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An Efficient Distribution Transformer Fleet Modernization Strategy for a Rapid Transition toward the Active Electric Networks

A modernization strategy associated with the replacement of the No-Load Tap Changer (NLTC)-fitted distribution transformers having low-performance standards (the commissioning year being before 2000) with the On-Load Tap Changer (OLTC)-fitted Tier 2 transformers according to the Eco-design European Directive, to ensure the flexibility of the voltage control in the PV-rich active electric distribution networks (AEDNs) has been proposed to increase the energy efficiency, decrease the energy losses, and improve the voltage quality. The strategy includes two stages: the first corresponds to the sizing process of the transformers (the determination of optimal rated power), and the second takes care of the sizing process of the associated OLTC (optimal number of tap positions and the minimum time between two tap changes). An AEDN supplied from an electric distribution substation (EDS) equipped with a transformer commissioned in the year 1979 (rated power of 250 kVA and NLTC with 3 taps) from a peripheral urban area belonging to a Romanian Distribution Network Operator (DNO) has been used to demonstrate the efficiency of the strategy, obtaining an optimal solution associated with a Tier 2 transformer having rated power of 160 kVA and OLTC having 17 taps which led to an energy-saving of 4.5 MWh/year and improved the voltage quality quantified through the smallest value of the mean squared value of voltage for a time between two changes of the tap position of 15 minutes, keeping all voltages from the nodes between the allowable limits.