Research On Dynamic Characteristic And Parallel Circulating Current Restrain Of Three-Phase Inverters

The three-phase inverters, which behave as the main ac power supplies, have played an important role in many areas such as official automatic system, medicine, telecom, national defense, and so on. The parallel systems of three-phase inverters have been researched more and more deeply because of their great markets and wide applications. In recent years, the parallel systems based on high performance PWM single-phase inverter have grown to mature, so the parallel systems of high performance PWM three-phase inverters are taken more attention by the researchers of power supplies. However, the filter inductances and output transformers in three-phase inverters are all three-pole cores for considerations of the cost and size, and this lead to the magnet couples between three phases and different characteristics from single-phase inverters. The dynamic models of three-phase inverters are very complex due to the magnetic coupling, so they are always ignored in the recent researches on three-phase inverters. This dissertation analyses the dynamic performance and the design method of three-phase PWM inverters parallel systems based on the automatic theory and instantaneous symmetric component transformation theory. New systemic theory basis for high quality PWM three-phase inverter design is provided. Novel parallel control schemes are proposed. Firstly, based on the experiments of parallel systems of three-phase inverters, this dissertation analyses the circulating currents caused by the dead time effect. We found that the traditional inverter models could not explain these circulating currents, because the dead-time effects were equivalent to resistant. In fact, the dead time difference between inverters could cause considerable low order harmonic circulating currents. This dissertation established the single-phase inverter model where the dead time effect is considered as a harmonic disturbance. This inverter model interpreted the distortion of inverter output waveform caused by the dead time effect and nonlinear loads, and moreover, the influences of instantaneous voltage feedback regulations on the distortion are researched. From this model, we further established the dynamic model of circulating current between parallel single-phase inverters. Based on the model, the yield of harmonic circulating currents can be explained and then the restrain of them is researched. Furthermore, The stability of circulating current is discussed. To research the dynamic characteristics of three-phase inverters, this dissertation established dynamic model of three-phase inverters in K frame for the first time while referring to the models of three-phase ac motors and three-phase power systems. Compare to the instantaneous models in abc stationary frame and dq0 rotation frame, this model is the simplest and uncoupled, so it is easy to solve the dynamic procedure of three-phase inverters, and it is the main method used in this paper. For the convenience, the Laplace transformation to instantaneous symmetrical components is discussed, and we can study three-phase inverters in s domain. Based on the instantaneous symmetrical components model of three-phase inverters, we researched deeply the dynamic characteristics of three-phase inverters in unbalanced status (such as the single-phase step load) and in fault status (such as the shortcut between phases). Based on the full analytical solution, this paper conclude the facts that lead to the unbalance of three-phase inverters and the restrain of them, which can be referred to when we design and research three-phase inverters. Based on the harmonic disturbance model of single-phase inverters and the instantaneous symmetrical components model of three-phase inverters, this dissertation established for the first time the harmonic disturbance model of three-phase inverters. With this model, we analyzed the dynamic characteristics and the stability of three-phase inverters adopting the instantaneous voltage feedback regulations. The stability judgments of three-phase inverters, which take account of the magnet coupling between phases, are derived. Based on the harmonic disturbance model of three-phase inverters, we further established the dynamic model of circulating currents in three-phase inverters parallel systems. The coupling and stability of three-phase circulating currents are analyzed. The three-phase harmonic circulating currents and the restrain of them are discussed. The zero-sequence circulating currents are a special phenomenon in three-phase inverters parallel systems. Based on the instantaneous zero-sequence component model of three-phase inverters, we found that the magnet coupling between three phases have great influences on zero-sequence circulating currents when the three-pole reactors and transformers are used. These influences are researched comprehensively in this paper.Based on all the above researches on three-phase inverters and their parallel systems, we complete the design and test of 50 kVA and 300 kVA multi-UPS parallel systems. The design considerations of three-phase inverter parallel systems are discussed in this dissertation. Fully considering the intrinsic dynamic characteristics of three-phase inverters, two parallel inverter systems achieved good safety, stability and dynamic quality. At the end, the experiment results are given.