Research On Transformless Photovoltaic Grid-connected Inverters With Active Power Filter

Solar energy is an inexhaustible and pollution-free green energy, which makes it the appropriate andurgent new energy for world’s economic development. Photovoltaic grid system Grid system is an effectivemeans of energy conversion, and it can convert solar energy into electrical energy. PV grid system has twoforms: the isolated and the non-isolated. The isolated system includes a transformer and can isolate the gridand solar panels to protect the staff and equipment, however, its bulky volume, heavy weight and high costresult in a low efficiency. Though the non-isolated PV grid system has solved this problem, it brings aboutlarge common-mode currents that do harm to people and equipment. So it is a trend to produce anon-isolated PV grid system to eliminate or suppress the common-mode currents.With the development of science and technology, more and more power electronics have becomeusing in factories and private households. It is convenient for people to live and work, meanwhile it alsobrings about more power harmonics endangering the power grid and equipment. The Active Power Filter isan effective means of harmonic suppression, and it is active, controllable and flexible compared to thepassive filter, but its initial high costs limit its wide use.Firstly, this article introduces research status of photovoltaic technology and active filter, andanalyses the causes of common-mode currents in non-isolated PV grid-connected system and the principleof common-mode currents suppression, on the basis of which operating principles of a novel H6non-isolated PV grid system are elaborated. Then, the relation of common-mode currents, circuitparameters to switching frequency is discussed, providing a theoretical basis for the circuit design for anon-isolated topology in which transistor’s parasitic diodes act as clamped diodes. A H6inverterexperimental prototype is built with the designed circuit parameters, and verifies its correctness and validityof common-mode currents suppression. On this base, a H7topology, in which voltages are clamped by theDC line capacitor, is researched to better suppress the common-mode current. A H7experimental prototypeis also built to verify its validity.Secondly，similarities as well as the combination theory of PV system and APF are analyzed. So howto detect harmonics becomes the key technique in this combination to detect harmonics becomes the keytechnique in this combination. Summarizing the common methods of single-phase APF harmonic detectionand studying a ip-iqharmonic detecting technique with phase-locked loop based on instantaneous powertheory. This detecting technique ensures that the load’s harmonic and reactive components can still be fastand effectively detected on the condition of voltage distortion.At last, the Unified control strategy is studied based on H6PV converter with APF. For economicconsiderations, the strategy aims at transmitting maximum photovoltaic array powers. A current and voltagecontroller has been designed, and the circuit model is built on simulation tool Matlab.