A comparison between harmonic indices in low voltage grid-tied photovoltaic system

Abstract

Power quality (PQ) of low voltage grid-tied rooftop PV system is regarded as an important step towards achieving the quality of supply. The deployment of a rooftop PV system is expected to increase in South Africa especially in urban areas. However, it is important for low voltage customer to install a PV system with low PQ disturbances. One of the most common PQ disturbance in power system is the distortion of current and voltage which is called harmonics. In this paper, the harmonics content is best presented as total harmonic distortion (THD) and total demand distortion (TDD). THD is the index of the most common harmonic that has been used by most of the literature. It has been observed that the measured THD is higher when the power consumed by the loads and solar irradiant is low. This could result in a situation whereby the low voltage customer with a PV system might be penalized by the utility. At higher solar irradiance and high-power consummation by loads the THD values of the grid is very low and is within the acceptable limits according to the standard. The PV system discussed in this study does not inject unwanted harmonics to the grid. The TDD result of the grid-tied PV system remains at a lower level regardless of whether the solar irradiance and the power consumed by the loads are lower or higher. The paper concludes that THD analysis can be misleading and TDD analysis are reliable and convincing. However, the study recommends the practice of TDD in PQ analysis.