Virtual Capacitors for Single Phase Power Electronics Converters

Nowadays more and more electronic loads are connected to single phase grids. All this electric appliances requires a conversion stage at the design stage to transform the alternating current supplied by the network to a direct current. The classic method of single-phase AC/DC conversion involved the use of a full-wave rectifier which allows the passage of the positive half-wave, as well as the passage of the inverted negative half-wave, obtaining an output voltage exclusively positive with period double that of the network. The output voltage can subsequently be filtered to reduce its ripple and "smoothed" by one or more capacitors. Although this method is one of the simplest and cheapest, it has some criticalities that cannot be ignored. The amplitude of the ripple, and therefore the average voltage on the DC bus, is strictly related to the capacity of the DC-link and to the load current. Furthermore, since there is no conduction by the diodes as long as the grid voltage is lower than the bus voltage, the current absorbed by the network has a pulsed and strongly distorted form, with a high THD, which therefore causes harmonic pollution in the network. Through the use of active rectifiers part of these problems can be solved: an active bridge with PFC and boost stage allows a cleaner AC/DC conversion from the point of view of the network, with an output voltage that is adjustable, and maintaining a unity power factor. The dependence between the ripple amplitude and the capacity of the dc bus and load remains, but in this type of converter the ripple is mostly at twice the grid frequency, and the average value is fixed at the one imposed by the control algorithm. In both cases, one or more capacitors are used to reduce the low-frequency ripple. However, this involves an increase in the size of the converter and therefore a lower power density, which is becoming an index of fundamental importance for the new generations of converters. The most used capacitors for this function on the dc link are usually of the electrolytic aluminum type, because these guarantee a good compromise between cost and energy density. However, these capacitors are known to have a relatively low reliability, therefore making it necessary to search for a more effective solution, which allows for a higher power density, with a more reliable type of capacitors and with reduced capacitance. Several active methodologies have been explored over the years to deal with second harmonic ripple of the AC side in single-phase applications, namely ripple eliminator, ripple reduction circuit, active capacitors, virtual infinite capacitor. Each of these methods has an auxiliary circuit and an auxiliary energy storage device. The goal of this thesis is to design a similar device and study a control strategy to apply to it.