Modeling and voltage method for Modular multilevel converter

Modular multilevel converter (MMC) is a promising converter in medium and high voltage applications due to its modularity and easiness to meet any voltage level by cascading the sub-modules. However, large amount of modules not only increase the failure probability of the devices, but also increase the difficulty of the fault diagnosis, which have brought a challenging for popularization and application of the MMC. Therefore, an extended state observer based fault diagnosis strategy for MMC is proposed in this paper. The extended state observer is designed based on the circulating current equation. The output of the extended state observer is compared with the theoretical value calculated by the modified switching models to locate the fault. The simulation is modeled and carried out in Matlab/Simulink. The effectiveness of the proposed strategy is verified by the simulation results.

This paper presents a robust sensor fault reconstruction method for modular multilevel converter (MMC) based on sliding mode observer (SMO). An uncertain system with sensor failure is firstly established by resorting the MMC state space model. The main idea of sensor fault reconstruction is to introduce an appropriate filter and convert the sensor fault into a fictitious actuator fault form by constructing an augmentation system. Robust sliding mode observer is designed with convex optimization theory and linear matrix inequality (LMI) techniques. The sliding mode motion is maintained via equivalent output control method and the fault information is obtained directly to realize the fault information of the sensor. Finally, taking the parameters of the MMC system model as an example, the effectiveness and reliability of the proposed approach are verified by simulation.

The great number of power modules (PMs) imposes great computational challenge on the voltage balancing of modular multilevel converter (MMC) as the sorting of the PM capacitor voltages is usually adopted. This paper presents a distributed control method for PM capacitor voltage balancing in MMC. With this method, all PMs in one arm are grouped and the sorting algorithm of the voltage balancing in each group is implemented in decentralized control unit. To keep the voltage balancing among different groups, a close-loop control system is proposed, using the average capacitor voltage of each group as the feedback. With this method, the computational load is reduced compared to the conventional method while the voltage balancing effect and the switching frequency are almost the same. Real-time experimental results on a ±350 kV/1000 MW MMC based on RT-LAB (OP7020 and OP5607) verify the feasibility and effectiveness of the proposed method.

SummaryThe intrinsic characteristics such as modular structure, filterless operation, and absence of DC‐link capacitor have made the modular multilevel converter (MMC) superior to the conventional multilevel converters. However, the submodule (SM) capacitor voltage balancing is crucial. This paper presents a voltage balance control method for MMC. The proposed method generates 2N + 1 levels (where N is the number of SMs per arm) which halves the step size of output voltage and improves the harmonic performance as compared to N + 1 level generation. It requires only one carrier per phase and no voltage and current measurements are needed for the implementation. Therefore, the proposed method makes the system cost‐effective and reduces the implementation issues related with carrier‐based methods substantially. It also alleviates the non‐uniform charge distribution problem with the phase disposition pulse width modulation (PDPWM) technique. Further, a detailed mathematical analysis is presented which enhances the understanding of balancing algorithm. In addition to this, the circulating current with 2N + 1 modulation is analyzed and a control technique is presented. The proposed voltage balancing method is simulated comprehensively in MATLAB/SIMULINK software and validated by the experimental results obtained from a laboratory prototype.

This paper presents a new converter topology for voltage source converter based high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three level structure and consists of series-connected IGBT bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor stacks. The IGBT bridge stacks are leveraged to generate the output the voltage waveforms and to commute the thyristors. The basic operation principle and the methods to commutate the thyristors are explained in detail. Due to the three level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. The simulation results proved the analysis and verified the feasibility and benefits of the proposed converter topology.

This study proposes a new fault detection method for modular multilevel converter (MMC) semiconductor power switches. While in common MMCs, the cells capacitor voltages are measured directly for control purposes, in this study voltage measurement point changes to the cell output terminal improving fault diagnosis ability. Based on this measurement reconfiguration, a novel fault detection algorithm is designed for MMCs semiconductor power switches. The open circuit and short circuit faults are detected based on unconformity between modules output voltage and switching signals. Simulation and experimental results confirm accurate and fast operation of the proposed method in faulty cell diagnosis. Implementation simplicity, no need to extra voltage measurements and multi faults detection ability are some other advantages of the proposed method.

This article proposes a mission profile-based reliability prediction method for modular multilevel converters (MMCs). It includes key modeling steps, such as long-term mission profile, analytical power loss models, system-level and component-level thermal modeling, lifetime modeling, Monte Carlo analysis, and redundancy analysis. Thermal couplings and uneven thermal stresses among submodules are considered. A case study of a 15-kVA down-scale MMC has been used to demonstrate the proposed method and validate the theoretical analysis. The outcomes serve as a first step for developing realistic reliability analysis and model-based design methods for full-scale MMCs in practical applications.

Fractional frequency transmission system (FFTS) is a promising solution to offshore wind power integration. The back-to-back modular multilevel converter (MMC) and modular multilevel matrix converter (M3C) are universally regarded as two promising ac/ac converter candidates of the next generation. This paper performs a detailed comparison between back-to-back MMC and M3C with special emphasis on their total capacitor and semiconductor ratings. The arm current and capacitor ripples are first analyzed, then comparison results of a 220 kV, 500 MW case is given. In terms of peak-current-based and RMS-current-based total semiconductor ratings the M3C is 42.56 % and 22.97 % larger respectively, while its total capacitor rating is merely 33.15% of the back-to-back MMC. The better candidate can be determined according to the quantitative comparison results and the latest prices of semiconductors and capacitors.

Accurate evaluation of power losses in a modular multilevel converter (MMC) is very important for circuit component selection, cooling system design, and reliability analysis of power transmission systems. However, the existing converter valve loss calculation methods using the nearest level modulation (NLM) method and the traditional sorting-based capacitor voltage balancing strategy are inaccurate since the submodule (SM) switching logics in the MMC arms are uncertain. To solve this problem, the switching principle of the SMs in the sorting-based voltage balancing strategy is analyzed. An accurate MMC power loss calculation method based on the analysis of loss distribution of various SM topologies, including half-bridge submodule (HBSM), full-bridge submodule (FBSM) and clamp double submodule (CDSM), is proposed in this paper. The method can accurately calculate the losses caused by the extra switching actions during the capacitor voltage balancing process, thus greatly increasing the calculation accuracy of switching losses compared with existing methods. Simulation results based on a practical ± 350 kV/1000 MW MMC-HVDC system with variety of MMC topologies with different voltage balancing strategies demonstrate the effectiveness of the proposed method.

To implement the STATCOMs for transmission and distribution applications, high power converters are needed. A multilevel converter is a logical choice for such applications as it can meet the imposed line side current and voltage harmonic distortion standards without extending its device ratings. Several types of monolithic multilevel converters have been proposed based on different structures of a DC link voltage (or current) to generate staircase output voltage (or current) levels. The multilevel current source converters (CSC) are also a potential configuration for STATCOM. Multilevel CSCs using fully controllable switches have the advantages of low output harmonics and controllable active and reactive power. During the last decade, modular multilevel converters have shown a breakthrough and have made their way to commercial high power applications. Modularity, in general, refers to a technique to develop comparably large systems by combining smaller subsystems. For power converter topologies, this means, a cascaded connection of converter cells seems to be an interesting solution to reach high voltage and high quality waveforms with a minimum complexity. This chapter provides an overview on monolithic and modular multilevel converter topologies, targeting high-power applications and in particular STATCOM. Modular multilevel converters can be built either straightforward by applying the modular building block cells or by a combination of cells with monolithic multilevel topologies. The monolithic and modular CSCs can also be realized by employing the duality concept on each corresponding Voltage Source Converter (VSC). This chapter summarizes the most recent developments made for multilevel converter topologies, by covering new promising topologies and operational issues. In addition, emerging trends, challenges, and possible future directions of the multilevel converter technologies are outlined.

ABSTRACTThe modular multilevel converter (MMC) has attracted significant attention in the fields of high and medium‐voltage applications in recent years. Traditional indirect finite control set model predictive control (IFCS‐MPC) strategies for MMC require voltage measurements from each submodule (SM) for the voltage control loop. As a result, a considerable number of sensors is required to achieve energy balancing, leading to increased circuit complexity and higher hardware costs. This paper proposes an IFCS‐MPC method for MMC with a reduced number of sensors approach. In the approach, an SM capacitor voltage observer is designed based on the discrete‐time model of capacitor voltage, utilizing the measured arm current and SM switching state. Only one SM voltage sensor is needed per arm, which is used to correct the observed voltage errors caused by the discrete‐time model and the accuracy of the parameters. Subsequently, the observed values of the SM capacitor voltages are substituted for the measured values in the voltage sorting algorithm of IFCS‐MPC to reduce voltage sensors. The simulation and experiment results are carried out to demonstrate the effectiveness of the proposed control system and the feasibility of reducing the number of sensors.

Modular multilevel converters (MMCs) have become one of the most attractive topologies for high-voltage and high-power applications. A double-carrier phase disposition pulse width modulation (DCPDPWM) method for MMCs is proposed in this paper. Only double triangular carriers with displacement angle are needed for DCPDPWM, one carrier for the upper arm and the other for the lower arm. Then, the theoretical analysis of DCPDPWM for MMCs is presented by using a double Fourier integral analysis method, and the Fourier series expression of phase voltage, line-to-line voltage and circulating current are deduced. Moreover, the impact of carrier displacement angle between the upper and lower arm on harmonic characteristics is revealed, and further the optimum displacement angles are specified for the circulating current harmonics cancellation scheme and output voltage harmonics minimization scheme. Finally, the proposed method and theoretical analysis are verified by simulation and experimental results.

Accurate and efficient evaluation of losses in modular multilevel converters (MMC) is of great significance for the design and reliable operation of high‐voltage direct current transmission systems. In this paper, an efficient online loss evaluation method for MMC is proposed. First, a switching frequency surface is constructed based on the determined modulation and capacitor voltage balancing strategy, which enables the estimation of the switching frequency under any steady‐state operating condition. Then, an arm average value model (AAVM) is employed to estimate the upper bounds of the switching frequency and associated switching losses. Moreover, combined with the switching frequency surface, an approximate and efficient estimation of the actual switching losses is achieved, which ensures a balance between computational accuracy and efficiency. In addition, the thermal network model is employed to represent the temperature‐dependent characteristics of losses. Considering the impact of losses on simulation results, the energy absorbed by a controlled voltage source in series with the arm is used to characterize the losses. Subsequently, a switching loss injection method based on a decay function is proposed, which mitigates voltage spikes and enhances the effectiveness of loss injection. Finally, an AAVM considering loss injection is developed in PSCAD to verify the computational efficiency and accuracy of the proposed method.

This paper proposes a novel fault-tolerant control method for modular multilevel converter (MMC) with an improved level-shifted carrier PWM, based on the concept of virtual voltage. First, an improved phase disposition level-shifted (PD-LS) PWM with only one carrier in each arm is adopted. Then, based on the virtual voltage idea, a simplified control method is proposed to realize fault-tolerant operation without carriers reconfiguration. Compared with traditional fault-tolerant control methods, due to the reduced carriers generation and decreased comparisons between carriers and modulation waves, the modulation process of the proposed scheme can be dramatically simplified during both the pre-fault and the post-fault process. The effectiveness of the proposed method is verified by simulation and experiment results.

In this paper a generalized control approach for modular multilevel converters is applied to a converter connecting a three-phase and a five-phase system directly without any central DC link. This new 3 × 5 modular multilevel matrix converter offers an alternative to other multilevel converters applied to five-phase machines, but foremost acts as example to prove the generality of the control. The cascaded control structure consists of an inner current control and an outer branch energy balancing control. Both are systematically derived and verified by simulation.