Integration of Photovoltaic into grid using Modular Multilevel Converter
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- Institutt for elkraftteknikk 
The integration of solar energy into the power grid is a challenging task. Many configurations and control methods have been studied. In this thesis, the Modular Multilevel Converter is adopted to integrated Photovoltaic (PV) into the grid. The main focus of this thesis is divided into two parts. The focus of the first part is to investigate how to integrate photovoltaic into the grid by using the MMC. First the topology of the MMC is introduced, and form this a mathematical model is derived. This is derived in order to later be able to implement the MMC into Matlab/Simulink. Then different aspects considering the control of the MMC is discussed, such as capacitor voltage balancing and suppression of circulating current. The modulation technique applied to the MMC is the phase- shifted pulse width modulation. This section is followed by an introduction of PV systems. A review over different configurations in order to integrated PV into the grid is given. This is followed by looking into how this integration done by using MMC. One specific configuration is chosen to investigate more closely. This configuration is a multistring configuration, where one connects the PV array at the DC link of the submodule with and DC/DC buck converter between the PV array and the submodule. This configuration is implemented in Simulink, with the MMC connected to a typical European low-voltage grid, having nominal voltage of 400 V and a frequency of 50 Hz.In the second part of the thesis different maximum power point tracking (MPPT) algorithms is studied in detail. PV panels have a characteristic nonlinear I_V curve with a distinct MPP. In order to extract maximum power out of the PV array, the PV panels have to operate at their MPP despite changes in environmental conditions. The MPPT algorithms that are the Perturb and Observe (P\&O), Incremental Conductance (IC), Ripple correlation control (RCC) and Fractional Open- Circuit Voltage(FOCV).In order to find the static and dynamic efficiency of this four MPPT algorithms, the algorithms is simulated with both constant and variable irradiance. The temperature is kept constant in both the simulations, since it is the effect of irradiance change that is studied. The dynamic efficiency is measured with two kind of irradiance profiles. One with a trapezoidal and one with a staircase irradiance profile. This simulations is done with the PV array connected to a buck converter, which is connected in the submodules of the MMC. This approach makes it possible to study the behavior of the MMC while the irradiance is chaining.