Neutral Point Potential Research Articles

A Modified SVPWM Strategy for Reducing PWM Voltage Noise and Balancing Neutral Point Potential

PWM (pulse width modulation) is the most widely applied current conversion technology, but the high-frequency harmonics it causes have a significant negative impact on inverter system performance. This paper focuses on the three-phase T-type three-level inverter as the research object and addresses existing PWM voltage noise and midpoint potential imbalance issues by proposing an improved random SVPWM strategy, named Neutral Point Potential Balance Random Space Vector PWM (NPB–RSVPWM). The NPB–RSVPWM strategy includes three main steps: (1) introducing a midpoint potential balancing control loop to adjust the synthesis timing of the effective vectors to generate pulse signals, optimizing midpoint potential balance; (2) employing a randomly varying carrier frequency in place of the carrier used in the SVPWM strategy to generate the driving signals for switching devices; and (3) controlling the inverter through the driving pulse signals. This strategy optimizes the synthesis sequence of traditional SVPWM strategy vectors and incorporates random frequency modulation techniques. The mathematical model analyzes PWM harmonic expressions corresponding to fixed switching frequencies, and a random frequency carrier is chosen to suppress these PWM harmonics. The effective vector’s equivalent circuit is analyzed, proposing a technique for optimized vector synthesis timing. The simulation and experimental results verify that the NPB–RSVPWM technique can disperse PWM harmonic energy, reduce voltage noise, and optimize midpoint potential balance. Under the NPB–RSVPWM strategy, the line voltage spectrum becomes uniform, the maximum harmonic content is greatly reduced, and the fluctuation in the DC side midpoint potential is significantly improved. Compared with the traditional SVPWM strategy and random PWM strategy, the NPB–RSVPWM strategy has a lower voltage noise, smaller total harmonic distortion, and a more stable midpoint potential. The effectiveness and feasibility of the NPB–RSVPWM strategy are verified by simulation and experimental results.

A Mathematical Structural Modulation Method for Neutral-Point Potential Balancing of Three-Level Neutral-Point-Clamped Converters

To achieve the fast neutral-point potential balance of three-level neutral-point-clamped (3L-NPC) converters, a mathematical structural modulation method is presented in this paper, featuring fast balance capability, low computation, and easy implementation. In this method, the duty cycle is decomposed into the differential-mode component (DMC) and the common-mode component (CMC). The CMC is used to synthesize the AC output voltage while the DMC is adopted to control the neutral-point potential. Based on the fractional-order sliding-mode control theory, the DMC is constructed. As a result, neutral-point potential balancing in a pre-calculated time is achieved. Finally, the effectiveness of the proposed method is verified by experimental results.

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.

Bidirectional PWM Converter to Interface AC Part to DC Microgrid with Neutral Potential Balancing

This research presents a description of a bidirectional pulse width modulated converter that functions as a rectifier by using the direct power control strategy and space vector modulation technology. In contrast to the voltage-oriented control strategy, this method minimises the number of internal current loops and requires more tuning. While functioning as an inverter, it applies the space vector modulation technique as well. The approach, on the other hand, is not the same as rectifiers since it calculates the duty ratio of the neighbouring voltage vectors in each small triangle that is generated in each sector that has a period of 60 degrees. In both situations, the neutral point potential has been brought back into equilibrium.

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.

A Fault-Tolerant Control Strategy for Three-Level Grid-Connected NPC Inverters after Single-Arm Failure with Optimized SVPWM

Three-level NPC inverters have been widely used in grid-connected systems due to their superior performance compared with two-level inverters, but more switches lead to high fault probability. Meanwhile, the neutral point potential (NPP) fluctuation of the DC link is an inherent problem of three-level NPC inverters. To keep the three-level NPC inverter running stably after single-arm failure, a fault-tolerant control strategy based on an optimised space vector pulse width modulation (SVPWM) is proposed in this paper. Firstly, the common-mode voltage (CMV) of the postfault three-level NPC inverter is analysed and then the preliminary synthesis principles of the reference voltage vector are determined. Then, in order to ensure the NPP balance and the quality of the grid-connected currents, the reference voltage vector synthesis rules are optimised, a low-pass filter (LPF) and a hysteresis comparator are designed, respectively, to ensure the quality of grid-connected currents and effectively decrease the DC link NPP deviation. Finally, the simulation results show that the proposed fault-tolerant control strategy can realize the stable and reliable operation of the grid-connected three-level NPC inverter after single-arm failure, and the CMV can be reduced significantly, the quality of grid-connected currents is also improved. The proposed fault-tolerant strategy also shows good performance when the grid-connected currents change.

Space Vector Pulsewidth Modulation Strategy for ANPC-5L Inverter Based on Model Predictive Control

SVPWM strategies for active neutral-point-clamped five-level (ANPC-5L) inverters have multiple control degrees of freedom and control objectives. However, traditional SVPWM strategies are hard to realize optimal control of multiple objectives due to the objectives are coupled with each other. To address this problem, an SVPWM strategy based on model predictive control (MPC) is proposed. In the pulse generation process of the proposed SVPWM, all control degrees of freedom are utilized to achieve optimal control of multiple objectives through MPC. Firstly, voltage vector sequences that meet the requirements of CMV are selected. Then, a cost function is constructed to realize optimal control of the neutral point potential (NPP) and three floating capacitors (FCs) voltage. The proposed strategy has advantages such as high control accuracy and producing low CMV. Finally, the effectiveness of the proposed strategy is validated by a 150kW three-phase ANPC-5L inverter prototype.

Duty-Cycle Correction-Based Model Predictive Current Control for PMSM Drives Fed by a Three-Level Inverter With Low Switching Frequency

In this paper, a duty-cycle correction-based model predictive current control (DC-MPCC) is proposed for permanent magnet synchronous motor (PMSM) supplied by a neutral-point clamped three-level voltage source inverter (NPC-3LVSI). Unlike the conventional MPCC, which evaluates the impact of basic voltage vectors on the concerned state variables, the proposed DC-MPCC modifies the output voltage levels with optimized duty-cycle corrections. Firstly, the last three-phase voltage levels are assumed to be kept during the next control period. Then, the current tracking error and neutral-point potential are predicted. After that, the voltage levels are modified with zero, one, or two state changes, which formulate seven candidate solutions. Subsequently, the duty-cycle corrections of the modified voltage levels are computed based on the principle of minimizing current tracking error and neutral-point voltage drift. Finally, the optimal switch sequence is generated by evaluating and sorting a cost function with a penalty on switch actions. The proposed DC-MPCC features variable switching instants, relatively lower sampling frequency, and satisfactory performance under low switching frequency. Experimental tests carried out on a NPC-3LVSI fed PMSM drive, with 100 Hz fundamental frequency and 400 Hz switching frequency, accompanied by a video demonstration, validate the effectiveness of the proposed method.

An Improved FCS-MPC Based on Novel Sector Optimization and Capacitor Charge Balance Algorithm for T-Type 3P-3L Converters

Conventional FCS-MPC suffers from high computational burden and cumbersome weight factor design, especially for three-phase three-level (3P-3L) converters. In this paper, an improved FCS-MPC is proposed for T-type 3P-3L converters to reduce computational burden, eliminate weight factor and improve system performance. Three improvements are as follows. First, a sector optimization method based on effective vector radiation range is proposed to determine the optimal vector for current tracking without vector enumeration, and combined virtual vector is adopted to supersede basic vector and avoid affecting neutral-point (NP) potential. Second, a capacitor charge balance algorithm is proposed to determine the optimal vector for NP potential balance without weight factor, which minimizes negative effects on current tracking. Third, a hybrid vector output mode is proposed to give full consideration to current tracking and NP potential balance performance. Finally, advantages of proposed MPC are verified in simulation and experiment, where code execution time and current THD are reduced by 16.1 μs and 1.27%, steady-state current error and capacitor voltage deviation are experimentally reduced by 0.8 A and 0.6 V, and transient DC voltage recovery time and capacitor voltage deviation are experimentally reduced by 2 ms and 0.4 V compared with conventional FCS-MPC.

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.

A simplified space vector pulse width modulation method for a three‐level four‐leg inverter considering the neutral point potential balance

AbstractThis paper presents a simple three‐dimensional space vector modulation method in ghγ coordinates for a three‐level four‐leg neutral‐point‐clamped inverter. The idea uses redundant vectors as centre points to establish two‐level space vector diagrams simplifying the original space, the first sector of which is designated as subspaces, where a modulation process is implemented. The duty cycle and all possible switching sequences in a tetrahedron can be calculated automatically without manual storage and can avoid a full‐scale search process. Additionally, the neutral point potential can be balanced by three types of charge through cancellation. Such features enable the inverter to demonstrate good performance under closed‐loop control. Moreover, the proposed method can be applied to a multilevel inverter. Simulation and experimental results demonstrate the effectiveness of the proposed concept.

Research on the Coordination Suppression Strategy of Neutral Point Potential and Common Mode Voltage for NPC Three-Level Inverter

A Neutral Point Clamped (NPC) type three-level inverter has the advantages of a low switch voltage and a high-quality output waveform. However, there are inherent problems, such as neutral point potential asymmetry and common mode interferences, which easily affect the symmetry of the three-phase output voltage and the safe operation of the inverter. Firstly, according to the different modulation degrees, the fluctuation of midpoint potential can be suppressed by adding an action time correction factor to the traditional space vector pulse-width (SVPWM) modulation strategy and virtual space vector pulse-width (VSVPWM) modulation strategy, respectively. Secondly, considering the suppression of common mode voltage, a new virtual space vector pulse-width (NVSVPWM) modulation strategy is proposed, which uses a set of new vector synthesis methods. Finally, the simulation and experiment prove that the NVSVPWM strategy with an action time correction factor can suppress the fluctuation of the neutral point potential and common mode voltage at the same time, and the effect is significant.

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 neutral-point potential balancing control method with third harmonic injected pulse width modulation for three-level converters

The unbalance of neutral-point potential (NPP) is an important problem in three-level converter. This paper proposes a neutral-point potential balancing control method with the third harmonic injected pulse width modulation (PWM) for three-level converters, which can a) realize better steady and dynamic performance of restraining the NPP unbalance; b) realize the similar performance with the space vector pulse width modulation (SVPWM) but with less amount of calculation; c) be applied in multiple operating modes such as grid-tied/off-grid mode, voltage-source/current-source mode. Finally, simulation results are presented, which validate the performance of the proposed method.

Model Predictive Control With Constant Switching Frequency for Three-Level T-Type Inverter-Fed PMSM Drives

Finite control set model predictive control (FCS-MPC) shows superior dynamic performance with simple structure for motor drives. However, conventional FCS-MPC method presents unfixed switching frequency, making it intractable for the design of heat sinks and output filters. To address this issue, this article is devoted to developing a new FCS-MPC strategy with constant switching frequency for permanent magnet synchronous motor (PMSM) drives fed by three-level T-type inverter. The switching frequency regulation is realized by extending the period control technique, where the periods are measured between each rising edge and falling edge of the gate signals. The proposed algorithm can be embedded in the iterative process of the FCS-MPC, which can easily achieve the fixed switching frequency with balanced neutral point potential. The performance of this control strategy is numerically evaluated and experimentally verified by an 11-kW PMSM drive platform.

Improved Model Predictive Current Control of NPC Three-Level Converter Fed PMSM System for Neutral Point Potential Imbalance Suppression

For the neutral point clamped (NPC) three-level converter fed permanent magnet synchronous motor (PMSM) system, the performance of the conventional model predictive current control (MPCC) algorithm will be deteriorated if the amplitude of the neutral point potential (NPP) is large. Additionally, the adjustment process of the weighted coefficients of the conventional MPCC algorithm is complex because of numerous control terms in the cost function. To solve the above issues, an improved MPCC algorithm is proposed in this paper. Firstly, Newtonian iteration is used to transfer the stator current into stator voltage in the cost function. Then, the NPP term in the conventional cost function can be eliminated by introducing the partition control of the NP potential, which also eliminates the whole adjustment process of weighting coefficients. Finally, based on the amplitude of the NPP, the amplitude and phase angle of medium and small vectors are modified to improve the control performance of the torque and flux. Experimental results show that the fluctuation of the neutral point potential can be suppressed rapidly. Meanwhile, the performance of the torque, flux and current are also improved compared with the conventional MPCC.

An Improved LMSVM Method for Leakage Current Suppression and Neutral-Point Voltage Control in Transformerless NPC Three-Level Inverters

Transformerless three-level inverters have many advantages over two-level inverters in photovoltaic (PV) systems. However, in transformerless systems, except for the inherent unbalanced neutral voltage of the three-level inverter, the leakage current also needs to be limited strictly. Concerning the defects of the conventional common-mode voltage (CMV) suppression methods, an improved LMSVM (ILMSVM) method is proposed. Unlike the conventional CMV suppression method, the ILMSVM method no longer just abandons the high CMV small vectors to limit the range of CMV to ±1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6U_{\mathrm {dc}}$ </tex-math></inline-formula> , but limits CMV variation to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta 1/6U_{\mathrm {dc}}$ </tex-math></inline-formula> in all switching cycles. So, high CMV small vectors also can be used in some sequence to help controlling neutral point (NP) while keeping leakage current well suppressed. Moreover, medium vector sequences are also used as an independent control sequence to help achieving bidirectional control on NP potential. Compared with LMP/NSVM strategy, the ILMSVM method has advantages in regulating NP potential, and leakage current spikes of the LMP/NSVM method are also eliminated. In comparison with the conventional SVPWM (CSVPWM) method, leakage current in the ILMSVM method is also well restrained. Simulations and experiment results verify that the ILMSVM method can well restrain leakage current and maintain good control of NP in comparing with LMP/space vector modulation (SVM) method and CSVPWM method.

Model predictive direct power control strategy for leakage current suppression on a grid‐connected three‐level inverter

AbstractThe nonisolated grid‐connected photovoltaic inverter system suffers from the problem of leakage current. Therefore, a leakage current suppression control strategy based on the model predictive direct power control is proposed for the three‐level four‐leg grid‐connected inverter. At first, the system model is established to analyze the factors that affect the leakage current and the potential of the neutral point. Second, the first three legs, inverting part, use model predictive direct power control, and the fourth leg of the inverter is used to maintain a constant common‐mode voltage to achieve the suppression of the leakage current. The fourth leg is indirectly controlled by the switch state of the first three legs; thus, the number of optimization times is reduced. Finally, by simulation, the leakage current is compared and analyzed with the three‐level three‐leg inverter under conditions with different parasitic capacitance values. The results show that the proposed strategy suppresses the leakage current by more than 60% without affecting the neutral point potential and has a fast optimization speed.

Three-Level Indirect Matrix Converter With Neutral-Point Potential Balance Scheme for Adjustable Speed Drives

To improve the performance and explore a new solution of adjustable speed drive systems, a T-type three-level indirect matrix converter (T3LIMC) topology and its modulation scheme are presented in this article. The presented topology is composed of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> input filter, a current-source rectifier, and a T-type three-level inverter, where the split dc source voltage of the inverter is formed by two input line-line voltages and the neutral-point voltage. A multicarrier-based modulation scheme for the inverter and a specific zero-sequence voltage are also developed, and the desired neutral-point current of the inverter is constructed to compensate for the input currents. Consequently, the balanced neutral-point potential and symmetric sinusoidal input currents are achieved without additional control effort. In addition to advantages such as bidirectional power flow, sinusoidal input–output currents, controllable input power factor, no dc-link energy storage elements, and compact structure, the T3LIMC also improves the output waveform quality by generating the three-level output voltages. Meanwhile, the zero current switching (ZCS) operation of the rectifier helps to improve efficiency. Finally, the functionality and effectiveness of the presented topology and modulation scheme are verified by experimental results.