Investigation of the Effects of DC Current Injected by Transformer-Less PV Inverters on Distribution Transformers

Rooftop transformer-less photovoltaic inverters (TLPVIs) have the potential to inject a very small dc current into the distribution network. The dc current quantity can be very high in a residential power distribution system where several TLPVIs are installed. Distribution transformers (DTs) supplying power to the customers in the networks can be adversely saturated with this high dc current accumulated from the small dc current injected by individual TLPVI. In this paper, a systematic investigation is carried out on the dc current injection potential of commercially available TLPVIs. Furthermore, the influence of this aggregated dc current on the DTs are analyzed through modelling, simulation, and laboratory testing. A three-phase three-leg transformer is used to analyze the effects of the high dc injection from the TLPVIs. A finite element analysis is carried out in COMSOL Multiphysics environment to explore the dc current injection effect. The results show that the injected dc current from TLPVIs highly affect the DTs and a solution is needed to overcome this challenge before the problem spreads widely and causes a barrier to the growth of the integration of renewable resources to the power grids.