Neutral Point Voltage Research Articles

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.

Millman’s Theorem revisited

In this work we present some results bearing on Millman’s Network Theorem (MNT for short) with a specific focus towards its application to three phase power systems. First we present an original, insightful proof of the Theorem that simultaneously provides the foundation for the Universal Single Phase Equivalent Circuit (USPEC) framework. Next we introduce two modifications that generalise the standard MNT and render it better adapted to the analysis of three-phase power circuits. The first result is a vectorised version based on the USPEC method which allows a very concise calculation of load voltages and the second is aimed at the calculation of neutral point voltage unbalances in more general power circuits. We illustrate the practical application of these results with the help of several commented examples and besides we provide the Matlab/Octave scripts used to perform the analyses

New Space-vector hysteresis current control with neutral point voltage balancing applied on three-level VSI for wind power conversion

This paper presents a new space-vector hysteresis current control (SV-HCC) with neutral point voltage balancing applied on a three-level NPC-VSI. The last supplies a rotor winding of the doubly fed induction generator (DFIG). The proposed control technique controls the power delivered from the wind generator to the electrical network by controlling the rotor currents of the DFIG. It is based firstly on the detection of area and sector in which the error vector tip of the rotor current can be located, secondly, on the neutral voltage sensing. And then on the application of an appropriate voltage vector among the 27 voltage vectors of the three-level NPC-VSI to accomplish two main goals: 1- Controlling the active and reactive powers delivered by the wind generator to the electrical network. 2- Balancing the neutral point voltage of the three-level VSI.

Improved virtual space vector modulation for neutral point voltage oscillation and common-mode voltage reduction in neutral point clamped three-level inverter

Improved virtual space vector modulation for neutral point voltage oscillation and common-mode voltage reduction in neutral point clamped three-level inverter

Ground-Fault Recognition in Low-Voltage Ships Based on Variation Analysis of Phase-to-Ground Voltage and Neutral-Point Voltage

Ground-Fault Recognition in Low-Voltage Ships Based on Variation Analysis of Phase-to-Ground Voltage and Neutral-Point Voltage

A Modulation Strategy to Operate Five-Level HC Inverter at Wide Power Factors With Enhanced Neutral Point Voltage Balancing Capability

A Modulation Strategy to Operate Five-Level HC Inverter at Wide Power Factors With Enhanced Neutral Point Voltage Balancing Capability

Improved Carrier-Based DPWM for Vienna Rectifier Considering Oscillated and Unbalanced Neutral-Point Voltages

Improved Carrier-Based DPWM for Vienna Rectifier Considering Oscillated and Unbalanced Neutral-Point Voltages

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.

High-Resistance Connection Diagnosis of Doubly Fed Induction Generators

The high resistance connection fault of the stator is a common fault in doubly fed induction generators, which causes a three-phase imbalance in the stator circuit. Since the stator winding is directly connected to the power grid, interference from the asymmetric power grid must be eliminated in order to achieve the accurate diagnosis of stator resistance imbalance faults. Therefore, a new diagnosis method based on filter shunt capacitor banks is proposed in this paper. By introducing shunt capacitor banks, an artificial neutral point is constructed to replace the neutral point of the power grid. Then, the neutral point voltage of the stator winding relative to the artificial neutral point is selected as a fault characteristic signal. In this paper, the change in three-phase stator currents after a high-resistance connection fault is analyzed in detail, and by comparing the fault characteristic signal with three-phase stator currents, the fault phase location and fault severity of high-resistance connection can be accurately obtained. Finally, simulations are carried out via the field-circuit coupling method to validate the effectiveness of the proposed method.

Super-twisting sliding mode control for neutral point voltage of three-phase four-wire inverter based on SiC/Si hybrid switch

The three-phase four-wire voltage source inverter (3P4W VSI) is widely used in applications like uninterrupted power supply (UPS) and bidirectional onboard charger. The increasing power density demand requires higher switching frequency and lower switching loss. To fulfill the conflicting objectives, two-fold methodology is proposed in this paper: 1) SiC/Si hybrid switches (HyS) together with recently reported gate trigger are reported for the first time in the 3P4W VSI; 2) the natural point voltage is controlled to track a sinusoidal voltage with the frequency equal to 3 times of fundamental frequency in order to achieve higher DC-bus voltage utilization rate, and further reduce the switching loss. The traditional PI controller is very hard to achieve desired performance due to both the nonlinearity and the variant reference of the natural point voltage control system. Thereby, the super-twisting sliding mode control (ST-SMC) is proposed in this paper to achieve desired tracking performance and fast dynamic response. The effectiveness and superiority of the system are verified by both simulation and experiment comparison with the existing methods using a 5 kW prototype.

Improved virtual SVPWM algorithm for CMV reduction and NPV oscillation elimination in Three-Level NPC inverter

An improved virtual space vector pulse-width modulation (IVSVPWM) strategy is presented to address simultaneously the problem of common mode voltage (CMV) with neutral point voltage (NPV) fluctuations in wind power converters. The proposed IVSVPWM method suppresses the amplitude of the common mode voltage to Udc/6 by discarding the positive small vector. Meanwhile, the new virtual vector is synthesized by selecting a vector whose neutral point current sums to zero among the retained voltage vectors. In addition, a feedback mechanism is established in the proposed strategy. According to the feedback result, the distribution coefficients of each basic vector in the virtual vector are recalculated to suppress the NPV fluctuation. Through the above control strategy, the dual effect of suppressing the CMV and balancing the NPV is achieved in the full modulation. The feasibility and effectiveness of the strategy are verified by simulation and experiment.© 2017 Elsevier Inc. All rights reserved.

An improved model predictive control of back-to-back three-level NPC converters with virtual space vectors for high power PMSG-based wind energy conversion systems

In this study, an improved virtual space vector (VSV)-based two-stage model predictive control (MPC) scheme is presented for neutral point clamped (NPC) converters in high power-rated permanent magnet synchronous generator (PMSG)-based wind energy conversion system applications. The presented MPC scheme utilizes the VSVs and involves two-stage prediction for effectively reducing computation complexity. In stage-I prediction, a virtual space vector is identified based on the optimum cost function. Then, a set of sub-sector vectors are placed in stage II prediction based on the capacitor voltage levels. From this, the cost function predicts the optimum switching state. At the same time, the conditional selection of small voltage vectors in stage II prediction eliminates the neutral point voltage balancing constraint. Finally, the presented method is verified by the simulation of the back-to-back NPC converter of a high-power-rated PMSG-based wind energy conversion system. Further, the adaptability of the presented scheme for maximum power point tracking, reactive power control, and dc-link voltage control is also verified by simulation.

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 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.

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 Model Predictive Control Scheme for Flying Capacitors and Neutral Point Voltages Control in Nested Piloted Neutral Point Clamped Inverter

Five-level nested piloted neutral point clamped (5L-NPNPC) inverter is a new version of nested neutral point clamped multilevel topologies. In 5L-NPNPC, voltage fluctuations in the flying capacitors (FCs) and neutral point (NP) increase as the output frequency decrease, which is not desirable for the safe operation of the inverter. In this paper, a new model predictive control-based solution is proposed to tackle this problem. First, feasible combinations of space voltage vectors are determined through which the reference voltage of the inverter is synthesized. Then, a two-step optimization process is proposed to select the optimal switching states. The voltages of FCs and NP are the control objects in the first and second optimization steps, respectively. The proposed control scheme mitigates FCs and NP voltage fluctuations in the entire frequency range while keeping the switching frequency fixed. Also, the peak value of common-mode voltage (CMV) is limited to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_dc</i> /6. Furthermore, the proposed control scheme divides the optimization process into two steps, leading to a significant reduction in the computational burden of the processor. The mentioned achievements are important in medium-voltage (MV) motor drive applications. The simulation results and scaled-down experiments are provided to support the above claims.

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.&#x0D; 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.&#x0D; 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.&#x0D; 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.&#x0D; 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.

Advanced sensorless nonlinear control and stability analysis of single-stage PV systems connected to the grid via a 3L-NPC inverter

This paper proposes a novel control strategy for a single-stage photovoltaic (PV) system consisting of two PV panels connected to the grid via a three-level neutral point clamped converter topology with an LCL filter. This control method is based on a developed 5th order model containing sum and difference terms of the voltages of the two input capacitors. This allows decoupling the issues of maximum power point tracking and power factor correction from the issue of balancing the power exchange generated by the panels, which facilitates control design and improves system performances. The control problem under consideration is dealt with using a non-linear controller composed of three loops: (i) an inner loop is developed, based on backstepping and Lyapunov approaches, to correct the power factor by forcing the grid current to be sinusoidal and in phase with the grid voltage; (ii) an outer loop is designed, using a filtered proportional-integral controller, to regulate the DC bus voltage to a climate-dependent reference; (iii) a balancing loop is designed, using a proportional-integral controller, to cope with the neutral point voltage balancing problem. The proposed controller also includes a state observer that provides on-line estimation of the network state variables that are not accessible to measurements. Another important aspect of this work is the development of a formal, complete and rigorous analysis in order to describe the performance and analyse the stability of the closed-loop system using various analytical tools, including averaging theory, Routh criteria and indirect Lyapunov stability. A simulation in MATLAB/SIMULINK environment shows, on the one hand, the efficiency and robustness of the proposed nonlinear controller against changing climatic conditions and, on the other hand, the superiority of this control strategy compared to the one based on a PI linear inner loop controller for the studied system with an l- filter and an LCL filter.