The modular multilevel converter (MMC) has become popular for medium-voltage applications. During MMC operation, unavoidable circulating currents flow through its arms and cause extra losses in the converter system. To generate a reference for circulating current, different current injection methods to control the circulating current have been reported. The conventional current injection techniques used so far relies on a mathematical model that excludes the effect of arm inductor voltages in its design. For high-voltage applications with many submodules (SMs), the above exclusion does not make much difference. But for medium-voltage applications with few SMs, the applied arm inductor voltages may be significant for a given dc-link voltage. In this article, inductor voltages are included in the proposed circulating current model, and a new reference for circulating current is obtained resulting in reduced circulating current through MMC arms. The proposed method is analyzed in detail on both seven-level and nine-level three-phase MMC, where advantages of the 2 N + 1 level phase disposition pulsewidth modulation (PD-PWM) scheme are retained. The simulation study is carried out in PLECS blockset to show the reduction in the rms magnitude of circulating current. The validation of the proposed technique in a lab-scale hardware prototype shows the effectiveness of the proposed method.

The Modular Multilevel Converter (MMC) is widely used for high and medium voltage applications due to its several advantages over other multilevel topologies. The number of voltage levels that can be obtained at the AC output can be increased at the expense of greater number of submodule (SM) capacitors. As the capacitor voltages need to be monitored for voltage balancing, the MMC system requires a large number of voltage sensors. This increases capital investment for the MMC system and failure points due to increased hardware interfaces. So far, a few works have been reported that uses observation techniques to either completely eliminate or reduce the number of sensors. This paper presents a Discrete-Time Sliding Mode Observer (DTSMO) for estimating the capacitor voltages and thus possibly eliminating all the voltage sensors of the SM capacitors. A switching function based discrete mathematical model is used for the design framework of the proposed DTSMO. The errors in observation are taken as important performance indicators under both transient and steady state conditions. It has also been observed that the proposed DTSMO delivers robust performance in presence of parametric uncertainty of capacitance values. The control algorithm is simulated in MATLAB/Simulink to validate the performance of the proposed observer. It is further validated through experimental results in a hardware prototype.

Modular multilevel converter (MMC) has achieved its popularity in wide range of voltage applications over other multilevel topologies due to its modularity, scalability, and ease of design. A typical MMC is composed of many capacitors and the voltages across them are required to be measured for the control of system variables like the circulating current and for capacitor voltage balancing. The number of capacitors and their associated measurement devices increase with higher voltage levels. Hence, to reduce the economical and design burden, several techniques for the observation of capacitor voltages have been presented till date. In this article, capacitor voltages of the submodules are observed using a simple Lyapunov-based observer (LBO) and the convergence proof of the observation error is established mathematically. The performance of the proposed LBO is studied in simulation using MATLAB/Simulink and also experimentally validated. The proposed observer is also compared in simulation with two other existing observation techniques from literature.

The Modular Multilevel Converter (MMC) that consists of several submodules (SMs) in its structure generates high quality output voltages. The controller of the MMC requiring all the SM capacitor voltages to be sensed makes it expensive and less reliable in terms of field implementation. In practical scenario, Sliding Mode Observer (SMO) can be used as a robust estimation technique in sensing the SM capacitor voltages. This paper proposes a Discrete Time Sliding Mode Observer (DTSMO) that estimates the capacitor voltages of all the SMs of an MMC based on discrete time model approach. This generalized technique can be suitably adopted for a MMC with any phases and any number of submodules. The proposed observer is simulated for medium voltage application in MATLAB/Simulink and results show its effectiveness in estimation. The problem of dissimilar initial charging among SM capacitors is also studied and the observer exhibits stable performance in such conditions. The MMC structure with this proposed observer can eliminate a large number of sensors particularly in high voltage applications.

The Modular Multilevel Converter (MMC) has proved to be a revolutionary topology for application in high voltage domain. Model Predictive Control (MPC) has garnered a lot of attention in the recent years for the control of multiple input multiple output (MIMO) systems like the MMC. Though highly advantageous in terms of simplicity of implementation and fast dynamic performance, the computational complexity of MPC increases as the number of submodules (SMs) increase. Moreover, issues like the design of weights in cost function and variable switching frequency hinder the use of MPC for the MMC. To take care of this problem, a unique Modulated Model Predictive Control (M 2 pC) strategy has been implemented for the MMC using a simple search algorithm. The proposed approach takes care of the above issues in addition to ensuring control of output current, circulating current and capacitor voltages. The proposed method ensures fixed switching frequency with reduced calculations. The performance of the method is examined using simulation studies for a 10 kV/0.7 MW three-phase MMC system connected to a passive load.