Technical evaluation of retrofitting solid-state transformers and battery energy storage systems for peak shaving within the footprint of ageing line-frequency distribution transformers in Aotearoa New Zealand

Abstract

Aotearoa New Zealand’s distribution network faces significant strain from a 250% increase in distributed solar photovoltaic installations between 2013 and 2022. These trends increase vulnerability to failure of line-frequency transformers (LFT). The objective of this paper is to investigate the technical feasibility of retrofitting existing LFT footprints with integrated solid-state transformers (SST) with battery energy storage systems (BESS).  Using MATLAB/Simulink to simulate an 11 kV radial feeder, the study evaluated performance under strenuous dual-peak winter demand periods with transformers loaded to 110% of their rated capacity. The research demonstrates the potential recovery of up to 80% of the physical volume compared to LFTs allowing the footprint to be repurposed for decentralised BESS. The findings indicate that the medium density storage technologies (0.15-0.35 kWh/L) are the best option. The technology enables  a load curtailment of 20% reducing the transformer loading from 110% down to the safe 90% threshold. However, the dual-peak demand profile for Aotearoa New Zealand’s often depletes storage during morning cycles, while restricted charging windows limites utilisation of higher-density storage. Furthermore, while successful at the 0.4 kV busbar level, the aggregate impact of the main 11 kV feeder remaines minimal below 7.6%. It is recommended that future research focus on predictive control methods to optimise storage state of charge across successive peaks and the integration of downstream renewables to expand charging windows.