Chunshui Du

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.

Z

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.

-Source Three-Level T-Type Inverter

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.

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

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.

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

Paralleled inverters have the merits of high power rating, improved reliability and convenient maintenance, but zero-sequence circulating current (ZSCC) will occur and lead to current distortion and system loss. The ZSCC control is more complicated when adopting three-level topology and increasing paralleled number. This paper addresses the suppression of ZSCC in multi-paralleled three-level T-type inverters (3LT2Is). A new equivalent model is developed by reorganizing the ZSCCs into another form. The ZSCC controller based on the new model is easier to implement. Therefore, the number of paralleled inverters can be increased without complicating the control scheme and the inverters could be switched on or off optionally. Besides, feed-forward (FF) strategy is adopted to eliminate the ZSCC spikes caused by symmetrical three-level space vector modulation (SVM). The effectiveness of the proposed control scheme is verified by both simulated and experimental results.

Amplitude limiting for the photovoltaic (PV) grid-connected inverter with the function of active power filter

Parallel three-level T-type inverters (3LT2Is) can increase power rating and reliability, but the circulating current problem will arise. Unbalanced conditions including unbalanced filter inductances and unbalanced grid voltage have negative effects on the operation of parallel 3LT2Is. This paper presents an improved repetitive control scheme for circulating current suppression in parallel 3LT2Is under unbalanced conditions. An average model of parallel 3LT2Is in dq0 reference frame is derived to analyze the circulating current. It can be seen that unbalanced filter parameters generate additional disturbance in zero-axis current system. A repetitive controller is designed to mitigate the disturbance caused by unbalanced filter parameters, and feed-forward terms are introduced to eliminate the remaining disturbances. The proposed control scheme can effectively suppress the circulating current in parallel 3LT2Is under unbalanced conditions. Experimental results are presented to demonstrate the effectiveness of the proposed scheme.

Modeling and suppression of circulating currents for multi-paralleled three-level T-type inverters

Parallel three-level T-type inverters (3LT2Is) can increase the capacity of the distributed generation system. In this situation, the zero-sequence circulating currents (ZSCCs) will occur in the common dc-link voltage of parallel 3LT2Is. This paper presents a novel ZSCC reduction method based on the developing selective harmonic elimination pulse-width modulation (SHEPWM) for parallel 3LT2Is. And each inverter is made to operate with a different PWM pattern. Compared to the conventional methods, the proposed method can remove about p times the number of harmonics with the same switching frequency. The performance and effectiveness of the proposed method were confirmed by experimental results.