Neutral Point Potential Fluctuation Research Articles

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.

Integral Sliding Mode-Based Model Predictive Current Control With Low Computational Amount for Three-Level Neutral-Point-Clamped Inverter-Fed PMSM Drives

This article proposes a novel integral sliding mode (ISM)-based model predictive current control (MPCC) strategy with low computational amount for three-level neutral-point-clamped (3L-NPC) inverter-fed PMSM drives. For conventional MPCC for 3L-NPC inverter, 27 voltage vectors are evaluated for prediction and calculation in each control cycle, therefore computational burden is a major hurdle for practical implementation of MPCC. With only calculation of reference voltage and determination of its sector position, a revised MPCC strategy with neutral point potential (NPP) balancing is proposed, which not only dramatically reduces the number of candidate voltage vectors involved in cost function calculation but also effectively suppresses NPP fluctuation of 3L-NPC inverter. To enhance MPCC robustness against motor parameter variations, the integral sliding mode (ISM) is embedded into MPCC for PMSM drives by enforcing sliding mode throughout the entire system response, and thus a novel ISM-based MPCC strategy is put forward. Furthermore, its robust stability is analyzed. The results of simulation experiments confirm that the proposed ISM-based MPCC strategy with NPP balancing for 3L-NPC inverter can enable PMSM drive system to have satisfactory control performance and strong robustness against motor parameter variations.

Space Vector Based Capacitor Voltage Balancing for a Three-Level NPC Traction Inverter Drive

A dc-link capacitor voltage balancing topology based on virtual space vector is proposed for traction drive application in the field-weakening region. Four different strategies are implemented and their duty ratios are individually derived. The system is then connected to an interior permanent magnet synchronous machine (IPMSM) generally used for traction applications. A comprehensive investigation on the capacitor voltage variation in the field-weakening region is studied and the main contributing vectors are identified. All the required duty cycles of the vectors are analytically calculated based on the three nearest available voltage vectors. All the four proposed strategies helped to minimize the neutral point potential fluctuation (NPPF) considerably in comparison to the earlier proposed schemes. The proposed scheme is then supported by the simulation and experimental verification with a 6.0-kW IPMSM.

Online Feedback Dead Time Compensation Strategy for Three-Level T-Type Inverters

The insertion of dead time will lead to the distortion of output waveforms, which seriously affects the characteristics of the ac induction motor control system. In order to reduce the output distortion of three-level T-type inverters caused by dead time insertion, an online feedback dead time compensation method is developed. In this method, the compensation time is obtained and further corrected by the online feedback of duty cycle. Meanwhile, current polarity judgment is accurately realized by sampling the neutral point (NP) potential and three-phase output terminal voltage potentials. In addition, in this article, we take the NP potential fluctuation into account to approximate the true situation. The developed method can be applied to both open-loop and closed-loop motor drive systems. To verify the effectiveness of the method, essential simulation and experiments are carried out. Compared with the traditional methods, the developed method can prominently reduce the output waveform distortion. Simulation results emphasize that the proposed scheme can significantly improve the performance of three-level inverters, and the identical conclusion is obtained on 210 V/2.24 kW motor platform.

Low Switching Frequency Model Predictive Control of Three-Level Inverter-Fed IM Drives With Speed-Sensorless and Field-Weakening Operations

This paper proposes a single-vector-based model predictive control (MPC) for an induction motor drive supplied by a three-level neutral-point-clamped inverter operating at a low switching frequency. Unlike the torque and flux control in the conventional MPC, the proposed MPC tries to minimize the error between the applied voltage vector and a reference voltage vector obtained based on the principle of deadbeat control, thereby reducing the number of weighting factors in the cost function. To reduce the computational burden and restrict the high jumps in both phase and line voltages, two switching tables are proposed and compared for the preselection of candidate voltage vectors. The neutral point potential fluctuation and switching frequency are also included in the cost function to achieve the balance between the upper and lower dc voltages and a relatively low switching frequency. Furthermore, a speed adaptive stator flux observer with a novel gain matrix is proposed to achieve speed-sensorless operation, which has a higher speed and flux estimation accuracy than conventional fixed gains. Finally, the proposed MPC is extended to the field-weakening operation by adjusting the torque and stator flux references online, which significantly widens the speed range and improves the practical value of the MPC. The effectiveness of the proposed method is confirmed by the experimental results obtained at an average switching frequency of less than 600 Hz.

A Current Harmonic Minimum PWM for Three-Level Converters Aiming at the Low-Frequency Fluctuation Minimum of Neutral-Point Potential

When the selected harmonic elimination pulsewidth modulation (SHEPWM) is adopted to three-level neutral-point-clamped converters to reduce the switching frequency, the neutral-point potential (NPP) problem becomes more severe compared with other modulations. It is because there is no degree of freedom for SHEPWM to control the NPP. SHEPWM realizes a better harmonic performance by calculating the switching angles offline based on harmonic requirements. Since it is harsh to solve the angles online, we cannot change the angles to control the NPP. Aiming at the low-frequency NPP fluctuation minimum, a novel current harmonic minimum method based on the optimal third-order/ninth-order harmonic is proposed in this paper. The optimal third-order/ninth-order harmonic is derived based on the relationships among the third-order/ninth-order harmonic and the NPP. They can be utilized to realize the low-frequency NPP fluctuation minimum without changing any switching angle online. Moreover, some comparisons, such as the solution ranges, weighted total harmonic distortion, the initial value of iteration calculation, show that the proposed method has a good output performance. Finally, the experimental results are given to verify the validity of the proposed method.

Influence of Digital Delay on the Zero-Sequence Voltage Injection Neutral-Point Potential Balance Algorithm for Three-Level NPC Inverters and Its Compensation Method

Three-level neutral-point-clamped (NPC) inverters need to ensure the balance of the neutral-point (NP) potential during operation. The zero-sequence voltage injection balance algorithms based on the carrier-based pulse width modulation (CBPWM) have been proposed in many papers and proved to be effective in controlling the NP potential. In industrial applications, the algorithms are mostly implemented by digital controllers, which have inherent computation digital delay. However, there is little attention to how the digital delay influences the balance algorithms. In this paper, the influence of the digital delay on the zero-sequence voltage injection balance algorithm has been studied in detail. It is found that the digital delay causes a fluctuation of the NP potential. The frequency of the fluctuation is 1/6 control frequency, and the amplitude of the fluctuation is related to the control frequency, the modulation index, the output current, and the dc-side capacitance. Furthermore, in order to eliminate the NP potential fluctuation caused by the digital delay, a digital delay compensation method is proposed in this paper. The correctness of the theoretical analysis and the effectiveness of the compensation method have been verified by the simulation and experiment results.

Low-Complexity Model Predictive Control of Single-Phase Three-Level Rectifiers With Unbalanced Load

The neutral-point potential fluctuation in single-phase three-level rectifiers leads to coupling between the line current regulation and dc-link capacitor voltage balancing, deteriorating waveform quality of the line current. For addressing this issue, this paper proposes a low-complexity model predictive current control (MPCC) with constant switching frequency, which achieves a decoupling control of the line current and the neutral-point potential under the unbalanced load condition. The switching frequency is fixed by combining the MPCC scheme with predefined switching sequences. The boundary of the durations for voltage vectors in switching sequences is presented to balance the capacitor voltage without worsening the line current quality. The optimal switching sequence and the corresponding optimal durations of voltage vectors in the switching sequence are readily derived without the assessment of the cost function, which dramatically simplifies the complexity of the MPCC scheme. Finally, simulations and experimental results are conducted to verify the effectiveness of the proposed MPCC scheme.

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.

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.

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.