Neutral Point Potential Control Research Articles

Model predictive control for Vienna rectifier with constant frequency based on inductance parameters online identification

When the finite control set model predictive control (FCS-MPC) is applied to the three-level converter, there are problems such as large current harmonics, high requirements for the computing efficiency of the microcontroller, complex multi-objective optimization and limited output vector switching. In addition,the mismatch of inductance parameter may directly affect the observation accuracy of FCS-MPC. To solve the above problem,a double vector model predictive control strategy with constant frequency strategy based on the inductance parameters on-line identification is presented.First, a single objective cost function based on the direct power is constructed by optimizing the redundant vector which is selected to balance the neutral-point potential, the design of weighting factor is avoided.At the same time, in order to effectively reduce the grid current ripple under single vector modulation, space vector pulse width modulation (SVPWM) is realized by combining with zero vector control mode. And, a parameter identification method based on model reference adaptive system(MARS) is proposed to overcome the effect of model parameter mismatch.Finally, the results show that the proposed F-MPCCF has good steady-state and dynamic performance from the static, transient and neutral-point potential control.

Capacitor sizing of three‐level neutral point clamped voltage source inverter for electric vehicles: Effects of modulation and motor characteristics

AbstractA three‐level neutral point clamped (3L‐NPC) voltage source inverter (VSI) topology can be advantageous in electric vehicles with a high DC‐link voltage and a high switching frequency. A bulky DC‐link capacitor is not an option; thus, the DC‐link capacitor's sizing considering the traction system characteristics is an important design step. This paper investigates how the DC‐link capacitor size of a 3L‐NPC VSI gets affected by the combination of the interdependent characteristics of an electric drive, such as its power factor, modulation index, current, and fundamental frequency, with the effects of the modulation methods. Five pulse width modulation (PWM) methods, of which three of them have an active neutral point potential control, are compared in terms of their neutral point potential (NPP) oscillations. Then, the size of the DC‐link capacitor is determined for each PWM method so that the NPP ripple is kept under desired limits at all operating conditions. It is shown that both the modulation technique and the electric machine characteristics influence the capacitor size. For example, electric machine design modifications can introduce more than a 30% reduction in capacitor size. Finally, DC‐link and NPP oscillations with different PWM methods are experimentally validated in a scaled‐down 3L‐NPC inverter.

Virtual Vector-Based Direct Power Control of a Three-Phase Coupled Inductor-Based Bipolar-Output Active Rectifier for More Electric Aircraft

Direct power control (DPC) has gained increasing attention in recent years as a simple and efficient control strategy for pulse width modulation (PWM) rectifiers. In this paper, the idea of DPC is introduced into the three-phase coupled inductor-based bipolar-output active rectifier (TCIBAR) for the first time, and a virtual vector-based direct power control (VVB-DPC) strategy is proposed for TCIBAR to realize the bipolar DC power supply for more electric aircraft (MEA). First, the mathematical model of the TCIBAR is deduced, and the basic principle of the classic DPC strategy is reviewed. On this basis, the limitations of the classic DPC strategy in TCIBAR control are analyzed. Second, a set of virtual vectors are derived to establish a novel virtual-vector switching table. Based on the virtual-vector switching table, the hysteresis power control of TCIBAR can be realized without affecting the DC-side neutral-point potential of TCIBAR. Finally, a neutral-point potential control method based on DPC architecture is studied and integrated into the VVB-DPC strategy to maintain the bipolar DC voltage balance of TCIBAR under unbalanced load conditions. The VVB-DPC strategy is experimentally studied on a TCIBAR prototype, and the experimental results verify the feasibility and effectiveness of the proposed strategy.

Dead-Time Compensation in Active NPC Three-Level Inverters Considering Current Ripple

Existence of dead time that makes the ultimate terminal voltage being inconsistent with the modulation voltage, is recognized as the main reason for leading output voltage distortion, as well as the corresponding low-frequency current harmonics through the filter inductance in inverters. The distortion becomes particularly severe in high power-density high efficiency applications where the wide bandgap semiconductors are gradually being adopted, providing higher switching frequency with small junction capacitance. In order to explain this phenomenon, the mechanism of three-phase current distortion in the vicinity of phase current zero-crossing point is thoroughly studied by vector analysis in the space-vector (SV) plane. The relevant factors that lead to the degradation of current quality are also summarized. Improved compensation strategy has been proposed to ameliorate the current distortion. Compared with the conventional method, an additional item for current ripple compensation is introduced, the value of which is obtained by the deadbeat predictive method. This simple scheme avoids the compensation error caused by current ripple and significantly reduces the current total harmonic distortion (THD) value under the case of small inductance. Furthermore, neutral-point (NP) potential balance control is also considered in conjunction with the scheme. Simulations and experiments are carried out in an active NP clamped (ANPC) inverter platform, which demonstrates the validity of theoretical analysis and the proposed method.

Hybrid Modulation Strategy With Neutral Point Potential Balance of T-Type Three-Level Converter Under Switch Open-Circuit Fault

With the loss of a large number of space vectors under switch open-circuit faults, the neutral point potential (NPP) control of T-type three-level converters is faced with a huge challenge. It becomes harsh to balance the NPP and maintain the symmetry of line voltages under switch open-circuit faults simultaneously. In order to improve the NPP control performance and stable operation of T-type three-level converters, this article presents a hybrid modulation strategy that can enhance the NPP control ability even under switch open-circuit faults. Through a carrier-based analysis tool, two candidate modulation strategies are first evaluated from the perspective of the adjustment range of the neutral point current. The first one maintains the conventional three-level modulation in two nonfault phase arms. The second one degrades the mode of modulation from three-level modulation to two-level modulation in two nonfault phase arms. By making full use of the NPP oscillation characteristics of the two candidate methods, a hybrid modulation strategy based on transition duty-cycle and enabled bridge-arm adjustment is put forward to achieve a comprehensive optimal control performance. Numerical analysis and experiments are carried on to prove the correctness and feasibility of the proposed method.

A novel neutral-point potential balance control method based on voltage feedback for neutral-point clamped three-level inverter

Neutral-point potential imbalance is not only an inherent problem of neutral-point clamped three-level converters, but also the key factor affecting their performance. The traditional capacitor neutral-point potential balance control methods based on space vector modulation, such as VSVPWM, have several shortcomings including the uncompensated region of neutral-point voltage and the slow dynamic response. A novel voltage feedback control method on neutral-point potential balance is proposed in this paper. A coefficient is introduced to define the degree of neutral-point potential imbalance. The composite signal composed of this coefficient and neutral point current is designed. Based on the improved reasonable sectors’ division, the suitable small vectors are selected by this composite signal to obtain the switching sequence of power devices. The voltage feedback control method is employed to realize accurate neutral-point potential balance of NPC three-level inverter. The experimental results show that this method can quickly balance the neutral-point potential fluctuation caused by disturbance. It has the advantages of low switching loss, stable output voltage and current waveform, and good dynamic performance.

Model-Based Design and Experimental Validation of Control System for a Three-Level Inverter

Considering the disadvantages of the traditional development pattern in an embedded control system for an inverter, that is, the single-process thinking and separation of software and hardware, a novel method, which is a model-based design, for developing a double closed-loop control system for the diode-clamped three-level inverter was proposed. System control models, including the PWM control algorithm model, the voltage control model, the neutral-point potential balancing model, and the frequency control model, were built with the MATLAB platform. The code-generation capacity and the operation effect were verified through a series of tests. The inverter with diode clamp and neutral-point potential control was developed. Codes were generated automatically and downloaded to the eZdsp28335 control chip. Experimental waveforms of the phase voltage, line voltage and current were analyzed under regulating the voltage and frequency. The experimental results demonstrate that the models and the generated codes are correct. Further studies have proven the feasibility of the system development model.

Harmonic Analysis and Neutral-Point Potential Control of Interleaved Parallel Three-Level Inverters for Flywheel Energy Storage System

Flywheel energy storage system is a popular energy storage technology, in which inverters are the center of electrical energy conversion, directly affecting the power capacity. Parallel operation of three-level inverters is an effective approach to achieve larger motor drive power and the interleaved operation can improve the harmonic characteristics. However, harmonic analysis models of the interleaved parallel three-level inverters are rare in the literature and how the neutral-point potential imbalance affects the harmonics characteristics has not been discussed. This article establishes the harmonic calculation for balanced and unbalanced neutral-point potential through the five-level voltage capability of the interleaved parallel three-level inverters. Moreover, a neutral-point potential control method based on zero-sequence voltage injection is proposed. The implement process of the method is proposed, and how the operating frequency affect the ability of the neutral-point potential balance is studied. Finally, the simulation and experiment results verify the feasibility and practicability of the established harmonic analysis models and the neutral-point potential control method.

A Novel Neutral-Point Potential Balance Control Method Based on Voltage Feedback for Neutral-Point Clamped Three-Level Inverter

A Novel Neutral-Point Potential Balance Control Method Based on Voltage Feedback for Neutral-Point Clamped Three-Level Inverter

Research on three-phase four-wire NPC converter for Tokamak low-frequency resonance suppressor

During the operation of Tokamak power system, a large number of low frequency harmonic currents with complex spectrum will be generated. Low-frequency resonance suppressor is an effective solution to suppress this low-frequency harmonic currents. The low-frequency harmonic suppression theory of low-frequency resonance suppressor is analyzed in this paper. Meanwhile, the state space equation of three-phase four-wire neutral-point-clamped rectifier is established in the LFRS. According to the mathematical model, the double closed-loop and zero sequence current control strategies are proposed to improve power factor operation on AC side and neutral point potential balance control on DC side of three-phase four-wire converter. The results show the correctness of the theoretical analysis and provide a technical reference for suppression of low-frequency harmonics of other Tokamak devices.

Research on Control Method of Neutral Point Potential Balance of T-Type Three-level Inverter Based on PLECS

The T-type three-level inverter topology has the advantages of low electromagnetic interference, high efficiency, and low output harmonic content. This article combines constant power inverter, independent control of active and reactive power output, Analyzed and studied the neutral point potential balance control of the T-type three-level inverter topology. Through PI adjustment control on the amount of charge of the capacitor, the midpoint voltage of the capacitor is always maintained in a balanced state, and the fluctuation of the midpoint voltage is controlled within ±0.23%. This method can effectively avoid the influence of the difference of capacitance parameters on the DC side on the midpoint voltage. The PLECS software simulation verifies the reliability of the capacitor voltage equalization circuit under the condition of the voltage imbalance at the midpoint of the DC side voltage source supply capacitor voltage equalization.

Analysis of Three Phase 3-Level NPC Voltage Source Converter for AC-DC Conversion

The use of power electronic converters influences the generation of harmonics and reactive power flow in power system. Therefore, three-phase multilevel improved power quality AC-DC converters are gaining lot of popularity in power conversion applications. This work deals with critical problem of multilevel structure i.e neutral point potential (NPP) variation. In this paper, a simplified current controlled scheme is presented to ensure unity power factor operation. Neutral point potential (NPP) of three-phase, 3-level NPC AC-DC converter is controlled by modifying control signal in the controller using NPP regulator. An auxiliary circuit is being presented in this paper as an alternative option for controlling the neutral point potential of the converter. Comparison has been carried out between these control techniques in terms of power quality. A complete mathematical model is presented for better understanding of both techniques used for NPP control. The presented control techniques has been verified through simulation investigations and validated

Novel model predictive direct power control strategy for grid‐connected three‐level inverters

Conventional switching table-based direct power control (DPC) usually suffers from higher power ripples due to the heuristic nature of the switching table. This study presents a novel model predictive DPC (MPDPC) strategy for grid-connected three-level inverters. In the proposed MPDPC strategy, a new synchronous frame-based predictive model is first developed and the conjugate of the complex power is then chosen as the control variable to simplify the analysis. Based on the newly developed predictive model, new principles of the voltage vector selection are further derived with the strict analytical proof to match all the control requirements, including the active and reactive power control, neutral point potential balance control, as well as the avoidance of the high voltage jumps. Finally, experimental results are provided to support the theoretical study and confirm the effectiveness of the proposed MPDPC strategy, as well as to demonstrate better performance in both steady-state and dynamic responses over the conventional DPC.

Neutral point potential balancing method for three‐level power converters in two‐stage three‐phase four‐wire power conversion system

Two-stage power conversion system (PCS) for energy storage systems has been considered in islanded operation mode. A three-level T-type three-leg three-phase four-wire topology (3LT2 3L3P4W) is employed as AC/DC part and a three-level buck/boost converter is used as DC/DC interface. This study is mainly focused on balancing the neutral-point potential (NPP) of the PCS with asymmetrical load connection. An NPP low-frequency ripple model of 3LT2 3L3P4 inverter is proposed considering various load conditions firstly, which includes detailed expression of low-frequency ripple components at fundamental frequency and fundamental frequency multipliers. Then, a novel NPP balancing method based on front-end three-level buck/boost converter under proportional resonant control is presented, in which the resonance points are set at fundamental frequency and triple fundamental frequency to suppress the dominant components of NPP ripple. Furthermore, a small-signal model of three-level buck/boost NPP control system is established and stability analysis is carried out. Simulation and experimental results have been provided to verify the accuracy of the NPP low-frequency ripple model and the effectiveness of the NPP balancing method.

Research on neutral-point potential control of a three-level inverter

<p>Diode-clamped three-level inverters have been widely used in high voltage and high power fields because of their unique advantages. Nowadays, diode-clamped three-level inverters have become a research hotspot. In order to reduce the content of energy harmonics injected into the power grid by the inverter system, the neutral point potential needs to be controlled. This paper proposes a control method based on a proportional controller. The voltage sector was redefined and the design of the proportional controller was completed. In combination with the introduction of a new PWM technology, a smooth control of the midpoint potential was achieved. The effectiveness of the method is verified by simulation in MATLAB<em></em></p>

Optimal Two Degrees-of-Freedom Based Neutral Point Potential Control for Three-Level Neutral Point Clamped Converters

Optimal Two Degrees-of-Freedom Based Neutral Point Potential Control for Three-Level Neutral Point Clamped Converters

A neutral point potential control for three‐level neutral‐point‐clamped converters based on the search optimization method of the two degrees of freedom

To realize the neutral point potential (NPP) control for three‐level neutral‐point‐clamped (TL‐NPC) converters, a novel NPP control based on the search optimization method of the two degrees of freedom is proposed in this paper. It can overcome the low‐frequency NPP fluctuation problem that appears under nearest‐three‐vector pulse width modulation (PWM)‐based NPP control under some operating conditions. Although the traditional dual modulation wave method can also solve the low‐frequency NPP problem, the zero‐sequence voltage is limited under this modulation. It decreases the control domain of the neutral point current (io), which causes serious NPP drift problem especially in some special operating conditions, such as a long dead time, a strong asymmetrical load, or serious switch device inconsistency. The proposed method can expand the control domain of io by defining a new degree of freedom, thus lifting the limits of the zero‐sequence voltage and searching for the optimal two degrees of freedom. The wider control domain of io corresponds to a stronger NPP control ability. Experiments are carried out finally to prove the merits of the proposed method, by comparing the switching frequency, the NPP control speed, and the output performance. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Neutral Point Potential Control for Three Phase 3 - level Neutral Point Clamped Active Front End Converter

Neutral Point Potential Control for Three Phase 3 - level Neutral Point Clamped Active Front End Converter

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 Hybrid PWM-Based DC-Link Voltage Balancing Algorithm for a Three-Level NPC DC/AC Traction Inverter Drive

This paper presents a hybrid pulsewidth modulation (H-PWM) technique, for a three-level neutral point clamped electric vehicle (EV) traction inverter drive. The proposed strategy depicts the advantages of both classic space-vector PWMs (SV-PWMs) as well as carrier-based PWM (CB-PWM). The duty cycles for the traction inverter switches are calculated by the CB-PWM, to reduce the computational time and to control the complexity of the system. The redundancies of the switching states are then used to balance the two dc-link capacitor voltages, similar to the SV-PWM-based strategy. The proposed scheme is capable of maintaining the difference between the two dc-link capacitor voltages for a wider range of machine speed-torque variations. Furthermore, a single carrier is used for PWM, instead of multiple carriers, which further reduces computational complexity. A detailed comparative study is carried out, to prove the performance difference between the low switching loss-based SV-PWM techniques, which is previously proposed by authors and the present proposed H-PWM strategy. In addition, total harmonic distortion of voltage and current (%THD $_{{{v},i}}$ ), duty cycles of the switches, total inverter power loss, as well as dc-link voltage balancing capabilities are also compared. The balancing ability of the proposed strategy by changing the neutral point potential control is also demonstrated. Detailed simulation and experimental studies are carried out to prove the performance of the proposed control strategy with a 6 kW, surface-mounted permanent magnet synchronous machine. Simulation and experimental test results prove the desired performance of the proposed scheme.