Research On Key Techniques Of Solar Grid-connected Power System

Grid connected inverter is the most important power conversion unit in a photovoltaic electricpower generation system. This paper mainly deals with the key techniques such as voltage and currentcontrol, maximum power point tracking and anti-islanding protection of small scale single phasephotovoltaic grid connected inverter, which contributes to the performance and reliability of thesystem.It gives the basic electric circuits, control methods and product design examples in chapter2.Aiming at the problem that the dc input voltage range of the single-stage inverter is very wide and theoptimum design of the control system is difficult, this dissertation put forward that pulse widthmodulating carrier or proportional-integral controller parameters of current loop vary with DC inputvoltage, and the inverter can work well in the full voltage range. The transient voltage overshoot ofthe ac output when the grid is suddenly gone is also solved by this method. Double-close loop of inputvoltage and current control system is analyzed based on the work mode and circuit model of thefront-stage Boost converter of a two-stage inverter.Maximum power point tracking is a basic function of the solar inverter. The equivalent circuitmodel, voltage-current characteristic of solar module and static and dynamic maximum power pointtracking efficiency evaluation in related standard are introduced in chapter3. The Disturb-Observemethod is designed, and the realization and efficiency analysis are given out to meet the requirementsof voltage step and period of cycle. The regulation of PV voltage is achieved by closed loop controlbased on maximum of the power sent to the grid. The cycle of voltage disturb and power estimationwith good stablilty and high efficiency is designed according to the dynamic performance of theclosed voltage control. For the single-stage inverter, there is a power loss because of the voltage rippleof double line frequency with the dc-bus voltage. The relationship between the power loss and thedc-voltage and ac voltage ripple is given out.Anti-islanding is one of the most important protective functions of small scale grid connectedinverter. The calculation and simulation method of none detection zone of frequency protection arepresented in chapter4, which are analyzed based on the load impedance characteristic and the failurereason of frequency detection method. It introduces the working principle of regular frequency driftand phase shift method for islanding detection, and sets forth the effects of over and under frequencylimits and frequency or phase interference strategy on the none detection zone. Furthermore, thisdissertation illuminates the effects of the load impedance characteristic, the tracking error of phaselock loop and phase error of closed current control loop on islanding frequency, and the result isverified with simulations and experiments. According to the phase formula of islanding system, three methods of intermittently frequency or phase interference, bi-directional frequency drift or phase shiftand increasing of frequency interference cycle are presented to decrease none detection zone ofanti-islanding protection and to improve waveform of current injected into the grid.There may be many power generation systems at one common power point with the applicationenlargement of photovoltaic grid connected system. This dissertation gives the math model of currentcontrolled multi-inverter grid connected power system. It also analyzes synchronization strategy offrequency or phase interference and the effects on the islanding detection. Based on this, thisdissertation also illustrates an anti-islanding protection strategy of multi inverter grid connectedsystem and verifies the result by simulation and experiments.