21st Century Control of Large-Scale Solar and Storage Sites

The 205 megawatt (MW) Tranquility solar facility in Fresno County, California has been in operation since 2016. In 2021, the large-scale site was upgraded with two battery energy storage systems (BESS) totaling 72 MW / 288 MWh, to help mitigate intermittency issues and improve overall site efficiency. The addition of battery storage necessitated a redesign of the site control mechanism which, while managing the more than 800,000 solar PV modules mounted on tracking tables, now also had to deal with the integration of the load /battery discharge and meet strict parameters imposed by both the California Independent System Operator (CAISO) and a power purchase agreement.

Controller requirements are complex. The controller’s ability to offer both independent and aggregated operation and control of site assets is a necessity. Requirements included:

  • Manage both solar and storage as separate resources for energy transfer as well as CAISO and buyer dispatch purposes
  • Prevent combined solar and storage generation from exceeding interconnection capacity and potentially damaging substation transformers
  • Manage curtailment in a way that prioritizes curtailment of storage generation over solar generation
  • Aggregation of separate meters on storage and solar assets

Typically, such a system configuration would require multiple hardware controllers based on remote terminal units (RTUs) and/or individually programmed programmable logic controllers (PLCs). Ensuring that such a complex system of individual units operates consistently and efficiently is a huge challenge that requires significant resources to optimize and troubleshoot.

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

Power plant controller in action

The PPC provides synchronized and coordinated control. This ensures that the power flow and voltage at the interconnection point and at each substation transformer meets all commercial requirements and remains within the technical limits of the system.

One way to achieve this is to actively monitor the output power of the BESS and solar PV to ensure that the power is below the 110 MVA of the transformers. Using a 100ms feedback control loop sweep, the PPC sends actual power setpoints to the BESS Energy Management System (EMS) and Solar PV SCADA. If the BESS is commanded to discharge and the amount of discharge will result in over 110MVA overshoot, the controller will reduce the solar PV and then discharge the BESS or limit the discharge of the BESS so that the total discharge of the BESS and solar PV is less than 110 MVA.

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

21st century controls for 21st century problems

The Tranquility site uses a software-based approach to solve the complex problems of managing large-scale solar energy and storage. Hardware solutions of the past are no match for today’s AI-assisted technology, which surpasses performance in speed, accuracy and efficiency. A software-based PPC offers a scalable and scalable solution that is prepared for the complexities introduced by 21st energy markets of the century.

To learn more about the Tranquility Solar and Storage Site, download the case study.