Safety by design: PCS Considerations in mixed PV and battery energy storage systems

As solar and battery energy storage systems (BESS) become increasingly common, installers frequently encounter projects that combine equipment from different manufacturers. While these systems may operate independently without issues, integrating them into a shared electrical infrastructure introduces important safety challenges that designers must address.

The challenge of different ecosystems

Consider an existing PV system paired with a newly installed battery from another manufacturer. Although both systems may be individually certified, they do not necessarily communicate or coordinate their power output.

The battery’s controller may monitor existing PV production through current transformers (CTs), but it may not have the ability to reduce or shut down the third-party inverter’s output.

This becomes a significant concern when both systems can simultaneously supply power to the same electrical infrastructure. Without proper engineering, their combined output could exceed the allowable current ratings of shared conductors or busbars.

One of the most overlooked design issues is assuming that monitoring an independent energy source is equivalent to controlling it. When equipment operates outside a common ecosystem, designers must account for the potential output of every independently operating source.

The role of PCS in mixed-manufacturer systems

A Power Control System (PCS) provides a means of managing power flow to maintain defined electrical limits, offering greater flexibility when integrating multiple energy sources.

NEC 705.13 in the 2023 edition establishes requirements for using listed PCS equipment to limit current on busbars and conductors. However, the presence of PCS functionality does not automatically guarantee that all connected energy sources are controlled.

In mixed-manufacturer installations, a battery PCS may have direct control over its own inverter while having no authority over the existing PV system. In these situations, the independently operating PV system must be accounted for when establishing the PCS operating limits.

For example, a properly configured PCS may maintain the required electrical limits by adjusting battery charging and discharging in response to PV production, without directly controlling the third-party inverter.

The critical consideration is whether the PCS has sufficient control authority to maintain the required limits under all relevant operating conditions.

Proper PCS application and system safety

Correct PCS application begins with identifying the electrical equipment requiring protection and evaluating the maximum potential contribution from every connected energy source.

Designers must establish a control strategy that accounts for simultaneous PV production and battery discharge while ensuring that the ratings of shared conductors, busbars and other electrical equipment are not exceeded.

CT placement is particularly important. Sensors must be installed at the appropriate measurement points to provide the PCS with accurate information about the power flowing through the equipment being protected.

Equally important is understanding the PCS’s control limitations. When the primary controller cannot regulate an independent PV source, the system must be designed to maintain the required limits through the equipment it can control or through another approved protection method.

Designers must also evaluate how the system responds to communication failures, incorrect measurements and loss of control. The proposed configuration should be supported by applicable equipment listings, manufacturer instructions and the adopted electrical code. Proper commissioning must verify that the PCS operates as intended.

Conclusion

Integrating PV and BESS equipment from different manufacturers offers significant flexibility, particularly when adding battery storage to existing solar installations.

However, these configurations require careful consideration of independently operating energy sources.

The presence of a PCS alone does not guarantee a safe installation. Designers must understand which sources can be controlled, account for those that cannot, and verify that the control strategy maintains the required electrical limits.

Ultimately, safe system integration depends on designing around the actual capabilities of the PCS rather than assuming that every connected energy source operates within a coordinated ecosystem.

By Vahe Hayrapetyan. Hayrapetyan is an engineer and Managing Director of Energy Solar, Inc., a California-based solar EPC company working on residential and commercial PV and battery storage projects.