Alian Chen

Space-Vector-Modulated Method for Boosting and Neutral Voltage Balancing in

In this paper, a new modulation method is proposed for a Z-source three-level T-type inverter (3LT2I). The Z-source 3LT2I combines the merits of the Z-source two-level inverter and the advantages of the Z-source neutral-point-clamped inverter. Although the previous literature has proved that the space vector modulation (SVM) technique for Z-source 3LT2I produces the desired voltage transfer gain with minimized switching loss, it cannot deal with the neutral-point (NP) potential balance issue. Therefore, an improved SVM method addressing this problem for Z-source 3LT2I is proposed. The method simplifies the space-vector diagram of a Z-source 3LT2I into that of a Z-source two level inverter. In order to obtain NP potential balance, the pattern of the conventional symmetric SVM is changed by properly adding or subtracting a percentage of the minimum gate-on time. The upper and lower shoot-through states can be inserted into the improved symmetric SVM, thus offering av few advantages, including boosting ability, NP potential balance, and low switching loss. The considerate stable NP voltage and the boosting gain have been experimentally proven by results from a prototype inverter.

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Parallel three-level T-type inverters (3LT2Is) can effectively increase the power rating. However, when multiple 3LT2Is are connected to a single dc bus, the zero-sequence circulating current (ZSCC) will occur. The ZSCC will distort the output currents and increase the system loss. To overcome this drawback, this paper proposes a novel deadbeat control scheme for the ZSCC suppression. A generalized model for analyzing ZSCC in parallel 3LT2 Is is developed for designing the deadbeat control method. The proposed deadbeat control scheme is realized by real time adjusting the dwell time of small vectors in each space vector modulation. Compared to the conventional PI control method, the influence of different sector is eliminated, thus, improving the output current waveform and reducing the ZSCC. The effectiveness of the proposed method is verified by simulation and experiments.

-Source Three-Level T-Type Inverter

This paper discusses a novel single-phase flyback inverter, which is composed of a Boost converter and a flyback inverter suitable for ac PV module systems. Multilevel pulse train technique and DCM operation is used for Boost converter to generate a stable dc voltage. The output voltage ripple of the converter can be reduced effectively and the dynamic response can be faster. And a reduction in the total capacitance on the Dc-Link is realized. For the flyback inverter, a peak-current mode control scheme is employed to draw a sinusoidal output current into the AC grid. The simulation results show that the proposed inverter has the properties of high power factor, low cost, low current distortion and small volume.

Space Vector Modulation for Circulating Current Suppression Using Deadbeat Control Strategy in Parallel Three-Level Neutral-Clamped Inverters

Selective harmonic elimination pulse-width modulation (SHEPWM) is a well-known switching technique applied in power electronics field for eliminating low order harmonics. According to symmetrical pulse patterns over the quarter of the period in SHEPWM, the neutral-point voltage could be balance naturally in a certain range. However, the neutral-point voltage fluctuation would be enlarging as the output current increases. In addition, when neutral-point voltage is unbalanced seriously, it would cost much time to recover balance on the classical SHEPWM modulation. Therefore, a novel neutral-point voltage balancing control method for three-level T-type inverter (3LT2I) using SHEPWM is presented, which installs a cascade small-vector control system behind the conventional three-phase SHEPWM modulation. In order to reduce the switching losses, the small-vector control system unit can be disabled or enabled according to deviation level of the neutral-point voltage. Moreover, the novel control method does not affect the output line-to-line voltage waveforms of SHEPWM inverter. Therefore, it not only has the same effect with the conventional SHEPWM on eliminating low order harmonics, but also can minimize the fluctuation of the neutral-point voltage and restore the neutral-point voltage balance rapidly compared with the conventional SHEPWM. Simulation and experimental results confirmed the performance and effectiveness of the proposed method.

High-frequency DC link flyback single phase inverter for grid-connected photovoltaic system

The topology of parallel three-phase voltage source inverters (VSIs) has been widely utilized to raise system power rating, but zero-sequence circulating currents (ZSCCs) are generated by control effect and hardware parameter differences. ZSCCs could lead to current distortion and impact the system stability. The model predictive control (MPC) method has been applied to the inverters to get high robustness, fast dynamic response and low switching frequency. However, the MPC method is rarely used in parallel inverters because of the ZSCCs problem. This paper proposes an improved MPC algorithm for parallel system to track the reference currents as well as suppress the ZSCCs. The contribution of each space vector to ZSCCs is analyzed and the cost function is redesigned in the new method. The cost function will pick out the optimal vectors to guarantee the control requirements. Experimental results verified that the improved algorithm is effective and performs well in both current tracking and ZSCCs suppression.

Neutral-point voltage balancing control for three-level T-type inverter using SHEPWM

In this paper, the Z-source three-level T-type converter (3LT2C) is proposed for low-voltage applications. The Z-source 3LT2C basically combines the positive aspects of the Zsource two-level converter with the advantages of the Z-source neutral point clamped (NPC) converter. The traditional space vector modulation (SVM) technique for Z-source 3LT2C produces the desired voltage transfer gain with minimized switching loss, but cannot deal with neutral-point potential (NP) balance issue. Therefore, an improved SVM method considering this coupling problem for Z-source 3LT2C is proposed. The method simplifies the space-vector diagram of a Z-source 3LT2C into that of a Z-source two-level converter. In order to obtain NP balance, the time of the conventional symmetric SVM is changed by properly adding or subtracting a percentage of the minimum gate-on time. The upper-shoot-through and lower-shoot-through states can be inserted into the improved symmetric SVM, which gives a large amount of benefits, both in boosting ability, NP balance and switching loss. The considerate stable NP voltage and the boosting gain have been proven in experimental from a prototype converter.

A novel model predictive control algorithm to suppress the zero-sequence circulating currents for parallel three-phase voltage source inverters

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.

Space-vector-modulated for Z-source three-level T-type converter with neutral voltage balancing

The three-level T-type inverter has outstanding performances and better in the switching device selection than a two-level inverter. However, the problem of common-mode voltages and neutral-point potential (NP) unbalance is generated. This paper proposes a model predictive control method for reducing the common mode voltage and balancing the neutral-point potential (NP). Based on the relationship between the switching states and common-mode voltage (CMV), the 19 voltage switching vectors are adopted to reduce the CMV. In addition, the P-type and N-type small vectors are selected to control the NP balance. Without utilizing the redundancy zero and small vectors, the common-mode voltage controlled by the proposed MPC algorithms can be restricted within ±Vdc/6. Furthermore, the proposed technique can balance the DC-link voltage with reduced CMV. The proposed method has been verified by the lab experiment.

Fault-tolerant control strategy for T-type three-level inverter with neutral-point voltage balancing

Gird-connected photovoltaic (PV) inverters with the function of active power filter (APF) not only inject the active power into the electric network, improve the power quality, but also make full use of their capacity, especially in the cloudy day or the night. An improved implementation of the amplitude limiting control of the three-phase grid-connected PV inverters is presented. To achieve the optimally compensation within the limited inverter switch rating, the amplitude limiting method on the load type is introduced. a novel maximum power point tracking (MPPT) strategy of the modified incremental conductance (INC) algorithm is also presented in this paper, Which improves the tracing speed and improves the GCI system stability. A one-cycle digital current controller shows good robust, quick response. A 30kW prototype is built and tied together using the proposed control strategy. Experimental results showed the effectiveness of the strategy.

Model predictive control method to reduce common-mode voltage and balance the neutral-point voltage in three-level T-type inverter

To combine the voltage-buck and voltage-boost capabilities of Z-source inverter and the advantages of three-level inverter in high power applications, a novel Z-source hybrid clamped (Z-HC) three-level topology with one isolated DC voltage source is proposed in this paper. Through the analysis of its operation states and the charging and discharging loops of clamping capacitors, a modified phase disposition modulation method in which phase current and phase voltage are introduced to generate PWM signals is designed to reduce the switching losses by minimizing commutation count. Besides realizing the neutral-point voltage balance automatically, for Z-HC inverter, only one shoot-through state is desired to boost voltage that makes shoot-through signals inserted easily. The designed inverter has been tested in experimentation using a laboratory prototype to prove the validation with the proposed modulation method.