Static and rotary transformers for renewable energy storage systems

The introduction of Renewable Energy Sources (RESs) to electrical power grids creates concerns over grid power imbalances. 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, maintaining grid energy balance and supporting overall fault levels. It also leads to issues with reactive power provision, often needing compensation 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 storage systems to provide bi-directional power flow to maintain AC power grid transient 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 AC connection of the battery storage system via a 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 presents a sizing study for the static transformer in either a TESS or BESS system, as rated for operation and lifetime longevity. The static transformer is then compared with a rotary transformer system 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 conversion from the switched DC system, and the energy transfer, the rotary transformer component also provides transient fault current to the grid during grid disturbances.