Incorporating Floating batteries into a grounded solar array?

Solar + Storage are gaining popularity as we look towards innovative ways to firm up the production of solar energy and turn solar into becoming a familiar energy resource. There is an urgent need to optimize the circuit, to provide voltage and frequency stabilization, as more renewables come onto to a grid. One of the easiest ways to do that is to utilize energy storage.

Problem & Solution

Typically, installed PV systems are negatively grounded at the suggestion of the PV panel manufacturer. Also, the National Electrical Code (NEC) and certain AHJ rulings also encourage PV arrays to be grounded, as opposed to being allowed to “float.” Currently, large-scale battery energy storage systems are generally “floating” or not grounded. The DC-coupling approach to combine Solar + Storage requires both the battery and the PV system be placed on a common DC-bus.  Battery and the PV system components must have a common ground, which is not at all the case when the PV is grounded, and the battery is floating.

Installing batteries into an existing PV array using DC coupling always looks easier on paper but dealing with the wiring practices of the existing site and all the physical infrastructure is not an easy task. Overcoming that challenge requires the installation of a galvanically isolated DC optimizers to create a floating ground between the PV systems and the batteries. This is not generally an issue with AC-coupled solar + storage systems as the individual inverters behind the PV and batteries create this grounding isolation automatically.

Secondly, battery storage systems need to have floating grounds for safety purposes too. This requirement is necessary with large battery systems because having a floating ground allows for two faults to occur for a serious safety situation to arise. By allowing for two faults, the first fault can be detected by the on-board fault detection systems and safely disconnect the battery before a problem occurs.  

Galvanic isolation is used where two or more electric circuits must communicate, but their grounds may be at different potentials. Galvanic isolation is an effective method of breaking ground loops by preventing unwanted current from flowing between two units sharing a ground conductor. When DC coupling solar and storage, a DC-DC converter is required to map the voltage differences between the PV system and the battery. A galvanically isolated DC-DC converter serves the dual purpose of mapping PV voltage to battery voltage while isolating the differential grounding schemes that could be present.

Adding storage to solar projects is the next major step in the clean energy revolution, but it’s not without technical challenges. We are hoping that commercialization and day to day emerging advance technologies will help us to solve these problems.