21st century control of utility-scale solar and storage sites
The 205-megawatt (MW) Tranquillity solar facility in Fresno County, California has been in operation since 2016. In 2021, the utility-scale site was retrofitted with two battery energy storage systems (BESS) totaling 72 MW / 288 MWh, to help mitigate intermittency issues and improve the site’s overall efficiency. The addition of battery storage required a re-thinking of the site’s control mechanism, which while managing the more than 800,000 solar PV modules mounted on tracking tables, now also had to contend with integrating battery charging/discharging and adhere to strict parameters imposed by both the California Independent System Operator (CAISO) and a power purchase agreement.
The controller requirements are complex. The ability of the controller to offer both independent and aggregated operation and control of the site’s assets is a necessity. Requirements included:
– Managing both solar and storage as separate resources for energy shifting as well as CAISO and off-taker dispatching purposes
– Preventing the combined output from both solar and storage from exceeding the interconnection capacity and potentially damaging substation transformers
– Managing curtailment such that it would prioritize curtailing storage output over solar production
– Aggregating separate meters on the storage and solar assets
Typically, such a system configuration would require multiple, hardware-based controllers relying on individually programmed remote terminal units (RTUs) and/or programmable logic controllers (PLCs). Ensuring such a complex system of individual units functions consistently and efficiently is a tremendous challenge that requires significant resources to optimize and troubleshoot.