Tomomichi Ito

Harmonic current reduction control for grid-connected PV generation systems

Integration of renewable energy sources (RES) with significant capacity, including photovoltaic (PV) generation systems, into the grid is expected within ten years. Such RESs bring to the grid not only their benefits but also some technical problems, as increase of harmonic currents. Compliance with the technical requirements for harmonic current is necessary for successful increase of RES capacity. A scheme to reduce harmonic current for grid-connected PV generation systems was developed. This control scheme effectively reduced harmonic current in the grid current of the PV generation system caused by voltage distortions at the grid. Experiments using a prototype of the power conditioning system (PCS) showed its validity. 400-kW PCSs with the control scheme have been installed and have been in service since the end of 2009.

An envelope-based detection method for resonance damping in grid-connected converters

As the penetration of renewable energy sources (RES) increases, variation of system impedance seen from grid-connected converters becomes much larger than expected. In this situation, stable operation of converter becomes more challenging. One of the issues caused by system impedance is resonance harmonic problem. Typically active damping control is used and designed based on the known conditions of power grid, that is, characteristics of harmonic resonance do not change too much. However, as penetration of RES increases, the power capacity of RES becomes comparable to power grid. In this situation, the per-unit impedance seen from RES system becomes small, and system operation like switches of transmission line will make the percentage of system impedance becomes much larger than before. When large variation of percentage of system impedance occurs, the aforementioned characteristics of harmonic resonance is not fixed anymore. This could cause the failure of conventional active damping control. Therefore, resonance damping function should response with this change to maintain the stable operation of converter. For this purpose, a new method for resonance damping is proposed in this paper. The proposed method on-line identifies the occurrence of resonance and detects the resonance frequency at the same time. These functionalities improve the conventional active damping control and enable converter having more stable operation in the power grid with wide system impedance variations.