Controlling Solar and Storage for Optimal Results

The role of SCADA in an effective DC-Coupled Solar + Storage System

Mike Crawford
Vice President, NLS Engineering

Hanan Fishman
President, Alencon Systems

SCADA systems, also known as plant controllers, have played an important role in grid connected Solar systems for years. With the rise in interest in deploying DC-coupled Solar + Storage systems, the role of such systems is growing further in importance.

As a leading power electronics technology provider for DC-coupled Solar + Storage systems, here at Alencon together with our SCADA partner NLS Engineering, we thought there would be value in sharing our observations on SCADA considerations for your next Solar + Storage project.

SCADA Basics for Utility Scale Solar
Let’s start with some basics.  SCADA is an acronym for Supervisory Control And Data Acquisition. In a traditional utility scale PV plant, SCADA systems consist of software and hardware components that allow a plant owner to:

- Directly interact and control various pieces of equipment including inverters, meters, trackers, meteorological    stations and other smart devices through human-machine interface (HMI) software

- Monitor, gather, and process real-time data

- Record events into a log file

The role of SCADA in grid-connected, PV only plants is to help the plant owner attain their contracted commitments, operate the plant safely, support grid stability through functions like the management of reactive power and curtailment while maximizing the plant’s energy production. For a deeper understanding of what a SCADA system does in a traditional, utility scale PV plant, click here.

The Importance of SCADA for DC-Coupled Solar + Storage

As you move from designing and implementing conventional PV systems to those that include both Solar and Storage, particularly when coupling PV and batteries on the DC side inverter, the importance of the SCADA system increases much further.  A DC-coupled Solar + Storage system can consist of a number of key pieces of technology, including the PV panels, DC-DC optimizers, batteries and corresponding battery management system (BMS) and grid connected inverters. As an analogy, you can think of a Solar + Storage system as an orchestra, where the aforementioned pieces of technology represent the various musical instruments.  The SCADA, or plant controller, is the conductor.  It serves as the technology to assure all of those elements work in concert to assure proper operation of the battery energy storage system, or BESS.

The SCADA interacts with the elements of the BESS to assure the correct charging and discharging of both the battery as well as dispatch of energy to the power grid.  The SCADA will send commands to the various components in the BESS to direct them to take certain actions to achieve the overall desired behavior of the system.  These commands are typically sent via a standard communications protocol such as MODBUS.

In a Solar + Storage system, it is important not to confuse the SCADA with the Battery Management System (BMS).  A battery management system (BMS) is the electronic system that manages the battery racks inside the container and provides such functions as protecting the battery from operating outside its safe operating area, monitoring its state of charge and balancing charge among battery cells and racks.   The job of the SCADA is to interact with the BMS as it pertains to the rest of the system, i.e. the inverter, DC-DC optimizers and power coming from the PV panels.

Another important role of SCADA in a DC-coupled Solar + Storage system can be to command the curtailment of PV production when the battery is full. This is a particularly important function in a DC-coupled system, because one of the primary benefits of such an approach to pairing solar and storage is to capture energy that might have otherwise been clipped as a result of a high DC to AC ratio. Of course, it is possible to have too much of a good thing, i.e. there can come a point where the battery can be full and PV production will need to be curtailed.  It is SCADA’s job to make that determination and command the appropriate action to make sure curtailment occurs correctly based on the functionality of the equipment in the BESS.

Making the Right Choice in SCADA Partners

As a leader in the emerging field of DC-coupled Solar + Storage, here at Alencon we often find ourselves helping our customers put together the various elements of their BESS, from of course the selection of our DC-DC optimizers to the storage medium to the SCADA system. We strongly encourage our customers to think about the SCADA system as early on in the design process as possible, as this choice is one of the most important decisions you can make as it prescribes so many other decisions prior to commissioning the project.  We have successfully partnered with NLS Engineering in executing multi-megawatt DC-coupled Solar + Storage projects. As such, the strongest recommendation we can make when selecting a SCADA partner is to chose one with real, hands on experience in successfully deploying Solar + Storage projects.  The reality is, the field of DC-coupled Solar + Storage is still in its infancy and even many experienced, general purpose SCADA developers will not have had practical experience in this domain. Particularly if you are deploying your first DC-coupled Solar + Storage project, such expertise could prove to be invaluable.