 10kV three-phase medium voltage to a lower DC-voltage in the range of 100…400V to provide pure DC power to applications such as electrolyzers for hydrogen generation, data centers with a DC power distribution and DC micro grids. Modular SSTs which can be interpreted as modular multilevel converters with an isolated DC-DC output stage per module, are designed with redundant modules to increase reliability. Usually, each of the three arms operates independently, and therefore, only a fixed number of faulty modules can be compensated in each arm, even if all modules are operational in the remaining two arms. With the proposed zero-sequence voltage injection, up to 100% more faulty modules can be compensated in an arm by employing the same hardware. In addition, module power imbalances are nearly eliminated by utilizing a fundamental frequency zero-sequence voltage. A dominant 3rd harmonic zero-sequence voltage injection in combination with the 5th, 7th and several higher order harmonics with adaptive (small) amplitudes minimize the required arm voltages at steady-state. For nominal operation or symmetrical faults, the proposed technique is equivalent to the well known Min-Max voltage injection, which already reduces the peak arm voltage by 13.4% (to $\frac{{\sqrt 3 }}{2}$) compared to a constant star point potential. A statistical analysis proves, that the expected number of tolerable faulty modules of the 1MW SST increases by 12% without the need for additional hardware.

Thermal stresses induced by high root-mean-square (rms) currents in dc-link capacitors significantly affect their lifetimes and compromise the overall reliability of converter systems. This article proposes a novel zero-sequence voltage (ZSV) injection method to reduce the rms current in dc-link capacitors for back-to-back three-level converters. The proposed method adjusts the overlapping intervals of dc bus currents by injecting a differential ZSV on either the rectifier or inverter side, while a common ZSV is injected on both sides to control the neutral point voltage. Simulation and experimental results demonstrate that the proposed method reduces the capacitor rms current by up to 40%–50%, with minimal effect on the total harmonic distortion of the output currents. The proposed method improves system reliability by reducing power losses in the capacitors and extending their operational lifetimes, making it highly suitable for applications that demand both high efficiency and reliability.

Hybrid transmission systems associated with line commutated converters (LLC) and voltage source converters (VSC) have gradually become a particular solution for high voltage direct current (HVDC) transmission. The full-bridge submodule (FBSM) based modular multilevel converter (FB-MMC) has gained significant attention for its dc voltage reversal and fault ride through capabilities. However, large capacitance is essential for high power transmission of FB-MMC, increasing cost and size rapidly. This paper analyzes the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC with zero sequence voltage injection. The design principles of the FBSM capacitance are also presented compared with the one without zero sequence voltage injection under equivalent voltage ripple conditions. Finally, comparative simulation results on a FB-MMC system verify the correctness of the analyses.

High frequency and high amplitude common-mode voltage (CMV) can induce a number of problems on the reliable operations of back-to-back two-level converters, such as shaft voltages and bearing currents. To restrain the CMV for back-to-back two-level converters, a simple and effective CMV reduction method based on zero-sequence voltage (ZSV) injection is proposed in this article. By injecting the optimal amount of ZSV on either the rectifier-side or the inverter-side, the corresponding pulse overlapping areas that result in high CMV amplitudes can be eliminated with the proposed method, then the amplitude of CMV can be reduced by half, and the pulse edges of CMV can be reduced by one-third. Simulation and experimental results are presented to verify the effectiveness of the proposed method in CMV reduction, while some comparisons are also conducted to demonstrate the superiority of the proposed method.

Cascaded H-bridge converter is a promising candidate for large scale grid connected solar PV plant. However, non-uniform power among the phases of the converter occurs due to non-uniform irradiation and temperature of the cells. This causes unbalanced grid currents which is not acceptable according to the standard grid codes. A fundamental zero sequence voltage is therefore injected to balance the grid currents. But, high degree of unbalance in power generation among the phases leads to over modulation in the cells. This can happen during failure of PV modules. To avoid the over modulation, a novel zero sequence voltage is proposed in this paper. It increases the power balancing capability of the converter. The zero sequence voltage injection is implemented through a novel PWM clamping method. For obtaining the novel zero sequence voltage, a control scheme is also presented. The proposed technique is verified through simulation in MATLAB Simulink and through experiment in the lab prototype.

To address the limited speed and scale of the simulation of modular multilevel converter (MMC) in electromagnetic transient simulation program (EMTP), this paper put forward a MMC fast electromagnetic transient model based on rotation transformation. The crucial advantage of the rotation transformation is to reduce the fundamental frequency of the original signal via the rotation transformation of coordinate system. With proposed method implemented in the EMTP of MMC, the simulation step size can be relatively increased, and the fast electromagnetic transient simulation can be achieved. In this paper, the equivalent model of MMC switch function model expressed by the state equation is derived. Based on this, the novel MMC model based on rotation transformation is further derived combined with the concept of rotation transformation. Then, the state equation of proposed novel MMC model is calculated in Matlab, and the simulation results and simulation time-consuming are compared with which of 51-level topological MMC electromagnetic transient model established in PSCAD/EMTDC. The results of comparative analysis show that the simulation duration of the MMC model based on rotation transformation is far less than the traditional MMC electromagnetic transient model without sacrificing accuracy.

The full-bridge submodule (FBSM)-based modular multilevel converter (FBSM-MMC) offers promising performance due to its ability to output negative FBSM voltage. This paper studies this ability of the FBSM-MMC under zero-sequence voltage injection (ZSVI) and second-order harmonic current injection (SHCI). The focus of the research is to redistribute the FBSM powers by injecting an additional power degree of freedom, resulting in a smaller FBSM capacitor voltage ripple, while keeping the maximum AC output voltage for a given fundamental frequency component of the arm voltage reference. Accordingly, a control strategy was developed, based on non-sinusoidal ZSVI, and SHCI is proposed for further improving the performance of the FBSM-MMC. The proposed non-sinusoidal ZSVI contains a higher sinusoidal third-order harmonic component than that of pure sinusoidal third-order voltage injection (THVI) when operating under the same maximum AC output voltage. By implementing this solution, a smaller amplitude of the injected second-order harmonic current can be achieved, producing a lower power loss in the FBSM-MMC. Considering the proposed solution, the relationship of the arm powers and FBSM capacitor voltages are also discussed. Finally, the simulation results and experimental results are presented to verify the effectiveness of the theoretical analysis of the proposed method.

This paper presents the techniques for regulation of system balancing in modular multilevel converter (MMC) based HVDC system under the AC grid double line fault condition. Due to the special structure of MMC, the balancing of the cell capacitor voltage is crucial for any operating circumstance of the system. Also, the responses against the grid faults of HVDC system based on MMC are different to those based on the conventional voltage sourced converters. Therefore, even under the nonpermanent fault in AC grid, the balancing of the cell capacitor energy of MMC has to be guaranteed continuously. Among AC grid fault modes, the double line fault causes one of the most severe transients in MMC. In this paper, a balancing control method of MMC based on an offset voltage injection strategy in the output reference voltages is proposed. By the virtue of the proposed method, the HVDC system based on MMC can have the enhanced fault ride-through capability against the double line fault in AC grid. The validity of the proposed method is verified by simulation results of a ±200kV, 400MVA point-to-point HVDC system.

DC voltage balance based on zero sequence voltage is commonly used for star-connected cascaded H-Bridge (SCHB) STATCOM to compensate negative sequence current. However, zero sequence voltage injection brings about side-effect of increasing STATCOM output voltage and thus the Negative sequence Current Compensation Range (NCCR) is significantly limited to avoid overmodulation. To extend the NCCR, improving the dc voltage utilization by injecting third harmonic component is a typical method. Compared with third harmonic injection (THI), square wave features lower peak-to-average ratio. Unfortunately, the capability of square wave injection (SWI) for the NCCR extension has not been well explored. In this article, to further extend NCCR, not only the SWI is employed for SCHB STATCOM, but also it is compared with THI in term of NCCR by establishing the relationship between the maximum STATCOM voltage and negative sequence current. The comparison results reveal that the THI and SWI both have their own strengths and weaknesses in NCCR, which highly depends on the amplitudes and phases of positive and negative sequence currents. Furthermore, the criteria associated with the amplitudes and phases of the positive and negative sequence currents are derived to determine the harmonic injection method with larger NCCR by using symbolic-graphic combination method and MATLAB tools. As a result, a new harmonic injection method is proposed by selecting the THI or SWI with larger NCCR flexibly according to the amplitudes and phases of positive and negative sequence currents. Finally, the proposed method is verified by experimental results on a 400 V/7.5 kVar SCHB STATCOM prototype.

The parallel three-phase grid-connected converters (GCCs) with common dc and ac buses can increase the system power level effectively. However, zero sequence circulating current (ZSCC) will occur inevitably. In this paper, the mechanisms and characteristics of ZSCC in two parallel three-phase GCCs are first investigated based on three typical cases. From the analysis, it was found that not all ZSCC can be suppressed. Generally speaking, ZSCC caused by inconsistent modulation voltages between modules cannot be effectively handled. And ZSCC caused by different injected zero sequence voltages (ZSVs) or asynchronous carriers can be suppressed. Based on the theoretical analysis, identical ZSV injection and carrier synchronization methods without communication are, thus, proposed to suppress ZSCC at the source, which are easily implemented without adding any hardware. Finally, the theoretical analysis and the effectiveness of the proposed methods on ZSCC suppression are well-verified by experiments.

A significant problem of three-level neutral-point clamped (NPC) converters is the fluctuation of the neutral-point voltage. A SVPWM strategy with neutral point voltage balancing for single phase three-level converters adopted widely in railway traction drive system is proposed in this paper. The relationship between the adopted SVPWM and carrier-based pulse width modulation (CBPWM) with a zero-sequence voltage injection (ZSVI) is analyzed in details. Theoretical analysis shows that SVPWM is a special type of CBPWM-ZSVI methods. Performance of the proposed SVPWM scheme is verified by simulation and experimental test.

Grid emulators provide efficient manners to test grid-connected distributed power generators. They test the power generator such as photovoltaics, wind turbines without hardware damages. Recent studies investigated conventional voltage source inverters such as a two-level voltage source inverter and a three-level voltage source inverter as grid emulators. However, they have limited voltage capabilities and weak transient responses. This paper presents a technique for the control of a modular multilevel converter as a grid emulator in fault scenarios, where voltage sags, harmonics distortions, and frequency deviations are emulated. The steady-state performance in these scenarios is analyzed and compared with that of a two-level voltage source inverter. Energy balancing between the sub-modules in a modular multilevel converter is always a crucial topic in terms of stability, particularly under fault conditions. In this paper, the transient response of the point of common coupling voltage and the sub-module energy in the modular multilevel converter with a delta-wye configuration is analyzed and compared with a two-level voltage source inverter. Simulations of a 24 sub-modules modular multilevel converter verify the control method with S70 V at the dc-link and a 400 V line-to-line ac voltage at the point of common coupling. The modular multilevel converter grid emulator with a delta-wye transformer configuration shows better steady-state performances than a two-level voltage source inverter. The energy stored in each sub-module is well balanced during the fault scenarios, which indicates strong stability.

Submodule (SM) fault diagnosis is necessary for the reliability of modular multilevel converters (MMC) and the complex operating environment of MMC may cause missing data and noise in signal detections, where some of the existing open circuit fault diagnosis methods can be disabled. An artificial intelligence (AI) diagnosis method for open circuit faults in MMC based on extreme gradient boosting system (XG-Boost) is proposed in this paper which can diagnose the open circuit fault quickly and accurately in the mentioned conditions. Compared with neural network diagnosis method, the proposed method has higher accuracy with the same non-ideal data. Faster speed and better processing ability of the proposed method are also validated by simulation of a three-phase MMC in MATLAB/Simulink.

Voltage sag in a power system is an unavoidable power quality issue, and it is also an urgent concern of sensitive industrial users. To ensure the power quality demand and economical operation of the power system, voltage sag management has always drawn great attention from researchers around the world. The latest research that realizes the power quality conditioning has used dynamic voltage restorers (DVRs), static VAR compensator (SVCs), adaptive neuro-fuzzy inference systems (ANFISs), and fuzzy logic controllers based on DVR to mitigate voltage sag. These devices, methods, and control strategies that have been recently used for voltage sag mitigation have some limitations, including high cost, increased complexity, and lower performance. This article proposes a novel, efficient, reliable, and cost-effective voltage sag mitigation scheme based on a modular multilevel converter (MMC) that ensures effective power delivery at nominal power under transient voltage conditions. The proposed method, the MMC, compensates for the energy loss caused by voltage sags using its internal energy storage of the submodules, and ensures reliable power delivery to the load distribution system. Furthermore, control strategies are developed for the MMC to control DC voltage, AC voltage, active power, and circulating current. Detailed system mathematical models of controllers are developed in the dual synchronous reference frame (DSRF). Validation of the results of back-to-back MMC for dynamic load distribution system is analyzed which proves the effectiveness of the proposed scheme for voltage sag mitigation.

FPGA can be used to implement nanosecond step size simulation of Modular Multilevel Converter (MMC). However, the design of the solver limits its application in simulation realization. This paper presents an equivalent model simulation realization. Backward Euler method is used to discretize the bridge arm inductance and the sub-module capacitance. Thevenin’s theorem is used to get the bridge arm equivalent circuit. The method take both the calculation accuracy and the simulation rate of the model into account and the design difficulty of the model solver is reduced. Model solution with the 100ns-step simulation is implemented in the FPGA. Comparing and analyzing the results between primary simulation and hybrid simulation of equivalent model from the effects and errors, two simulation waveforms are highly compatible and the paradigm error is within 0.5%. Moreover, FPGA-based Hardware-in-the-loop can simulate the complete operation process of the MMC switch, which confirms the effectiveness of the method.