Modelling, Design and Implementation of D-Q Control in Single-Phase Grid-Connected Inverters for Photovoltaic Systems used in Domestic Dwellings.
This thesis focuses on the single-phase voltage-source inverter for use in photovoltaic (PV) electricity generating systems in both stand-alone and grid-tied applications. In many cases, developments in single-phase PV systems have followed developments in three-phase systems. Time-variant systems are more difficult to control than time-invariant systems. Nevertheless, by using suitable transformation techniques, time-variant systems can often be modelled as time-invariant systems. After the transformation, the control signals that are usually time-variant (often varying sinusoidally in time) become time-invariant at the fundamental frequency, and are hence much easier to deal with. With this approach, synchronous rotating frame control techniques have been previously proposed for high performance three-phase inverter applications. The transformation theory cannot be applied directly in single-phase systems without modification, and the d-q components would not be time-invariant in situations where harmonics, resonances or unbalance is present. Single-phase inverter controller designs based on the use of a synchronous rotating reference frame have been proposed, but such designs do not always perform as well as expected. This thesis aims to improve single-phase voltage-source inverters. The main objective is to address, in terms of cost, efficiency, power management and power quality, the problems found with single-phase designs based on a synchronous rotating frame single-phase inverter controller. Consequently, this thesis focuses on a novel controller approach in order to obtain a more reliable and flexible single-phase inverter. As the first step, this thesis investigates the single-phase inverter switching gate-drive algorithms and develops a form of space-vector pulse-width-modulation (SVPWM) in order to reduce total harmonic distortion. The results of the new SVPWM algorithm demonstrate its superior performance when compared with sinusoidal pulse-width-modulation (SPWM) which is often used with single-phase inverters. The second step, which is further reviewed and presented in this thesis, is the modelling of the single-phase inverter control based on the synchronous rotating frame. A mathematical analysis is conducted to determine the mechanism of the coupling that exists between the voltage phase and amplitude terms, and a new transformation strategy is proposed based on using the voltage phase as a reference at the Park transformation stages, and the current phase as a reference for the current at the transformation stages. The line-frequency components of the feedback signals are transformed to time-invariant components, thus eliminating the ripple and reducing the computational burden associated with the controller stage. Consequently, the inverter feedback controller stage is designed so that the coupling terms are decoupled within the controller itself. The effectiveness of the techniques proposed in this thesis are demonstrated by simulation using the MATLAB/SIMULINK environment. The proposed technique was also investigated through a practical implementation of the control system using a Digital Signal Processor (DSP) and a single-phase inverter. This practical system was tested up to 1 kW only (limited by the available inverter hardware). Nevertheless, the correlation between the simulation and the practical results is high and this gives confidence that the developed mechanism will allow the 2.5kW goal to be achieved. Practical test cases illustrate the effectiveness of the models. In addition, the comparisons between experimental and simulation results permit the system’s behaviour and performance to be accurately evaluated. With the development of the new controller, small-scale single-phase renewable energy systems will become more useful in the field of power quality management through their ability to separately control the phase and amplitude of the output voltage. Consequently, incorporation of this type of generator within the national electrical distribution network, as distributed generators (DG) at low-voltage level, can assist with power quality management at the consumer side of the grid. In addition, such a generator can also operate in stand-alone mode if the grid becomes unavailable. The third step in this thesis investigates small-scale single-phase renewable energy systems operating as decentralized distributed generators within a local network. This operation is achieved by controlling the inverter side using the quantities measured at the common coupling point between the grid and the inverter, without requiring other extensive communications. Thus, the small-scale single-phase renewable energy distributed generator systems will contain only a local controller at each installation.
- PhD