The Study Of Grid Connected H-bridge Cascaded Multi Level Filter Based On Hierarchical Control

Today the application of electronic products in power network is more and more widely, especially some non-linear loads,which bring great distress to the power grid. Active Power Filter (APF) is a dynamic and real-time filtering device of harmonic and reactive power. It can be timely tracking and improve the power quality without affecting the nature of load.For this superior performance it has been widely concerned.The switching frequency of APF under middle and high voltage is always low,which leads to large switching loss and large output harmonics.To solve the problem we find the H-bridge cascaded circuit structure is the best choice. It not only has the advantage of simple structure, but also has a great advantage in flexibility. Therefore, this paper focuses on cascaded H-bridge.Firstly,in this paper harmonics and reactive power are both defined, then telling their harms.Because of these shortcomings,the power system need filter device,so the active power filter were detailed introduced.Multilevel H-bridge cascaded filter has a lot of advantages and the common PWM strategy is not feasible. After many years of research, scholars have found a suitable control strategy, that is, carrier phase shift pulse width modulation (CPS-SPWM) technology, this method is introduced in this paper, and the formula is deduced.The main content of this paper includes three parts:the first is regulating the voltage of DC side.Cascaded H-bridge has multiple independent modules and each module contains independent capacitance, when the device is running the voltages of DC side may serious different.It is really dangerous.In order to stabilize the capacitor voltage, the hierarchical control strategy is studied in this paper.The second is three phase coupling analysis.The main circuit topology of cascaded H-bridge is based on ordinary three-phase half bridge structure(without neutral), and in the three-phase half bridge structure the neutral is coupling. Therefore, it is inferred that cascaded H-bridge structure is coupling too.In this part,the formula is derived and demonstrated.The third is the selection of part devices.Containing two parts:(1) Selecting output filter which can meet the output performance requirements and the control performance requirements; (2) Selecting the capacitors of DC side which can meet the ripple suppression requirements.Finally, this paper set up H-bridge cascaded system simulation model in simulink environment. The actual effects of the proposed method are tested by the actual output waveforms. The simulation results show that the modulation and control strategies proposed are correct and feasible.