Neutral Point Voltage Balance Research Articles

Hybrid Virtual Coordinate-Driven CBPWM Strategy of Three-Level T-Type NPC Converters for Electric Aircraft Propulsion Applications

Green airborne transportation driven by the state-of-the-art turboelectric propulsion technology has received considerable attention from the aviation industry. To generate electrified thrust, three-level neutral-point-clamped (3L-NPC) converter-fed motor drives are one of the most promising candidates for the powertrain of future aircraft. However, the neutral-point (NP) potential ripple and deviation lead to harmonics and imperil the lifespan of semiconductors and capacitors. Besides, excessive power losses on the converter can limit propulsive power and shorten flight distance. To solve these issues, a new hybrid modulation strategy is proposed in this paper. With respect to the takeoff and climb featuring NP voltage drift-prone regions, carrier-based (CB) coordinate-driven modulation waves are exploited to enable virtual-space-vectors (VSVs). For operating points with high power factor, such as cruise, by reconstructing modulated signals, not only can the NP voltage balance be realized, but also the switching loss is reduced using active discontinuous pulse trains. The less computational burden is attributed to implementations in the sextant-coordinate system. The effectiveness of the proposed algorithm is verified by simulation results from a Simulink/PLECS model and experimental results obtained from a 200 kVA silicon-carbide (SiC) based T-type 3L-NPC prototype with a variable output fundamental frequency.

A Novel Virtual Space Vector Modulation With Optimized Neutral-Point Voltage Control Capability for Ten-Switch Three-Phase Three-Level Inverter

Ten-switch three-phase three-level inverter (TP-TLI) is receiving more attentions owing to the reduced system cost compared to traditional TP-TLIs. However, it still faces serious neutral-point voltage (NPV) balance problem. This article proposes a novel virtual space vector modulation (NVSVM) with optimized neutral-point voltage control (NPVC) capability. A set of virtual vectors are selected to generate zero NP current. By analyzing the vector synthesis principle over the full modulation areas, a new control degree, which is utilized to optimize the NPVC capability and reduce line voltage THD, is introduced. Besides, a dead time compensation method based on volt-second balance is also given to reduce the switching loss. The NVSVM not only improves the NPVC capability without dc offset and obvious ac oscillations in NPV under full range of modulation index and power factor (PF) conditions, but also reduces line voltage THD and the switching loss. Theoretical analysis shows that the available NP current for the proposed NVSVM is about 2.75 times greater at most than that for traditional VSVM. Simulation and experimental results are given to validate the effectiveness of the NVSVM and control strategies.

Comprehensive Analysis of Vienna Rectifiers for Renewable Resources

The issue of energy has emerged as a significant subject of concern in contemporary discourse. The significance of the advancement of novel energy sources is progressively growing. Relevant strategies should aid in the advancement of power electronics. The rectifier is a commonly used electronic component that finds application in many circuits involving electronic equipment. Its primary function is to convert alternating current into direct current, which is then supplied to the load. The Vienna rectifier is a widely used rectifier that has garnered significant attention and scrutiny from professionals in the field of power electronics on a global scale. This article begins by illustrating the use of power electronics technology, specifically focusing on the charging pile circuit. It proceeds to discuss the classification and current research status of rectifiers. Subsequently, the article provides an overview of the circuit layout and operational principles of the Vienna rectifier circuit. This section provides an overview of the approaches and features used to optimize the operating performance of Vienna rectifiers in four key areas: current control, voltage control, neutral point voltage balance management, and harmonic stability control. This research will ultimately provide a forecast about the future development trend of Vienna rectifiers, together with a comprehensive overview of all improvement attempts.

A Modified Carrier-Based PWM Strategy for Common Mode Voltage Elimination and Neutral Point Voltage Balance in a Unidirectional Three-Level Converter for AC Motor Drives

A Modified Carrier-Based PWM Strategy for Common Mode Voltage Elimination and Neutral Point Voltage Balance in a Unidirectional Three-Level Converter for AC Motor Drives

Discrete space vector modulation and optimized switching sequence model predictive control for three-level voltage source inverters

This paper proposes a discrete space vector modulation and optimized switching sequence model predictive controller for three-level neutral-point-clamped inverters in grid-connected applications. The proposed strategy is based on cascaded model predictive control (MPC) for controlling the grid current while maintaining the capacitor voltage balanced without weighting factor. To enhance the closed-loop performance, the external MPC evaluates 19 basic and 138 virtual vectors (VV) of the proposed space vector method. The optimal control voltage is then selected using an extended deadbeat method to reduce the execution time of the proposed control algorithm. By using the discrete space vector modulation principle, the VV are synthesized based on switching sequence (SS) and are divided into negative and positive SSs considering their impact on the neutral point (NP) potential. The inner MPC evaluates both types of SSs and selects the one that keeps the capacitor voltage balanced. Various controllers are evaluated and compared against the proposed control strategy. The results show that the proposed strategy improves performance without weighting factor, while maintaining a total harmonic distortion of current to be less than 2%. Compared to the modulated MPC which provides the same fixed switching frequency, the proposed controller reduces the computational burden by over 50% while also providing better NP voltage balance accuracy.

An Asymmetrical Active-Neutral-Point-Clamped Five-Level H-bridge Inverter for Open-Winding Motor Applications With Its Modulation Strategy

Asymmetrical active-neutral-point-clamped five-level H-bridge (ANPC5L-HB) inverter has reduced switch count and fewer high-frequency switches compared with other existing five-level H-bridge topologies. However, multiphase ANPC5L-HB inverter for open-winding motor applications is rarely researched. In this paper, a three-phase ANPC5L-HB inverter for open-winding motor applications is proposed. The modulation and neutral point (NP) voltage balance strategies for the inverter are also proposed. By using the proposed modulation strategy, transitions between different switching states are constrained to avoid unexpected intermediate states. Meanwhile, zero voltage switching (ZVS) of low-frequency switches could be achieved. For the NP voltage balance problem, optimal switching states are selected by using the MPC approach, and a better NP control performance is realized compared with the classical method. The effectiveness and high efficiency of the inverter are validated by a 25kW experimental prototype. And a peak efficiency of 98.685% is confirmed under 10kHz switching frequency. The good performances of the proposed strategies have been fully verified.

A Novel Modulation Scheme for Simultaneous Common-Mode Voltage Reduction and Neutral-Point Voltage Balance in the Reduced Switch Count Quasi-Z-Source Three-Level Inverter

The reduced switch count quasi-Z-source three-level inverter (RSC-QZS-TLI) not only maintains the advantages of continuous input current in the conventional quasi-Z-source three-level inverter (C-QZS-TLI), but also reduces the number of power switches and system cost. However, it inevitably faces the challenges of high common-mode voltage (CMV) and neutral-point voltage (NPV) imbalance, which threaten system safety and reliability. Moreover, due to the inherent features of this topology, the medium vectors cannot be generated, which means that the traditional modulation method is not applicable any more. To address the above challenges, a novel modulation method is proposed. The pre-selected basic vectors are optimized primarily, which restrict the CMV magnitude within one-sixth of dc-link voltage. Then, for the particularity of RSC-QZS-TLI, the distribution factor of small vector and its limitation range are introduced for the first time, and an indirect calculating approach is developed to obtain the duty cycles of the pre-selected four vectors, which maintains the correct ac output voltage and dc voltage boosting ability. Meanwhile, according to sector number and output of NPV controller, the distribution factor is modified to control the NPV balance. Compared with the traditional modulation method, the CMV magnitude of the proposed method is reduced by 50 %, and the balanced NPV is controlled actively. Simulated and experimental results verify the feasibility of the proposed scheme.

An Improved Reference Voltage Decomposition Method Based on Three-Level NPC Converters With Neutral-Point Voltage Balancing and Common-Mode Voltage Reduction

In this paper, an improved reference voltage decomposition method (RVDM) that takes account of neutral-point (NP) voltage balancing and common-mode voltage (CMV) reduction for neutral-point-clamped three-level converter (NPC-TLC) is proposed. Based on the analysis of space vector diagram (SVD), CMV is simply reduced by injecting zero-sequence voltages at two-level SVD. Then, according to the principle of adjusting zero-state duty cycles to change the NP current, the switching states of a certain phase or two phases are adjusted from two to three. Therefore, two improved methods that take the balancing control of NP voltage and the reduction of CMV into account are introduced and named method I and method II respectively. Both two proposed improved methods are simple in principle and easy to implement. Considering reliability and output performance, method II is selected as the improved strategy. Finally, the superiority of the improved strategy is verified by experiments, and experimental results are consistent with the theoretical analysis.

A Resonant Switched-Capacitor Converter for Neutral-Point Voltage Balancing of Three-Level Inverters

A Resonant Switched-Capacitor Converter for Neutral-Point Voltage Balancing of Three-Level Inverters

Influence of switching sequences on the neutral point voltage balance in a three-level voltage source invertor

BACKGROUND:Ensuring the voltage balance of the neutral point of the DC link within acceptable limits is one of the mandatory requirements during the operation of a three-level autonomous neutral point clamped voltage source inverter. It is known that neutral point voltage imbalance can adversely affect the operation of the inverter and the load at all, lead to the failure of both power switches and capacitors in the DC link. Neutral point voltage imbalance mostly often occurs due to the asymmetry of the nominal values of DC capacitors, inconsistent properties of switching devices, asymmetric three-phase load, and also due to the imperfection of the converter control algorithm.
 AIMS:Selection of the optimal switching sequence that ensures the smallest deviation of the neutral point voltage and the number of switching of power switches in a three-level voltage source inverter.
 METHODS:For an objective comparison of the considered switching sequences and their influence on the neutral point voltage balance, a simulation model of a three-level neutral point clamped voltage source inverter was developed in theMATLAB/Simulinksoftware. To control this inverter, a space vector modulation method was used with three different switching sequences: five-segment, seven-segment and standard.
 RESULTS:The topology of a three-level neutral point clamped voltage source inverter is presented. The experimental dependencies of the neutral point voltage deviation error was taken with a change in the modulation coefficient, the frequency of the fundamental harmonic at the inverter output, and load characteristics for three different switching sequences.
 CONCLUSIONS:In this paper, the influence of the basic vectors and the influence of switching sequences on the neutral point voltage balance are studied. Combinations of states of large and zero basic vectors do not affect the neutral point voltage. For the medium basic vectors, the neutral point voltage can increase or decrease depending on the operating conditions of the inverter. The small basic vectors significantly affect the neutral point voltage. Considering the abovementioned, the seven-segment sequence that ensures both the optimal balance of the neutral point voltage and the level of switching losses should be considered as the best switching sequence for a three-level inverter.

Impartial Sequential Model Predictive Control of Parallel T-Type Rectifiers for Power Sharing and Circulating Current Elimination

A parallel T-type rectifier connection is often required to improve the system capacity and stability in high-power applications. However, the parallel rectifier faces the challenges of zero-sequence circulating current (ZSCC) elimination, neutral point (NP) voltage balance, DC voltage stability and power sharing. First, to solve the multi-objective optimization problem of two parallel three-level T-type rectifiers with LCL filters (LCL-3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Rs), a double closed-loop control scheme is adopted. In this scheme, an impartial sequential model predictive control (ISMPC) is proposed as the inner-loop controller for ZSCC elimination and NP voltage balance, and an adaptive droop control with voltage feedforward is designed as the outer-loop controller to achieve DC voltage stability and power sharing for each rectifier based on its capacity. Second, the main influencing factors of ZSCC in LCL-3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Rs are analyzed, and ZSCC elimination is considered as one control objective in ISMPC. Finally, ISMPC is proposed to solve the problem that sequential model predictive control (SMPC) could not select the optimal solution due to the fixed priority and the excessive process only controlled a single objective. The proposed method is tested on a prototype hardware platform of a 10-kW and a 5-kW parallel rectifier. Experiment results demonstrate the superiority of this method over existing methods under several typical scenarios.

Low-Complexity Multistep Sequential Model Predictive Current Control for Three-Level Inverter-Fed Linear Induction Machines

This paper proposes a low-complexity multistep sequential model predictive current control (M-SMPCC) for three-level inverter-fed linear induction machine (LIM) drives to further improve the steady-state drive performance at low switching frequency. In order to implement multistep MPC for performance improvement, the idea of reducing the search space to fewer voltage vectors (VVs) is adopted to reduce its dramatically increasing complexity. However, there are some problems of the existing simplified methods directly applied for long prediction horizons. First, the reference VV in existing methods cannot consider the performance of long prediction horizons, so a reference VV sequence considering current tracking performance and switching frequency is derived to guide the optimal VV search. Second, with the existing search methods, some VVs that yield better performance in long prediction horizons may be excluded in advance. Hence, a more detailed region division and search method is designed. Moreover, to address the inherent neutral point voltage (NPV) fluctuation in the topology, one sequential optimization strategy is proposed, in which the current tracking performance and NPV balance are guaranteed by two separate cost functions. Finally, the proposed method has validated through comprehensive simulation and experiments, showing lower current harmonics and thrust ripples at low switching frequency.

Low-complexity model predictive control of a four-level active neutral point clamped inverter without weighting factors

Abstract The four-level active neutral point clamped (ANPC) inverter has become increasingly widely used in the renewable energy industry since it offers one more voltage level without increasing the total number of active switches compared to the three-level ANPC inverter. The model predictive current control (MPCC) is a promising control method for multi-level inverters. However, the conventional MPCC suffers from high computational complexity and tedious weighting factor tuning in multi-level inverter applications. A low-complexity MPCC without weighting factors for a four-level ANPC inverter is proposed in this paper. The computational burden and voltage vector candidate set are reduced according to the relationship between voltage vector and neutral point voltage balance. The proposed MPCC shows excellent steady-state and dynamics performances while ensuring the neutral point voltage balancing. The efficacy of the proposed MPCC is verified by simulation and experimental results.

Topology and Space Vector Modulation Method for the Reduced Switch Count Quasi-Z-Source Three-Level Inverter

This paper proposes the topology and space vector modulation (SVM) method of a reduced switch count (RSC) quasi-Z-source (QZS) three-level inverter, which can realize voltage buck-boost operation by adopting ten power switches. Compared with the QZS three-level T-type inverter, the presented inverter can further reduce the number of power switches. The SVM method for the proposed topology is designed, which introduces a distribution factor to balance the neutral-point voltage (NPV) actively. In addition, the up-shoot-through (UST) and down-shoot-through (DST) states are inserted within the durations of redundant small vectors to boost the dc input voltage, while the normal ac output voltage is not affected. Detailed comparisons of the RSC QZS three-level inverter and the existing impedance-source-based three-level inverters are conducted, and design guidelines and recommendations for selecting different components are given as well. Simulated tests based on MATLAB software and experimental tests based on a hardware-based test rig are conducted to validate the operating performance of the proposed topology and modulation method, which reveal that single-stage power conversion is achieved. Moreover, the NPV balance is actively controlled by using the proposed method.

Hybrid Active Modulation Strategy for Three-Level Neutral-Point-Clamped Converters in High-Speed Aerospace Drives

In the aircraft electric starter/generator system, the three-level neutral-point-clamped converters play a crucial role in driving turbofan engines and delivering onboard electrical power. However, the conventional pulsewidth modulation (PWM) strategies face the challenge of capacitor voltage deviation, large common-mode voltage (CMV), and extra switching losses. Regarding the characteristics of the studied wide-speed range aerospace drives, the modulation scheme needs to be designed according to its operating conditions. To tackle the above demerits, a hybrid active modulation approach is, hence, proposed in this article. By the coordinate-based PWM, the nearest-three-vector is used in the startup process as the neutral-point (NP) voltage balance can be realized with fewer switching intervals; when the drives run in generation mode, an enhanced carrier-based virtual-space-vector modulation technique is involved, which aims to eliminate NP voltage fluctuation, suppress CMV, and simplify the modulation process. With the help of bias-offset injection in the time and voltage domain, capacitor voltages can be effectively kept at a balanced state even though the imbalance exists. The validity of the presented algorithm is proved by simulation and experimental results obtained from a 45 kW, 32 kr/min aircraft starter/generator test rig.

A Single-Phase Common-Ground Five-Level Transformerless Inverter With Low Component Count for PV Applications

Multilevel inverters (MLIs) featuring reactive power ability and common ground (CG) have become much popular in transformerless grid-tied photovoltaic applications. A switched-capacitor (SC) based five-level transformerless inverter configuration is thus proposed in this article, which requires only six switches, one diode, three capacitors, and one input dc voltage supply. Notably, only four power switches are operated in high frequency, thus, the switching losses are reduced. Moreover, the presented topology can effectively tackle the leakage current problem using a CG architecture. A multicarrier phase disposition pulsewidth modulation strategy is employed to achieve reactive power regulation and SC voltage self-balancing. To achieve the neutral point voltage balance of the presented five-level inverter, a simple closed-loop voltage-balancing control method is proposed. The operation principles with modulation strategy and voltage self-balancing of SC are discussed in depth. Comparisons between the proposed and the state-of-the-art MLIs are presented in detail. Finally, experimental tests on a single-phase 1.1-kW prototype validate the appropriate performance of the proposed inverter.

A Modulation Method to Eliminate Leakage Current and Balance Neutral-Point Voltage for Three-Level Inverters in Photovoltaic Systems

In photovoltaic systems, common-mode voltage (CMV) generates leakage current, which shortens the insulation lifespan and causes safety problems. As for neutral-point-clamped (NPC) inverters, zero CMV PWM (ZCMV PWM) can be implemented to eliminate leakage current, but unable to balance neutral-point (NP) voltage in dc side. This article presents a novel modulation technique for NPC PV inverter. The strategy contains ZCMV PWM and another two modulation modes which keeps CMV constant in one switching period, so as to eliminate leakage current. In order to balance NP voltage, NP current is adjusted by selection of optimal modulation mode and the leakage current is still well eliminated during the process of balancing NP voltage. Compared with conventional ZCMV PWM, the proposed scheme can remove both leakage current and NP voltage deviation. The output performance and limitations of the proposed method are also introduced in the paper. Experiments are provided to verify the validity of the proposed method.

Neutral Point Voltage Balancing Using MOPSO based Weighting Factor Tuning for FCS-MPTC of Three Level T-Type VSI Fed IM Drive

This paper proposes designing and implementing an offline optimization of weighting factors associated with the Finite Control Set Model Predictive Torque Control (FCS-MPTC) for a T-Type Multilevel Inverter (MLI) fed three-phase induction motor drive. Since the IM drive driven by T-Type MLI requires neutral point voltage balancing for reliable and acceptable performance, a regularization term with a weighting factor is added to the cost function of FCS-MPTC involving the neutral point voltage balancing. The optimized weighting factors were obtained by applying the Vikor decision-making method on the Pareto front obtained from the Multi Objective Particle Swam Optimization (MOPSO) algorithm. The improvement is shown by comparing the results of the proposed method with the conventional FCS-MPTC applied to the same IM drive. The proposed optimization methodology also showed better dynamic performance, particularly during loading. The results from MATLAB/Simulink environment show significantly improved flux ripple and almost perfect neutral point voltage balancing with the proposed optimized version.

An Improved Model Predictive Control Method Using Optimized Voltage Vectors for Vienna Rectifier With Fixed Switching Frequency

The Vienna rectifier is an attractive topology for industrial applications. In order to obtain fixed switching frequency (FSF) while maintaining lower current distortion, an improved model predictive control (MPC) method using optimized voltage vectors and switching sequences is proposed in this article. First, the space vector diagram of the Vienna rectifier is divided into six sectors according to the input currents. Then using finite-set MPC (FS-MPC), the sector can be further divided into subsectors, which reduces the number of the candidate vectors from 19 to 7 simultaneously. Second, to achieve decoupling of the neutral-point (NP) voltage balancing and current distortion, the small regions are recombined into novel N- or P-type regions to compensate for the absent vectors. More importantly, the FSF is realized by the multiple vectors determined together by the fluctuation of the NP voltage and the cost function without weighting factors, which improves the current quality and it is beneficial for filter design. Finally, based on FSF-MPC, sequence optimization between adjacent sampling periods is further considered. Compared with FSF-MPC, the improved FSF optimized (FSFO) MPC can significantly reduce the switching loss. The effectiveness of the proposed FSF- and FSFO-MPC methods is verified by simulation and experiment.

A Hybrid Modulation Strategy With Neutral Point Voltage Balance Capability for Electrolytic Capacitorless Vienna Rectifiers

Electrolytic capacitorless Vienna rectifiers have many advantages, such as longer lifetime and higher power density. However, the reduced dc-link capacitance aggravates the voltage stress due to the neutral point (NP) fluctuation. In this article, a hybrid modulation strategy combining the modulations of redundant vector and compression vector is proposed, which can realize the NP voltage balance for electrolytic capacitorless Vienna rectifiers. On the basis of analyzing the limitation of NP-balance capability of conventional space vector pulsewidth modulation (PWM) strategy in the region of high modulation index, the negative influence of reduced capacitance on the NP fluctuation and the input current harmonics is revealed. To suppress the NP fluctuation, the compression coefficient is introduced to redistribute the duty cycles of voltage vector, which can reduce the negative effect of medium vector on the NP voltage. In addition, the phase voltage error is modeled to reduce the input current harmonics with the appropriate adjustment factor. The implementation of the proposed strategy is realized by the equivalent carrier-based PWM, which can avoid the sector judgment and angle calculation. By using the proposed strategy, the NP voltage balance and the input current harmonics reduction can be realized simultaneously. The effectiveness of the proposed strategy is verified on an electrolytic capacitorless Vienna rectifier platform.