Column: Transformer lifecycle
Operation of transformers: Impact of harmonic loading on transformer losses
Due to the increasing number of electronic devices with non-sinusoidal power supplies applied in today’s grids, transformers are being subjected to enhanced internal heating. Also, additional heating of transformer coils is caused by current waveform distortions due to non-linear loads, which in turn induce additional harmonic components to the fundamental (power frequency) current. The additional heat production, together with the normal heat from the transformer under load, leads to accelerated insulation degradation and reduction of the transformer lifespan. This paper discusses possible mitigation solutions for harmonics on transformers, ranging from specification of the transformer to monitoring and operational measures.
Keywords: harmonic loading, additional heating, loss of transformer life
Among various power quality problems, it is mainly the presence of harmonics in the network that interacts adversely with the network components and causes inconveniences to the network operators and consumers or producers connected to the grid. The operation of power electronic devices produces harmonic currents that lead to additional harmonic power flow and increase the network’s total apparent power demand while decreasing the true power factor of the network. Large harmonic currents can also cause overloading and additional power losses in network components. It may lead to high thermal stresses and early aging of the transformer.
In the future electricity infrastructure, which is characterized by the usage of multiple power electronics devices, harmonics can become a problem for the network manager. Various costs due to harmonics include operational costs (e.g. increased power losses), aging costs (e.g. reduced lifetime cost) and costs due to equipment’s maloperation. This paper presents the sources of harmonic loading, the impact of harmonics loading on the losses of a transformer, the additional rise of the transformer temperature and the additional loss of life due to the elevated temperature. Also, mitigation of the impact of harmonic loading on the transformer is discussed.
2. Harmonics sources
In the last decade, there has been a growing concern about harmonic distortion and the effects of harmonics in power systems. In Europe, the transmission and distribution grids are designed to carry the fundamental 50 Hz frequency current. Harmonic frequencies are integer multiples of the fundamental supply frequency, i.e. for a fundamental of 50 Hz, the third harmonic would be 150 Hz and the fifth harmonic would be 250 Hz.