Step-Up Transformers for PV Plants: Load Loss Estimation under Harmonic Conditions

Abstract

In 2010, South Africa introduced the Renewable Energy Independent Power Producer Procurement Programme to invigorate low-carbon energy generation using renewable energy resources. Amongst the development of wind and hydro plants; solar photovoltaic plants then took off rapidly. The volume of solar PV plants connected to the grid in 2015 and 2016 was unparalleled. However, the solar PV electric power distribution network is challenged with the presence of unacceptable harmonics and distortion content at the point of common coupling due to the switching action of inverters and operation of non-linear loads. It therefore essential to precisely estimate the resultant increased load losses in order to estimate the hot spot temperature and total losses during service.In order to demonstrate the significance of these challenges, this paper provides a comprehensive harmonic analysis of an oil-filled transformer for the supplied geometrical data and harmonic spectrum of an oil-filled transformer. The load losses are evaluated under fundamental and harmonic conditions. The load losses are classified into I2R and stray losses. The I2R losses can be easily obtained by analytical calculations and also through practical measurement. On one hand, analytical calculation of the stray losses cannot give an optimized estimation and practically their immeasurable. In this paper, a 2D FEM model that is dynamic to any transformer geometrical data is developed to produce a pragmatic loss estimation by using knowledge of the magnetic flux density. Generally, analytical formulations neglect the radial magnetic flux density component. During operation, this component becomes predominant towards the winding ends and cannot be disregarded.