Larger systems need care in design and commissioning
In larger sites (Single phase systems with more than one inverter/charger in parallel, or three-phase systems with 6 or more units in total) - special rules apply to the design and wiring of such sites.
These special rules are documented here:
...and those rules are really important to follow.
In part, the rules specify that the AC inputs and AC outputs on each parallel-wired unit should be hardwired together - with no circuit breakers between those paralleled units on each phase.
If a site of this size is commissioned where the Redflow batteries start out totally empty (and this is the way they are usually shipped), there is a risk of imposing high stress (potentially capable of damaging the AC transfer relay) depending on how the clustered system is first powered up (and before the cluster has been configured).
Considerations with Redflow batteries when large sites are initially powered up
A potential issue exists in large sites, if the DC bus starts out completely de-energised (all Redflow batteries completely empty) at the point where AC energy is first introduced to the inverter/chargers (and before the Victron Energy devices have been firmware-updated and cluster-configured).
In this scenario (no DC bus energy present when AC power is first applied), there is a significant current inrush through the AC input (and through the AC input transfer relay), as the capacitors inside the inverter/charger unit charge up from the AC source.
This current inrush does not occur if the inverter is supplied with DC power before the AC power is connected. For conventional batteries, that is what normally happens - batteries arrive on site with some energy already in them... and the DC power is introduced to the cluster before the AC source is turned on.
However: Redflow batteries usually arrive on site in a totally empty and 'off' state.
Note: This is routine situation for Redflow ZBM2 batteries - to arrive on site 'empty and off'. They have no particular shelf-life limitations when 'empty and off'. No damage or stress accrues to an empty Redflow battery at all, and there is no risk of external electrical shock on arrival on site, because there is no electrical energy in the battery whatsoever!
On a site with a single Victron inverter/charger, this AC current inrush not been observed to create any practical issues other than occasionally causing input circuit breakers to trip at the very first power-up event.
However, in larger setups (multiple units per phase), and where where one unit gets turned on first (and this is a routine step - including for initial firmware upgrades), that unit can suffer a greatly magnified level of current inrush.
Because that first unit is wired (via the AC output side) to all other units on that same phase, the capacitors in all parallel-wired units get charged at once through that single AC input path - and through that single AC transfer relay.
The resulting current inrush - now multiplied by the number of inverter/chargers present in that phase - is sufficiently high that damage can potentially occur to the AC transfer relay in the unit concerned.
This damage presents as an 'Error 24' on subsequent unit startups. Error 24 in this context means the transfer relay testing has failed - usually in this scenario it is now permanently stuck 'on'. The unit concerned then requires repair or replacement to resolve the damage to that AC transfer relay.
The Solution: Charge The Batteries First
In all sites with parallel-wired Victron inverter/chargers, steps should be taken to charge one or more of the Redflow batteries on the DC bus, and that DC energy should then be used to supply power to the entire Victron inverter/charger cluster before AC power is introduced to the site.
Once the units are fully powered up (and inverting to their AC outputs), AC input power can then also be applied if desired (or - even better - initial cluster upgrade and configuration can be completed on DC power alone, if sufficient DC power is present to do so).
Initial on-site charging of the Redflow batteries
This initial on-site battery charging for a large site can be done in multiple ways, including:
- Using a small AC power supply of suitable capability (for instance, a MeanWell RSP 750-48, which can be adjusted to supply 55V or more - enough to charge one Redflow battery at a low but effective rate).
- Wiring just one of the Victron Energy inverter/chargers to the battery (with the AC output wiring not connected to any other unit) - in effect using it as a simple battery charger. Once the battery system is charged and running, the connection of the parallel wired AC output paths in the cluster can be completed.
- Using another DC energy source (e.g. DC MPPT units, if present in the installation ) to either put initial charge into the batteries and/or to boot and charge the capacitors in the inverter/chargers.
While the DC bus is energised and the batteries are charging, this is a great opportunity to perform the initial configuration of the BMS and of the batteries concerned, as per the guide noted below, while you wait:
Once there is (say) at least 10% charge in the battery system, and once has been interfaced properly with the BMS ... and the BMS CANBus path wired to the Victron GX system ... then battery power can be used to boot the Victron Energy cluster and the configuration of that cluster can proceed.