Research On Cascade H-Bridge APF Under Unbalance Grid

With the development of modern industrial economy, technology, access to the power system of a large amount of non-linear load and impact load imbalance, as harmonics, voltage sags, grid voltage asymmetric three-phase power quality problems such as deteriorating as well. Active Power Filter (Active Power Filter APF) as a new type of harmonic and reactive Power compensation device, to realize dynamic compensation, improve Power quality.However, the current domestic and foreign studies of APF mainly concentrated in the field of low voltage, APF used in high pressure areas face many of the key problems to be solved. This paper discusses several kinds of main circuit of APF topology, and compare performance from multiple aspects. Because the cascade multilevel structure has good output harmonic characteristic, is easy to be modular and can be divided into phase control, high reliability, etc. So this paper finally adopted cascade level as the main circuit of APF topology structure. On the basis of combining the operation mode of the cascaded H bridge structure, the use of carrier phase shift modulation technology of frequency doubling, good improves the equivalent switching frequency of device, better solved the switch frequency and inherent conflict between the capacity of APF.Second, the present study are in power grid voltage under ideal condition, but the reality eccn voltage unbalance is ever-present, how to solve the grid voltage unbalance of a series of influence on the device is one of the key technologies needed to resolve. Based on cascaded H bridge was deduced in detail the APF in ABC stationary coordinate system of the mathematical model on the basis of the analysis of the main circuit parameters selection; Also analyzed the grid voltage unbalance affect APF device.In this paper, the unbalanced power grid when cascaded H bridge of APF control strategy is studied. Based on instantaneous reactive power theory, USES the IP-IQ algorithm for real-time harmonic current detection, and designed the closed-loop PI current tracking and control system; On the analysis of the impact of dc voltage unbalance of system using the mathematical formula is deduced on the basis of the cascaded H bridge caused by the mechanism of dc side voltage imbalance. Finally, adopt hierarchical control pressure of dc side voltage stability. The hierarchical structure of the upper control using a new kind of double loop control strategy not only realize the overall unbalanced power grid power and negative sequence voltage control, and suppresses the unbalanced grid impact of negative sequence voltage of APF, the device to ensure safe and reliable operation. Middle zero sequence voltage injection method is used to eliminate the phase to phase voltage imbalance, the underlying control adopts voltage supplementary volume method is used to realize the pressure between modules.Finally in the Simulink environment a lot of simulation experiments, the simulation results verify the validity and feasibility of the proposed control strategy in this paper.