Discrete Wavelet Transforms for Synchronization of Power Converters Connected to Electrical Grids

Abstract

Electronic power converters connected to electrical grids allow industrial processes, traction applications and home appliances to be improved by controlling the energy flow depending on the operation conditions of both the electrical load and the grid. This is the case of variable frequency drives, which can be found in pump drives or ship propulsion systems (Bose, 2009) maintaining the electrical machine in the required operation state while ensuring a proper current consumption from the electrical grid. Recent researching and developing efforts on grid-connected power converters are due to the integration of renewable energy sources in electrical grids, which requires the implementation of new functionalities, such as grid support, while maintaining reduced current distortion levels and an optimal power extraction from the renewable energy source (Carrasco et al., 2006; Liserre et al., 2010). In the most general case, a grid-connected power converter consists of power and control stages which ensures the appropriate energy management (Erickson & Maksimovic, 2001; Mohan et al., 2003). In the first one, electronic power devices, such as power diodes, thyristors, insulated gate bipolar transistors (IGBTs) or MOS-controlled thyristors (MCTs), and passive elements (inductances and capacitors) are found. The switching state of the power devices allows the voltage or/and current across the passive components to be controlled. Resistive behaviors must be minimized in order to avoid conduction power losses. The second stage, in case of controlled semiconductor devices, consists of a signal conditioning system and the required hardware for implementation of the converter controller (Bose, 2006). Recent advances in field programmable gate arrays (FPGAs) and digital signal processors (DSPs) allow the complexity and functionalities of the controllers employed in power converters to be increased and improved (Bueno et al., 2009). In grid-connected power converters these functionalities include, in most cases, the synchronization with the electrical grid, the evaluation of the reference current amplitude at the grid-side and current control (Kazmierkowski et al., 2002). The amplitude and phase of the grid-side current depends on the reference current evaluation and the synchronization subsystems while the current controller ensures that the current waveform matches the reference one. The implementation of these subsystems depends on the application characteristics. Other functionalities, such as grid support (Ullah et al., 2009) or detection of the islanding condition (De Mango, Liserre & D’Aquila, 2006; De Mango, Liserre, D’Aquila & Pigazo, 2006), can be added if it is required. These controller functionalities can be implemented by applying diverse approaches, such as digital signal processing techniques, i.e. Fourier Transforms (McGrath et al., 2005), Kalman 11