We have a site set up in a microgrid configuration. 2x10kW Quattro 130kWh lithium storage, 3kW DC coupled PV, 2x 6kW Fronius, 1x3kW Fronius all connected to AC out1. We have just set up a multi plus 5kW connected to 48x1000 AH lead carbon and are using this as backup storage feeding it into the AC out1 of the Quatro’s as well.
The lead batteries were recently installed at this site but was not sufficient storage, so we upgraded to the lithium bank. But with the lead carbons virtually new and already in place it seemed a waste not to utilise them.
The multi plus feeds into the system when the quattro lithium are low. The multi plus then charges again when the lithium’s are near full. This has given us an extra 40kWh of back up storage that will only get drawdown a handful of times a year in bad weather conditions.
This is controlled with nod red through with a cerbo on each of the inverters. The Quattro’s cerbo send the system state of charge and consumer load (from a separate power meter) to the the other cerbo via modbus. The other cerbo then instructs the multi plus power level using ESS mode 3 to discharge to meet the consumer load then to charge of the micro grid with lithium batteries are batteries are near full.
This has been working well and stable with the multi plus feeding in and matching the required consumer load . This results in the quattro’s having close to 0W load while working.
The only problem occurs when the Fronius are providing power to the system. The multi plus will start to somewhat resonate sending power back and forth to the grid. This occurs in both of the states with of the multi plus being set to charge or discharge. It even occurs when the lead batteries are full so the multi plus would usually be sitting idle with only small float power being drawn. The inverter will send a significant amount or power back and forth. Screen shot below from VRM showing unstable resonating when Fronius kicks with full lead batteries , another showing stable discharge another.
I know this is not a supported configuration but am interested to know why this may be happening and if there is a way to resolve? I can see the use for this configuration in future applications. Any insite or help appreciated
Also added a screen shot of the multi plus input voltage and current when the Fronius is providing power to the system.
