Yes it’s possible to perfom the AC coupling (adding an ongrid inverter in the AC load or AC flex of the
inverter).
AC coupling is the principle of using separated battery inverters and PV inverters in the same system. The different elements are connected via the AC lines, hence the name AC coupling.
In islanded mode, the battery inverter creates the voltage/frequency of the local grid and the solar grid inverters synchronize and connect to that local grid as if they were on the normal grid. In this kind of system, the solar production is directly consumed by the AC loads. If the solar production is bigger than the loads, the excess of power goes back to the next3 and recharges the battery. When the battery is full, the grid inverter must reduce/stop its production. The frequency shift control uses the properties of grid inverters that must reduce the power production when the frequency of the grid increases in order to participate to the grid stability. The frequency between 50 and 50.2Hz is used for primary control of the grid. The solar inverter must reduce linearly their power production between 50.2 and 51.5Hz.
With the next inverter-chargers, the solar inverters are connected on the load side, either on AC flex connection or AC loads connection. A dedicated AC flex connection is the best option because it is measured separately. That way the grid inverter has a monitoring of its full production independently of the loads and it is easier to understand what happens in the system.
The AC coupling can be useful in various situations.
- To add solar on an existing next system directly in AC.
- To update an existing solar system with more solar and batteries with the use of the next inverters. The grid inverter is left in place and some additional solar is connected to the new next inverter. It is a premium choice for the case where microinverter or optimizers are used because it would be complicate to rewired the solar roof.
- It is also interesting for large systems, as it is not always wanted to have the full power with the backup inverter. Per example a 50kW solar system with 30kW of backup capacity would have 2 next3 in parallel and a grid inverter with up to 30kW of solar.
Studer Innotec advises the following design rules for AC-coupling systems:
- The solar power in AC coupling should be smaller than the battery inverter power.
- The grid inverter should have the modern function of power production reduction in function of the frequency of the grid to work in island mode. Else use to the simplified AC-coupling configuration.
- The battery capacity should be sufficient to absorb all the AC coupled power.
- At least C/5 power with lead acid battery
- At least C/2 power with lithium battery
- For robustness of the system use a mix of AC coupling and DC coupling.
- This allows for black start of the units after an undervoltage of the battery.
- With AC coupling, if the battery inverter is stopped, everything is stopped.
- In case of use of a genset on AC source instead of the grid, it is not allowed to have backfeeding to the genset. It may be causing troubles to the generator.
- In that case the designer must take countermeasures to avoid this. The simplest way is to disconnect the solar inverter when the genset is connected.
- The standard way to make AC coupling is to use of the AC flex connection with the proper settings. That way all the solar can be monitored automatically. This is important to understand later what happens in the system for the end user of the system.
To understand:
- The next cannot prevent backfeeding to the grid from the grid inverter. The power coming from the AC coupled solar inverter will directly supply the AC loads, then the excess is used to recharge the battery until it is full but then will go straight to the grid. The next cannot change the frequency when it is connected to an external source.
- The energy management strategies of the next are identical with AC coupling or DC coupling.
- It is not a standard situation to have power coming from the AC load or AC flex and this must be explicitly allowed with a setting, else a “back feeding” error will be raised.
Setup
The frequency control is available from software version 1.2.18.0. The main programming is done at the commissioning of the installation during the wizard. This is the quickest and safest way to program the system. The answer YES to the AC coupling question will set the proper values to all the individual settings. The question is asked during the AC configuration of the next inverter-chargers.