Three-phase photovoltaic grid injection using film capacitors

Solar photovoltaic energy is considered to be an important source of generating electric power, because it is a clean renewable energy source and is easy to install and maintain. Over recent years, as a result of photovoltaic cell and panel research, its efficiency has increased and its costs have been significantly reduced. Furthermore, a photovoltaic system is a set of electrical and electronic equipment that produces electrical energy from solar radiation. For the interface between the photovoltaic solar array (DC) and the three phase electrical grid (AC), the use of the three-phase inverter (DC-AC converter) is necessary, which should be optimised to improve the maximum power injected into the grid from the photovoltaic solar panels, guaranteeing electrical power quality. The lifetime of the inverter is typically factor 2–3 lower than the PV panels. This is mainly due to the presence of electrolytic capacitors and the heating of transistors, due to switching loss. In this thesis, a novel control strategy, called a 120-degree Bus Clamped PWM (120BCM), is presented. A 120-degree BC-PWM is a special switching sequence PWM technique, employing bus clamp sequences that is use only a single phase under a PWM every 60-degrees, while the other phases are clamped. This reduces the switching loss to a much lower level. The BC-PWM method was used to generate six PWM signals to control a three-phase inverter system every 60-degrees, with constant power input and a small DC link film capacitor. Hence, the inverter lifetime can be increased up to the operational lifetime of the photovoltaic panels. Thus, the Total Cost of Ownership (TCO) of the PV system will significantly decrease. For the grid synchronisation, a new three-phase synchronisation method for three-phase photovoltaic inverter is designed and simulated. This method can be referred to as the double integral synchronisation method (DISM). This is a new synchronisation method, presented on the basis of integrating the grid voltage twice to generate the reference signals of the current for the three-phase photovoltaic inverter. Generally, photovoltaic three-phase inverters have large electrolytic DC-Link capacitors. These capacitors are known for their large size and limited operating lifetimes, particularly in the case of systems with high ripple currents. In this thesis, an appropriate value of a DC-Link film capacitor for a grid connected three-phase photovoltaic inverter is evaluated. The low value film capacitor for the DC-Link allows higher operating temperature conditions. The final aim of using a film capacitor, rather than an electrolytic capacitor, is to extend the operating lifetime of the three-phase photovoltaic inverter to match the lifetime of the photovoltaic panel (roughly 25 years in countries with significant sand erosion). To further improve the power injected into the grid, a double integral synchronisation method was used. The performance of the proposed inverter was extensively verified through simulations and a practical system setup. The simulation results showed that the 120BCM approach significantly reduces switching losses, compared to state-of-the-art PWM techniques. The advantage in switching losses is eight times more than the centred PWM, and roughly four times more than the 60-degree bus clamped PWM, for switching losses proportional to current and frequency. In this dissertation, to achieve the above theories, a new transistor-leg gate-driver circuit was designed and implemented for the three-phase grid-tied photovoltaic inverter system using SiC-MOSFET during the power phase. The designed circuit has a dual gate driver for each leg; each circuit has an optocoupler-driver to turn the power transistor on and off, and a desaturation protection circuit. The design of this part was also possible without electrolytic capacitors. The control algorithm for the full system of the three-phase application runs on a TMS320F28335 DSP. A new smart voltage and current monitoring system (SVCMS) is also proposed in this thesis. It monitors a three-phase electrical system using the Arduino platform as a microcontroller to read the voltage and current from sensors, then send the measuring data by a Bluetooth device (HC-05) as a wireless communication to monitor the result using a new android smartphone application. This application uses open source software (MIT App Inventor 2). Finally, the work also has an academic purpose, as it is a platform on which new knowledge can be implemented and a demonstration of design and realisation, which is important for real applications in industry