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This article proposes a dual-layer modulation technique that combines two- and three-level modulation techniques to control the average neutral-point current (ANC) in neutral-point-clamped (NPC) converters. In the proposed technique, two modulation layers are employed to independently synthesize the desired ac-side voltage reference and ANC reference. The first (outer) modulating layer operates at a lower frequency and synthesizes the ANC reference from the three-phase currents. It determines the duration of applying each modulation technique, based on the ANC reference and the maximum ANC injection limits of the converter. The second (inner) modulating layer operates at the switching frequency and synthesizes the three-phase voltages of the NPC converter from the common dc voltages by comparing the calculated duty cycles and a carrier waveform. The proposed dual-layer modulation technique has advantages of faster dynamics and reduced mathematical computations compared to the conventional approach. The feasibility of the proposed modulation technique is demonstrated experimentally on a laboratory prototype. A detailed performance comparison among different modulation techniques is also presented.

This article presents a control strategy for a dual-input neutral-point-clamped (NPC) inverter-based grid-connected photovoltaic (PV) system to asymmetrically control the PV arrays without incorporating any additional external circuit. This is achieved by employing an analytically developed linearization block, which helps in generating the required neutral point (NP) current to counter for the difference in the PV currents. The analysis also provides the maximum NP current compensation limits of the NPC inverter. If the required NP current reference is less than the maximum limit, both PV arrays operate at their maximum power points. However, if the required current exceeds the limit, one of the PV arrays operates at its maximum power point, while the output power of the other PV array is reduced to satisfy the NP current limit. Experimental tests are conducted on a laboratory prototype to validate the performance of the proposed control strategy. currents. The analysis also provides the maximum NP current compensation limits of the NPC inverter. If the required NP current reference is less than the maximum limit, both PV arrays operate at their maximum power points. However, if the required current exceeds the limit, one of the PV arrays operates at its maximum power point (MPP) while the output power of the other PV array is reduced to satisfy the NP current limit. Experimental tests are conducted on a laboratory prototype to validate the performance of the proposed control strategy. Ministry of Education (MOE) Accepted version This work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant No: 2019-T1-001-168 (RG 80/19).

In this paper, a novel generalized circuit topology of the n-time quadrupled-hybrid-neutral-point-clamped (Qn-HNPC) converter and a new decomposed sensor-less modulation method are proposed to multiply the number of output voltage levels of the 5L-HNPC converter with a less number of components. The Qn-HNPC converter is realized by applying $n$ number of the proposed voltage-level multiplier modules (VLMMs) to the 5L-HNPC converter. Hence, the number of output voltage levels is n-time quadrupled in the attained Qn-HNPC converter, whereas a less number of components, as well as, low cost and size, are added to the main 5L-HNPC converter. Moreover, a novel generalized decomposed sensor-less modulation method is introduced to decompose the main 5L-HNPC and the proposed VLMMs reference signals in the attained Qn-HNPC converter. Consequently, only four low-voltage and low-power switches operate at switching frequency, whereas the remaining devices commutate at the low or fundamental frequency. Furthermore, employing the proposed modulation method in the attained Qn-HNPC converter causes self-balancing of all the capacitors voltage, doubling the first switching harmonic cluster frequency of the output voltage, as well as a remarkable decrease in the control complexity, the stored energy, and the size of the VLMMs capacitor. The presented simulation and experimental results verify the performance and viability of the proposed Qn-HNPC converter topology, as well as, the suggested decomposed modulation method.

Datos suplementarios: https://ars.els-cdn.com/content/image/1-s2.0-S0360544220309427-mmc1.mp4 Multilevel converters are nowadays an enabling key in the integration of electric power into the grid as they introduce less distortion and, thus, they are more compliant with the grid standards, among other benefits. A well-known topology is the three-level Neutral-Point-Clamped whose control requires to deal with the capacitors voltage unbalance. This paper presents a modulation approach where the injection of a common component in the modulated voltage is studied in order to achieve such voltage balance. An optimization problem that, apart from the voltage balance, aims the lowest number of commutations and can be solved very efficiently with up to five computations of the cost function is formulated. The main advantages of the proposed modulation strategy are its simplicity and its flexibility, since it is also valid for unbalanced grid conditions and, with little added complexity, for low (and even zero) power factor conditions. Simulation results under unbalanced grid conditions are provided in order to show its validity under this scenario. The strategy is evaluated and compared with a space-vector-based approach in an experimental setup, yielding similar total harmonic current distortion, a 30% reduction in the number of commutations, and better voltage balance performance for lower power factor conditions. Ministerio de Economía y Competitividad DPI2016-75294-C2-1-R Junta de Andalucía (Consejería de Economía y Conocimiento), Programa Operativo FEDER Andalucía 2014-2020, US-1264655.

This paper uses a novel approach for the control of three-level neutral-point-clamped (NPC) rectifiers in order to tackle the capacitor voltage balance problem. A distinctive feature of the new control approach is that it is based on a model which is written in terms of the duty ratios for each phase at each level. Hence, the system model presents nine duty cycle variables. Despite the fact that this formulation is different from the usual ones, it is shown that the control problem of currents and dc-link voltage can be formulated in a similar way to conventional methods. Furthermore, the control of the capacitor voltage balance can be expressed by means of equations that are decoupled from the currents and dc-link voltage dynamics, which results in a specific controller for the voltage balancing that does not affect the previous dynamics. A key point of the proposed approach is that part of the modulation stage is implicit in the formulation. Two particular controllers are compared in this paper. The first one fulfills the different control objectives at the expense of a large number of commutations. This problem is overcome in a new proposed controller, which presents similar performance and a satisfactory number of commutations. Experimental results are performed showing the effectiveness compared with a modified virtual space vector modulation with capacitor voltage balance capabilities. Agencia Estatal de Investigación (MINECO)/FEDER DPI2013-41891-R Agencia Estatal de Investigación (MINECO)/FEDER DPI2016-75294-C2-1-R

In the research field of multilevel converters, three-level NPC (neutral-point-clamped) converters, which unfortunately may cause the deviation of the neutral-point voltage of DC-link capacitors, are widely discussed. Theoretically, virtual space vector modulation (VSVM) could guarantee the balance control of the neutral-point voltage. However, there still exist some uncontrollable space vector regions. Based on VSVM, this paper proposes a varied virtual space vector modulation (VVSVM) method for three-level NPC converters. Under complete modulation conditions, this method can control the balance of the neutral-point voltage of DC-link capacitors by adjusting the duty cycle of small vectors and regulating the current generated by virtual medium vectors. Compared with commonly used VSVM methods and mixed modulation strategies, this method is simpler and more practical. The effectiveness and validity of this method are verified by simulations and experiments.

This paper addresses the problems associated with the dc-link capacitor voltages of the three-level neutral-point-clamped power converter: the imbalance of the capacitor voltages as well as the presence of an ac-voltage low-frequency oscillation in the dc link of the converter. In order to cope with them, a mathematical analysis of the capacitor voltage difference dynamics, based on a direct average continuous model, is carried out, considering a singular perturbation approach. The analysis leads to a final expression where a sinusoidal disturbance appears explicitly. Consequently, the two problems can be handled together using the ordinary formulation of a problem of regulating the output of a system subject to sinusoidal disturbances, applying classical control theory to design the controller. In this way, the controller is designed including the disturbance estimate provided by a Luenberger observer to asymptotically cancel the disturbance, while keeping also balanced the capacitor voltages. Experiments for a synchronous three-level neutral-point-clamped converter prototype are carried out to evaluate the performance and usefulness of the converter working as a grid-connected inverter under the proposed control law. MICINN-FEDER DPI2009-09661 Junta de Andalucía P07-TIC-02991

In this paper, a predictive control scheme is proposed for the low-voltage ride-through (LVRT) enhancement of direct-driven permanent-magnet-synchronous-generator-based megawatt-level wind turbines. The proposed method uses the turbine-generator rotor inertia to store the surplus energy during the grid voltage dips. The power conversion system is realized using a three-phase diode-bridge rectifier, a three-level-boost converter, and a neutral-point-clamped (NPC) inverter. The wind turbine requirements, such as maximum power point tracking, net dc-bus voltage control, balancing of the dc capacitor voltages, and reactive power generation, are modeled as the reference control variables. The generator- and grid-side cost functions are defined to deal with these control objectives. During each sampling interval, the control goals are achieved based on the minimization of cost functions. The coordination of boost and NPC converters and the exchange of reference control variables during normal and LVRT operation are formulated such that the power converters operate in a safe mode while meeting the grid code requirements. Simulation and experimental results are presented to validate the proposed strategy.

This paper presents a generalized pulse width modulation (PWM)-based control algorithm for multiphase neutral-point-clamped (NPC) converters. The proposed algorithm provides a zero sequence to be added to the reference voltages that contributes to improve the performance of the converter by: 1) Regulating the neutral-point (NP) current to eliminate/attenuate the low-frequency NP voltage ripples; 2) reducing the switching losses of the power semiconductors; and 3) maximizing the range of modulation indices for linear operation mode. The control method is formulated following a carrier-based PWM approach. Hence, dealing with complex space-vector diagrams to solve the modulation problem for multiphase converters is avoided. The recursive approach means that it can be easily extended to n-phase converters without increasing the complexity and computational burden, making it especially attractive for digital implementation. The proposed method allows regulating the NP voltage without the need for external controllers; therefore, no parameter tuning is required. The algorithm has been tested in a four-leg NPC converter prototype performing as a three- and four-phase system and operating with balanced and unbalanced loads.