STATE OF ART TECHNIQUES FOR CONTROL AND PROTECTION OF DER INVERTERS UNDER ISLANDED AND GRID CONNECTED CONDITIONS

Microgrid technology and its application is being widely used in the power system for different advantages compared to conventional power plants. Since inverter have pivotal role in converting DC power from renewable sources to power grid, this thesis fundamentally focuses on correct modeling, design, control and protection of three-phase inverters either in islanded condition or grid-connected. Islanding occurs when part of the electric power system is isolated from the main power grid. Islanding occurs mainly in preplanned manner or in emergency cases. The planned islanding is done for maintenance of the grid. In general scenario, interconnecting any conventional generator to the power network will create parallel path. This parallel path leads to decrease in net impedance and fault current increases. But the case is different in case of Inverter interfaced system where the fault current level is quite low.For control and protection, different methods have been adopted. This thesis broadly explains on two control methods which are hysteresis and Pulse width modulation techniques. In addition, this project explores on harmonic injection type protection. Using MATLAB/Simulink for analysis, hysteresis and Pulse width modulation technique are modelled for obtaining desired level of current. Hysteresis is simple, provides stability, very fast response and good accuracy. Hysteresis method is based on variable switching frequency and uses dq Park transformation for controlling actual current within the lower and upper hysteresis band. The Pulse width modulation method uses a fixed frequency concept and generates pulses using PWM generator