String inverter control design for photovoltaic applications

In the contemporary power system, the electric power generation is mainly provided by synchronous generators, which are regulated to preserve the correct operation of the electric grid. In particular, the active power balance is guaranteed by keeping the grid frequency at the reference value, e.g., 50 Hz. During a load power variation, the whole system reacts by decelerating or accelerating and transferring the inertial energy of the synchronous generators into electrical power. In a second phase, the primary and secondary regulations of the frequency operate to recover this energy and restore the reference frequency in the power grid. This control paradigm is severely challenged in case of a large penetration of converters connected to non-synchronous sources (e.g. solar PV). New control strategies are necessary to be used in the future perspective of grid controlled in a system with low inertia. This thesis is carried out at the at Huawei Nuremberg Research Center to develop a control model of a string inverter for the PV TRUST project, where it takes part in collaboration with ENEL to increase PV system reliability in large scale PV areas. At first, the state-of-the-art regulation system for synchronous generators and the impact on the system of the various system parameters (e.g. inertia constant) are observed. To obtain the aim of the thesis, different control strategies are investigated and developed using simulation software solutions such as MATLAB and PLECS. Especially, control strategies for grid following converters, which can adapt to the behaviour of the grid, grid forming, which can generate an electric grid in islanded operation, and Virtual Synchronous Machine control, that introduces the typical frequency regulation dynamics of a synchronous machine to a static converter, are analysed and developed. The block schematics of the control is built. The realised string inverter is the result of several voltage source inverters connected in parallel and interconnected to the grid with LCL filters. a lumped model of the string converter is built. Once the basics are in place, the cascade control that returns the voltage reference is built. Thanks to a PLL, the inner current control loop operates in the rotating dq-axis reference frame and it’s designed according to the passivity method to assure system stability. Saturation and anti-wind-up technique are integrated calculating an equivalent direct and feedback gains. The external DC-link voltage control is realised with energetic inputs controlling its square value to don’t introduce non-linearity between DC and AC part of the convert. Finally, the frequency control is designed. Virtual inertia energy quantity is introduced in the control to increase power system inertia and to operate with the same dynamic as synchronous generators. PLECS simulations are carried out to evaluate the converter's response to variations in reference DC voltage, power supplied by the PVs and load request when it works supplied by the photovoltaic source and together with synchronous machines at different power levels. Having highlighted functional limitations, an energy storage is added in the power circuit to support the DC-link voltage. Its activation control is developed. In conclusion, the designated control demonstrates the ability to work frequency regulation as desired under conditions of high and low load demand or photovoltaic output power, cooperating in state-of-the-art regulation.