Abstract

Solar photovoltaic (PV) panels and wind turbines are two commonly employed Renewable Energy Sources (RESs). Their main drawback is that they are stochastic and fluctuating sources. Roof-top PV is easier to install and operate in the urban environment. Due to the variable nature of RESs, an energy storage system (ESS) is used to solve the problem of mismatches between the variable power generation and load demand in standalone/islanded systems. The ESS is employed to store surplus energy and supply it back to the system when needed. The most common ESS applied in standalone PV systems is based on batteries. Batteries, in general, have high energy density but low power density, giving low charge/discharge rates. Based on the characteristics of the batteries, it will not be able to respond optimally to fast changing and high frequency power exchanges. One solution to this limitation is by combining batteries and supercapacitors (SCs), which can provide high power but have low energy storage capacity, to form a hybrid ESS (HESS). A battery-SC combination has been considered in this Thesis. The aim of this combination is to reduce the current stresses on the battery occurring due to sudden changes in PV generating and load demand. For that, an effective control method is essential for the optimal operation of the HESS with varying load demand and PV generation.
This Thesis discusses a control strategy for a HESS consisting of three proportional plus integral (PI) type controllers that control the currents of two bidirectional power electronic converters, one for the battery and the other for the SC, connecting them to a common DC bus, whose voltage should be regulated. The boost mode has been considered for modeling the class C interface power converters and designing the relevant PI controllers. For the sake of determining the reference currents for both converters, a low pass filter is used to split the HESS current, required to regulate the common DC bus voltage, into low dynamic components for the battery and fast dynamic components for the SC. First, the analysis of the ESS with a single element (battery) is used with the PV system and variable load to test the performance of the basic inner current and outer DC nano-grid voltage regulation loop. Later, the battery and SC units are used along with the PV system to verify whether the current split technique, with fast dynamic to the SC, can provide fast regulation of the DC bus voltage while reducing the current stress on the battery. A SC voltage control loop has also been designed to keep the SC voltage with a certain reference value. DC nano-grids can also be connected to an AC utility grid. This is usually done by means of a single-phase DC-AC converter that tends to produce a low order voltage harmonic in the DC bus. The usual solution based on a large electrolytic capacitor, presents the drawback that this component presents a short lifetime. The solution employed in this work was to use the HESS to perform an active filtering function, what was accomplished by adding a resonant (R) term to the PI of the DC bus voltage loop. The effectiveness of the control scheme is shown by simulation studies in MATLAB®/Simulink. Finally, experimental results are provided to verify those described by theoretical analysis as well as simulation results.