Is there anybody here with experience of a microgrid island what consists of several cooperative Multiplus ESS? Preferably all energy available in the microgrid will be produced by MPPT chargers without additional grid inverters or rotating alternators.
Obviously, exactly one of the ESS needs to be the master what provides the grid clock frequency by connecting its OUT1 to the microgrid line. Other ESS are slaves what connect its ESS input to the microgrid line.
Supervisors with access to all ESS could control and equalize the SOC of each battery in cooperative manner by adjusting the slave grid setpoints. Obviously this operator should regard something similiar like the 1:1 rule for the power between the master ESS and sum of all active slave ESS.
Is such operation feasable and somebody experienced with?
Yes this concept would work, and I’ve looked at this from the theoretical standpoint. Major concern is that the grid forming unit must be significantly more powerful than the satellite units, and must be able to cope with the maximum back flow of power… This type of system is most easily controlled through droop frequency, though this is difficult to arrange with the multi’s.
It is not the intention of ESS to feed power to the grid. Unlike normal inverters there is no need to use any P(f) characteristic curve to adjust the power by frequency.
Assume master ESS with high SOC and slave ESS runs out of battery. Automatically, the slave ESS tries to satisfy its loads by using the input from grid. As master ESS output is connected to this grid, the slaves loads become additional loads for the master ESS. This happens automatically without supervisiors actions of changing any parameters.
If master ESS runs down to low SOC with the risc of microgrid shutdown, it needs supervisor action. Supervisor selects a slave ESS with high SOC and changes its grid setpoint to feed in power.
Assume master ESS has 1kW loads and any slave ESS is instructed by grid setpoint to feed in 1kW, the loads seen by master ESS go automatically down to about zero. No frequency changes required. Only voltages and grid resistance according to the law of Ohm.
At this point I am unsure what happens if the slave ESS feeds in 2kW what is the double of the master ESS 1kW loads. As master ESS is only connected by its output, it is not possible to instruct the master ESS to consume the 1kW additional power by charging its battery (?) Possibly the Master Multiplus will consume the power automatically for charging as it tries to maintain the correct voltage level at ESS out? Possibly the voltage will rise above upper limits and system shuts down as Multiplus cannot go to charger mode without connected input (?) The situation is similar to external AC coupled grid inverter at the ESS output when AC coupled PV inverter are limited with increasing frequency. The question remains: Is frequency upshift only in case of full battery or is this always?
If always, we could add external charger from master ESS output to master ESS battery. Normally such wiring does not make any sense but it increases the master ESS loads what then can be feed by a slave ESS in same height.
Another possibility would be to switch the master role with another slave dynamically. This could be done by a early make - late brake contactor what swaps ESS input and output to the grid line. Do not know if this is practically feasable as this is a short circuit between ESS input and output for a short moment. The master ESS needs to feed its own input. If internal contactor closes for recognized sync, external contactor opens and ESS becomes slave.
For switching a slave to master role, external contactor needs to bypass the ESS input and output. When done, the input can be interrupted by additional contactor und ESS becomes master ESS. Obviosly, we must not have 2 masters as they run out of snyc sooner or later.
Maybe the Master ESS must not run in ESS mode at all to do what it should do in both directions?
I’ve not tried this but you’ve inspired me to set this up in our shop and see how things behave. I won’t have time to set up a test system like this in the near term, however.
Will the “master” system have a utility connection? It wasn’t clear to me in your post if the island was off-grid or not. If the “master” has access to the utility, do you want to feed back to the utility or not?
Assuming no grid connection or at least no feedback, then I would not use ESS on the “master”. Like @MikeD mentioned, you need to make the “master” large enough to handle all possible aggregate feed-in from the “slave” ESS systems. You could reduce the size of the “master” by limiting the feed-in from the slaves to the master in their ESS configs.
The master would need the PV Inverter assistant installed to accept backfeeding into its AC-Out.
What I’m not clear about is how the slave ESS units will respond to the frequency shifting of the master to slow down their backfeeding into the master. The grid code and grid settings will likely dictate that.
I’m sorry that I don’t have firm answers or experience with this. Someday I’m going to try it out in the shop
Yes, the system has public grid access. I run 2 independend ESS and have a free connection between the houses / ESS. Thats why I started thinking about the direction of using this cable. Of coarse, best is always automatic bidirectional use.
Grid might fail any time. Then both ESS go on working like intended. Unfortunately, load power and battery SOC might be in a unexpected state at grid failure moment. Thats why I like to have the 2 ESS in cooperative behaviour sharing batteries and loads inside a true microgrid. Its exactly the situation to run a island with several houses without public grid.
If this is working with master and one slave, it will work with any number of n slaves as long as the unbalanced power does not exceed the masters capacity. Therefore any operator with supervisor access needs to take care. It seems a feasable and lazy job, as battery SOC are changing only slowly.
The only unknown question seems for the moment: Is the master ESS able to consume power from its OUT1 to charge its battery. I am going to open another thread for this question to avoid specialists have to read too much stuff.
In the system I looked at, the entire microgrid did not have grid access. There are generators at the central inverter storage.
The concept was to have the satellite system mirror the soc of the central system, either by taking power from the microgrid, or feeding it back. In this fashion no satellite station would run too low on soc. The central system would be capable of running in parallel with at least on generator to support the entire load. As the central inverter was droop controlled using a p(f) function to control the satellites was feasible. The system also had distributed solar charging. This was not a small system, but 500kW central inverter with > 1MWh of storage.
Re your question, yes, Multiplus 2 and Quatro can take power in on AC out in order to charge the battery. That’s how they work with grid connected PV inverters on AC out.
Seems to work in both directions. Your answer is identical to the answer in above linked thread. Hopefully it will also work without rotating generator and without frequency shifts. At least frequency shift should only occur for master battery near 100% SOC if the operator missed some action or energy is too much in total for every ESS.
I am going to test this with my two ESS by connecting them with a 45kVA switchboard cabinet. That cabinet was made in Corona time from used material for similar purpose but then never used. It will take a while until I can play arround with the system but hopefully its doing what I expect. At least I your confirmations gave me the confidence to start with a schematic diagramm therefore.
Here is my first schematic to try. Everything 3 phase, drawn without control lines. On the left top there are 2 connections to outside: A public grid and a private microgrid. The public grid provides 2 electricity meters. One eHZ for the feed in and one Ferrais for the consumption. Both are supervised by additional current instruments T1 and T2. For all of the 3 sources there are contactors to couple them to the system what are eHZKoppel, FerrKoppel and GridKoppel.
At the right hand we have all the loads No. 18 supervised by current instrument T3. In the middle is the Multiplus ESS with ABB meter and of coarse many MPPT DC chargers not shown here in detail. ESS input and Out1 are coupled to a own busbar by the contactors ESS_In and 18_ESS. This is in fact the ESS output busbar but I named Out Busbar 18 as all loads No 18 are connected here in case of public supply without using the ESS Out1.
There are 4 diffrent use cases and its active contactors described in the table on the bottom.
Full Feed In to public grid. The ESS output is not in use and not coupled to the Out busbar. Loads No 18 are supplied by the public Ferrais meter.
Surplus Feed in. ESS supplies the loads No. 18. Additional power comes or goes to eHZ meter.
Use case 3 and 4 assume public grid failure. ESS now supplies its own loads no 18 from Out1. In Slave mode, ESS Input is connected to the microgrid. In master mode, ESS input remains unconnected and ESS Out1 supplies the microgrid using the ESS_bypass contactor to the input busbar.
Hi today I got my small microgrid working with ESS mode 3.
so the concept works, BUT you must be aware of what the grid codes programmed into the inverters allow you to do. Some grid codes don’t let you feed back power from DC sources - either PV or battery. So if you have a connection to the Public grid, then you need both utility approval, and to abide by their rules - which may negate the operation of the system. Far better to use a back up generator than the public grid.
IF you are strictly on a microgrid with no connection to the public network, then you can set you own rules. What makes this concept work is the control system’s transfer function.
This also depends on what the situation is with the peripheral ESS systems: whether these have local loads and or local PV charging - either AC or DC connected.
The key to how a transfer system works is the type of the central inverter: whether this is isochronous (fixed frequency) or droop mode (frequency is function of load) in operation.
in an isochronos system, the peripheral ESS systems must have reliable communication to the central control system, and receive their power setpoints from that central system. In a droop controlled system then a local P(f) transfer function can be used.
System stability and Loss of Mains:
To make this work, the central inverter of the microgrid must be of a higher power rating than the sum of the peripheral ESS units. Each of the peripheral ESS units must have loss of mains detection for safety.
Public grid is available but assumed to be diconnected for microgrid operation. Communication of several peripheral slave ESS with a central master ESS seems difficult why P(f) frequency droop control seems to be the only possibility to avoid crashs automatically.
Same difficulty is to respect the central master systems size if more than one peripheral slave is connected. Therefore it is not the power rating sum of peripheral slave ESS but the sum of their actual exported (or consumed) power.
To keep this in balance, any human operator needs to take care that battery SOCs of any connected system cannot run to its upper or lower limits. Equivalent to what the operators of transmission networks do with dispatch of power plants to keep production and consumption always in balance.
