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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.

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.

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.

In view of a three-phase/level boost rectifier, the performance of VIENNA rectifier is affected by the neutral-point (NP) voltage unbalance and oscillation. A novel strategy of zero-sequence component injection to balance the neutral-point voltage and suppress oscillation for VIENNA rectifier is proposed in this paper. Based on the mathematical expressions of neutral-point voltage, a simplified carrier-based PWM based on the equivalence of SVM is implemented to balance the neutral-point voltage. Analyzing mathematical relationship between the fluctuation of neutral-point voltage and the zero-sequence component, an optimized zero-sequence component is proposed to suppress neutral-point voltage oscillation. The validity of the theoretical analysis and feasibility of the proposed strategy are verified by simulation and experimental results.

In this paper, an improved fault-tolerant control strategy with neutral-point (NP) voltage balancing for T-type three-level inverter when an open-circuit fault occurs is proposed. Different from the existed fault-tolerant control strategies, this method can not only restore the output current but also balance the neutral-point voltage. The proposed strategy is illustrated by dividing fault into two conditions: the faulty condition of half-bridge switches and NP switches. When a half-bridge switch fails, the NP voltage is controlled by adjusting the amplitude of the three-phase reference voltages. In cases of the open-circuit fault in a NP switch, the balance and oscillations of the neutral-point voltage are considerably controlled by adding a proper offset value to switch turn-on times of three-phase reference voltages. The presented method does not require any additional hardware and complex calculations. Experimental results show the performance of the proposed control strategy.

A three-level converter has two DC-links and it makes output voltage with three levels. If the two DC-link voltages are not the same, the output voltage is distorted.

This study presents the development of a control system for permanent magnet motors embedded in railway wheels for integrated traction and steering of rail vehicles. The overall control scheme and the control strategies for the provision of traction and wheelset steering are presented, but a main focus of this study will be concerned with the development of a new sensor-less position detection technique that overcomes the difficulties of position detection at zero and very low speed using the back-emf- based methods. The new technique does not require the injection of high frequency harmonics, and detects the motor position from simple measurements of the neutral point voltage of the motor influenced by the variation of inductances with respect to rotor position. Both theoretical development and experimental verifications are provided.

Nowadays, an increasing number of grid is adopting resonant grounding method. Only accurately detecting the capacitive current, could we play the advantages of resonant grounding. The most popular method in the field is the tuning method, which includes two-point method and three-point method. This paper analyzes the principle of two-point method and identifies a major problem in the application of two-point method. A stall of two-point method stays in the state of overcompensation while another stall stays in the state of under compensation, which has not been taken into account. Finally, the paper verifies the effect of proposed improvements using MATLAB simulation.

This paper investigates the problem of low-frequency voltage oscillations that appear at the neutral point (NP) of a three-level neutral-point-clamped (NPC) converter. Starting with a detailed analysis of their origin, the paper derives the minimum amplitude of these oscillations that can be achieved by nearest-vector (NV) modulation strategies. It then proves that the criterion of the direction of dc-link capacitor imbalance, which is commonly adopted by NV strategies for performing the task of capacitor balancing, poses a barrier in achieving this minimum. A new criterion is proposed instead, together with an algorithm that incorporates it into existing NV strategies. For the case of NPC inverters operating as motor drives, the resulting reduction in the amplitude of NP voltage oscillations ranges from 30% to 50%. The approach has the advantage of avoiding the significant increment in switching losses and output voltage harmonic distortion, caused by other methods. Simulations in MATLAB-Simulink are used to illustrate its operation and verify that it offers the claimed benefit.

To improve the efficiency of the photovoltaic (PV) system using the three-level inverter, PV modules are separately connected to the DC-links. However, in this connection, the output current distortion can be caused by the unbalanced neutral-point voltage. Then, total efficiency is decreased. This paper proposes a high-efficiency switching strategy and a method for the neutral-point voltage control for a single-phase three-level inverter. In the single-phase three-level inverter, this switching method increases efficiency by reducing switch loss. In addition, the proposed method is implemented by changing space vector and modifying the reference voltages to control the neutral-point voltage. As a result, efficiency of the PV system is increased. The proposed method is proven by the simulation results.

This paper proposes an efficient neutral point control algorithm for three level T-type inverters. The proposed algorithm helps to get balanced neutral point voltage with high efficiency steady state operation. The proposed controller relies on using the hysteresis control for DC link capacitors' voltage balancing. The proposed algorithm is implemented by adjusting the type of redundant small vectors and its duration times in the nearest three vectors (N3V) space vector modulation (SVPWM). Lower switching losses have been achieved by using reduced transitions in the output voltages. The proposed neutral point voltage controller achieves multiple objectives as it increases significantly the ability of the T-type three level inverter to balance the voltage of neutral point with reduced switching events. The proposed method is simple to implement without need for complex calculation or additional hardware. The results of simulation and experimental work are shown in order to verify the feasibility and effectiveness of the proposed controller.

This paper proposes a new hybrid space vector pulse width modulation (SVPWM) strategy to minimize the neutral point voltage ripple of a three-level T-type converter. The origin of neutral point voltage ripple is analyzed in detail. Then, the controllability of a nearest three vectors (N3V) based SVPWM on the neutral point voltage balancing is investigated. In the uncontrollable regions of the N3V SVPWM approach, a non N3V based SVPWM is incorporated to balance the neutral point voltage with a switching cycle based profile. The non N3V based SVPWM is realized by replacing the medium voltage vector of the three nearest vectors (NVs) with a large voltage vector in an appropriate way. By introducing an offset vector, the reference voltage vector of the three-level converter can be synthesized in a way similar to that of a two-level converter. The effectiveness and the advantages of the proposed hybrid SVPWM scheme are verified by the simulation results in Matlab @ Simulink.

This paper explores the fundamental limitations of the neutral-point voltage balancing problem for different loading conditions of three-level voltage source inverters. A new model in the DQ coordinate frame utilizing current switching functions is developed as a means to investigate theoretical limitations and to offer a more intuitive insight into the problem. The low-frequency ripple of the neutral point caused by certain loading conditions is reported and quantified.

An auxiliary resonant commutated pole (ARCP) inverter can achieve not only so-called soft switching but also PWM control. The fluctuation of the neutral point voltage, however, causes commutation failure of the inverter.It was found by analytical and experimental results that flux vector following control for switching pattern generation gives larger value of the voltage fluctuation than sin-triangular wave comparison method. Then a new control scheme to maintain the neutral point voltage at a half of dc link voltage was proposed. The method utilizes the redundancy of zero voltage vector of the voltage source inverter. For the ARCP 3 phase inverter (AC output 200V, 4.5kVA) built and tested, the value of capacitors to keep the neutral point voltage with the control can be decreased to about 1/60 of that when no control is applied.

This paper discusses a frequency multiplying circuit, which consists of a multi-phase voltage-source inverter and multiple toroidal core. The principle of the frequency multiplying is to utilize the neutral point voltage fluctuation of the load. A multi-core transformer is used to obtain the voltage fluctuation. This paper discusses the design of the circuit, transformer and loss analysis of the proposed circuit. Besides, this paper clarifies the validity of the proposed converter in terms of simulation and experimental results.

This paper investigates the neutral point voltage balancing problem in three-level NPC PWM VSI theoretically. Current vector is introduced to model the disturbance of NP voltage due to middle voltage vectors and small voltage vectors. The ripple of NP voltage is consisting of two parts, one is low-frequency and the other is high frequency. The low frequency is three order of frequency of VSI output. The condition of modulation index and power factor angle that NP voltage can balance in one sample cycle is proposed. And in some condition, the NP voltage can balance in one sixth of VSI output cycle.

In this paper a modification of the neutral-point-clamped (NPC) converter, in which a fourth leg has been added, is presented. The main aim of this additional leg is to increase the reliability of the converter and thereby allow it to continue operating when a breakdown in one of the IGBTs occurs. In addition, during normal fault-free operation, the fourth leg also guarantees a correct control of the neutral point voltage, removing completely the low frequency voltage oscillations. A simple and computationally efficient control scheme is proposed for this purpose.

This paper analyzes the unbalancing problem of neutral-point voltage with light load in DC-Rail ZVT softswitching inverter. Description of the unbalance reason and the influence to the circuit operating process are detailed through theoretical analyses, and then the schemes of solving this problem are presented. Finally, the scheme feasibility is verified by relative experiments.

This paper describes the application of the vector control technique to a three level or NPC (neutral point clamped) inverter using a DSP (digital signal processor) TMS320C25. A newly developed PWM technique for this kind of inverter is applied. As a result the switching losses as well as the current distortions are reduced. A smaller fluctuation of the neutral point voltage was also obtained. The proposed method was applied to a 20 kW motor using an NPC converter and inverter. Theoretical and practical aspects as well as the experimental results are presented in this report.

The three-level neutral point clamp topology has attracted applications in medium-voltage high-power drive systems in recent years. Since the topology utilizes a neutral point voltage to achieve half bus voltage withstanding of power devices and three-level output voltage delivery, it is critical to balance the neutral point voltage in occasions of disturbed or even continuous unbalance load conditions. This paper proposes a couple of balancing techniques based on space vector modulation principle and experimentally compares their effectiveness against the existing method.

Two space vector control schemes are presented to improve current and neutral point voltage performance for a boost type three-level rectifier. While the exact vector scheme can make the AC line current follow the command current better, the average vector scheme can balance the DC neutral point voltage better. Both methods show much improved line current and neutral point voltage performance. Limitations and trade-offs of each scheme are also discussed in the paper. Simulation results are obtained with the proposed schemes.