For large-scale energy storage applications, variable-speed pumped hydro storage plants (VS-PHSP) are a proven solution due to their fast start-up and high ramping capabilities. Variable-speed operation is primarily dependent on power electronic converters. However, existing converter topologies often suffer from drawbacks, including a large number of semiconductor devices, the need for series-connected switches, and non-uniform voltage distribution, which result in increased losses, complex cooling requirements, and overall system cost. To overcome these challenges, this paper proposes a back-to-back configuration of a new four-level converter for VS-PHSP systems. The proposed topology eliminates the requirement for series device connections and ensures uniform voltage sharing among switches. Subsequently, it reduces the overall device count and their cost. In addition, a model predictive control for the proposed back-to-back configuration is implemented and validated through MATLAB simulations.

Due to the low computational burden, sequential model predictive control (SMPC) methods have emerged as promising solutions for multilevel inverters (MLIs). Over the years, several SMPC approaches focusing on either load current or load voltage as the primary objective have been explored for MLIs. These methods have adopted philosophies such as cost function with weighting factors, offline switching vector selection or inherent strategies to minimize common-mode voltage (CMV). However, the literature lacks a comprehensive analysis of SMPC methods with CMV minimization and their pros and cons on system performance. To fill the research gap, this paper presents the philosophies behind various SMPC methods for four-level inverters (FLIs), along with the mathematical models utilized in SMPC implementation and case studies to highlight their performances. Finally, a comparative analysis of SMPC methods is provided, aiding researchers in selecting suitable methods for their applications.

This paper proposes a new four-level multilevel converter for medium-voltage, high-power applications. The proposed topology requires a lesser number of components, eliminates series connection of devices, and suitable for back-to-back operation due to the presence of common DC-link. Furthermore, the proposed topology does not require isolated DC source thereby the phase-shifting transformer can be eliminated to reduce the system cost and size. In addition, a simple capacitor voltage balancing approach based on multicarrier-based pulse width modulation scheme is proposed. The proposed approach selects a right switching state to maintain each flying capacitor voltage at their nominal value by considering the current direction, instantaneous value of capacitor voltage, and voltage level. The performance of the proposed four-level multilevel converter and voltage balancing approach is verified through simulations at steady-state and transient operating conditions.

HVDC systems are used for bulk power transfer where reliable operation of the system is very important. A novel fault tolerant method under sub module (SM) failure is proposed in which the faulty SMs are bypassed and system is operated with reduced number of SMs using simple phase disposition pulse width modulation (PD-PWM) until the faulty SMs are replaced. The number of levels in the modulation signals for the faulty phase are adjusted depending on the number of faulty SMs in each arm. Once the fault is cleared, the number of levels in modulation signals will revert to normal mode, bringing the MMC to normal operation. This approach provides fault tolerant operation with full power handling capacity without significant performance degradation of the system during fault. Compared to conventional fault tolerant (FT) methods, proposed approach results in fast and simple changes in PWM signals, better efficiency, higher reliability with least number of spare or redundant SM units. The proposed FT strategy is verified through simulation. The simulation results are presented.

Model predictive control is a promising approach to control a multi-objective modular multilevel converter (MMC). In this approach, the control objectives of MMC are included in a single cost function and evaluated for all possible switching states by using weighting factors. The complexity of weighting factor selection process increases with the number of control objectives, and it affects the performance of MPC as well. To reduce the dependency on the weighting factors, a new model predictive control with zero-sequence voltage injection is proposed. With the proposed approach, some of the control objectives like reduction of submodule capacitor voltage ripple and circulating current can be achieved without using a cost function. The proposed approach also reduces the total harmonic distortion of the output voltage and current waveforms. The performance comparison of the proposed and existing MPC approach has been verified through MATLAB simulations on a three-level flying capacitor (3L-FC) based MMC system.

The multilevel submodules (SMs) bring the added advantages of compact size and improved efficiency over standard submodules to the modular multilevel converter (MMC). These SMs have more than one floating capacitor, which stores the arm energy. Each arm of MMC has several SMs connected in series to meet the required operating voltage. The SM capacitors voltage or energy must be controlled to ensure the safe and reliable operation of MMC. This objective can be achieved by using either voltage balancing or energy balancing approaches. When all the SMs have equal capacitance value, then the voltage and energy balancing approaches have identical performance. However, the SM capacitance deteriorates over a long run, which affects the performance of MMC. This paper presents the performance comparison of voltage and energy balancing approaches for MMC with equal and unequal SM capacitance. The performance of balancing approaches is evaluated in terms of active and reactive power loading of individual SMs, rating of IGBT devices and SM capacitors, and output voltage and current harmonic distortion.

The computational complexity is one of the major issues in the implementation of direct model predictive control (D-MPC) for modular multilevel converters (MMC). The computational complexity is significantly reduced by combining the conventional voltage balancing approach with the MPC. This approach is referred as indirect model predictive control (I-MPC). The D-MPC and I-MPC approaches have higher voltage harmonic distortion and current ripple. In this paper, an improved indirect model predictive control (II-MPC) is proposed to overcome the drawbacks of D-MPC and I-MPC approaches. To implement the II-MPC strategy, a detailed three-phase MMC model is presented. The simulation studies are conducted on a three-phase half-bridge based MMC to validate the dynamic and steady-state performance of II-MPC strategy.

The use of conventional half-bridge submodules in the Modular Multilevel Converters brings important benefits such as the reduction in the power losses and semiconductor devices. However, the system with half-bridge submodule lacks the capability of blocking DC-faults currents. In this paper, a 5-level submodule with DC-fault blocking capability is presented. The proposed submodule allows to operate with different internal capacitances in order to reduce the cost of the system. Furthermore, a control and balance strategy for the novel 5-level submodule is proposed.

Increased use of variable frequency drives in industry has given rise to power quality problems. This paper addresses the power quality problems especially the harmonics problems in modern oil drilling rigs with AC drives. These power systems are isolated one, with local generator supplying the loads. Some of the operating constraints like harsh operating conditions and limited space have direct effect on the choice of the filters. This paper discusses the various solutions to mitigate the harmonic problems for the drilling rigs. These solutions are evaluated based on practical constraints such as performance, space, system modifications, size, reliability, maintenance, cost and THD.