Analytical Assessment of DC Components Generated by Renewable Energy Resources with Inverter-Based Interconnection System due to Even Harmonics

This paper deals with the assessment of DC components generated by renewable energy resources with inverter-based interconnection system to the electric grid. DC injection is a critical issue related to power quality of distribution network systems with high penetration of inverter-based interconnection systems. This type of interface systems may improve the performance of the electric generation unit and affect positively or negatively the power quality of the distribution network depending on the proper or improper designation. The investigation of the various causes of DC components and the analytical assessment of their maximum levels are crucial for the proper operation of inverter-based interface systems and the limitation of DC injection. A method based on analytical calculations using a computer software has been implemented for the assessment of DC components contained on an inverter's output voltage when even harmonics are present on the network voltage. Moreover, a simulation package was used to demonstrate the existence of DC components under various conditions. It was proved by the current analysis that the amounts of DC components generated when even harmonics are present on the network voltage can be high under abnormalities on the power grid but they are not considerable under normal operating conditions. 1. Introduction The need for analysis and assessment of DC components on voltage and/or current, which may appear at electric grids, aroused mainly due to the significant problems they result in. Power transformers are the power system element which is mainly affected by the presence of DC components. Saturation, generation of harmonics, power losses, reduction in life cycle, and increasing noise levels are some of the effects on transformers introduced by the presence of DC components. Moreover, the saturation of current transformers used by measurement devices of control and protection of electric grid equipment may cause inaccurate measurements with considerable consequences. In cases where many DC injection sources are connected to the grid, the need for limitation and standardization becomes urgent. Such cases include the interconnection of several distributed generation units at Medium-Voltage (MV) or Low-Voltage (LV) level through inverters based on the technology of power electronics. Those inverters may generate DC components on the output voltage during abnormal conditions that will lead to DC current injection flowing to the electric grid if no power transformer is used between the inverter and the grid (transformer-less