Abstract

Supercapacitors are energy storage devices that can contribute with a fast varying current for the regulation of DC grids. However, their power electronics interface should present fast dynamic response as well, what is not exactly the case for the conventional 2-switch buck-boost bi-directional DC-DC converter operating with conventional dual-state modulation scheme. Besides, if the DC grid voltage happens to fall below the supercapacitor voltage, the anti-parallel diodes of the converter conduct and one loses control of the current injected into the DC grid. The alternative considered in this thesis is a 4-switch bi-directional DC-DC converter with an intermediate inductor, which allows the implementation of a tri-state logic that eliminates the RHP zero of the transfer function Iout/D, thus allowing the design of a fast acting controller. However it does not provide a solution for the earlier problem of losing control when grid voltage falls below the supercapacitor voltage. One alternative is to use the same 4-switch converter in the buck-boost mode using tri-state logic, which allows the input voltage to be lower or higher than the grid voltage while eliminating the RHP zero at the same time.
This work presents a dynamic model for the 4-switch tri-state converter operating in the boost and buck-boost mode and connected to a droop-controlled DC micro-grid. The tri-state has essentially 3 stages, wherein the Off state length is kept constant. Based on this control approach, there are 2 possible sequences, for which a thorough analysis has been done. Further, the component sizing, selection of various parameters and control loop design for a tri-state system operating with boost mode of operation for higher voltage gain requirement (supercapacitor voltage is significantly lower than DC bus voltage), and with buck-boost mode of operation for low voltage gain requirement (supercapacitor voltage is slightly lower than, equal to or higher than DC bus voltage), is discussed. The simulation result for both modes of operation is presented as well showing an improved performance when compared to the conventional dual-state scheme.
Finally, an experimental implementation of the converter is done and results for the same are provided to verify those described by theoretical analysis as well as simulation results.