Optimización del comportamiento de un convertidor de tres niveles NPC conectado a la red eléctrica

Abstract

This Thesis is contextualized in the fleid of the electric power distribution; its aim is the optimization of VSCs (Voltage Source Converter) behaviour when they are used as interface connected to the grid of electricity generation systems, in order to deliver or to demand energy to the utility grid with the best possible quality. To achieve this objective, on one hand the design of more robust converters is proposed, especially, under the utility grid disturbances; and, the other hand, new control algorithms are presented to improve the VSC behaviour under possible system disturbances. At the design framework, in this Thesis a back-to-back NPC three-level converter is developed and a LCL-filter is proposed for connecting the converter to the utility grid. The chosen topology optimizes the current harmonic components, as the first harmonic group in the converter output voltages are concentrated around double of the commutation frequency. On the other hand, the LCL-filter, to be a third order one, gets high attenuations at the converter commutation harmonics. From the point of view of the control algorithms, the Thesis studies the control system of the VSC connected to the grid. In this context, a vectorial current controller for the VSC with LCL filter is proposed, and the number of sensors is decreased using a state observer. In addition, a grid voltage synchronization method is studied, and its behaviour inside the global control system is analysed for different configurations of the synchronization algorithm. Lastly, a DC-bus voltage controller is presented, where two lineal equations are developed to design it. The stability of the VSC connected to the grid is analysed according to its operation mode (rectifier or inverter), and the DC-bus capacitor value. Different conditions can disturb the correct functionally of VSCs connected to the grid. They may be connected to grids where the impedance is unknown; grid filter components may suffer temporal drifts; and the grid may experience disturbances such as unbalanced, harmonics, dips, etc. Different algorithms are presented in this Thesis, to identif’ and to compensate these problems, which cause steady-state errors and even instabilities. In this context, an algorithm to identify both the grid inductance and resistance is presented, and also a method to compensate the generated errors. In the identification process of temporal drifts of the grid filter components, the response of two different algorithms is analysed, by means of simulations, adding the noise of the current sensor measurements. The used algorithms are: a direct method based on the grid filter model, and a stochastic method based on the EKF (“Extended Kaiman Filter”). To achieve the compensation of the grid voltage harmonics, a method called “predictive feedforward” is designed. Therefore, the final system only delivers fundamental hannonic currents to the grid, and consequently the grid active power is constant. Finally, a dual vectorial current controller is used to compensate permanent and transient unbalanced voltages. The proposed current controller has been tested for VSCs connected to the grid through an L-filter and LCL-filter with passive and active load in the DC-bus.