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. 

By contrast, aggregating control into a single software-based controller that centralizes control over the entire site is a much more precise, scalable and efficient solution. This is what the site owners opted for when they installed a PXiSE Renewable Power Plant Controller (PPC). 

Power Plant Controller in action

The PPC provides synchronized, coordinated control. This ensures power flow and voltage at the point of interconnection and at each substation transformer meet all commercial requirements and remain within the system’s technical limitations.

One way it achieves this is by actively controlling the output power of both the BESS and the solar PV to ensure power is below the 110 MVA ratings of the transformers. Using a feedback control loop scanning at 100ms, the PPC sends real power set points to both the BESS energy management system (EMS) and the solar PV SCADA. If the BESS is ordered to discharge and the discharge amount will cause an exceedance of more than 110 MVA, the controller will either curtail solar PV and then discharge the BESS or limit the BESS discharge so that the total discharge from the BESS and solar PV is below 110 MVA.

Such autonomous decision-making on the controller’s part, informed by the customer’s business priorities, is one of several benefits realized through the controller’s optimization features. Instead of being locked-in to a specific time-of-day charge/discharge pattern, the controller uses forecasts and artificial intelligence (AI) to make real-time decisions in the customer’s best interest that stay within regulatory and power purchase agreement boundaries.

21st century controls for 21st century problems

The Tranquillity site uses a software approach to solve the complex issues related to managing utility-scale solar and storage. The hardware-based solutions of the past are no match for today’s AI-aided tech which outperforms on speed, precision and efficiency fronts. A software-based PPC offers a scalable, future-proof solution that is prepared for the complexities introduced by 21st century energy markets.

To read about the Tranquillity solar and storage site, download the case study.