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For the open-end winding permanent magnet synchronous motor (OEW-PMSM) fed by dual 3-level inverters with common DC bus, it is necessary to balance the neutral point voltage and suppress the zero-sequence current to improve system performance. The traditional methods to solve these two problems are usually contradictory, and are difficult to obtain satisfactory results. This paper proposes a novel modulation strategy to deal with this dilemma. In this strategy, the neutral point voltage balance is achieved by automatically equalizing the distribution proportion of the reference voltage of OEW-PMSM between the two inverters. At the same time, the strategy makes full use of the control freedom brought by dual inverters to control the three phase voltages of OEW-PMSM independently, so as to ensure that the average value of zero sequence voltage in a sampling period is zero in any case, and thus greatly reducing the ZSC. The experimental results verify the effectiveness and feasibility of this strategy.

Virtual space vector pulsewidth modulation (VSVPWM) provides a new approach to neutral point voltage (NPV) control and common mode voltage (CMV) suppression. Few values of coefficients configured in the virtual vectors cannot adjust the NPV continuously. An improved VSVPWM (IVSVPWM) with active neutral point voltage control (ANPVC) and CMV suppression is proposed in this article. Continuously variable coefficients configured in the virtual vectors adjust the NPV actively and continuously for any load power factor over the full modulation indexes. Failure of CMV suppression is analyzed, and the undesired vectors with high CMV are avoided by limiting the range of the variable coefficients. The limited variable coefficients and the selected basic vectors with low CMV suppress the CMV effectively. Simulation and experimental results are given to validate the proposed novel IVSVPWM, which can achieve quicker ANPVC and reduce CMV to half of the conventional VSVPWM simultaneously.

This article proposes a novel duty ratio regulated virtual voltage vector (VV) model predictive torque control (MPTC) strategy for post-fault eight switch three-phase inverter fed induction motor drive with reduced neutral point (NP) voltage fluctuation. The proposed method contributes to substantially improve the steady-state torque control performance under post-fault conditions. Furthermore, compared with the conventional MPTC, it reduces the torque ripples as well as the NP voltage fluctuations. The mathematical model of eight-switch three-phase post-fault inverter fed induction motor is presented. Based on this, the synthesis of virtual VV is analyzed. Simulation and experimental results are presented to validate the effectiveness of the proposed method.

Nowadays, neutral point voltage (NPV) balancing control and leakage current (LC) reduction methods are considered to improve the reliability of transformerless three-level T-type (3LT2) inverter. Medium vector pulse width modulation (MVPWM) method can control the inverter operation to eliminate the LC; however, is powerless to balance the NPV. Alternatively, discontinuous PWM (DPWM) methods can be implemented to balance the NPV. In this article, a hybrid method for transformerless 3LT2 inverter is proposed to simultaneously minimize the LC and accomplish balancing control in case of NPV unbalancing. The proposed method applies MVPWM switching patterns to eliminate LC while the NPV is balanced. If the NPV becomes unbalanced, it applies specific switching patterns of the DPWM methods as alternative patterns to balance the NPV with minimal LC. Accordingly, the proposed method not only maintains the LC elimination advantage of the MVPWM method but also realizes the NPV balancing capability without any topology modification or adding electrical components. Noteworthy, even the NPV balancing is accomplished with minimal flow of the LC. The proposed method is carrier-based PWM, which shortens execution time and simplifies practical implementation. Experimental tests have been accomplished and results demonstrate the effectiveness of the proposed method on adding the NPV balancing capability with minimal LC to the MVPWM method.

Three-level neutral-point-clamped (NPC) inverter is used in many industrial applications due to its attractive advantages in terms of harmonics content, achieved power level, and electromagnetic interference reduction. However, the main concern in this topology is the imbalance of the neutral point (NP) voltage in the dc side of the inverter. Indeed, the NP voltage distorts the output voltage of the inverter and increases voltage stress on its switching devices. In this article, a new NP voltage balancing control based on modified three-level space vector pulse width modulation (SVPWM) is proposed. The core idea of this method consists in adjusting the application times of redundant vectors in such a way that the NP voltage is kept balanced. In this method, the appropriate adjustment direction is determined by measuring only the capacitor voltages and the output currents of the inverter. The performances of the proposed method are validated and compared through various experimental tests, and the obtained results show an excellent NP voltage balancing regardless of the modulation index values.

Neutral-point voltage imbalance is one of the main problems of five-level neutral-point clamped inverters that impedes their industrial application. In order to solve this problem, a closed-loop neutral-point voltage balancing method based on carrier-overlapped pulsewidth modulation (COPWM) is proposed in this article. The four dc-link capacitor voltages are balanced by three steps: voltage balancing between the two outer capacitors, voltage balancing between the two inner capacitors, and voltage balancing between the two inner and the two outer capacitors. The relationship between the neutral-point currents and the reference voltage is studied, and the top and the bottom dc-link capacitor voltages are balanced by zero-sequence voltage injection. The two inner capacitor voltages are balanced by adjusting the width of switching signals. Simulation and experimental results are presented to confirm the validity of this method.

Neutral-Point clamped (NPC) three-level inverters have a broad application prospect. However, the voltage imbalance of the capacitors and the drafting of its neutral-point voltage will generate voltage stresses on the switches and even increase the total harmonic distortion (THD) rate in their output. In this paper, an improved repetitive control method based on the idea of equivalent input disturbance (EID) compensation is adopted. By adding an additional neutral-point branch, the neutral-point voltage control problem is transformed into a disturbance suppression problem. The case studied are tested using MATLAB/SIMULINK software and the experiment results show that this proposed control strategy can overcome both periodic and non-periodic disturbance, restraining the voltage fluctuation at the neutral point to a lower level.

In order to ensure the safe and reliable operation of three-phase four-wire (3P4W) three-level grid-connected converter (TL-GCC) from the point view of neutral point voltage (NPV) self-equilibrium, a sufficient condition based on carrier-based pulse width modulation (CBPWM) strategy is presented. The analysis of NPV self-equilibrium based on the sufficient condition is presented while different forms of current are outputted. Then, a novel active control method based on producing zero sequence direct current is proposed to balance NPV. The theoretical analysis of NPV self-equilibrium as well as the effectiveness of the proposed active NPV control method are verified by experimental results.

Tertiary winding provided in Star-Star transformer to stabilize neutral point voltage and to provide a path for circulation of zero sequence currents, is the innermost (hidden)winding. Tertiary winding, if not used for any auxiliary supply or compensation purpose, is generally less prone to fault as far as system fault levels aren’t on higher side, as it remains almost idle i.e. no significant current flows in normal balanced network conditions. In one variant of such cases, tertiary winding delta is formed inside the tank and only one end is brought out to earth it and therefore, individual testing of tertiary winding is limited. This paper presents a case study showing how a fault in tertiary winding, with such configuration, echoed (reflected) in the field test results

A novel single-phase hybrid nine-level converter topology is proposed in this paper. the operation principles and one-dimensional space vector modulation strategy of the proposed topology are analyzed, and the integrated balance control strategy of the floating capacitor voltage and the neutral point voltage based on the redundant switch state selection is studied; Finally, the feasibility of the topology and the effectiveness of the control strategy are verified by simulation.

In photovoltaic (PV) systems, transformerless three-level inverters have been widely used because of the simple structure and high efficiency. However, in addition to the inherent neutral-point voltage unbalance problem in three-level inverters, leakage current issues also should be carefully regulated in transformerless systems. But conventional common-mode voltage (CMV) reduction methods cannot well deal with the two problems. According to the principle of generation of leakage current, an improved large-medium-small-vector modulation (ILMSVM) method is proposed in this paper based on the LMP/NSVM method. Unlike conventional CMV suppression method, small vectors with high CMV are also used in the proposed method to control the neutral-point voltage while leakage current is well suppressed. Variation of CMV is kept within 1/6Udc in each switching cycle. And leakage current spikes are also avoided between the switching sequences. Compared with LMP/NSVM method, the neutralpoint voltage is adjusted faster and leakage current spikes is avoided in applying the ILMSVM method. Compared with conventional SVPWM method, ILMSVM method has little disadvantages in neutral point voltage control but can well suppress the leakage current. Effectiveness of the proposed method is verified by comparing the results with conventional SVPMW method and LMP/NSMV method in simulation.

Three-phase VIENNA rectifier with carrier-based pulse-width modulation (CBPWM) is widely used in various fields for its simple structure and high reliability. However, continuous high switching frequency of very semiconductors leads to considerable switching losses. The carrier-based discontinuous modulation (CB-DPWM) method is a preferable method to avoid the switching losses and achieve high efficiency. However, it results in extra issues around the zero-crossing area, causing current distortion. Therefore, a compensation strategy is deployed to address this problem. Moreover, a retrofit zero sequence component injection method is proposed to enhance the neutral-point potential balance ability of the CB-DPWM method. The feasibility of the employed methods are verified by simulation.

In this paper, Vienna rectifier has been inspected as one of the prominent topologies among three-level switched-mode rectifiers. A fast processing discontinuous modulation strategy has been proposed with the aim of improving the rectifier’s performance. The proposed method not only takes advantage of special properties available in three-phase three-level rectifiers, but also reduces the control design complexity and the switching loss. Moreover, neutral point voltage balancing is inherently realized. To achieve these goals, the rectifier is decoupled into two 2-level boost converters in every defined region, leading to simple modulation algorithm and fast processing control strategy. The validity of the proposed technique has been verified via simulation and experimental results conducted on a laboratory prototype.

It is important to maintain the neutral-point (NP) voltage balanced for the three-phase four-wire three-level neutral-point clamped (NPC) inverter. In this paper, after detailed discussion, a mathematical model of the neutral-point voltage are derived. Then a novel control strategy is proposed based on the disassembly of zero level (O Level) to maintain the neutral-point potential. A variable named neutral-point control margin (NPCM) is defined to represent the disassembly margin of each phase. The zero level of one of the three phase is disassembled quantitatively after calculation. Furthermore, the neutral-point voltage can keep balanced while the average output voltage remains unchanged. The proposed control strategy is verified by simulation and experiments.

Owing to its high performances in terms of minimization of the harmonics and the switching losses as well as its ability to generate variable amplitude AC voltage for the same DC bus, the SVPWM is still widely used in the control of multi-level inverters. However, the requirement of the linear transformations and the large number of trigonometric functions is an important drawback of this algorithm, which increases its cost in terms of computation time. Consequently, the trade-off between the signal quality and the algorithm simplicity is still appears. In this way, a new technique is proposed to do this, an efficient SVPWM algorithm with feeble computational overhead based on voltage-second balance principle was needed, this technique for its simplicity on the one hand (simplify the calculation and reduce the computation time) on the one hand and on the other hand afford a contribution to the problem of neutral-point voltage balancing in three-level NPC converter in contrary conventional SVPWM algorithm who involves complex mathematics and requires more hardware resources to implement and takes more time for its execution. in this paper a novel space vector pulse with modulation (SVPWM) algorithm with direct computation based on the neutral-point balancing problem in a three- level inverter analyzed using a redundant algorithm is presented ,the proposed control strategies will be examined by simulation and real time implementation using Matlab/Simulink software with real time interface based on dSpace 1103 board ,the performance of the proposed control technique is demonstrated through simulation and experimental investigation.

This study presents two neutral-point balance control methods for a hybrid VIENNA rectifier, which is composed of a parallel system of a single-switch three-phase Boost rectifier and a VIENNA rectifier. Each rectifier in this topology processes part of the output power, therefore, a highly efficient and reliable performance is achieved in this rectifier system. However, the unbalance voltage of two DC-link capacitors in the VIENNA-type three level rectifier is also reflected in this hybrid rectifier. The fluctuation reason in neutral-point has been analysed using a mathematical model, meanwhile, the two methods with a hybrid algorithm and a simplified method based on zero-sequence component injection are proposed to control neutral-point fluctuation in this hybrid rectifier system. Furthermore, the performance comparison between these two methods is provided. The effectiveness of the two control methods has been verified using experimental results.

Since a T-type three-level PWM converter has several advantages in terms of harmonics and conduction loss, it has been widely adopted for various low voltage applications. However, a neutral point voltage control is necessarily required for stable system operation, and an offset voltage can effectively provide the required neutral point current under unbalanced load conditions. Nevertheless, all types of unbalanced loads cannot be accommodated, in other words, there is a limitation on how much unbalanced load conditions can be allowed. Therefore, this paper analyzed the maximum allowable unbalanced load conditions in the T-type three-level PWM converter. This result can be properly utilized for an effective design verification considering unbalanced load conditions as well as a comprehensive approach for the stable system operation. The feasibility of the analytical result is verified through simulation and experimental tests.

Insulation monitoring is crucial to personal safety and equipment operation in the power distribution system. Recently, with the rapid development of distributed generators, the three-phase voltage-source PWM converter (VSC) is widely used as the bidirectional interface between the dc microgrids and the distribution networks. This paper presents the relationship between the response of insulation monitoring device (IMD) and the neutral-point voltage deviation (NPVD) of VSC, by analyzing the disturbances of the switching function and the dc-side insulation balancing. Then, a method of detecting the NPVD for grid-connected VSC is proposed, based on the dc signal injection and the Duffing oscillator. Furthermore, the proposed method takes the mean and spectrum of the neutral-point voltage as the primal characteristics. Thus, the dc signal injection is adopted to detect the mean of the neutral-point voltage, and the Duffing oscillator is adopted to determine the frequency disturbance of the neutral-point voltage, respectively. The test result confirms the existence of the neutral-point voltage deviation and verifies the availability of the proposed method. Hence, the proposed method could be used to improve the response of IMD.

This paper proposes a novel switching method with the neutral-point voltage control in a single-phase neutral-point-clamped three-level inverter (SP-NPCI) used in photovoltaic systems. A proposed novel switching method for the SP-NPCI improves the efficiency. The main concept is to fix the switching state of one leg. As a result, the switching loss decreases and the total efficiency is improved. In addition, it enables the maximum power-point-tracking operation to be performed by applying the proposed neutral-point voltage control algorithm. This control is implemented by modifying the reference signal. Simulation and experimental results provide verification of the performance of a novel switching method with the neutral-point voltage control.

In the paper by Chen et al. ( IEEE Trans. Ind. Electron ., vol. 66, no. 4, pp. 2837–2846, Apr. 2019), the authors proposed two fault-tolerant control strategies for T-type three-level inverters considering neutral-point (NP) voltage oscillations. However, some basic problems are worth being discussed about the proposed fault-tolerant control methods. This letter presents some comments and results concerning the paper by Chen et al .

With the rapid development of electric vehicles (EV s), the lack of the charging infrastructure has become the bottleneck of the development of EVs. Aiming at the problem of the low charging efficiency and power capacity of AC network, the DC distribution system has been introduced for a settlement. However, a large amount of EVs charging would inevitably result in the DC neutral point voltage imbalance, especially using bipolar structure. In this paper, the main topology structures and neutral grounding modes of DC distribution network is researched and analyzed. Based on the structural feature of DC bipolar distribution network, the influencing factors of DC neutral point voltage, such as network parameter and load distribution, is studied. Then, aiming at the problems of the voltage unbalance, a new coordinated control method is designed to suppress the voltage fluctuation. Finally, a study case of DC distribution network is simulated and analyzed to verify the proposed method, and the results show that the unbalanced voltage could be depressed effectively.

This paper carries out a study on the use of back Electromotive Forces (EMF) zero crossing detection to obtain speed estimation on Brushless Direct Current (BLDC) motor. The study covers the control strategy, changeable speed input and gain changing on Proportional-Integral-Derivative (PID) speed controller. For that purpose, direct back-EMF detection method is used to obtain back-EMF value. This method compares the detected zero crossing with neutral point voltage. This method is re-studied because it is inexpensive and simple whether in its circuit implementation or equation algorithm.

This paper presents an accurate control method of neutral current for neutral point voltage balancing in three-level neutral point-clamped (NPC) inverters considering digital control and PWM delay. A stable and reliable operation of NPC inverters demands balancing in two DC capacitor voltages. Since the voltages are determined by the neutral current, the neutral current should be controlled accurately. The neutral current is synthesized with three-phase currents over a PWM cycle. Due to the digital control and PWM, their delays are included in the synthesis of the neutral current. These delays make the neutral current control inaccurate and affect voltage balancing control in DC link. In this paper, the delays were analyzed and compensated so that the accurate control of neutral current was achieved. Simulations and experimental results verify the effectiveness of the proposed method.

This paper proposes a new two-step robust identification method for the transformer sequence parameter based on median estimation, the method combines multiple sets of data of different time to identify the transformer's ratio and positive sequence parameters step by step and uses the median method to realize identification. First, this paper establishes equivalent circuit model of the transformer which depends only on the PMU (Phasor Measurement Units) measurements and the identified parameters on each side. Second, the actual ratio can be estimated by the median method, then this paper combines the estimated ratio, least square method and median criterion to identify the parameters. This method can effectively reduce the impact of noise and biased data. It is worth noticing that this paper eliminates the neutral point voltage in the model so as to solve the problem that the neutral point and the excitation branch of the transformer are not measurable or the low voltage side of the three-winding transformer is not measured. Finally, the simulation results show the effectiveness of the proposed method and the anti-noise and robustness of the algorithm, and the practicality in engineering is proved by the field PMU data.

Based on the space vector pulse width modulation (SVPWM) theory, this paper realizes an easier SVPWM strategy, which is equivalently implemented by CBSPWM with zero-sequence voltage injection. The traditional SVPWM strategy has no effect on controlling the neutral-point voltage balance. In order to solve the neutral-point voltage unbalance problem for neutral-point-clamped (NPC) three-level inverters, this paper proposes a neutral-point voltage balance controller. The proposed controller realizes controlling the neutral-point voltage balance by dynamically calculating the offset superimposed to the three-phase modulation waves of an equivalent SVPWM strategy. Compared with the traditional SVPWM strategy, the proposed neutral-point voltage balance controller has a strong ability to balance the neutral-point voltage, has good steady-state performance, improves the output waveforms quality and is easy for digital implementation. An experiment has been carried out on a NPC three-level inverter prototype based on a digital signal processor-complex programmable logic device (DSP-CPLD). The obtained experimental results verify the effectiveness of the proposed neutral-point voltage balance controller.