Neutral-point Potential Balancing Research Articles

Improved Modulation Mechanism of Parallel-Operated T-Type Three-Level PWM Rectifiers for Neutral-Point Potential Balancing and Circulating Current Suppression

In high-power applications, parallel-operated T-type three-level pulse width modulation (PWM) rectifiers (T3LPRs) are widely employed to improve the power capacity and system reliability. For parallel-operated T3LPRs, there are two important issues need to be properly addressed: 1) the neutral-point potential (NPP) balancing, and 2) the zero sequence circulating current (ZSCC) between the common ac and dc bus. In this paper, the fluctuation of NPP in a T3LPR and the generative mechanism of ZSCC in parallel-operated T3LPRs are first investigated. The zero sequence voltage (ZSV) difference between the parallel-operated T3LPRs is identified as the main excitation source of the ZSCC problem. Meanwhile, the disconnection of the neutral point and the control effect difference of NPP balancing in each T3LPR also affect the ZSCCs within the parallel-operated T3LPRs. To adjust the ZSV of T3LPRs, the dwell time of the positive small voltage vector in each switching period of the converter, which is usually adjusted to balance the NPP, is also required to be controlled to suppress ZSCC. Therefore, in this paper, to avoid the possible conflict, an improved modulation mechanism is designed to guarantee simultaneous ZSCC suppression and NPP balancing. The neutral points of each T3LPR are connected. Then, by adjusting the dwell time of the positive small voltage vector, the charging time of two dc capacitors is adjusted to balance the NPP, and the ZSV differences between different T3LPRs are adjusted to suppress ZSCCs. To avoid the control effect difference of independent NPP balancing in different T3LPRs, all T3LPRs share the same value marking the difference between two dc capacitor voltages. Thus, a communication with a greatly reduced baud rate requirement is needed for NPP balancing to transfer the difference between the two dc capacitor voltages. Moreover, no extra circuit is needed for ZSCC suppression. These approaches make the proposed improved space vector PWM strategy much more practical and easy to employ. The proposed method is verified with experiments on two 3-kW T3LPR prototypes.

A Coordinated Strategy Providing Zero-Sequence Circulating Current Suppression and Neutral-Point Potential Balancing in Two Parallel Three-Level Converters

In two parallel three-level T-type converters (3LT2Cs) with the sharing dc bus and ac bus directly, the neutral-point (NP) potential balancing and the zero-sequence circulating currents (ZSCCs) have to be considered to ensure normal operation of parallel system. In the conventional control scheme, the distribution factor of small vectors with space vector pulse width modulation (SVPWM) strategies (or the injected zero-sequence voltage with SPWM strategies) has to carry out two tasks simultaneously: NP potential balancing and ZSCCs suppression. However, the common challenge is how to coordinately control ZSCCs and NP potential due to only one controllable degree of freedom for two different targets. In this paper, the conventional scheme is investigated comprehensively. Theoretical analysis shows that the control authority over ZSCCs and NP potential is limited. Furthermore, a coordinated control strategy is proposed as follows: the distribution factor of one converter is in charge of the common NP potential balancing with the auxiliary NP bus, while the distribution factor of the other converter is in charge of ZSCCs regardless of NP potential. Based on the strategy, the distribution factor of every individual converter only needs to control one target, achieving the independent control of NP potential and ZSCCs. Theoretical analysis shows that NP potential balancing and ZSCCs suppression can be achieved simultaneously in two parallel 3LT2Cs. Finally, the effectiveness of the proposed strategy is verified by experimental results.

Neutral‐point potential balancing control strategy of three‐level active NPC inverter based on SHEPWM

This study proposes a novel control strategy for a three-level active neutral-point clamped (3L-ANPC) inverter using selective harmonic elimination pulse width modulation (SHEPWM). The proposed control strategy can effectively control the neutral-point potential (NP) balancing and eliminate the selective harmonics simultaneously. The three-phase SHEPWM line-to-line voltages of the inverter are considered as switching-states. Among these switching states, the six pairs of redundant switching-states generate the same line-to-line voltages and thus the same harmonic spectrums with each pair having opposite effects on NP balancing. As a result, the pair of switching-states can be swapped to control the NP while still maintaining the same inverter output. The decision to make such swap will take into account directions of output currents, polarities of NP offsets and exclusion of directly switching between P and N states. The simulation and experimental results on a Matlab/Simulink model and prototype of a three-phase 3L-ANPC inverter are used to verify the effectiveness of the proposed control strategy.

Neutral point potential balance algorithm for three‐level NPC inverter based on SHEPWM

In order to obtain the capability of neutral point potential balancing with low switching frequency, a novel balance control algorithm for neutral-point-clamped (NPC) three-level inverter based on Selective harmonic elimination pulse-width modulation (SHEPWM) are proposed. On the basis of the switch sequences and voltage vector action angle corresponding to the three-phase SHEPWM waveform at different modulation index, the three kinds of redundant vector adjustment schemes are designed. Then, load current and neutral point potential feedback to the controller, and the three-phase pulse-width modulation signal adjusted directly by the hysteresis controller to realise neutral point potential balance. Experimental results show that different redundant vector adjustment schemes have different neutral point potential balancing speed and average switching frequency, but do not affect the harmonic elimination ability of the line voltage.

A new type of single-phase five-level inverter

At present, Neutral Point Clamped (NPC) multilevel inverter is widely applied in new energy field. However, it has some disadvantages including low utilization rate of direct current (DC) voltage source and the unbalance of neutral potential. Therefore, a new single-phase five level inverter is proposed in this paper. It has two stage structure, the former stage is equivalent to three level DC/DC converter, and the back stage uses H bridge to realize inverter. Compared with the original central clamp type inverter, the new five level inverter can improve the utilization of DC voltage, and realize the neutral point potential balance with hysteresis comparator.

An Improved Virtual Space Vector Modulation Scheme for Three-Level Active Neutral-Point-Clamped Inverter

This paper presents an improved virtual space vector modulation (IVSVM) scheme for a three-level active neutral-point-clamped (3L-ANPC) inverter to balance neutral-point potential (NP), reduce switching loss, and suppress common mode voltage (CMV). In the IVSVM scheme, an improved virtual medium vector (VMV) is synthesized by the original medium vector and the adjacent two pairs of the small vectors since there are always two pairs of the small vectors in any regions of the new space vector diagram. The improved VMV provides the flexibility to control the NP balancing under any modulation indexes and power factors in one switching cycle while achieving SL reduction and CMV suppression. This is achieved by the proper selection of two pairs of small vectors in terms of NP charge and the pulse sequence. The experimental platform based on a 3L-ANPC inverter is established and results obtained to verify the effectiveness of the IVSVM control strategy.

A Novel SVPWM Algorithm Considering Neutral - Point Potential Balancing for Three - Level NPC Inverter

For three-level inverter, complexity of control strategy and neutral-point potential imbalance problem of DC side is the bottleneck restricting its application. In order to solve the problem, a simplified implementation of three- level space vector pulse width modulation (SVPWM) considering neutral-point potential balancing is proposed in this paper. The proposed SVPWM algorithm is based on judging of three phase voltages and voltage-second balance principle, which does not need to perform the sine and cosine c alculations, and thus it is more convenient and effective than traditional SVPWM algorithm. Also, the neutral-point potential balancing can be realized conveniently and effectively. The proposed algorithm can not only effectively simplify the calculation and reduce calculation time greatly, but also achieve the same control effect as traditional SVPWM. It has certain reference significance and can be used to shorten sampling time and improve the inverter performance. Finally, the proposed SVPWM algorithm is verified by simulation and experimental results.

Neutral-point Potential Balancing Method for Switched-Inductor Z-Source Three-level Inverter

Switched-inductor (SL) Z-source three-level inverter is a novel high power topology. The SL based impedance network can boost the input dc voltage to a higher value than the single LC impedance network. However, as all the neutral-point-clamped (NPC) inverters, the SL Z-source three-level inverter has to balance the neutral-point (NP) potential too. The principle of the inverter is introduced and then the effects of NP potential unbalance are analyzed. A NP balancing method is proposed. Other than the methods for conventional NPC inverter without Z-source impedance network, the upper and lower shoot-through durations are corrected by the feedforward compensation factors. With the proposed method, the NP potential is balanced and the voltage boosting ability of the Z-source network is not affected obviously. Simulations are conducted to verify the proposed method.

Torque Ripple Reduction in Three-Level Inverter-Fed Permanent Magnet Synchronous Motor Drives by Duty-Cycle Direct Torque Control Using an Evaluation Table

In this paper, a direct torque control algorithm with novel duty cycle-based modulation is proposed for permanent magnet synchronous motor drives fed by neutral-point clamped three-level inverters. Compared with the standard DTC, the proposed algorithm can suppress steady-state torque ripples as well as ensure neutral-point potential balance and smooth vector switching. A unified torque/flux evaluation table with multiple voltage vectors and precise control levels is established and used in this method. This table can be used to evaluate the effects of duty-cycle vectors on torque and flux directly, and the elements of the table are independent of the motor parameters. Consequently, a high number of appropriate voltage vectors and their corresponding duty cycles can be selected as candidate vectors to reduce torque ripples by looking up the table. Furthermore, small vectors are incorporated into the table to ensure the neutral-point potential balance with the numerous candidate vectors. The feasibility and effectiveness of the proposed algorithm are verified by both simulations and experiments.

Design of Speed Control System for PMSM Based on Three-Level Structure

三相永磁同步电机在节约电能方面具有非常广阔的前景，它具有结构简单、体积小、效率高、易于散热和维护简单等优点。相比传统的两电平逆变器，三电平逆变器具有很多优点：单管耐压低、开关损耗降低、谐波含量小等，因此将三电平技术应用到永磁同步电机调速系统中，具有重要的理论意义和实用价值。本文主要研究了基于三电平中点箝位型(NPC)逆变器的永磁同步电机(PMSM)调速系统。电机采用转子磁场定向的矢量控制。为了达到控制目的，在三电平逆变器控制系统中引入了零序电压注入法。这种方法很好地控制了中点电压的平衡，以期降低逆变器输出电压谐波含量，改善电机定子电流波形，这将减小电机转矩脉动，改善电机控制性能。本文介绍了永磁同步电机的数学模型和矢量控制方法，阐述了中点箝位型三电平逆变器的拓扑结构和换流模式，分析了中点电位平衡算法和调制策略，最后通过实验验证了系统的可靠性和稳定性。 The three-phase PMSM has a very broad prospect in saving electrical energy. It has the characte-ristics of simple structure, small volume, high efficiency and good heat radiation. Compared with conventional two-level inverters, three-level inverter has a series of advantages such as low vol-tage-proof for single transistor, low switching loss and low harmonic content, etc. Therefore, it is of great theoretical significance and practical value to apply the three-level inverter to the speed regulating system of permanent magnet synchronous motor (PMSM). In this paper, the speed regulating system of permanent magnet synchronous motor on the basis of three level neutral point clamped (NPC) inverter is studied. Rotor flux oriented vector control method is applied on the motor. In order to implement the control process, zero sequence voltage injection method is introduced in the proposed three-level inverter control system. With this method, the balance of the neutral point voltage can be well achieved to reduce the harmonic content of the output voltage of the inverter and improve the stator current waveform, which will further minimize the motor torque ripple, and improve motor control performance. The mathematical model and vector control method of PMSM are introduced in this paper, both the topology and commutation mode of three-level inverter with neutral point clamping mode are described, and the neutral point potential balance algorithm and modulation strategy are also analyzed. Finally, the experiment verified the reliability and stability of the system.

A Hybrid Modulation Strategy with Reduced Switching Losses and Neutral Point Potential Balance for Three-Level NPC Inverter

In this paper, carrier-based pulse width modulation (CBPWM), space vector PWM (SVPWM) and reduced switching losses PWM (RSLPWM) for the three-level neutral point clamped (NPC) inverter are introduced. In the case of the neutral point (NP) potential (NPP) offset, an asymmetric disposition PWM (ASPDPWM) strategy is proposed, which can output PWM sequences correctly and suppress the lower order harmonics of the inverter effectively. An NPP balance strategy based on carrier based PWM (CBPWM) is analyzed. A hybrid modulation strategy combining RSLPWM and the NPP balance based on CBPWM is proposed, and hysteresis control is adopted to switch between the two modulation strategies. An experimental prototype of the three-level NPC inverter is built. The effectiveness of the hybrid modulation is verified with a resistance-inductance load and a permanent magnetic synchronous motor (PMSM) load, respectively. The experimental results show that reduced switching losses and an acceptable NPP can be effectively achieved in the hybrid modulation strategy.

Model Predictive Direct Power Control of DC Micro Grid Rectifier Based on Neutral Point Potential Balance

Aimed at the problem of DC micro grid rectifier control delay and the DC bus voltage stability, the method of two-step model predictive direct power control (TMPDPC) combined with neutral point potential control is proposed. Model predictive direct power control (MPDPC) is designed for rectifier. A cost function is then used to evaluate the active and reactive power ripples, from which the vector that generates the lowest power ripple will be applied during the next sampling interval. Two-step predictive control is designed to compensate for the delay of one-step predictive control. On the basis of this, the two-capacitor voltage unbalance problem in DC side is considered, and the neutral point potential control is added. In the Matlab/Simulink simulation, compared with control effect of direct power control (DPC) and one-step model predictive direct power control (OMPDPC), the control strategy of neutral point potential added to TMPDPC can make the system stability and control accuracy better. The validity of this scheme was validated by physical simulation at last.

A simplified three-level SVM-DTC induction motor drive with speed and stator resistance estimation based on extended Kalman filter

In this paper, a speed-sensorless direct torque control (DTC) drive for induction motors (IMs) fed by three-level neutral-point clamped (NPC) inverter has been investigated. The main objective of the study is to solve two different problems in DTC drive with three-level NPC inverter. First, the conventional switching table fails to consider some limitations caused by the topology of NPC inverter. As a solution to this problem, a new switching method based on simplified three-level space-vector modulation (SVM) technique has been developed by fulfilling both neutral-point potential balance and soft commutation requirements needed for three-level NPC inverters. Other problem is to make accurate estimates, especially in very low and zero speed operation. To address this issue, an extended Kalman filter (EKF)-based observer has been utilized for the estimation of rotor speed, load torque, and flux. The estimated load torque is used to improve the response of speed estimation in both transient and steady states. The stator resistance has also been estimated simultaneously to make more robust and reliable flux estimation. The proposed drive system has been tested under some challenging conditions and verified by the simulation results.

Model Predictive Optimization Control Strategy for Three-level AFE Converter

In view of the fact of traditional MPC prediction for three-level AFE converter with numerous switching vectors, time-cost computation and complex control, a simplified model predictive control algorithm is proposed in this paper. The multiple current prediction is transformed into a single virtual reference voltage vector prediction according to the inverse procedure of the model current prediction, and vector distribution method is adopted which can screens out the optimal vector. In the process of rolling optimization, multi-objective control is carried out by adding neutral point potential balance and reducing switching losses and other constraints to the cost function. Also the control delay of the algorithm is compensated. Finally, simulation experiments of three-level AFE converter under steady-state and dynamic conditions are provided. The results have verified correctness and practicability of the strategy.

A Novel Virtual Space Vector Modulation Strategy for the Neutral-Point Potential Comprehensive Balance of Neutral-Point-Clamped Converters

A novel Virtual Space Vector (VSV) modulation strategy for complete control of potential neutral point (NP) issues is proposed in this paper. The neutral point potential balancing problems of multi-level converters, which include elimination of low frequency oscillations and self-balancing for NP dc unbalance, are investigated first. Then a set of improved virtual space vectors with dynamic adjustment factors are introduced and a multi-objective optimization algorithm which aims to optimize these adjustment factors is presented in this paper. The improved virtual space vectors and the multi-objective optimization algorithm constitute the novel Virtual Space Vector modulation. The proposed novel Virtual Space Vector modulation can simultaneously recover NP dc unbalance and eliminate low frequency oscillations of the neutral point. Experiment results show that the proposed strategy has excellent performance, and that both of the neutral point potential issues can be solved.

Space-Vector-Modulated Method for Boosting and Neutral Voltage Balancing in Z-Source Three-Level T-Type Inverter

In this paper, a new modulation method is proposed for a Z-source three-level T-type inverter (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I). The Z-source 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I combines the merits of the Z-source two-level inverter and the advantages of the Z-source neutral-point-clamped inverter. Although the previous literature has proved that the space vector modulation (SVM) technique for Z-source 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I produces the desired voltage transfer gain with minimized switching loss, it cannot deal with the neutral-point (NP) potential balance issue. Therefore, an improved SVM method addressing this problem for Z-source 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I is proposed. The method simplifies the space-vector diagram of a Z-source 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I into that of a Z-source two level inverter. In order to obtain NP potential balance, the pattern of the conventional symmetric SVM is changed by properly adding or subtracting a percentage of the minimum gate-on time. The upper and lower shoot-through states can be inserted into the improved symmetric SVM, thus offering av few advantages, including boosting ability, NP potential balance, and low switching loss. The considerate stable NP voltage and the boosting gain have been experimentally proven by results from a prototype inverter.

A Novel Neutral-Point Potential Balance Strategy for Three-Level NPC Back-to-Back Converter Based on the Neutral-Point Current Injection Model

The neutral-point (NP) potential balance control in three-level neutral-point-clamped (NPC) back-to-back converter is a research nodus. Its current strategies are the same as the strategies of a single three-level NPC converter. But the strategies do not give full play to its advantages that the neutral-point current can only flow through the connected midlines in both sides of the converter but does not flow through the DC-bus capacitors. In this paper, firstly the NP potential model based on the NP current injected is proposed. It overcomes numerous variable constraints and mutual coupling in the conventional model based on the zero-sequence voltage injected. And then on this basis, three NP-potential balance control algorithms, unilateral control, bilateral independent control, and bilateral coordinated control, are proposed according to difference requirements. All of these algorithms use the midlines rather than the DC-bus capacitors to flow the NP current as much as possible. Their control abilities are further quantitatively analyzed and compared. Finally, simulation results verify the validity and effectiveness of these algorithms.

A single-phase nine-level inverter for renewable energy systems employing model predictive control

In this paper a single-phase cascaded Multilevel Inverter (MI) consisting of full NPC bridges for renewable energy systems is investigated. The development of the introduced control strategy is based on the application of Model Predictive Control (MPC) technique meeting the required specifications of the individual voltage level regulation of each separate DC source and neutral point potential balance under unity power factor. Illustrating the effectiveness of the developed controller, two separate PV strings are controlled while being subject to transient phenomena related to asymmetric string conditions in terms of solar radiation and reference voltage variation. The inverter topology of full NPC bridges in cascade allows low current THD factor to be achieved using a simple inductor instead of complicated LCL filters benefiting also from low capacitor banks in the DC-link bus. The proposed MPC performance has been validated by experiments carried out on a cascaded MI topology delivering power from two separate PV system strings.

Neutral Point Potential Balance of Dual Three-Level Converter Based on Fuzzy Logic Control Strategy

In dual three-level converter system, the rectifier and inverter have unified problems of the neutral point potential imbalance.This paper analyzed the reasons of imbalance of neutral point potential in detail and summarized the relationship between vector status and four-quadrant status of three-level converters.Then a method based on fuzzy logic control (FLC) is proposed,which could be automatically adjust the balance factor.Combined the balance factor with the status of the converter the duration of positive and negative small vectors action could be obtained.Finally the balance of the neutral point potential is achieved.This method could be generally used on rectifier and inverter.At last, the simulation was done on both inverter and rectifier side.The results verified the feasibility of the proposed method.

Three‐phase interleaved high‐step‐up converter with coupled‐inductor‐based voltage quadrupler

This paper proposes a high-efficient DC-DC solution with the features of galvanic isolation, high-voltage gain, zero voltage switching operation, low input current ripple and high-power density. The converter is implemented by a three-phase coupled inductor bridge to remove the bulky input electrolytic capacitors. The introduced active clamp circuit recycles the energy stored in the leakage inductance and absorbs the voltage spikes on the main switch voltage. The series configuration with a voltage doubler at the second side contributes to a high-voltage gain and reduces the voltage stress across the rectifier devices. The output diode reverse-recovery problem is naturally mitigated by the leakage inductance of the coupled inductors. Moreover, the active control with a neutral-point potential balance and a phase-deficient operation of the proposed converter are also studied. A converter prototype is designed and evaluated to verify the theoretical analysis and demonstrate a superior performance over the prior studies.