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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. [ABSTRACT FROM AUTHOR]

Selective harmonic elimination–pulsewidth modulation (SHE–PWM) is a modulation technique widely used to improve the efficiency and harmonic content in medium-voltage high-power converters. Among these converters, the three-level neutral-point-clamped (3L-NPC) converter is one of the most utilized topologies. However, the 3L-NPC converter requires a proper balancing of dc-link capacitor voltages. This article presents a control technique, based on pulse shifting method, to regulate the neutral point voltage when SHE-PWM is utilized. The proposed technique performs well with any power factor and it can also be directly applied to T-type converters and easily extrapolated to other multilevel topologies. The proposed method is experimentally validated in a scaled-down power converter and a full-scale medium-voltage 3L-NPC converter. [ABSTRACT FROM AUTHOR]

In this article, a nonlinear high-gain observer (NHGO)-based second-order sliding mode (SOSM) control strategy is proposed for the three-phase three-level neutral-point-clamped (NPC) converter. This controller applies the advanced SOSM algorithm both in the voltage regulation loop and in the power tracking loop, which provides a fast dynamic for the dc-link voltage, and also assures a good steady-state behavior for the NPC converter. Additionally, an NHGO technique is implemented in the voltage regulator combining with the SOSM algorithm. The conventional observer-based controllers suffer from the destructive effects of measurement noise, and it can only be addressed by diminishing the observer gain, which sacrifices the observer property. The NHGO technique adopts a time varying gain, that is, high gain in transient while low gain in steady state, which minimizes the adverse influence of measurement noise. The tuning method of the proposed NHGO-based SOSM controller is given to simplify the implementation process. Finally, the simulation and experimental results of the proposed control scheme for the NPC converter are given and compared with the conventional PI controller as well as the well-known linear extended state observer-based control method, which validates the feasibility and superiority of the proposed controller. [ABSTRACT FROM AUTHOR]

Single-phase symmetrical five-level converter offers a low-harmonic power interface, whose attractiveness is nevertheless affected by dc-link voltage imbalance. Fast rebalancing methods have therefore been proposed, if the natural balancing speed of the converter is intolerable. One possible method is to add a separate parallel RLC branch between the converter and its filter. Being parallel, the branch does not change spectral properties of the converter and filtering properties of the filter, in addition to its robust sensor-less requirement. However, being separated, it does not properly integrate with the filter to explore dual usabilities of the components. This article therefore proposes a few differential-mode (DM) filters with each having a common-mode (CM) capacitor as the balancing accelerator. The accelerator does not conduct and hence consume no power during the normal balanced condition. It conducts only when an imbalance occurs and directs only those harmonics with fast balancing ability to the dc link. Its design is simple and can conveniently be performed with an appropriate CM harmonic model. Its performance has ultimately been verified by simulation and experimental results obtained with two different modulation schemes. [ABSTRACT FROM AUTHOR]

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.

In this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme. [ABSTRACT FROM AUTHOR]

This article presents a new modulation technique with feedforward compensation for the three-phase three-level neutral-point-clamped converter. With the proposed technique, the capacitor voltage ripples are allowed to vary within certain limits. This enables an optimized design of the converter since the maximum capacitor voltages are predefined. Furthermore, the proposed modulation technique is able to reduce switching power losses compared to modulation techniques that eliminate capacitor voltage ripples completely. The proposed technique is therefore a tradeoff solution between aforementioned techniques and traditional modulation techniques where the capacitor voltage ripples are not limited. In the proposed technique, if the capacitor voltages are within the tolerable specified range, all the phases switch in two consecutive voltage levels in steady state. When the capacitor voltages go beyond the specified limits, one of the three phases is chosen to switch in three levels to inject an appropriate neutral point current for capacitor voltage balance. As the capacitor voltages are allowed to oscillate, the modulation technique is provided with feedforward compensation to avoid producing low-frequency distortion on the output voltages of the converter. Experimental results are presented operating under different loading conditions. [ABSTRACT FROM AUTHOR]

Power electronic converter plays a significant role in the hydropower generating systems of both fixed and variable speed types. These converters are used in the form of a simple rectifier for the static dc excitation in the fixed speed system and in the form of the ac excitation (ac–ac or ac–dc–ac) in the variable speed system. The parallel-connected multiple three-level neutral-point-clamped (3L-NPC) megawatt back-to-back (BTB) converters have been adopted in recently commissioned variable speed pumped storage units (VSPSU). For such configurations, reliability and efficiency are considered to be prominent. This article proposes a methodology for effective utilization of a parallel-connected BTB power converter for VSPSU of large capacity by performing the load-dependent activation /deactivation of converters. A vector control strategy for the active and reactive power deliveries of VSPSU is formulated. This article also focusses on the losses and thermal behavior of converters at different speeds of generator in accordance with water head variations. The proposed control system is designed in such a way to decrease overall losses in the power converter system. The effectiveness of the proposed method is explained on a real scale of 250-MW doubly fed induction machine with 5 × 5 MW power converters using MATLAB/PLECS software, and experimental validation is performed through a downscaled 2.2-kW laboratory prototype. [ABSTRACT FROM AUTHOR]

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.

The neutral-point voltage-balancing control is a critical issue of a neutral-point-clamped converter. The conventional control strategy is to inject the specific zero-sequence voltage to regulate the redundant vector, but it is unacceptable to the three-phase four-wire system. This paper provides a neutral-point voltage-balancing control by the dipolar modulation, and the control freedom of the zero-sequence voltage is flexible. Therefore, the proposed method is suitable for both three-phase three-wire and three-phase four-wire systems. Besides, the proposed method improves the control dynamic in a reactive power system. The proposed neutral-point voltage-balancing control is verified by the mathematical analysis and experimental results. The control dynamic, power loss estimation, and the output current performance are analyzed in this paper. [ABSTRACT FROM PUBLISHER]

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.

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. [ABSTRACT FROM AUTHOR]

Model predictive control (MPC) schemes have become popular in the field of power electronics due to their intuitive formulation, flexibility, and ease of implementation. Typically, these schemes have been implemented with the prediction horizon limited to one time-step, and extension of the prediction horizon over multiple time-steps remains an ongoing area of research. In this paper, a variant of the MPC strategy is proposed wherein the slope of the output trajectories is used to emulate long prediction horizons. Each of the outputs, e.g., current, voltage, torque, or flux, is regulated within a set of symmetrical bounds. When switching is necessitated due to collision with a bound, the switching state that yields the set of output trajectories with the minimum slope, relative to the reference trajectory, is applied to the converter. The key benefit of this approach is its ability to achieve low switching frequencies with a minimal level of computational burden. The feasibility of the scheme, which can be adapted easily to different case studies, is demonstrated through simulations of both a medium-voltage induction machine drive and a grid-connected converter. Experimental results, which are presented for a 1.68 kVA prototype grid-connected neutral-point-clamped converter, further demonstrate the practical viability of the proposed strategy. [ABSTRACT FROM PUBLISHER]

In this paper, a new medium voltage power converter topology using a diode rectifier, three-level boost (TLB) converter, and neutral-point-clamped (NPC) inverter is proposed for a high-power permanent magnet synchronous generator-based wind energy conversion system. The generator-side TLB converter performs the maximum power point tracking and balancing of dc-link capacitor voltages, while the grid-side NPC inverter regulates the net dc-bus voltage and reactive power to the grid. A significant improvement in the grid power quality is accomplished as the NPC inverter no longer controls the dc-link neutral point voltage. A model predictive strategy is proposed to control the complete system where the discrete-time models of the proposed power electronic converters are used to predict the future behavior of control variables. These predictions are evaluated using two independent cost functions, and the switching states which minimize these cost functions are selected and applied to the generator- and grid-side converters directly. In order to comply with the high-power application, the switching frequencies of the TLB converter and NPC inverter are minimized and maintained below 1.5 and 1 kHz, respectively. The proposed topology and control strategy are verified through MATLAB simulations on a 3-MW/3000-V/577-A system and dSPACE DS1103-based experiments on 3.6-kW/208-V/10-A prototype. [ABSTRACT FROM AUTHOR]

Abstract: 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 also keeping the capacitor voltages balanced. 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. [Copyright &y& Elsevier]

This paper investigates the problem of low-frequency voltage oscillations that appear at the neutral point (NP) of a three-level neutral-point-clamped (NPC) converter. Starting with a detailed analysis of their origin, the paper derives the minimum amplitude of these oscillations that can be achieved by nearest-vector (NV) modulation strategies. It then proves that the criterion of the direction of dc-link capacitor imbalance, which is commonly adopted by NV strategies for performing the task of capacitor balancing, poses a barrier in achieving this minimum. A new criterion is proposed instead, together with an algorithm that incorporates it into existing NV strategies. For the case of NPC inverters operating as motor drives, the resulting reduction in the amplitude of NP voltage oscillations ranges from 30% to 50%. The approach has the advantage of avoiding the significant increment in switching losses and output voltage harmonic distortion, caused by other methods. Simulations in MATLAB-Simulink are used to illustrate its operation and verify that it offers the claimed benefit. [ABSTRACT FROM AUTHOR]

Nearest vector (NV) modulation strategies for the neutral-point-clamped converter are known to generate low-frequency neutral point (NP) voltage oscillations. Non-NV strategies can eliminate these oscillations, but at the expense of higher switching losses and output voltage harmonic distortion. This letter proposes a simple way of creating hybrid strategies, as combinations of NV and non-NV strategies, which are also able to eliminate NP voltage oscillations. The approach minimizes the participation of non-NV strategies and hence their drawbacks, while it can be applied to any type of load (nonlinear and/or unbalanced). Simulations in MATLAB–Simulink are used to illustrate its operation. [ABSTRACT FROM AUTHOR]

This paper describes the experimental results of a medium-voltage modular 35-MW drive for oil and gas applications realized by interconnecting voltage-source three-level converters equally sharing the motor current. Drives rated for tens of megawatts are increasingly needed as torque helpers for gas turbines in the oil and gas industry, and one envisions that they will replace them completely in the future. These drives must be very reliable, often capable of reversible power flow and high power quality when connected to weak grids in remote areas. The pronounced torsional eigenmodes of large compressors and gas turbines demand for very low torque harmonic content. [ABSTRACT FROM AUTHOR]

This paper presents an optimal voltage-balancing compensator for a specific modulation technique applied to a neutral-point-clamped converter. The technique uses two modulation signals per phase, and it is called double-signal pulsewidth modulation. It completely eliminates low-frequency oscillations in the neutral-point voltage. However, it does not provide natural voltage balancing; therefore, a compensation loop is required. The proposed control generates a feedback compensation signal that correctly modifies the three-phase modulation signals. The optimal compensation signal is calculated by a dynamic limiter according to the intrinsic limitations of the system related to the variability range of the modulation signals. It significantly improves the voltage balancing under all operating conditions of the converter. In addition, this compensation strategy does not increase the switching frequencies of the power devices. The algorithm is tested and verified using both simulation and experimentation. [ABSTRACT FROM AUTHOR]

This paper presents a new low-loss modulation technique for the hybrid three-level four-leg converter. The total losses of the converter are reduced by about 18% on average compared to the standard three-leg neutral-point-clamped converter. Furthermore, the low-frequency voltage oscillation in the neutral point is completely cancelled, and the maximum benefit of the dc-link voltage is obtained. All these facts, together with the fault-tolerant ability due to the fourth leg, make this topology very interesting for applications such as wind generation, in which it is important to maximize efficiency and reliability. Some experimental results confirm the good performance of the proposed modulation technique. [ABSTRACT FROM AUTHOR]

This paper presents some modified topologies of the neutral-point-clamped converter. In all of them, the main change consists of adding a fourth leg, which is based on the flying-capacitor converter structure. The aim of this additional leg is to provide the converter with fault tolerance. Furthermore, during normal operation mode, this leg is able to provide a stiff neutral voltage. Consequently, the low-frequency voltage oscillations that appear at the neutral point of the standard three-level topology in some operating conditions no longer exist. As a result, the modulation strategy of the three main legs of the converter does not have to take care of voltage balance, and it can be designed to either achieve optimal output voltage spectra or improve the efficiency of the converter. Simulation and experimental results are presented to show the viability of this approach both under normal operation mode and in the event of faults. [ABSTRACT FROM AUTHOR]

This paper investigates low-frequency beat and harmonics in grid-connected three-level pulsewidth-modulated (PWM) voltage-source converters with low switching frequencies. The impact of switching frequencies and switching sequence, as well as mitigation techniques, are studied. Different from some recent results, the analysis confirms the advantages of selecting switching frequency, as an odd-triplen multiple of the grid operating frequency, for eliminating imbalance and undesirable even-order harmonics, which can negatively impact the dc-link neutral-point balance in a three-level converter. Tests and analysis show that low-frequency subharmonic beat can occur in a carrier-based PWM converter when the switching frequency is low and is an imperfect multiple of the grid frequency. Recognizing the beat as interaction of the switching sequence and switching frequency, mitigation techniques to reduce the beat and associated harmonics are investigated and verified through experiments. A simple displacement of switching frequency from the odd-triplen multiple of the grid frequency can effectively suppress the beat with an asynchronous PWM scheme. [ABSTRACT FROM AUTHOR]

This paper presents a novel modulation strategy for a neutral-point-clamped converter. This strategy overcomes one of the main problems of this converter, which is the low-frequency voltage oscillation that appears in the neutral point under some operating conditions. The proposed modulation strategy can completely remove this oscillation for all the operating points and for any kind of loads, even unbalanced and nonlinear loads. The algorithm is based on a carrier-based pulsewidth modulation. Nevertheless, it can generate the maximum output-voltage amplitudes that are attainable under linear modulation, such as space-vector modulation. Furthermore, this technique can be implemented with a very simple algorithm and, hence, can be processed very quickly. The only drawback of this strategy is that the switching frequencies of the devices are one third higher than those of standard sinusoidal pulsewidth modulation. A control loop for balancing the voltages on the dc-link capacitors is also proposed. This balancing strategy is designed, so that it does not further increase the switching frequencies of the devices when it is applied to the converter. The proposed modulation technique is verified by simulation and experiment. [ABSTRACT FROM AUTHOR]

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. • A modulation approach for capacitors balance in NPC converters is proposed. • The advantages are simplicity and validity for unbalanced-grid-voltage conditions. • The number of commutations is optimized. • An enhancement is proposed to work under low-power-factor conditions. • Experimental comparison with virtual space vector is performed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]