Power Decoupling Methods In Building-integrated Photovoltaic (BIPV) System

As an important auxiliary unit of the utility grid, Building-integrated photovoltaic(BIPV) system can improve the efficiency of photovoltaic system and the power quality ofthe grid. During the power transmission from the photovoltaic modules to the grid, thepower decoupling technique is very important. The passive power decoupling methodsneed many electrolytic capacitors, which weakens the system reliability. While in the activepower coupling methods, only some switches and a little capacitors are used to reduce theripple power. Hence, this kind of methods have advantages on system size, cost andreliability.In this paper, based on the composition principle of component-multiplex three-portconverter, a series of AC&DC three-port topologies competent to decouple power isderived through combining the power transfer basic unites. In consideration of thedifference of the inverters in all kinds of photovoltaic systems, the suitable powerdecoupling methods are different. In the BIPV system based on AC modules, the power ofeach inverter is small, and the size and cost is limited, therefore, the four-switch three-portconverter can work well in this application. In the system based on DC modules, thecentralized inverter’s power is large, and the reliability and the generality is the focus ofsolution selection. Under this condition, the power decoupling method based on DC Activepower filter (DC APF) is fit for this application. In addition, the power decoupling unit inthe method based on Source-series Y-type DC-DC-AC three-port converter is low, which isnecessary for the photovoltaic-storage hybrid system.This paper proposed a dual inductor current control method for the four-switchDC/DC/AC three-port converter. The voltage of the decoupling capacitor contains DCcomponent, which enriches the harmonics of the capacitor voltage and current, thus thepower control is much more complex. By analyzing the equivalent model of this topology,choosing one duty cycle and the difference of two duty cycles as the regulating variables isable to control the power of two ports independently. In the proposed dual inductor-currentcontrol method, the two controllers can be designed respectively and the implementationmethod is easy. The effect of power decoupling could be better with more than onceharmonic compensation. The simulation and experimental results verified the feasibility ofthe proposed method. In order to use Source-series Y DC-DC-AC three-port converter to condition thepower of the photovoltaic-storage hybrid system, a AC&DC direct current control methodis proposed in this paper. Although the dual inductor-current control method is simple andpractical, the dynamic performance of such method is undesirable. By controlling thepower of the DC port and the AC port directly, both the steady and the dynamicperformance can be superior. Meanwhile, no extra power calculation units are necessary inthis proposed method. Moreover, the controller design becomes easier by choosing theregulating variables rationally as well. The simulation results demonstrated that the analysisand presented control scheme is correct.Finally, this paper employed a control method based on virtual capacitor for DC APFto improve its performance. The power decoupling method based on DC APF is suitable forlarge-power photovoltaic system because of its reliability and commonality. In order toreduce the capacitor of DC APF, the dynamic performance is an important optimizationindex. By introducing an integrator in the feedback loop which is so called virtual capacitor,the process of picking up low-frequency harmonic is omitted. This comparison results insimulation and experiments verified the superior steady performance and dynamicperformance of the method based on virtual capacitor.