In this paper, the effect of min-max zero-sequence voltage (ZSV) injection on the maximum average neutral-point (NP) current control limits of a neutral-point-clamped (NPC) converter is analyzed. These current limits determine the stable operating range of the NPC converter in applications that impose a nonzero average NP current, like- NPC converters feeding multiterminal dc loads, back-to-back-connected NPC converter-based wind and high voltage dc transmission systems, etc. Previously, a dual-mode modulation technique was implemented in the literature which provided the maximum average NP current limits without considering the ZSV effect. As the ZSV injection affects the NP duty cycles, and in turn the NP current, it is necessary to analyze and quantify its effect. In this paper, the effect of ZSV on the current limits is analytically calculated. Simulation results are obtained in MATLAB/Simulink environment to validate the derived analytical equations.

Photovoltaic (PV) grid support functionality (frequency support) can be accomplished with the aid of flexible power point tracking (FPPT) technology. However, the convergence rate of FPPT algorithms needs to be fast, to provide grid support under transients. To fulfil this purpose, a Newton’s method-based (NM-B) FPPT algorithm is proposed in this paper that offers quadratic convergence, which is a higher convergence rate compared to the existing methods. Moreover, the search algorithm requires only a single initialization point, thus eliminating the difficulty of setting up the search boundary as in the case of the binary search-based and secant method-based (SM-B) FPPT algorithms. Simulation validation is performed for the proposed NM-B FPPT algorithm and the results are compared with the SM-B FPPT algorithm.

In a modular multilevel converter (MMC) based battery energy storage system (BESS), a fault tolerant design ensures uninterrupted operation of the MMC when a given number of submodules (SMs) have faulty batteries or no batteries. This paper quantitatively investigates the fault tolerance improvement in MMC-based BESSs with different numbers of redundant SMs, which is commonly practiced to improve the fault tolerance in modular converter topologies. Simulation results are obtained to verify the proposed analysis. The presented analysis provides guidelines for designers to choose an appropriate number of redundant SMs to achieve a desired fault tolerance.

Despite some recent efforts towards multi-objective design optimization of multilevel converters, design optimization of solid-state-transformers (SSTs) are not presented much in the literature mainly because of the lack of computationally feasible techniques. This paper is dedicated towards a computational feasibility study of multi-objective design optimization techniques for medium-voltage (MV) grid-connected SSTs. After defining the application and scope of SST design optimization problem, a brief description of the possible solution techniques are discussed which shows the merits of semi-numerical/hybrid design optimization techniques. Subsequently, a machine learning (ML) aided hybrid optimization technique is executed for a 15 kVA single-stage SiC-based SST design. Suitable component modelling is presented and a strong agreement is observed between theoretical optimization and experimental results. Finally, a comparative evaluation of the analytical, numerical, standalone hybrid and ML-aided hybrid optimization techniques (deployed for the same 15 kVA SiC-based SST design) reveals that the ML-aided hybrid strategy is best suited for SST design optimization as it requires feasible computational time for <5% error.

Air-conditioning and mechanical ventilation (ACMV), power distribution systems, elevators, and lighting systems are critical electrical assets of a smart building. Based on electrical components of these assets such as electrical motors, transformers, solid-state lighting and power electronic converters, a detailed analysis of failure mechanisms and statistics is presented in this paper. Further, suitable electrical and mechanical signals along with prevalent condition monitoring methods are suggested for condition monitoring of these assets based on the presented statistical analysis. This paper lays down the necessary foundation to understand the operating model of critical electrical equipment in buildings and provides a holistic picture for implementation of a predictive and preventive maintenance paradigm. This will enable the building operators to identify potential failures of building equipment and their evolution in real-time, driving decisions on scheduling repair or replacement of components to minimize downtime, save maintenance costs, improve occupant satisfaction, save energy, and maximize resilience.

Cascaded H-bridge (CHB) multilevel converter is widely used for medium voltage motor drives, Static Synchronous Compensator (STATCOMs) and solid-state transformers because of its advantages such as modular design, low harmonic distortion of input current and lower grid-side filter requirements compared to two-level converters. Among the failures in active devices, detection and localization of open-circuit (OC) faults either due to bond-wire lift off (device-side failures) or malfunction of gate driver (drive-side failures) are extensively studied. In this paper, fault detection and localization methods proposed in literature for CHB multilevel converters are analyzed and compared with respect to the time taken for fault localization and additional sensor requirements. For ease of understanding, the methods are classified as model-based, feature-based and hardware-based, and suitability of each classification is recommended for different applications.

This paper proposes a Modular Solid-State-Transformer (SST) topology consisting of bidirectional Matrix-Based AC-DC Dual-Active-Bridge (DAB) submodules. Unlike the conventional SST topologies, the proposed topology offers a single stage Medium Voltage AC (MVAC) - Low Voltage DC (LVDC) conversion. The discussion is focused on the AC-DC DAB submodule of a 1 MVA SST for the Energy Control Centre (ECC) of a microgrid/nanogrid. The possible modulation modes along with the corresponding power transfer limits are presented and a suitable steady-state modulation scheme is identified for the required operating point. The modeling and small-signal analysis of the AC-DC submodule is comprehensively explained. Simulation results including the steady-state operation and the frequency response of the 17.6 kW submodule are presented.

A modular multilevel converter (MMC) with traditional cascaded PI control structure suffers from poor steady state and dynamic performance. This is due to the limited harmonic rejection capability of PI controller thus failing to alleviate even order harmonics in circulating current and due to the slower outer average voltage control loop resulting in significant deviation in submodule capacitor voltages during load transients. This paper presents a new control structure with a repetitive controller (RC) in conjunction with a Lyapunov function-based controller (LFC) in the inner circulating current loop and an additional load current feed forward loop in addition to the outer voltage loop to improve both steady-state and dynamic performance of MMC. RC has a good steady-state harmonic suppression capability therefore can effectively alleviate the circulating harmonic currents, where as LFC ensures a stable operation and superior dynamic performance of the converter against load disturbances. Analytical equations governing the design of both the controllers are presented. Model of a single phase, 5-level MMC is developed on a PLECS RT box real-time simulator for hardware-in-the-loop (HIL) testing, it operates along with the external Texas Instruments DSP controller hosting the developed control scheme. Real-time simulation results are presented to substantiate the improvement in steady state and dynamic performance of MMC with the proposed repetitive and Lyapunov function based circulating current controller.

In Smart Grid there are abundant number of grid connected Voltage Source Converters (VSC) in the form of Electric Vehicle (EV) battery chargers, Renewable Energy Source (RES) fed grid connected inverters etc., that can be used for active and reactive power support to the grid. The VSCs remain idle when the load in case of EV charger or the energy source in case of RES fed grid connected inverters is not available. This work presents a single-phase PQ theory based control strategy to improve the utilization factor of such converters by using them to support the reactive power to the local loads when they are idle. The proposed control scheme is implementable with less computational effort. The system is further analyzed by calculating appropriate minimum DC bus voltage for a given amount of reactive power to be supported. The analysis for the designed system is validated using MATLAB-Simulink based simulation and a scaled down laboratory based experimental setup. The results are in good agreement with the analysis.