Neutral Point Voltage Research Articles

A Fast-Processing Predictive Control Strategy for Common-Mode Voltage Reduction in Parallel Three-Level Inverters

Model predictive control (MPC) is an attractive control scheme, in particular, for multilevel converters. However, MPC suffers from high computational burden and tedious tuning of weighting factors especially for parallel three-level inverters. Moreover, the parallel three-level inverters will unavoidably face the challenge of maintaining low circulating current, reducing common mode voltage (CMV), and balancing neutral point (NP) voltage in practice. Therefore, in this article, a fast-processing predictive control strategy is proposed for reducing the zero-sequence circulating current (ZSCC), NP voltage imbalance and CMV simultaneously in parallel three-level inverters. Based on the selected sector by the reference voltage vector, the proposed fast-processing predictive control takes feasible five or six candidate vectors into account; therefore, the computational burden, which is a disadvantage of the predictive control for inverters, will decrease. In addition, the proposed method eliminates the weighting factor, which avoids tedious tuning work. In the proposed control scheme, the candidate vectors are selected for ZSCC, NP voltage imbalance, and CMV reduction, and then, among them, the optimized switching vector is selected by optimizing the cost function. The performance of the proposed method has been validated by the simulation and experimental results.

A Hybrid Control Method for Neutral-Point Voltage Balancing in Three-Level Inverters

Neutral-point (NP) voltage fluctuation is an inherent problem in three-level inverters. Without any control measures, the NP voltage will deviate, and the dc bias and ac oscillation can be observed, which may lead to the failure of power devices and capacitors, so as to damage the system security. In this letter, a hybrid control method is proposed, which it combines zero-sequence voltage regulation with the decomposition of modulation waveform organically. This carrier-based method is simple with lower switching loss due to the unchanged switching events per switching cycle. Experiments are carried out in a 10-kVA three-level platform, which demonstrate the validity of proposed method.

Optimal switching sequence model predictive control for three‐level NPC grid‐connected inverters

In order to concentrate the frequency spectrum of the output voltage and improve the quality of grid currents for the three-level neutral point clamped inverter with the model predictive control, this paper proposes an optimal switching sequence model predictive control algorithm. Based on the increments of grid currents and the neutral-point voltage, the predictive model of the inverter is established in αβ frame. Moreover, including grid currents and the neutral point voltage tracking, the cost function is designed and simplified by arranging the voltage vector sequence in a sampling cycle appropriately. Meanwhile, the calculation of the optimal dwell time for each voltage vector sequence is derived by Lagrange multiplier method, and its solving process is simplified to reduce online computations. Furthermore, according to different voltage vector sectors, a voltage vector sequence preselection principle is introduced in this paper, besides, considering redundant small vectors in a pair have opposite affections on the neutral point voltage, the amount of voltage vector sequence that needs to be verified is further cut down, for obtaining the optimal voltage vector sequence. Finally, the experimental results show that the proposed method can concentrate the output frequency spectrums, which maintains a good quality of grid currents with a small neutral-point voltage fluctuation.

A Virtual Space Vector PWM With Active Neutral Point Voltage Control and Common Mode Voltage Suppression for Three-Level NPC Converters

Virtual space vector pulsewidth modulation (VSVPWM) provides a new approach to neutral point voltage (NPV) control and common mode voltage (CMV) suppression. Few values of coefficients configured in the virtual vectors cannot adjust the NPV continuously. An improved VSVPWM (IVSVPWM) with active neutral point voltage control (ANPVC) and CMV suppression is proposed in this article. Continuously variable coefficients configured in the virtual vectors adjust the NPV actively and continuously for any load power factor over the full modulation indexes. Failure of CMV suppression is analyzed, and the undesired vectors with high CMV are avoided by limiting the range of the variable coefficients. The limited variable coefficients and the selected basic vectors with low CMV suppress the CMV effectively. Simulation and experimental results are given to validate the proposed novel IVSVPWM, which can achieve quicker ANPVC and reduce CMV to half of the conventional VSVPWM simultaneously.

An Advanced Modulation Technique Featuring Neutral Point Voltage Ripple Suppression of Three-Level Converters in High-Speed Drives

This paper introduces an advanced space vector modulation technique for three-level neutral-point-clamped (3L-NPC) converters. The studied 3L-NPC converters within aircraft electric starter generator (ESG) systems normally operate at high modulation index, low pulse ratio and full power factor range for most of the operation time. Conventional modulation techniques exhibit large neutral point voltage ripple and possible voltage imbalance under such conditions. The increase of neutral point voltage ripple implies significant increase in converter capacitor size and decrease in capacitor reliability. The modulation technique introduced in this paper achieves improved neutral point voltage balancing within each switching period through enhanced applications of small vectors and restriction on medium vectors. The advanced implementation of small vectors is achieved using a sharing factor aided by a robust incremental current prediction procedure. With the predicted machine currents, the proposed method enables the optimum application of small vectors to balance the neutral point voltages. Simulation results based on a PLECS/Simulink model and experimental results obtained from a 45kVA, 32krpm aircraft electric starter generator prototype platform validate the reduction of the neutral point voltage ripple.

A Carrier-Based Discontinuous PWM Strategy for T-Type Three-Level Inverter with Reduced Common Mode Voltage and Neutral Point Voltage Control

A novel carrier-based discontinuous pulsewidth modulation (CB_DPWM) strategy for T-type three-level converter is proposed in this article. Because one significant advantage of the proposed CB_DPWM is the reduction of common mode voltage (CMV), it is named as RCMV_CB_DPWM hereafter. It also can reduce switching loss and remove dc offset from the neutral point voltage (NPV) simultaneously. RCMV_CB_DPWM has two design freedoms, one is modulation wave and the other is carrier wave. By injecting zero sequence voltage properly, the corresponding clamping mode can be acquired. Moreover, reduced CMV can be achieved by arranging carrier wave flexibly. The integrity of RCMV_CB_DPWM is analyzed as well as its NPV self-equilibrium ability, and an active NPV control method is then introduced. Moreover, its realization is discussed briefly. Finally, the experiments are carried out to verify the effectiveness of the proposed method.

Hybrid Modulation Strategy for the Vienna Rectifier

The Vienna rectifier can produce three voltage levels, however only the connection to the neutral-point is fully controllable using the bidirectional switches. When the neutral-point voltage is not imposed in a phase-leg, the polarity of the pulses generated depends on the diode that is conducting in that phase-leg, which is defined by the current direction. As a result, the voltage pulses generated can go in the opposite direction to what is demanded by the controller, and the current becomes distorted. This can be mitigated by injecting a particular zero sequence so that the reference voltage is clamped to the neutral-point during the intervals, where the current of a phase-leg has the opposite sign than its reference voltage. However, the converter is at risk of entering into overmodulation when operating with high modulation indices. In this article, a hybrid modulation strategy is introduced where a calculated minimal amount of reactive current depending on the operating conditions is injected to avoid overmodulation. The reduction of reactive current required compared to the state-of-the-art solutions is up to 100% in certain operating conditions. With the proposed method, the Vienna rectifier can operate in a wide range of power factors without compromising the quality of the source currents. The concept is verified by simulation and in a 1-kW Vienna rectifier experimental setup.

Model Predictive Controllers With Capacitor Voltage Balancing for a Single-Phase Five-Level SiC/Si Based ANPC Inverter

Employing both high bandwidth (HBW) controller and wide bandgap (WBG) devices in the structure of converters improve the system size, performance, and efficiency. In this paper, HBW model predictive controllers (MPCs) are proposed, with both fixed and unfixed switching frequencies, to control a single-phase five-level hybrid active neutral-point-clamped (ANPC) inverter. A hybrid modulation technique is considered in this paper, in which some of the switches are modulating with high frequency. Therefore, Silicon-Carbide (SiC) MOSFETs are employed in the converter structure to increase the switching frequency and consequently reduce the filter size and increase converter power density. To have the functionality of multilevel output voltage, some restrictions are defined in the adopted MPC with unfixed switching frequency. In the MPC with the constant switching frequency, predefined switching sequences are employed for all sectors. Moreover, to control the neutral point (NP) voltage, the applied times of both small voltage vectors are sets through a cost function. Finally, the simulation and experimental results prove the ability of both proposed methods to control the voltages of the load and NP.

A Simple Space Vector Modulation Method With DC-Link Voltage Balancing and Reduced Common-Mode Voltage Strategy for a Three-Level T-Type Quasi-Z Source Inverter

Single-stage converters based on an impedance source (IS) network merged with a multilevel inverter remain a trending research topic, from both the topological and control perspectives, because of their potential in applications involving renewable energy. This paper presents a new, simplified space vector pulse-width modulation (SVPWM) for a three-phase three-level T-type quasi-impedance source inverter (3L-T-type qZSI). The benefits of the proposed modulation strategy include functionalities, such as boosting the input voltage and controlling the neutral-point (NP) voltage using the redundant small vectors, to mitigate the inner capacitors voltages imbalance. The proposed SVPWM guarantees a minimum common-mode voltage while preserving a very simple implementation, thus reducing the complex computations and cascaded proportional-integral (PI) control loops of the conventional SVPWM methods. The proposed algorithm can be applied to different multilevel inverter configurations combined with an IS network. A comprehensive simulation study and experimental results demonstrated the good performance of the proposed control method for dynamic response and imbalance conditions.

Modulation Strategies for Anisotropy-Based Position Estimation of PMSMs Using the Neutral Point Voltage

Information obtained about magnetic anisotropy via neutral point voltage measurements can be used to estimate the position of synchronous and induction machines at all speeds including standstill. Due to its usually high signal-to-noise ratio, this estimation method can be an attractive alternative to approaches that acquire anisotropy information via current measurements. The position estimation method looked at in this paper relies on anisotropy information in the form of position-dependent inductance ratios. These are obtained through measurement of the voltage between the machine's neutral point and an artificial neutral point during the application of different voltage vectors via a two-level switching inverter. We analyze the conditions placed on and the consequences of implementing necessary modifications to a standard space vector modulation. Five different modified modulation strategies are compared and investigated, including two newly proposed strategies which use only a minimal set of active and zero voltage vectors for measurement of the inductance ratios and therefore allow high utilization of available voltage and a high update rate of the estimated position. Experimental results for three low-power three-phase permanent magnet synchronous machines are presented which suggest that modulation strategies that use active measurement vectors in all three instead of only two axes of the machine are less susceptible to systematic deviations in the position estimation that presumably result from nonlinear machine properties. As part of the machine model, a normalizing inductance variation ratio is introduced, which simplifies expressions and supports the comparison of motors.

Fixed frequency Finite Control Set Model Predictive Control For Three-level APF

Active power filter can compensate harmonic and reactive power, Three level neutral point clamp (ANPC) has the characteristics of low output harmonic and low device loss. The control of three-level active power filter needs to consider multiple objectives. Finite control set model predictive control (FCS-MPC) is a new method which can add constraints and multi-objective control. According to the limit of the switch state, it can track the reference current, which has the characteristics of fast dynamic response and good compensation effect. This method not only keeps the neutral point voltage balance, but also solves the problem of unstable switching frequency.

A Hybrid Method for Three-Level T-Type Inverters to Eliminate Neutral Point Voltage Ripple and Control DC-Link Capacitors’ Voltage for Separate MPPT

Nowadays, photovoltaic (PV) systems based on centralized topology with a three-level T-type (3LT2) inverter are used to decrease investment costs and increase conversion efficiency. In this topology, the split capacitors of the DC-link can individually connected to PV arrays. Accordingly, DC-link capacitors’ voltage should separately controlled to track PV arrays’ maximum power point (MPP) voltage. However, the asymmetric control of capacitors’ voltage causes line voltage to unbalance and output current to distort. In this paper, a hybrid method based on dipolar variable injection and zero-sequence voltage injection concepts is proposed to simultaneously control the DC-link capacitors’ voltage of the 3LT2 inverter and eliminate their voltages ripple. In the presence of partial shading, the proposed method can control the capacitors’ voltage of the DC-link to realize the separate MPP tracking while ensuring the balance of line voltages. Furthermore, the proposed method eliminates the voltage ripple on DC-link capacitors, inherently caused by the 3LT2 inverter, to improve the DC-link lifetime and the PV system reliability. The simulation studies and practical experiments have been performed, and their results prove the validity of the proposed method, whether the PF is unity or non-unity.

An Overmodulation Algorithm With Neutral-Point Voltage Balancing for Three-Level Converters in High-Speed Aerospace Drives

In this article, a virtual space vector based overmodulation algorithm is presented for three-level neutral-point (NP) clamped converters in high-speed aerospace motor drives. With the proposed inscribed polygonal-boundary compression technique, the output voltage capability is enhanced under a lower crossover angle and compression coefficient. As a result, it brings an opportunity for the operation of the studied aircraft electric starter/generator (ESG) systems easily extending from the linear modulation range into the overmodulation region. Furthermore, an active capacitor voltage balancing control method is investigated to recover NP potential imbalance in the case of high modulation index and low power factor operating conditions. To simplify the digital implementation of the algorithm, a fast calculation approach is adopted in this work. The modulation performance of the proposed strategy is verified by both simulation and experimental results with a 45 kW, 32 k r/min ESG prototype system.

Hybrid Discontinuous Space Vector PWM Strategy for Three-Level Inverters under Two-phase Loads Condition

The output performance of three-level inverters will be deteriorated under two-phase loads condition if conventional discontinuous PWM (DPWM) strategies are adopted. To solve the above issue, the amplitude and the phase angle of basic voltage vectors are modified in this article. Accordingly, sectors and triangles of the space vector diagram are reconsidered, duty cycles of basic vectors synthesized the reference vector are recalculated. And then four types of DPWM applicable for two-phase load are put forward. Although the harmonic distortion of the output waveform is reduced, discontinuous intervals are not optimum distributed. To further improve the output waveform quality, the amplitude of average current ripple vector is introduced as the evaluation index to redistribute discontinuous intervals of phase voltage in each fundamental period. Then, a hybrid DPWM strategy is presented and the harmonic distortion of the output waveform is minimized. Meanwhile, the neutral-point voltage ripple and the switching loss are also suppressed. The effectiveness of the hybrid DPWM strategy is verified by both simulation and experimental results.

Common-Mode Voltage Reduction Method for Three-Level Inverter With Unbalanced Neutral-Point Voltage Conditions

The three-level inverter has been widely researched and adopted in industry. This article further proposes a carrier-based modulation method to reduce the common-mode voltage (CMV) and obtain high-quality output currents for three-level inverter with unbalanced neutral-point voltage conditions. In this method, the zero-sequence voltage is calculated by three-phase modulation waves, three-phase output currents, and voltages across the dc-link capacitors. The limitations of the zero-sequence component are carefully considered in order not to generate the basic voltage vector with high CMV magnitudes. In addition, a deadbeat control scheme for controlling the dc-link voltages asymmetrically with the proposed pulsewidth modulation method is presented, and asymmetrical control of dc-link voltages can be realized with fast response. Compared with the conventional space vector modulation (SVM) and the virtual SVM methods, the proposed scheme can reduce the CMV by half. Simulated and experimental results validate the effectiveness of the proposed method.

A Novel Method to Evaluate the Influence of Vienna Rectifier Neutral-Point Voltage Fluctuation on Input Current Quality

The inherent neutral-point (NP) voltage fluctuation of the Vienna rectifier could have side effects on the input current quality. However, such effects have not been thoroughly analyzed in the literature yet. In this article, based on the three-level to two-level space vector modulation transformation, a novel method to evaluate the influence of the NP voltage fluctuation on the input current quality is introduced, which could optimize the NP control parameters. The proposed method superimposes the NP voltage fluctuation on the basis vectors, thereby establishing the correlation between the NP voltage fluctuation and the square root mean square of the input current ripple. The detailed model of the NP oscillation under two-level coordinates is presented. By combining the two-level modulation, NP voltage hysteresis control is adopted to simplify NP voltage control and suppress the NP low-frequency oscillation. The comprehensive influence of the NP compensation coefficient on the input current quality under NP hysteresis control is obtained, and the hysteresis control parameters are optimized by using the proposed evaluation method. The simulation and experimental results verify the correctness and effectiveness of the proposed evaluation method.

Generalised model predictive control scheme for three‐level converter under unbalanced grid voltage considering full power factor operation

With the increase of grid-connected renewable power generation units, the three-level converter (TLC) is widely utilized. However, when unbalanced grid voltage occurs, the converter should provide a certain of reactive power to satisfy the grid code, which decreases the power factor and degrades neutral-point (NP) voltage balance. The existing model predictive control methods for NP voltage balance in TLC mainly focus on high power factor operation, which are not applicable to low power factor operation. To solve this problem, this study proposes a generalized model predictive control (GMPC) scheme for the TLC. By analyzing the influence of P-type and N-type small vectors on NP voltage, it is indicated that the charging and discharging operation of the dc-link capacitors are determined by the grid currents directions with the defined small vectors. Since the directions of grid currents are changed with different power factors, the effect of one small vector on NP voltage is change. In the proposed GMPC scheme, the direction of grid currents and the capacitor voltage difference are simultaneously used to select the appropriate small vector to mitigate the NP voltage unbalance effectively in the full power factor range. The validity is verified by simulation and experimental results.

Hybrid Modulation Technique With DC-Bus Voltage Control for Multiphase NPC Converters

The article presents a novel carrier-based pulsewidth modulation technique for multiphase neutral point clamped converters. The technique is aimed to actively control the neutral point (NP) potential while supplying the desired set of line-to-line voltages to the load. Standard techniques are either based on the sole common mode voltage injection or on the sole multistep switching mode; contrarily, the proposed algorithm combines these two approaches to take advantage of their main benefits. The technique performs well for each number of phases, for each modulation index, and for each type of load. It can control in closed-loop the NP voltage to any desirable value with a reduced number of switching transitions. The proposed approach has been experimentally validated and compared with other carrier-based algorithms.

Operation of a Seven-Level T-Type Active Neutral-Point-Clamped Converter With Modified Level-Shifted PWM

This article presents novel control strategies for a seven-level (7L) T-type active neutral-point-clamped (T-ANPC) converter based on the level-shifted pulse width modulation (LS-PWM) technique. Compared with classical 7L topologies, such as NPC, flying capacitor converter, and cascaded H-bridge converter, the 7L T-ANPC converter can reduce the device requirements while reserving the advantages of high redundancy and low conduction losses. Furthermore, the internal switching state redundancy provides freedom to balance the flying capacitor (FC) voltage and realize the fault-tolerant operation flexibly. The control strategy with modified LS-PWM technique is developed for the T-ANPC to obtain desired outputs under normal and switch open fault modes. The proposed control strategy considers the complete balancing of dc-link neutral-point voltage and FC voltages. The feasibility of the proposed 7L T-ANPC and control strategy is evaluated by simulations and experimental tests under steady-state and several typical transient operating conditions.

Neutral-Point Voltage Balancing Methods of Series-Half-Bridge LLC Converter for Solid State Transformer

With the advancements in silicon carbide (SiC) power devices, the solid state transformer (SST)-based power delivery architecture shows advantages over traditional line-frequency transformer-based solution in terms of efficiency, power density, modularity, and scalability. This article presents a three-level converter cell for a modular SST, which consists of a neutral-point-clamped (NPC) H-bridge ac-dc stage and a series-half-bridge (SHB) LLC dc-dc stage. Compared with two-level cell-based SST, the total cell number is reduced to half, which means less system complexity and cost. The neutral-point (NP) voltage within each cell must be balanced for proper operation of the system. A simple and effective balancing method is proposed by adding a phase-shift angle to the modulation of the SHB LLC converter with zero-voltage switching. This idea is applicable to both two-level and three-level modulation modes for wide-range operation. Further, a novel NP voltage balancing method for burst-mode operation under light-load condition is proposed by short-circuiting the transformer secondary side during burst-off period and utilizing the resonant tank energy to balance the NP voltage without disturbing the load or the grid. The proposed method is experimentally verified on the SiC-based three-level converter cell operated around 200 kHz for SST.