Effective use of film capacitors in single-phase PV-inverters by active power decoupling

The lifetime and reliability of PV-inverters can be increased by replacing electrolytic capacitors by film-capacitors. Film-capacitors have a lower capacitance per volume ratio; therefore a direct replacement leads to very large and expensive solutions, especially for single-phase applications. This paper presents an active circuit which acts as an interface between the DC-link of a PV-inverter and an additional storage capacitor. The voltage ripple in the storage capacitor can be increased compared to the DC-link capacitor, allowing a more efficient use of the stored energy and thus a massive reduction of the overall installed capacitance. An especially promising application can be found in module-integrated PV-inverters, because here the most efficient and cheapest topologies suffer from big electrolytic capacitors which deteriorate the lifetime. The paper focuses on different possible control schemes of the decoupling circuit. Results from simulations are used for discussing the proposed control methods. Also results from an experimental efficiency comparison between systems with electrolytic and film-capacitors are given. Finally a lab-prototype of the decoupling-circuit is presented which will be used for further experiments.