Rupak Chakraborty

Wide Bandgap (WBG) devices like SiC-MOSFETs have become quite popular in recent times due to their superior switching characteristics, high current carrying capability and temperature stability. They are being adopted for many different applications and for a wide range of power levels. For the case of PV applications, manufacturers are considering moving to SiC-based topologies due to higher converter efficiencies and improved power density. However, the present industry largely uses hybrid approaches (IGBT + SiC-diode) to optimize system cost. The aim of this paper is to present a fair comparison of an industry-grade hybrid converter with another similar counterpart where only the Si device has been replaced with the SiC device. The effects of such a direct replacement on the efficiency and losses of the converter are studied under various power ratings. Both converters consist of two stages—a boost converter and a three-phase three-level DC to AC converter. Simulation and experimental results comprehensively indicate a higher efficiency (improvements of up to 8 percent points) for the full-SiC converter, and this is more prominent at low input voltages, where the boost converter is active. However, the gains in efficiency are moderate for high input voltages (1 percent point at nominal voltage), where the boost converter is bypassed, and the losses are almost entirely attributed to the inverter. When set in the backdrop of the Austrian inverter market, the use of SiC devices in PV inverters has the potential for an estimated savings of 37.5 GWh/year in terms of loss reduction.

Modular multilevel converter (MMC) gained popularity in modern power applications due to its attributes like scalability and modularity. But, the number of submodules of an MMC and its corresponding voltage sensing devices increase with the increase in voltage levels, which makes the design complex, bulky, and expensive. Hence, an observer for estimating the voltages across the capacitors makes the complete design inexpensive, simple, and compact. This article presents an exponential power rate discrete-time sliding mode observer (EPR-DTSMO) for the observation of voltage across the capacitors of the half-bridge submodule of an MMC. In this article, the hybrid nature of an MMC is investigated, and the MMC system is demonstrated to be observable using the $Z(T_{N})$ observability criteria for hybrid systems. The proposed EPR-DTSMO is then designed for the MMC, and its convergence for the observation errors is proved. The proposed observer is validated using an experimental setup after verifying with numerical simulations. Moreover, its performance is compared to other observer techniques present in the literature. Both simulation and experimental results show that the proposed EPR-DTSMO is well suited for the sensor-less operation of MMC without using any voltage sensors for submodule capacitors.

Modular Multilevel Converters (MMC) are increasingly gaining its importance in HVDC, drive applications due to several attributes such as modularity, scalability, low device stress, excellent output voltage waveform. For MMC operations, it is important to accurately measure the voltages across the individual capacitors of sub-modules (SMs). These voltage sensors, while upgrading for high voltage applications, may cause a few concerns in terms of cost, complexity, and data acquisition system. To avoid all these, a discrete time adaptive observer (DTAO) is proposed in this paper to observe the voltages across the capacitors of the SMs and use the observed values for closed loop operation of the MMC. The proposed observer is designed by considering the parametric uncertainty in the MMC and its convergence is proved mathematically. Moreover, the DTAO is compared with existing observation techniques for MMC SM capacitor voltages. The efficacy of the proposed observer is shown through simulation and validated using an experimental setup.

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) 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 Converters (MMC) are known for their high modularity and good quality output power. The problems of circulating current and capacitor voltage balancing for the MMC have been widely studied in available literature but little work is done towards grid current control with fast dynamic response. Conventional current controllers use PI or PR controllers which are slow and thus degrade transient performance of the MMC. Recently reported Model Predictive Control (MPC) methods for MMC aim to control the grid current but MPC suffers from computational burden and difficulties in adjusting of weights. Space Vector based Hysteresis Current Controllers (SCHCC) have been well established for drive applications. In this work, an SVHCC has been developed for a grid connected MMC system. Simulation studies show fast current control for cases of sudden change of active power reference and current reference. The controller exhibits quick tracking of the current reference in all cases. The proposed method has been discussed for a MMC system with three submodules, but it may be extended to any number of submodules and can be easily implemented in a digital controller with less computational effort.

Modular multilevel converters (MMC) are being widely used in high power applications due to its several advantages over other multilevel topologies. The MMC employs many Submodule (SM) capacitors in its arms and suffers from the problem of capacitor voltage deviations. To achieve voltage balancing of SM capacitors, controllers for the MMC require sensing of the capacitor voltages, which makes the entire system more complex and less reliable for practical applications. The problem increases when a large number of SMs are used in each arm of the MMC (particularly for high power applications). This paper aims to solve the problem of large number of sensors, by designing a variable gain discrete time observer (VGDTO) for estimation of the capacitor voltages. The proposed VGDTO requires tuning of only one parameter which makes it simple. MATLAB/SIMULINK® simulations for high voltage applications show the effectiveness of the VGDTO in estimating the SM capacitor voltages with a narrow error band. This method is also compared with previously published discrete time sliding mode observer (DTSMO) approach.

The modular multilevel converter (MMC) used in medium and high voltage applications due to its several advantages related to scalability and modularity. An MMC consists of several sub-modules (SMs) which house several capacitors in each arm of each phase. For the working of MMC, the voltage across these capacitors needs to be measured. High voltage applications demand more number of SMs, which needs more capacitors and its voltage sensing devices for operation. In this paper, a discrete time adaptive observer (DTAO) is designed for the estimation of capacitor voltages of SMs by taking the discrete model of an MMC. It handles the uncertainties due to the modeling error and parasitic dynamics and makes the design robust. This design approach is very simple as it serves the engineer to choose the design parameter in an easy way. This method is also compared with discrete time sliding mode observer (DTSMO) approach.