Solar power, clean and abundant, is considered as a vital contributor in the effort of transforming world energy-mix to pollution-free and natural-regenerative sources.   The solar micro inverters have gained greater visibility during the past several years due to their higher efficiency, greater performances, longer life expectancy and many other benefits. But, integrating small scale [<15kW] renewable energy sources, especially the low frequency switching solar inverters to the low voltage distribution grid has its own challenges due to their inability to generate reactive power to maintain the static voltage stability of the grid. Higher level of solar penetration has identified as a potential cause of low voltage grid instability due to lack of reactive power feeding and their tendency to keep on increasing the voltage higher than grid at the point of common connection [PCC] in order to inject the current to the grid. The studies and experience in voltage stability issues has resulted in introducing many new grid regulations to manage the grid voltage stability throughout the world. The new regulation, VDE-AR-N-4105-2011 is a German grid regulation standard specifically focuses on the low voltage grid connected power generators. This regulation has addressed the reactive power requirements in terms of power factor and supply management to maintain the grid static voltage variation less than 3% at the PCC, when connecting any type of distributed power generators to the low voltage network. This report discuss about the voltage stability issues related to low frequency switching inverters and present a solution to comply with low voltage grid regulation - VDE-AR-N-4105-2011; a SmartVar Generator concept, theory, design and functionality.

In order to inject solar power to the utility grid, among various types of inverters, Grid Connected Transformerless Solar Inverters (GCTSI) are mostly preferred for residential or commercial applications. This preference is because of the high energy efficiency and low cost due to the absence of a line frequency or a high frequency transformer. Peak value of the efficiency characteristics of GCTSIs can reach 98%, which are selected topology, component optimization, switching strategy and operating condition dependent. In spite of the attractive energy efficiency characteristics of GCTSIs, due to the lack of galvanic isolation, these inverters are vulnerable to leakage currents, which are prohibitive for the safety and the maintenance reasons. The purpose of this research is to analyze GCTSIs in terms of their leakage current and energy efficiency characteristics. In the research, the leakage current mechanisms of GCTSIs are identified and grid connected solar inverters are classified in terms of their leakage current characteristics including the GCTSIs. In addition to the existing ones, several novel topologies are proposed enriching the family of GCTSIs. The leakage current and the inductor current ripple performances of GCTSI topologies are analyzed and evaluated by detailed simulations for 3 kVA and 10 kVA single-phase systems. In addition, the energy efficiency characteristics of GCTSIs are investigated in these power levels by making use of Calculated Average Power Per Switching Cycle (CAPPSC) method. The efficiency studies with CAPPSC method provide design guidelines and comparison of the GCTSI topologies in terms of their energy efficiency characteristics.

For planning issues, it is useful to know the upper limit for photovoltaics (PV) in the electrical grid with current design and operation (defined as hosting capacity) and how this limit can be increased. Future costs for grid reinforcement can be avoided if measures are taken to implement smart grid technology in the distribution grid. The aim of this project is to identify challenges in Swedish electricity distribution grids with a high penetration of local generation of electricity from PV. The aim is also to help Swedish Distribution System Operators (DSOs) to better understand hosting capacity issues, and to see which room for PV integration there is before there is need for actions to maintain power quality. Three distribution grids are modelled and simulated in Matlab: Rural area, Residential area and City (Stockholm Royal Seaport). Since the project is a cooperation between Uppsala University and Fortum, three different representative grids from Fortum’s grid software ”Power Grid” have been used as input to a flexible simulation program developed at Uppsala University. The simulation includes Newton-Raphson power-flow computing but has also been improved with a model of the temperature dependency of the resistance. The results show that there is room for a lot of PV systems in the Swedish grids. When using voltage rise above 1.1 p.u. voltage as limitation, the hosting capacity 60% PV electricity generation as a fraction of the yearly load were determined for the rural grid and the suburban grid. For the city grid, which is very robust, the hosting capacity 325% was determined. When using overload as limitation, the hosting capacities 70%, 20% and 25%, were determined for the same grids.