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Multilevel inverter (MLI) design for high voltage and high power applications faces challenges such as unequal power losses and junction temperatures. To address the issues of decreased efficiency and reliability, this paper introduces two innovative pulse width modulation (PWM) approaches, such as Enhanced Level Shifted PWM (ELS-PWM) and Enhanced Phase Shifted PWM (EPS-PWM). They are designed for five-level neutral point clamped (NPC) H-Bridge Configurations (FLNPCHC), which are further cascaded to form MLIs. These methods leverage modified modulating waves and restructured triangular waves to improve power quality, balance power distribution, and ensure uniform power allocation across the inverter legs. The result is identical switch utilization rates, reduced switching times, lower junction temperatures, and enhanced overall efficiency of the FLNPCHC-MLI. These approaches are validated through MATLAB simulations and tested using a 1 kVA laboratory prototype. The outcomes demonstrate significant improvements in total harmonic distortion (reduced to approximately 2.3%) and efficiency (increased to approximately 97.8%), offering a promising solution for high-voltage applications.

In this article, we approach the problem of stability in nonlinear systems through a new perspective that views them as a combination of individual artificial systems carefully chosen to simplify the complex structure of nonlinear systems. This is achieved by recasting nonlinear vector fields as an algebraic sum of individual vector fields for which artificial systems with known invariant sets or at least in forms that allow for tractable approximation of their invariant sets. This attempt to restructure nonlinear systems stands out in comparison to other previous attempts like Lure' systems or network based models as a purely mathematical structuring technique that transcends the physical constraints and dependencies within dynamical models and allows the user to creatively construct artificial systems with the sole focus on the overall stability. The theoretical foundation is provided for a theorem about individual invariance to relate the invariant sets of individual artificial systems to the invariant set of their original system in a way that significantly simplifies the task of approximating regions of attraction. Several examples are used to demonstrate this theorem and we also evaluate the use this theorem for the challenging power system stability problems in both AC and DC grids. The proposed method is successfully applied to the IEEE 39-bus New England system, and a DC converter with constant power load giving accurate and realistic estimations of the critical clearing time and stability regions in comparison to state of the art approaches.

Harbor cranes particularly use induction motors (IM) as the main prime mover, which are powered by the grid through the machine side converter (MSC) followed by a grid side converter (GSC). To supply power to multiple cranes in parallel, double-circuit lines are utilized. Failure of a single feeder causes voltage instability in the load bus. To analyze the voltage stability on the load bus, this article proposes a comprehensive model of the GSC while simplifying the MSC as constant power loads (CPL). When used to describe the CPL behavior of the connected IM load, the proposed modeling shows how input admittance behaves as a negative incremental, growing voltage instability on the load bus. This study uses Nyquist-based stability analyzes to address the voltage stability issue caused by a double-circuit line failure and a negative incremental input admittance. The feasibility of creating a phase-locked loop (PLL) for such grid disturbances is investigated. The possibility of installing a static VAR compensator (SVC) with a battery energy storage system (BESS) on the load bus is explored if there is no equilibrium point in the $P_{e}$ - $\delta$ curves during line failure.

Due to high penetration of renewable energy sources, inter-area oscillations (IAOs) have become serious concern for power system stability. Hence, a wide-area damping controller (WADC) strategy is proposed for thyristor-controlled series compensator (TCSC) device to effectively damp IAOs. The feedback signals for the WADC design are selected by the modal sparse linear quadratic regulator (MSLQR) method, which are sole signals that modulate simple gains to achieve the desired damping performance. Moreover, the multi-model framework approach is adopted to ensure optimal damping corresponding to any uncertainties such as changing operating scenarios and time latency in feedback signals. However, the proposed primary control does not provide frequency regulation; hence, a secondary control based on the switched integral controller is applied in addition to primary damping control to further maintain the frequency at the steady-state level. Performance of suggested strategy is evaluated and verified on IEEE 4-machine, 11-bus system and 16-machine, 68-bus system. A comprehensive analysis shows significant improvement in the damping performance of concerned modes, leaving the local modes almost unaffected while maintaining the frequency at the prescribed steady-state level.

Inverter topologies for integrating a rooftop photovoltaic (PV) unit into a microgrid are becoming increasingly complex. This paper proposes a high-voltage boosting transformerless inverter (HVBTI) topology for enhancing such applications. The coupled inductor-based high voltage gain feature of the HVBTI configuration allows power to be delivered into the grid from a lower voltage PV source without using higher duties. In addition, HVBTI suppresses leakage current as the common connection shared between the $-ve$ terminal of the PV source and the grid neutral point. In comparison to existing topologies, the HVBTI topology uses a compact pulse width modulation strategy to control only six controllable switching devices. Again, using lower-rated switching devices is more cost-effective while increasing reliability and efficiency. In a laboratory prototype of a 1 kVA grid integrated system, the proposed HVBTI configuration is validated, and the maximum efficiency of the HVBTI is estimated at approximately 95$\%$.

This paper presents a mutual frequency support strategy for ac grids interconnected by multi-terminal high voltage dc transmission system. The proposed scheme has different strategies for ac side disturbance and converter outage. During ac side disturbance, the proposed scheme makes sure that the actual aggregated droop gains of the converters in the same ac grid is equal to the designed aggregated droop gains by effectively utilizing the available headroom (AvHR) of the converters. Moreover, the paper proposes a threshold compensation strategy to eliminate the effect of the frequency deviation threshold that is intentionally imposed on the frequency droop control of converters. Therefore, the proposed scheme can accurately achieve the operator pre-specified ratios among the frequency deviation of the ac grids. The aggregated droop gain of the converters in each ac grid is designed to achieve the pre-specified ratios amongst frequency deviation of the ac grids. Furthermore, during an ac side disturbance, converters in the disturbed ac grid operates in $P-f$ droop control mode and the converters in the supporting ac grids work in $P-V_{dc}$ droop control mode. Therefore, the impact of the frequency deviation on the dc voltage profile can be reduced. During converter outage, the proposed scheme minimizes the frequency deviation of the ac grids by trying to restore the power flow between the ac grids and their converters to a level before the disturbance. The effectiveness of the proposed scheme is verified through rigorous simulation tests.