The Research On Control Strategy Of Back-to-Back Three-Level Neutral-Point-Clamped Converters

Back-to-back neutral-point-clamp three-level converters have important research value and are utilized in medium/high voltage high power applications such as rolling, mine hoist, ship propulsion, wind power generation, etc. Neutral-point voltage balance control, dead-time and narrow pulse problem, SHEPWM modulation and coordinated control for back-to-back three-level converters are deeply discussed and studied in this article.For neutral-point voltage fluctuations of three-level NPC converters, it is pointed out that neutral-point voltage can be controlled by adjusting zero-sequence voltage based on the relationship among neutral-point voltage, average neutral-point current and zero-sequence voltage. Search optimization method obtains the optimal zero-sequence voltage through searching out all the zero-sequence voltage to calculate and compare the corresponding average neutral-point current and the required neutral-point current. The optimal zero-sequence voltage injection method gets the direct expression of average neutral-point current and zero-sequence voltage, and then obtains the optimal zero-sequence voltage by interpolation. The two methods can both inhibit the imbalance of neutral-point voltage effectively, but a low-frequency voltage fluctuation still exists. To restrain the low frequency fluctuations of neutral-point voltage, a double modulation wave method is introduced in this article. By refactoring modulation wave, it can always directly control average neutral-point current to zero in a switch cycle. So the low frequency fluctuation of neutral-point voltage is completely eliminated and would not be influenced by modulation ratio and the power factor.For dead-time problem and narrow pulse problem in medium/high voltage high power applications, three dead-time compensation methods are introduced and compared based on the analysis of dead-time effect of three level converters in this article. It is concluded that filtering current feedback control based on d-q coordinate system and zero current clamp straightening control based on y-8coordinate system can effectively solve zero current clamp problem. The traditional narrow pulse compensation method may lead three-phase unbalanced or bring new narrow pulses in other phases while compensating a phase. A narrow pulse processing method considering neutral-point voltage balance is proposed in this article, which directly calculates the limitations of narrow pulses to the modulation wave. There is little influence to the NP voltage balance control while avoiding narrow pulses completely.In response to the switching frequency limits in high-power traction drive system, a hybrid three-level PWM modulation strategy with wide speed range and low switching frequency is put forward, which uses asynchronous SPWM, synchronous SPWM and selective harmonic elimination PWM (SHEPWM) comprehensively. The basic principle of three-level SHEPWM modulation, angle calculation, curve fitting and DSP implementation is deeply introducted and necessary requirements for smooth transition among different modulations are analyzed.The coordinated control of three-level rectifier and inverter can guarantee a high performance of back-to-back systems. The power coordinated control and neutral-point voltage coordinated control of back-to-back converter are deeply discussed in this article. For the power coordinated control, two kinds of forward-feed control methods are presented and the ability to resist disturbance of loads and grid-side voltage is also explained based on the small signal analysis of feed-forward control methods. Neutral-point voltage fluctuation features of three-level converters are illustrated. It is pointed out that under the same modulation ratio, the lower the power factor, the greater the neutral-point voltage fluctuation amplitude and under the same power factor, the greater modulation ratio, the larger the neutral-point voltage fluctuation amplitude. Finally, full control domains of neutral-point voltage for three-level converters and three-level back-to-back converters are obtained and a series of neutral-point voltage coordinated control schemes are put forward and compared. The best way to coordination control is that mainly controlled by the inverter and auxiliary controlled by the rectifier.A three-level back-to-back converter for simulation and experiment is designed and set up. Simulation and experimental results show the validity and feasibility of various control methods which are put forward in this article.