Evaluating the harmonic effects on the thermal performance of distributed photovoltaic power generation systems

The rapidly increasing demand for Distributed Photovoltaic Power (DPVP) generation system transformers and the rise in the construction of solar photovoltaic plants in South Africa, present technical challenges in the latter involving high top-liquid and hotspot temperatures problems during their service lifetime. The winding Eddy current loss harmonic factors recommended by the standards do not take into account the skin effect due to the restricted penetration of electromagnetic fields on the conductors at high harmonic orders, which results in erroneous estimation of these losses as well as high temperature rise. In order to enable optimum estimation of the winding Eddy current losses, it is proposed in this paper a harmonic loss factor that considers the conductor skin effect under harmonic conditions by using the data supplied by the manufacturer for losses under rated conditions. Based on supplied harmonic spectrum, the service losses of a liquid-filled step-up transformer are estimated, top-liquid and hotspot temperature conditions are then realized. Lastly, the capacity of a transformer when supplying the given distorted load current profile is evaluated by using a technique ascribed to as derating. The use of this method is achieved by the computation of the maximum permissible current as a result of increased winding Eddy current loss and other structural losses. The results indicate transformer loading capacity increase to 97.6 % compared to 91.3 % when applying the harmonic factor approach recommended by the standards.