Comprehensive study of power quality criteria generated by PV converters and their impacts on distribution transformers

This paper investigates the impact of power quality criteria required by photovoltaic (PV) converters on legacy system transformers and its design procedure, since it is often argued that the use of traditional design for transformers in the PV integrated power systems is acceptable. The criteria studied in this paper are voltage amplitude variation (flicker), harmonics, frequency variation, power factor (PF) variation, DC bias, thermal cycling (loading), transients, inrush current, eddy current, and stray loss. Some of these problems will be simulated with a Voltage Source Inverter (VSI) and Current Source Inverter (CSI) which are frequently used in PV integrated power systems as Distributer Generation (DG) by decentralized controlled PV inverters. In this type of power system configuration, transformers may be utilized for isolation and/or filtering or as a part of the feeder facility. On the other hand, if the transformerless PV inverters are used in PV systems, then only the distribution transformers will be affected by the PV converters power quality problems. These issues can reveal or change the allowable penetration level at a specific bus location along the feeder. Furthermore, output voltage of VSI and output current of CSI of PV systems have a faster dynamic response and more distorted output than legacy power system voltage and current outputs. This means that the VSI and CSI need stiff controllers to ensure a suitable power exchange to the utility network, particularly to sensitive equipment like transformers that are designed for legacy unidirectional power transformations with a cleaner voltage and current than that provided by PV inverter based systems.