Masters of Electrical Engineering
School of Electrical, Computer and Telecommunications Engineering - Faculty of informatics
Latheef, Ahmed Ahsan, Harmonic impact of photovoltaic inverter systems on low and medium voltage distribution systems, M.Eng. thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2006. http://ro.uow.edu.au/theses/627
As residential customers become more energy conscious and environmentally aware, the installation of grid connected photovoltaic solar panels for small-scale electricity generation is expected to increase. However, the issue of quality of the electrical supply is as equally important as adopting sustainable energy. This thesis proposes a method to determine the quality of electrical supply based on the acceptable level of harmonic current that can be injected from a typical grid connected residential type photovoltaic inverter system (PVIS). The acceptable number of PVISs is based on not exceeding the recommended harmonic voltage levels in medium voltage (MV-11kV) and low voltage (LV-415V) distribution systems given in standard AS/NZS 61000.3.6-2001 and its application guide HB 264-2003. To undertake this study, an acceptable frequency domain model of a typical power system is developed, an appropriate model of a typical inverter spectrum is proposed and a method for allocating harmonic voltage distortion levels for PVIS in MV and LV systems by incorporating background distortion is suggested. The harmonic voltage distortion levels caused by the residential type PVIS are calculated based on conventional methods such as nodal analysis applied over the distribution network. A typical residential power system is adapted from the available literature. The LV distributors of the power system were modelled based on residential load and PVIS aggregation, and MV feeders are modelled based on distribution transformer aggregation. The distributors selected for LV systems study are based on overhead open-wire conductor, aerial bundled conductor and underground cabling types and the MV system feeders are based on an open-wire overhead conductor system. Residential load for harmonic studies is modelled based on the duration of equipment usage (with typical household ratings) during the power generation (active time) of the PVIS. Active time of the PVIS is estimated from field measurement data. Since the LV system is of multiple earth neutral (MEN) construction, an additional system study is required to investigate the effective neutral harmonic impedance. This study revealed the significance of the zero sequence impedance of the system to show the importance of representing the neutral current within the study. Consequently, the acceptable number of PVIS units is limited by triplen harmonic voltage magnitudes suggested by recommended harmonic voltage levels. Studying the available literature revealed that the development of a harmonic current spectrum to represent a typical photovoltaic inverter's line current is required. Hence, an adequate harmonic current spectrum was developed being selected from three distinct methods. The PVIS spectrums were modelled up to 40th harmonic, and an appropriate model was selected from among the three proposed models based on their compliance to recommended harmonic current emission levels, both individual and total, as suggested by standards. Examining the harmonic range up to 40th revealed that recent LV distribution network harmonic studies associated with PVIS are not wide enough in harmonic range to show some important network wide harmonic issues. Allocation of harmonic voltage distortion levels for the LV PVIS was based on the background distortion level and recommended harmonic voltage planning levels and the suggestion in standards to incorporate sufficient diversity for the MV and LV distribution systems contribution. Background harmonic voltage distortion levels were calculated based on published data related to field measurements from dedicated residential feeders in distribution systems. This study has proposed and identified a method to assess the harmonic distortion levels in MV and LV distribution systems, and related key issues, to assist the harmonic management of these systems due to grid connected PVIS.
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