This article explains how to operate a Victron+Redflow site safely where the desire is for the site to:
- operate mostly on-grid using Victron's ESS system
- be able to operate in off-grid mode during periods of grid failure
- be able to keep using the AC PV solar during grid outages to support site loads and charge batteries
This article also provides recommended steps and electrical configuration to improve the general resilience of sites - especially larger sites - operating in an on-grid scenario.
Connecting AC PV to the output ('load') side of a Victron inverter/charger setup, when on-grid, will lead to scenarios where the failure of the grid connection can cause the unexpected and immediate shutdown of the site.
This is due to a specific technical characteristic of Redflow batteries - namely that they prohibit charging, when full or when preparing for maintenance, using an electrically switched-in power diode.
The 'danger' scenario occurs:
- when all on-site Redflow batteries are all full (or blocking charge for other valid reasons)
- when the AC PV on the 'load' is exporting energy to the grid through the Victron system in the middle, and
- when the grid fails
WIth conventional batteries, the transition-period energy surge is directed, by the Victron inverter/chargers, into the battery array. To allow sufficient system capacity to do this, with conventional batteries, any AC solar array on the 'load' side can be sized based on the Victron "Factor 1.0 Rule".
However, with a Redflow based battery array, the 'Factor 1.0 Rule' does not apply ; There is no real safe size of AC solar array to have on the load side of a grid-interactive Victron+Redflow deployment when the grid is connected, unless some specific BMS logic is applied that can proactively shut down the grid-side AC solar under specific circumstances (read on).
The issue is that if the Redflow array is entirely full and/or otherwise in a 'charging not permitted' status when the grid fails, there is nowhere for the surge of surplus AC energy to go. The 'charging not permitted' Redflow batteries block charge electrically with a diode, which means there is no surge-absorbing capacity available.
Importantly: Even before the last Redflow battery is full (when grid-side AC definitely needs to be disabled for stability of the site), the available capacity to absorb surges of charging energy diminishes progressively as each Redflow battery in turn becomes totally full.
If the grid fails while the AC solar is exporting energy into the grid from the load side, in excess of the remaining charge capacity of the Redflow array, then the resulting high voltage AC spike can (and routinely does) cause the Victron system to shut down, because there is nowhere for the export energy flow to 'go' while the system readjusts.
Note that if the AC solar is installed on the grid side and left there, then there is no issue (other than that the AC solar is then unavailable as an energy source when the grid is down).
In addition to following the design noted here, site designers should also read and act upon all of the requirements noted in the general Victron off-grid configuration guide located here. In particular the AC PV Shutdown configuration noted on that page must be used as well, for stable site operation.
Automatic AC PV changeover using a Solar Automatic Transfer Switch (ATS)
Where it is desired to support AC solar charging in Redflow deployments both when on-grid *and* when off-grid, an Automatic Transfer Switch (ATS) is required.
This should be wired so that the output of the AC PV solar array is delivered to the 'grid' (input) side of the Victron inverter/charger(s) when the grid is alive, and such that it automatically switches to the 'load' side if the grid fails.
In large systems, recommended ATS units for this role include the Socomec ATyS series (see data sheet attached to this guide page), or the solid state product range from http://www.staticpower.com.au.
AC PV 'dual' metering to support the solar ATS
The Victron GX system and ESS control functions require that they have an accurate understanding of the physical location of attached AC PV Solar arrays. To allow the GX system to 'see' the AC PV as being in the right physical location in both on-grid and off-grid operating modes, the solution required is to deploy two energy meters, wired into the two output 'legs' of the Solar ATS unit.
This means that the GX system 'believes' that two solar arrays exist, with one configured in the GX to be located on the input side and one configured in the GX to be located on the output side.
In large systems these are typically Carlo Gavazzi EM24 CT based units (RS485 or Ethernet). In smaller systems they can be direct-wired EM24 meters.
A third meter is also required, as a grid input measurement meter (for all GX systems with grid-side PV, a grid meter is a mandatory additional requirement on the system).
With this metering arrangement in place, the Victron ESS system gets its 'maths right' in terms of accounting for AC PV production and correctly handling it, in both on-grid and off-grid situations.
Connecting three energy meters to a Victron GX device
Most Victron GX models only have a maximum of two physical USB ports able to be used for USB based energy meters.
Note: The Cerbo GX has three USB sockets but one is a 'USB-power-only' port and cannot be used for an energy meter.
To support the three physical meters needed to implement the solution noted here, the limited number of USB ports on most GX units leads to a need to choose your metering approach carefully:
- ET112 / ET340 meters require a dedicated USB-to-RS485 dongle per meter, meaning only 2 of them can be directly attached to most GX based systems. You can resolve this by using a small USB hub to expand the number of physical USB ports available to the GX.
- EM24-RS485 meters can be daisy-chained via RS485 (So you can connect two EM24 meters to a single USB-RS485 dongle, as explained here). This means you can connect three EM24's using just two USB ports. EM24's are three-phase capable but can be configured as single-phase meters.
- The EM24-Ethernet meters allow an arbitrary number of meters to be deployed as they all just plug into the local Ethernet LAN.
Disable GX automatic AC PV scanning before connecting AC PV to the system!
For AC PV that can be measured via Ethernet from the GX (e.g. Fronius etc), you must not measure the PV that way, when deploying this solution. Instead, you need to use the physical meters as the sole source of AC PV metering. If you do this, the PV production will be measured twice.
You must disable automatic AC PV scanning in the "PV Inverters" menu before attaching your AC PV to the local LAN, so the AC PV does not show up 'twice' on the system.
If you don't do this, and if the AC PV appears in the "PV Inverters" list due to the automatic scan, it is quite difficult to remove it again (it requires a unix shell login to be set up into the GX and to do hand editing of the GX's 'settings' xml file to remove those AC PV entries from the system as there is currently no 'delete' function in that menu). It is a lot simpler to just disable the automatic scanning first...!
Engage automatic 'AC PV Disable' when the battery array charging capacity is below a safe threshold
Even in this hybrid operating structure, you should still configure and use the "AC PV Disable" function that is documented on this off-grid operations article.
Doig this ensures the AC PV is turned off when the battery array is not accepting any charging energy.
This is essential to protect the site from potential outages if there is a large shift in energy load on the site while the solar is operating (e.g. if a large on site load is turned off in this circumstance).
While it may seem counter-intuitive to turn the AC PV off when the site is running off-grid, the reality is that dynamic AC production adjustment is not instant, and a Redflow battery site requires the AC PV to be disabled when there is insufficient charge capacity remaining in the Redflow battery array.
Of course, DC MPPT capacity can be added to the system that can remain operational and that can finish off the battery charging process to 100%, if desired.
As soon as enough charge capacity is available in the array, the AC PV can be re-enabled.
Note that this "AC PV Disable" functionality uses frequency-shifting, and as such it only operates when the grid is unavailable. When the grid is connected, the frequency-shifting mechanism is not available.
This means that for a grid-interactive system, you should either place all the AC PV on the 'grid' side (and accept loss of this solar charging capacity during grid failures), or take special measures as described in this article, to automatically 'switch' the AC PV to the load side - and to do that only during grid outages.
Additional ATS recommended for maximum site resiliency
One more ATS is recommended, especially (but not only) in large sites. This is an ATS that bypasses the entire Victron BESS inverter/charger cluster should that cluster fail (or if it is taken offline for maintenance / upgrades).
At the very least, a manual transfer switch should be installed to achieve this outcome, so that the site can always be reinstated using direct grid power in case of operational issues with the BESS system.
ATS Frequency Limit Configuration
The Socomec ATS units have a factory default 'valid frequency' limit of 105% of normal (i.e. 52.5 Hz vs a reference frequency of 50Hz).
The frequency-shifting energy regulation mechanism used by the Victron Energy cluster to regulate solar production has a maximum upper limit of 53Hz.
As a result, when configuring the ATS devices, you need to change the upper limit on acceptable AC frequency, raising it from the default of 105% to a higher value (e.g. 110% or more).
If this is not done, the ATS units will operate in 'unexpected' ways during periods when the Victron Energy system is commanding the Solar production to stop, because they will consider the energy signal to be invalid (frequency too high), and hence they won't be prepared to select the output of the Victron Energy cluster as a valid energy source.
This is a subtle issue, but very simple to resolve by just raising the upper valid frequency limit configured into the units. Note that the frequency limit is configured separately for the 'I' and 'II' inputs of the ATS, and you need to make sure you raise the limit on the correct input (or if in doubt, just raise it on both).
Example System Wiring Diagram using both a Solar Changeover and a 'Resiliency' ATS
The image below shows an example SLD for a site built using these principles and structures. It puts all these elements together and shows you how they interact.
Note the position and wiring of the two ATS units. There is one (the "Solar ATS") to switch the Solar between Grid and Load sites automatically, and the other unit (the "BESS ATS" is the one that will automatically bypass the Victron based BESS system in case of BESS system failure or maintenance.
There are three energy meters required in total (in this case, Carlo Gavazzi EM24 CT based meters).
- One for the grid input measurement ("Site CT Grid In")
- One for grid-side-solar-active energy measurement ("Site CT Solar 1")
- One for load-side-solar-active energy measurement ("Site CT Solar 2")
Here are the actual ATS units in MSB of the site concerned:
The image below shows what the Victron GX status display looks like when the system is running 'On Grid'. Note that the grid-side PV measurement is active and the load-side energy measurement is showing zero (as expected).
If the grid fails, the Solar ATS switches the solar to the load side of the system - with the grid side PV measurement then becoming zero and the load-side PV meter accounting for the off-grid-mode Solar production once the solar restarts in off-grid mode.
At the time this sample was taken, the battery array was already mostly charged and the charge rate was rolling off as batteries successively fill up, with plenty of PV energy being exported.