The introduction of Renewable Energy Sources (RESs) to electrical power grids creates concerns about the imbalance in the grid. Most RESs are non-dispatchable, meaning that natural elements have a significant impact on how much power can be produced, which leads to issues with frequency stability and maintaining grid energy balance. It also leads to issues with reactive power provision, often needing compensation in order to maintain voltage levels, and can influence short circuit fault currents when connected at the distribution level. Therefore, RES poses operational challenges and thus the interest in using electrochemical battery transient or steady energy storage systems (TESSs or BESSs) to provide bi-directional power flow to maintain AC power grid frequency stability (TESS) and support more sustained energy delivery (BESS).

The voltage level constraints of commercial DC-DC and voltage source converter (VSC) power electronic devices is limited to around 800V operational (1200V IGBT devices). Although higher voltage power electronic devices and VSC structures are being developed, to-date, the 1200V IGBT is the industrial benchmark device. This results in the 3-phase connection to the VSC of around 690 Vrms in practice. Thus, an interface transformer is necessary in the TESS or BESS system to connect to higher voltage, 3-phase electrical networks.

This paper provides an objective study of correctly sizing the static transformer for operation and lifetime longevity considering transient and longer term sustained energy transfer, essentially defining duty ratings for the transformer. The transformer conduction and magnetizing losses are considered and their impact on sizing specification discussed and quantified. Initial magnetization and the system start-up and shut-down operations are discussed and assessed.

The static transformer is then compared with a rotary transformer device comprised of a low voltage induction machine mechanically integrated with a high voltage, low per unit inductance synchronous machine that electrically connects to the utility power grid. While providing the necessary voltage step up from the switched DC system, and the energy transfer, the rotary transformer component also provides transient fault current to the grid during grid disturbances.

The paper will compare the static and rotary transformer options in both TESS and BESS systems.

Selected references

S. Ehsan, R. Ehsan, and R. Mohammad et. al.: “Impact of distributed generation on protection and voltage regulation of distribution systems: A review”, Renewable and Sustainable Energy Reviews, vol. 105, pp 157-167, 2019.

C. M. Affonso, M. Kezunovic, “Technical and Economic Impact of PV-BESS Charging Station on Transformer Life: A Case Study”, IEEE Trans. On Smart Grid, Vol. 10, No. 4, 2019.

A. Azmi, J. Jasni and N. Azis, et. al.: “Evolution of transformer health index in the form of mathematical equation”, Renewable and Sustainable Energy Reviews, 76, pp.687–700, 2017.

N. Etherden, M. Bollen, “Dimensioning of Energy Storage for Increased Integration of Wind Power”, IEEE Trans. on Sustainable Energy, vol. 4, iss. 3, pp. 546 – 553, 2013.

Y. Hu, et. al.: “Investigation of Transient Energy Storage Sources for Support of Future Electrical Power Systems”, IET Renewable Power Generation, Early Access, 2020.