This paper proposes an approach to control a photovoltaic (PV) cascaded H-bridge multilevel inverter with failed cells and changing meteorological conditions for large-scale grid-connected applications. The controller development is based on an analysis of the interaction between the inverter common-mode and differential-mode quantities, which is done in the time domain, supported by a space vector representation analysis. The proposed approach is able to produce balanced three-phase line-to-line voltages and currents even if there is a failed cell or if the fluctuating meteorological conditions cause uneven power distribution among the PV panels. This is achieved through the modification of the PWM reference phase voltage angles combined to the injection of a dynamic homopolar component. Such approach avoids tripping the system due to the protective functions #25 and #87 for per grid applicable codes and standards. Numerical simulations and laboratory experiments performed on a 7-level converter with different abnormal conditions to confirm the effectiveness of the suggested control strategy.

This article proposes a novel and generalized single-carrier (SC) pulsewidth modulation (PWM) scheme suitable for multilevel converters. Compared with the existing SC-PWM schemes, the proposed one presents a simpler approach to replicate the behavior of the phase disposition PWM method at any operating point. Besides having an easier digital implementation, the proposed SC-PWM scheme results in great control flexibility to optimize the operation of multilevel converters. Based on the rationales of the proposed SC-PWM scheme, an optimized switching strategy for a cascaded H-bridge (CHB) multilevel inverter architecture is developed. The proposed optimized switching strategy imposes appropriate switching sequences with a minimum number of power device commutations per cycle, leading to both half-wave and quarter-wave symmetries at the individual H-bridge cell level, which subsequently improves the converter harmonic performances and reduces the overall switching losses. The validity and effectiveness of both proposed modulation and switching strategies are demonstrated through the intensive simulation and experimental test results provided both in time and frequency domains. The experimental results are obtained on a three-phase seven-level CHB inverter for an 18-kW laboratory prototype.

This article proposes a high-level engineering guide to develop an integrated system model for power quality analysis in electrical submersible pump (ESP) applications with variable frequency drives (VFDs). Such analyses are troublesome in the industry due to the complexity of these systems. In this article, simple steps to perform system integration analyses of such arrangements, including torsional analysis, are developed. A simplified VFD-ESP model suitable for coupled electrical and mechanical analysis in steady state is proposed in all their configurations of practical interest. Such a model can be easily implemented in common simulation software, significantly reducing engineering efforts for implementation and analysis. The focus of the proposed model is the prediction of ESP failures that might result from a poor power quality caused by VFDs. Analytical expressions of different types of harmonics in these systems, as well as their accurate locations in the frequency domain, including their interharmonics and common-mode harmonics, are derived for this purpose. The effectiveness of the proposed model is verified through offline and real-time hybrid simulation results. Finally, a comparison between simulation results obtained using the proposed model and measurements collected on a down-scale laboratory prototype is carried out to demonstrate the accuracy of the suggested modeling approach.

A cascaded H-bridge multilevel converter topology is the ultimate solution for energy conversion in various industrial applications due to its exceptional features, such as high modularity and fault-tolerant capability. However, two circumstances can lead to unbalanced operation of the inverter, potentially causing a decrease in its reliability and survivability: unequal DC voltage sources and faulty cells. In recent decades, scientists and engineers have conducted intensive research and meaningful studies to propose control solutions capable of maintaining the stable and continuous operation of the inverter under these operational concerns. Typically, each challenge is addressed separately using a distinct compensation control scheme in the existing literature. The paper aims to offer a comprehensive review of the existing compensation control schemes appropriate for CHBMIs operating under unbalanced conditions. It overviews the most popular control schemes and summarizes their usefulness in such scenarios. The theoretical foundations of each control scheme are presented and discussed, including their operating principles, implementation schemes, advantages, and disadvantages. The paper concludes with suggested future trends that require further research for CHBMIs’ continued growth and adoption in various industrial applications.

This paper proposed a simplified modeling approach for a power quality (PQ) assessment of Electric Submersible Pumps (ESP) systems supplied by the two-level, the neutral-point-clamped three-level, and the cascaded H-bridge (CHB) multilevel inverter VFD topologies. The VFD switching function models and their analytical expressions are proposed to understand how they can create high-frequency components that might excite the resonance mode in a transmission cable or a rotating shaft system. Voltage, current, and motor airgap torque harmonics induced by each VFD topology in a balanced operation mode are derived and correlated to the PWM carrier and motor operating frequencies. The motor airgap harmonics are calculated based on Concordia’s transformation of voltages and currents in αβ-plan. These harmonic components are represented in the form of Campbell diagrams. An analysis of harmonics under unbalanced conditions was also conducted in a CHB VFD topology-powered ESP system with failed and bypassed cells. The investigated modulation technique is a neutral-shift PWM method that enables the system to operate balanced line-line voltages even if the line-neutral voltages are unbalanced. The effects of modifying the electrical spectrum using the neutral-shift PWM method on electrical and mechanical spectra are analyzed. The results of the Matlab/Simulink-based simulation show that the proposed full ESP system model is highly accurate in both normal and failure modes. The results are consistent with theoretical predictions and are graphically shown in the time and frequency domains for easy analysis. Hybrid experimental–numerical results on a reduced-scale laboratory setup are also discussed to confirm the correctness of the suggested developments.

This paper proposes a modified carrier-based PWM method for multilevel converters using a single-carrier. The existing scalar PWM strategies are based on the comparison between a sinusoidal reference waveform and multiple carriers on each phase. The major drawback of this approach is an increased number of carriers required when the output voltage of the multilevel converter increases; usually, n-1 carriers are needed for n-level converters. Consequently, higher computation effort is required, which increases the complexity of their practical implementation on low-cost digital control boards. To reduce the complexity of the implementation of the modulation strategy, a new single-carrier PWM strategy for multi-level converters is proposed in this paper. Only one carrier is required regardless of the number of output voltage levels to be generated by the converter. With simple manipulations, the sinusoidal references are transformed to fit the range of the triangle carrier; this is valid with and without an injected homopolar component. The simulation results show that the new strategy is theoretically equivalent to a multi-carrier PD-PWM method.The effectiveness of the proposed modulation scheme is applied to the 7-level and 11-level cascaded multilevel converters and successfully simulated at different operating points. Also, similar simulations are performed on a 5-level Neutral-Point-Clamped converter to demonstrate its wide application range