Xiangyang Xing

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

In this paper, a hybrid microgrid with series- and parallel-connected microconverters is proposed. A few series-connected low voltage microconverters are used to build a string converter with rated output voltage magnitude. Then, the parallel operation of string converters increases the power rating and redundancy of the microgrid. To realize both grid-tied and islanding microgrid control with proper power sharing, a hierarchical power regulation method is developed with simultaneous point of common coupling (PCC) voltage and microconverter voltage control. First, the well-understood droop control is adopted to obtain PCC voltage reference for power sharing between parallel string converters. Second, a power balancing regulation through microconverter local voltage control is used to redistribute output power demand among series-connected microconverters in a string converter. As the string converter line current measured by all microconverters is the same, it is utilized to develop a novel synchronizer to interconnect the string converter central controller and the microconverter local controllers with only low bandwidth communications. Due to the coordination of string converter central control and microconverter local control, the proposed hybrid microgrid can operate in grid-tied and islanding modes with real and reactive power sharing for each microconverter.

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.

Hybrid Microgrid With Parallel- and Series-Connected Microconverters

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

The Z-source neutral point clamped (NPC) inverters can realize buck-boost energy conversion with all satisfying advantages of the three-level inverters. A significant problem related to this topology is the fluctuation of the neutral-point potential (NP). However, the previous modulation techniques for this topology cannot achieve the considerate stable voltage. In this paper, based on the analysis of the relationship among the shoot-through duty cycle and the NP voltage, the average NP current, switching frequency capacitor value, a new feed-forward compensator by modifying the shoot-through duty cycle which can improve to balance the NP voltage balancing is proposed. Meanwhile, in consideration of a low voltage gain caused by the NP voltage balance method, a new control method based on a maximum constant boost control method is proposed, which brings the benefits in both high voltage gain and low-frequency ripple. The considerate stable NP voltage and the high voltage gain have been proven in simulation and through experimental results from a prototype converter.

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

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 novel control method for neutral point clamped inverters with a single Z-source network

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

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.