Neutral Point Potential Research Articles

Two‐stage carrier‐based PWM to mitigate neutral point potential oscillation in three‐level inverters

A two-stage carrier-based PWM (CB-PWM) strategy is proposed for a three-phase three-level neutral-point-clamped inverter to maintain its constant neutral point potential (NPP). The CB-PWM is a prevalent modulation method used to control inverters. However, the one-modulation-signal CB-PWM cannot mitigate the oscillation of the NPP. The two-modulation-signal CB-PWM, which can mitigate the oscillation of the NPP, requires many additional restrains with implicit principles. Besides, the capability to regulate the drift NPP in the second method is moderate. The proposed strategy in this Letter is to divide the modulation into two stages. The primary stage utilises the one-modulation-signal method. The secondary stage utilises the two-modulation-signal as the auxiliary to the former one. The modified strategy fixes the dc voltage imbalance and mitigates the oscillation of the NPP under any modulation index range and heavy load conditions. The results of simulations and experiments validate the proposed strategy as feasible and reliable.

Suppression strategy for output unbalance of eight‐switch three‐phase inverter

The fluctuation of neutral-point potential (NPP) is an internal problem of three-level inverter. The fluctuation of NPP is investigated when the inverter, eight-switch three-phase inverter is working in fault tolerant mode. It is proved that the NPP fluctuation will cause output unbalance if the inverter is still controlled by the conventional space vector pulse-width modulation (SVPWM) strategy. Thus, a novel SVPWM considering NPP fluctuation is proposed to suppress the output unbalance. The effectiveness of the proposed method is demonstrated by the 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.

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.

A Method for the Suppression of Fluctuations in the Neutral-Point Potential of a Three-Level NPC Inverter With a Capacitor-Voltage Loop

This paper investigates the problem of fluctuation of the neutral-point potential (NPP) in a three-level NPC inverter with a capacitor-voltage loop. The phase pulse width duty cycle disturbance pulse width modulation (PWM) method is proposed to suppress the NPP fluctuation efficiently. Based on the basic carrier-based phase-disposition PWM method, the average pulse neutral-point current model is established. Then, the frequency, amplitude, and equivalent initial phase of the NPP fluctuation are analyzed based on the current model. According to the alternating error of the dc-link capacitor voltages, a capacitor-voltage loop with a quasi-proportional resonant (PR) controller is presented. The control variable, which varies with the modulation index, phase current, load power factor, etc., can be obtained from the quasi-PR controller. Finally, an experimental three-level NPC inverter is described and the validity and feasibility of the proposed method are verified by experimental results.

Natural Balancing of the Neutral Point Potential of a Three-Level Inverter with Improved Firefly Algorithm

Modern power systems driven by high-power converters have become inevitable in view of the ever increasing demand for electric power. The total power loss can be reduced by limiting the switching losses in such power converters; increased power efficiency can thus be achieved. A reduced switching frequency that is less than a few hundreds of hertz is applied to power converters that produce output waveforms with high distortion. Selective harmonic elimination pulse width modulation (SHEPWM) is an optimized low switching frequency pulse width modulation method that is based on offline estimation. This method can pre-program the harmonic profile of the output waveform over a range of modulation indices to eliminate low-order harmonics. In this paper, a SHEPWM scheme for three-phase three-leg neutral point clamped inverter is proposed. Aside from eliminating the selected harmonics, the DC capacitor voltages at the DC bus are also balanced because of the symmetrical pulse pattern over a quarter cycle of the period. The technique utilized in the estimation of switching angles involves the firefly algorithm (FA). Compared with other techniques, FA is more robust and entails less computation time. Simulation in the MATLAB/SIMULINK environment and experimental verification in the very large scale integration platform with Spartan 6A DSP are performed to prove the validity of the proposed technique.

Carrier‐based generalised discontinuous pulse‐width modulation strategy with flexible neutral‐point voltage control and optimal losses for a three‐level converter

Owing to their capability to reduce switching losses, discontinuous pulse-width modulations (DPWMs) have been widely discussed in the last decades. However, when using the classical DPWMs in three-level (3L) converters, neutral-point (NP) voltage balancing may not be efficiently achieved, because the reference signals are clamped and saturated at regular intervals, and consequently it is difficult to inject the desired zero-sequence component (ZSC). In this study, as the foundation, the general implementation method of 3L carrier-based DPWM (CBDPWM) by injecting discontinuous ZSC (DZSC) is introduced. Following this, a corresponding NP voltage control method is presented and discussed. Finally, on the basis of output prediction and roll optimising method, a novel 3L generalised CBDPWM strategy is proposed which selects and injects the optimal DZSC. With this adjustment, the NP potential can be quickly and flexibly controlled within the given range, while the switching losses can be minimised. The proposed strategy is general and simple. It is applicable to any operating condition without complex calculation and extra efforts. The performance of the proposed strategy is comprehensively evaluated and compared with the traditional DPWMs, and the feasibility and effectiveness of the proposed strategy are validated by the experimental results.

Model Predictive Direct Power Control for Grid-Connected NPC Converters

This paper presents a model predictive direct power control scheme for high-power grid-connected neutral-point-clamped converters. The controller regulates the instantaneous real and reactive powers, as well as the neutral-point potential of the converter, within a set of symmetrical bounds while, at the same time, minimizing the switching frequency of the converter. The proposed approach is distinct from previous predictive power control strategies in that a long prediction horizon is used, allowing lower device switching frequencies to be achieved. A detailed model of the system is provided, and the control algorithm is discussed. Simulation results validate the applicability of the controller to a 3-kV 6.72-MVA system. Experimental results, which are presented for a 240-V 1.68-kVA prototype, show good agreement with those obtained in simulation and further validate the concept.

Capacitor Voltage Balancing of a Five-Level ANPC Converter Using Phase-Shifted PWM

Five-level active neutral-point clamped (5L-ANPC) converter is an attractive topology for high-power medium-voltage motor drives. This paper presents a capacitor voltage-balancing method for the 5L-ANPC converter, including the voltage balancing of dc-link capacitors and flying capacitors. In order to ensure that the series-connected or high-voltage switches of the 5L-ANPC converter are operated at fundamental frequency and the other switches are operated at a constant switching frequency, phase-shifted pulse width modulation is used to control this converter. The relationship between the average neutral-point current and zero-sequence voltage is investigated, and an optimum zero-sequence voltage is calculated to regulate the neutral-point potential. The voltage across the flying capacitor is also regulated by adjusting the switching duty cycles of two PWM signals, which varies the operation time of redundant switching states in each switching period. Simulation and experimental results are presented to verify the validity of this method.

A New Direct Power Control Strategy for NPC Three-Level Voltage Source Rectifiers Using a Novel Vector Influence Table Method

This paper proposes a novel direct power control (DPC) strategy for neutral-point-clamped (NPC) three-level rectifiers, to directly control the active power, the reactive power and the neutral point potential of the rectifiers by referring to three pre-calculated vector influence tables and minimizing an objective function. In the three vector influence tables, the influences of different voltage vectors on the active power, the reactive power and the neutral-point potential are shown explicitly. A conceptual description and control algorithm of the proposed controller are presented in this paper. Then, numerical simulations and experiments are carried out to validate the proposed method. Both the simulation and experimental results show that good performances during both the steady-state and transient operating conditions are achieved. As a result, the proposed strategy has been proven to be effective for NPC three-level rectifiers.

DC voltage balancing for neutral point clamped active PWM converter

This paper presents a unity power factor controller of three phase neutral point clamped (NPC) AC-DC PWM converter. A simplified current controlled scheme to control the DC link voltage, to ensure unity power factor operation and to control neutral point potential (NPP) of the converter is designed and developed for three-phase, three-level NPC AC-DC converter. The developed unity power factor controller (UPC) exhibits satisfactory performance under steady state and transient conditions. A supplementary circuit to control NPP has also been specified in this paper with its complete mathematical model. The developed control technique has been verified and confirmed through MATLAB simulation and experimental results.

Simplified SVPWM that Integrates Overmodulation and Neutral Point Potential Control

A simplified and effective space vector pulse-width modulation (SVPWM) algorithm with two and three levels for three-phase voltage-source converters is proposed in this study. The proposed SVPWM algorithm only uses several linear calculations on three-phase modulated voltages without any complicated trigonometric calculations adopted by conventional SVPWM. This simplified SVPWM also avoids choosing the vector sector required by conventional SVPWM. A two-level overmodulation scheme is integrated into the proposed two-level SVPMW to generate the output voltage that increases from a linear region to a six-step state with a smoothly linear transition characteristic and a simple overmodulation process without a lookup table and complicated nonlinear functions. The three-level SVPWM with a proportional-integral controller effectively balances the neutral point potential of the neutral point clamped converter. Results from the simulation in MATLAB/Simulink and the experiment based on a digital signal processor are provided to clearly demonstrate the validity and effectiveness of the proposed strategies.

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 three‐level one‐cycle controlled bidirectional ac–dc neutral‐point‐clamped converter without having voltage sensors

A three-phase three-level bidirectional ac–dc neutral-point-clamped converter (3 L-NPC) based on one-cycle control (OCC) is proposed in this study. It ensures that the 3 L-NPC converter operates stably during inverting mode of operation. A neutral point potential regulator is also presented which adds a dc offset to the three-phase modulating signals to nullify the deviation in the neutral point potential. First, a one-cycle controlled (OCC) 3 L-NPC converter which requires the sensing of instantaneous phase voltages of the utility is presented. Subsequently, a modified version of the OCC-based 3 L-NPC converter which does not requires sensing of the instantaneous phase voltages is proposed. The modified OCC-based 3 L-NPC estimates instantaneous phase voltages of the source from the pulse width modulation (PWM) switching sequence of the converter. The viability of the schemes is confirmed through a detailed simulation and experimental studies.

Simplified PWM Strategy for Neutral-Point-Clamped (NPC) Three-Level Converter

A novel simplified pulse width modulation(PWM) strategy for neutral point clamped (NPC) three-level converter is proposed in this paper.The direct output voltage modulation is applied to reduce the calculation time. Based on this strategy, several optimized control methods are proposed. The neutral point potential balancing algorithm is discussed and a fine neutral point potential balancing scheme is introduced. Moreover, the minimum pulse width compensation and switching losses reduction can be easily achieved using this modulation strategy. This strategy also gains good results even with the unequal DC link capacitor. The modulation principle is studied in detail and the validity of this simplified PWM strategy is experimentally verified in this paper. The experiment results indicated that the proposed PWM strategy has excellent performance, and the neutral point potential can be balanced well with unequal DC link captaincies.

A Space Vector Modulator for the High-Switching Frequency Control of Three-Level SiC Inverters

Operating three-level inverters at very high switching frequency improves the dynamics of servo drives. It reduces the cost and size of harmonic filters that lessen the dv/dt stress of drive motor windings and bearings. Low harmonic current distortion is a further benefit. Other applications are line-side front end converters, uninterruptable power supplies (UPS), and photovoltaic installations. The requirements of very high-switching frequency control are beyond the computing power of modern microcontrollers and digital signal processors. The problem is overcome by a three-level space vector modulation scheme that defines the switching sequence by taking a sequence of logic decisions instead of doing time-consuming arithmetic operations. The control of the inverter neutral point potential is an inherent part of the scheme and does not need additional algorithms. Implementation in an FPGA permits operating at switching frequencies up to 100 kHz. Experimental results are presented.

An Optimized Control Method Based on Dual Three-Level Inverters for Open-end Winding Induction Motor Drives

An optimized space vector pulse width modulation (SVPWM) method with common mode voltage elimination and neutral point potential balancing is proposed for an open-end winding induction motor. The motor is fed from both of the ends with two neutral point clamped (NPC) three-level inverters. In order to eliminate the common mode voltage of the motor ends and balance the neutral point potential of the DC link, only zero common mode voltage vectors are used and a balancing control factor is gained from calculation in the strategy. In order to improve the harmonic characteristics of the output voltages and currents, the balancing control factor is regulated properly and the theoretical analysis is provided. Simulation and experimental results show that by adopting the proposed method, the common mode voltage can be completely eliminated, the neutral point potential can be accurately balanced and the harmonic performance for the output voltages and currents can be effectively improved.

Modified one‐cycle‐controlled three‐phase pulse‐width modulation rectifiers under low‐output DC voltage conditions

The saddle pulse-width modulation (PWM) that can be obtained by injecting third harmonic current into sampled three-phase input currents is proposed to introduce in the conventional one-cycle control (OCC) strategy-based three-phase PWMrectifier on this study. In addition, for the three-level boost-type neutral-point-clamped structure rectifiers such as three-phase Vienna rectifier, the fluctuation in the neutral-point-potential (NPP) is analysed under modified OCC strategy. The proposed control scheme can not only improve the utilisation of DC bus voltage and the efficiency of converter without sacrificing the advantages of the conventional OCC scheme, such as no phase-locked loop and constant switching frequency, but also is beneficial to suppress the ripple in the DC-link voltage. Moreover, the neutral-point voltage loop is added in the proposed OCC scheme, and the neutral-point voltage controller is employed for controlling the unbalance DC voltage in the NPP which caused by the non-uniformity of parameters of components. The validity of the theoretical analysis and feasibility of the modified OCC strategy are verified by system simulation and experimental results based on a 5-kW prototype.

Carrier-Based Neutral Point Potential Regulator With Reduced Switching Losses for Three-Level Diode-Clamped Inverter

This paper presents the design and implementation of a simple neutral point potential (NPP) regulator for a three-level diode-clamped inverter employing a sine-triangle regulator in conjunction with a closed-loop controller with reduced switching losses. The regulator principle is based on adding a continuous variable offset voltage which regulates the midpoint potential of the dc bus. The novelty of the proposed NPP regulator is in the determination of the magnitude of variable offset voltage based upon the average value, peak-to-peak amplitude, total harmonic distortions, and third harmonic content in NPP. Aside from maintaining dc-bus voltage balance, the proposed regulator leads to a significant reduction in the voltage distortion at the NP, resulting in the reduction of the required dc-bus capacitance. It also reduces the switching losses of the inverter by inserting the “no-switching” zone within each half cycle of the fundamental voltage wave. Analytical, computer simulation, and experimental results verifying the approach are presented in this paper for various load power factor angles.

Modulation Methods for Neutral-Point-Clamped Wind Power Converter Achieving Loss and Thermal Redistribution Under Low-Voltage Ride-Through

The three-level neutral-point (NP)-clamped (3L-NPC) converter is a promising multilevel topology in the application of megawatt wind power generation systems. However, the growing requirements by grid codes may impose high stress and even give reliability problem to this converter topology. This paper investigates the loss and thermal performances of a 10-MW 3L-NPC wind power inverter undergoing low-voltage ride-through (LVRT) operation. A series of new space vector modulation methods is then proposed to relocate the thermal loading among the power switching devices. It is concluded that, with the proposed modulation methods, the thermal distribution in the 3L-NPC wind power inverter undergoing LVRT becomes more equal, and the junction temperature of the most stressed devices can be also relieved. Also, the control ability of the dc-bus NP potential, which is one of the crucial considerations for the 3L-NPC converter, is even more improved by the proposed modulation methods.