Neutral-point Fluctuation Research Articles

A Hybrid Discontinuous PWM Strategy for Current Ripple and Neutral-Point Fluctuation Reduction of Parallel Vienna Rectifier

Generally, discontinuous pulsewidth modulation (DPWM) strategy used in single Vienna rectifier will result in large current ripple and neutral-point fluctuation. When applied in interleaved rectifier, the issue of neutral-point voltage still exists. Besides, the input current around zero-crossing point is distorted due to the characteristics of unidirectional topology. Hence, a novel hybrid DPWM (H-DPWM) strategy for parallel Vienna rectifier is proposed in this article to reduce the distortion, current ripple, and neutral-point fluctuation. In this article, the characteristics of current ripple and neutral-point fluctuation with traditional DPWM (T-DPWM) are clarified at first. Then, the quantitative analyses on current ripple and neutral-point fluctuation with T-DPWM, classical space vector modulation, and optimized DPWM (O-DPWM) are presented in detail. To solve the issue of current distortion, the H-DPWM is proposed based on O-DPWM. Furthermore, the switching losses of these modulations are evaluated. At last, the analyses and performance of the proposed H-DPWM are verified by the experiments.

Design of 15 level reduced switches inverter topology using multicarrier sinusoidal pulse width modulation

In this proposed paper, multicarrier sinusoidal pulse width modulation (M-SPWM) method is implemented for design of 15 level reduced switches inverter topology. This inverter topology generates 15 level output-voltage with suitablelswitching pulse production using M-SPWM and altered level of voltages are attained with distinction of modulationlindex. The split inductor is used to diminish the harmoniclcontent and flatted output current. This type of system which contains different range of different range of voltage supplies. As a result, this inverter reduces the difficulty in gating time calculation and there is no neutral point fluctuation issue. This paper illuminates the modes of switching and minimization of stress in voltage and harmonic diminution are examined. The grades of the projected multilevel inverter (MLI) system are verified using Matlab/Simulink and dsPIC controller respectively.

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.

An Improved Control Strategy for the Bus Interlinking Converter Based on Three-level Topology in AC-DC Hybrid Microgrid

Considering the increasing requirements of the AC-DC hybrid microgrid (HMG) for the transmission power capacity and voltage level, the several bus interface converters (BICs) with the two-level PWM converter topology, need to be connected between AC bus and DC bus in parallel, which could generate circulating current and cycle power. The cost of the modular multilevel converter topology based BIC is too high. Hence, this paper proposes to apply the BIC with three-level topology to the HMG, and a droop control strategy with the midpoint potential hysteresis regulation link is designed, which can effectively suppress the fluctuation of midpoint potential without affecting the normal operation of the BIC in the grid connected or off mode. In addition, the principle of neutral point fluctuation and the process of hysteresis control are introduced, and the detailed design of dual-mode control strategy based on droop curve are also given. Finally, the effectiveness of the improved control strategy is verified by simulation.

Investigation and Comparative Analysis of Advanced PWM Techniques for Three-Phase Three-Level NPC-MLI Drives

The Multilevel inverter topologies (MLIs) are increasingly being used, which is the preferred choice in many industrial Medium-voltage (MV) applications. The inherent low switching frequency in medium voltage neutral point clamped MLI (NPC-MLI) fed industrial drives represent the power quality issues and filter-design challenges. In this paper, two multilevel pulse width modulation (PWM) methods; multi-carrier sine PWM (MCSPWM) and space vector PWM (SVPWM) has been considered for NPC-MLI. Both the methods are compared and analyzed based on different constraints such as voltage profile, total harmonic distortion (THD), common mode voltage (CMV) and neutral point fluctuation (NPF). The results convey that the SVPWM method is superior when compared with the MCSPWM algorithms. Analytical and simulation results achieved by using the MATLAB/Simulink for control systems are experimentally verified using FPGA-SPARTAN – III generation – 3AN –XC3S400 with a 2KW NPC-MLI fed adjustable speed – drive system.

Two methods to control neutral‐point voltage fluctuation for a hybrid VIENNA rectifier

This study presents two neutral-point balance control methods for a hybrid VIENNA rectifier, which is composed of a parallel system of a single-switch three-phase Boost rectifier and a VIENNA rectifier. Each rectifier in this topology processes part of the output power, therefore, a highly efficient and reliable performance is achieved in this rectifier system. However, the unbalance voltage of two DC-link capacitors in the VIENNA-type three level rectifier is also reflected in this hybrid rectifier. The fluctuation reason in neutral-point has been analysed using a mathematical model, meanwhile, the two methods with a hybrid algorithm and a simplified method based on zero-sequence component injection are proposed to control neutral-point fluctuation in this hybrid rectifier system. Furthermore, the performance comparison between these two methods is provided. The effectiveness of the two control methods has been verified using experimental results.

GCMT-249 Investigation and Performance Analysis of Direct Torque Control of 3Φ Induction Motor using 7 Level Neutral Point Clamped Multilevel Inverter

Objective: This work discusses with Direct Torque Control (DTC) of Induction motor using 7 Level NPC Multilevel inverter for reducing the complexity in conventional DTC method. Methods: The limitation of traditional DTC scheme is more distortion in torque profile and the mean torque output never meets the torque demand. To get better performance of the traditional DTC a Space Vector Modulation (SVM) technique is projected to meet the torque and flux demands at low speeds. Findings: By designing 7 level NPC for DTC of induction motor and thereby simplifying the classical DTC method and in very particular this paper is minimizing the complications in Torque and Flux control at minimal speeds by decoupling the torque and flux controls and also reduces the ripples in current and torque profile. 7 level NPC is introduced in this paper which helps in eliminating the torque disturbances into minimum. The proposed 7 level inverter uses less number of switches and neutral-point fluctuations are absent in this model. Detached DC supplies are employed for the DC links which carries the ripple current. The proposed inverter topology provided the required magnitudes and phases of voltage and current vectors to meet the desired load values with fewer harmonic content due to superior level inverter and less distortion in torque profile obtained in the closed loop DTC technique. 1 The DTC of Induction Motor simulated by implementing the SVM technique using 7 level NPC and the simulation results are experimentally validated. Improvements: The steady state torque and speed output have less ripple content which reduces the oscillation with a quick response at steady state was investigated.

A Closed Loop Current Control of PV-Wind Hybrid Source Fed Grid Connected Transformerless Diode Clamped-Multi Level Inverter

This paper presents a closed loop grid current control of Solar PV-Wind hybrid source fed split inductor based DC-MLI. Renewable energy sources generate significant proportion of global energy demand and will be the focal point of power generation by the turn of a few decades. Hybrid systems integrate the standalone efficiencies of PV and Wind generation to complement and reinforce individual output. A Diode Clamped or Neutral Point Clamped three level inverter reduces switching stresses and is apt for medium voltage high power applications. Space Vector PWM control of inverter reduces harmonic distortion besides eliminating neutral point fluctuation, capacitor balancing problem and common mode voltage reduction. It provides switching states for the inverter based on location of a space vector using reference voltage from grid. Using closed loop control the total harmonic distortion in grid current was reduced. The simulation and experimental results of the proposed renewable hybrid system were verified using Matlab/Simulink and FPGA.

Generalized PWM Technique for Dual Inverter Fed Induction Motor Drive

This paper presents a simplified Generalized Pulse Width Modulation (GPWM) algorithm to obtain various continuous and discontinuous PWM algorithms for a four-level inverter topology. The proposed four-level inverter is achieved by feeding the induction motor from both ends by a three-level cascade inverter from one side and by two-level inverter from other side. The proposed inverter configuration is capable to produce an output voltage of two-level, three-level and four-level in entire modulation range. In the proposed inverter topology neutral point fluctuations are absent and neutral clamping diodes are absent. With the application of various discontinuous PWM algorithms to the proposed inverter topology the switching losses are reduced when compared with continuous PWM algorithm. The simulation analyses are carried out in MATLAB/simulink environment and the results are presented.

A Novel Three-Phase Power Factor Correction with Digital Control

A new three-phase power-factor-correction (PFC) scheme is discussed using two single phase power factor correction circuit parallel connected. Two phase orthonormal voltage is produced by means of a auto transformer from a three phase input,fluctuation of neutral point by three single phase power-factor-correction circuit parallel connected is canceled,coupling interference is reduced among three phases,digitally controlled Power Factor Correction Boost Converters is adapted,input current wave sine with unity power factor suppress the secondary harmonic of input AC side is realized,voltage and low current stresses across each switch are reduced. Simulation and experimental results prove that it can achieve high power factor,low current,good capability of anti-interference and high reliability.

A Simple Space Vector PWM Scheme to Operate a Three-Level NPC Inverter at High Modulation Index Including Overmodulation Region, With Neutral Point Balancing

Multilevel inverters are being increasingly used for high-power medium-voltage applications. Three-level neutral point clamped (NPC) topology is most widely used topology of multilevel inverters. However, at higher modulation index, particularly in over-modulation region, the neutral point fluctuation deteriorates the performance of the inverter. Furthermore, operating the inverter at lower modulation index implies that it is operated at lower voltages only, and the installed DC link capacity is not fully utilized. This paper proposes a simple space vector PWM scheme for operating a three-level NPC inverter at higher modulation indices including over-modulation region, with neutral point balancing. Experimental results are provided.

Dual two-level inverter scheme for an open-end winding induction motor drive with a single DC power supply and improved DC bus utilisation

A dual two-level inverter fed open-end winding induction motor drive with a single DC-power supply is proposed. The proposed scheme produces voltage space vector locations identical to those of a conventional three-level inverter. In the conventional neutral-clamped three-level inverter, the series-connected DC link capacitors carry the load current, resulting in a fluctuating neutral point. Also, a three-level inverter requires bulky DC link capacitors. In the proposed scheme, the DC link capacitors carry currents of higher frequency and hence the neutral point fluctuations are reduced. A PWM switching strategy to suppress the zero sequence currents for all the voltage space phasor combinations is also proposed. This strategy is an improvisation of the one that uses a switched neutral to eliminate the triplen harmonic currents and results in an improved DC bus utilisation.

Three-level inverter configuration cascading two two-level inverters

A power circuit configuration to realise three-level inversion is proposed. Three-level inversion is realised by connecting two two-level inverters in cascade, in the proposed configuration. An isolated DC power supply is used to supply each inverter in this power circuit. Each DC-link voltage is equal to half of the DC-link voltage in a conventional NPC (neutral point clamped) three-level inverter topology. Neutral point fluctuations are absent, and fast recovery neutral clamping diodes are not needed. The proposed inverter scheme produces 64 space-vector combinations distributed over 19 space-vector locations as compared to 27 combinations in a conventional three-level topology. The present power circuit can be operated as a two-level inverter in the range of lower modulation, by clamping one inverter to a zero state and by switching the other inverter. When compared to the H-bridge topology, this circuit needs one power supply less. A space vector based PWM scheme is used for the experimental verification of the proposed topology.

Five-level inverter voltage-space phasor generation for an open-end winding induction motor drive

A topology for voltage-space phasor generation equivalent to a five-level inverter for an open-end winding induction motor is presented. The open-end winding induction motor is fed from both ends by two three-level inverters. The three-level inverters are realised by cascading two two-level inverters. This inverter scheme does not experience neutral-point fluctuations. Of the two three-level inverters only one will be switching at any instant in the lower speed ranges. In the multilevel carrier-based SPWM used for the proposed drive, a progressive discrete DC bias depending on the speed range is given to the reference wave to reduce the inverter switchings. The drive is implemented and tested with a 1 HP open-end winding induction motor and experimental results are presented.

A Space-Vector Modulation Scheme for a Dual Two-Level Inverter Fed Open-End Winding Induction Motor Drive for the Elimination of Zero-Sequence Currents

The dual two-level inverter fed open-end winding induction motor drive gives voltage space phasor locations similar to a three-level inverter fed induction motor drive. The dual inverter scheme renders advantages such as redundancy of the space vector combinations for the same number of space vector locations and the absence of neutral point fluctuations. In a dual-inverter scheme, a total of 64 space-vector combinations are distributed over 19 space vector locations compared to 27 space-vector combinations distributed over the same number of locations in a three-level inverter. The common mode voltages (the harmonics of the triplen order) need to be suppressed by transformer isolation or the use of harmonic filters. In this paper, a space vector modulation scheme for the suppression of zero sequence currents (the harmonics of the triplen order) for a dual two-level inverter fed open-end winding induction motor drive is described. The proposed scheme does not require bulky transformers or harmonic filters for the elimination of the zero sequence currents. The proposed space vector PWM is based on the observation that certain voltage space-vector combinations in the dual inverter scheme do not contribute to the triplen harmonics and hence the PWM scheme that exclusively employs these combinations is used to eliminate the triplen harmonic currents.