Multi-units system

1. Introduction

One next3 inverter is enough to supply up to 16kVA for 30 minutes to a three phased load (3x 5.3kVA). 

To power larger systems, there are two possibilities: 

  • Split the loads and power them with multiple independent next3 systems. 
  • Connect multiple next3 in parallel that will work together as a single larger inverter with synchronization. 

This second case is described in this application note. 

Two units of next3 can work in parallel creating a 32kVA inverter with a 80A transfer (55kVA). This is available from software version and higher. 

2. General system design

Up to two next3 units may be used on the same battery bank or on 2 battery banks to create a high- power inverter-charger system. 

The following points must be followed for the wiring: 

  • In multi-unit systems, next3 are interconnected via a communication with “Studer nx communication bus”. 
  • Units should be close to each other (in the same room). 
  • Each device must have the same software version, available on the update section.  
  • The nx-interface is mandatory for configuration. 
  • Use only one nx-interface per system. The system will not work properly with multiple nx-interface communicating at the same time on the bus. 
  • A reconfiguration of the system with the wizard is mandatory when you add new elements to the system. 
  • All the AC-loads connections of each next3 units of a system must be connected in parallel, for each respective phase (through a distribution panel for example). 
  • All the AC-Flex as loads must be kept separated. The different AC-Flex can be programmed separately. 
  • Only one AC-source connection will be used in a system. There is an identification process during the configuration of the system with the nx-interface (wizard). That means the maximum transfer is 80A and 80x230x3= 55.2 kVA. 
  • There is only one configuration possible in a system not a specific by device, except for: 
    • The PV inputs are independent.
    • The AC-FLEXs as load can be programmed independently, they must not be wired together.
    • The AUX relays and CMD IN are independent. 
  • The compatibility is only between next3 units. Don’t mix with other Studer products like the Xtender. This will not work and probably damages the devices. 

3. Communication between the units and identification of the units

The Studer communication bus is mandatory between the units and the nx interface.
Take care of the bus termination that is O (Open) in the middle of the chain and T (Terminated) at the end of the chain. This must be set properly before powering the system with the battery. It will be checked later during the wizard. 

When starting the system, the list of devices is done. Check the presence of all of them during the wizard process. 

During the wizard and later in the system menu it is possible to make all the LEDs blink for a clear identification of the unit (which one is device 1 and which one is device 2, ...). 

4. Battery management

In a multiunit system, one battery with BMS is communicating with one next3, they must be physically connected with the CAN-BMS cable, and it must be paired properly during the wizard process. 

It is allowed to have one common battery or multiple batteries.
With one common battery, the can-BMS is linked to one next3 only. It must be identified and configured during the wizard process. 

Don’t mix the communication bus of the batteries with the nx-bus. The communication with am BMS must be done with the battery wired to the same next. 

The communication bus to the battery BMS, the battery DC connection and the nx-bus are isolated from each others. There is no special requirement for the connections of multiple batteries. Per example, the minus (or plus) can be common, but this is not required. Each pole can be grounded or left floating. The installer is not constrained by the next3 but must respect the installation rules for his type of application. 

With lead acid batteries, each next must have its own temperature sensor. 

ATTENTION: In multi-unit systems, each next3 is connected to a common battery bank using its own protection device. All other DC consumers or sources must be connected directly to the battery using their own protection device. 

FOR YOUR INFORMATION: In multi-unit systems, the charge is synchronized, the charge current of each unit is automatically chosen by the PFD (Power Flow Dispatcher, which is a patented control algorithm). Just give the properties of each battery during the wizard process 

The possible number of batteries are automatically proposed up to the number of batteries.

Here the proper identification of which battery is on which inverter is done. It is especially important if you have batteries with different capacities. 

Then describe the batteries normally. 

4.1 Energy management of batteries

In multi-unit systems, the charge/discharge current of each unit is automatically chosen by the PFD (Power Flow Dispatcher, which is a patented control algorithm). Just give the properties of each battery during the wizard process. 

The use rules by PDF are: 

  • The charge/discharge is distributed proportionally to each battery capacity
  • The charege/discharge limits are used independently
  • Each SOC is managed independently (if they are not at the same level at start for example). 

Example: two next3 are in parallel and only one has solar connected to its MPPT entries. In that case the two batteries are charged anyway, proportionally to their battery size. The energy flows through the AC-Loads common connection. 

The batteries will be managed in function of their nominal capacity given during the wizard.
Per example with two batteries, one of 200Ah and one of 400Ah, the large battery will give 2 time more power than the small one. 

The configuration of each battery can be changed individually by accessing it from the dedicated screen of each battery.
It is possible to have different strategies for different types of batteries mixed. This is an open possibility, but it may become difficult to understand what happens in the system and predict how the system will age.

5. AC-source setup: grid and genset

There is only one grid and one genset connection in a system. The different AC-source of each next3 must be kept separated.
Below is the example for the standard grid-connected case. 

During the wizard, the proper unit that is physically connected to the grid must be selected.

When using a genset, it is advised to use the AC-source of the second inverter. One is connected to the grid and one to the genset.
That way all AC-Flex are available for flexible loads.

The wizard automatically proposes the free ports: 

This case with the grid and a genset in the system is presented below: 

6. Specific settings for mutli-unit systems 

With the nx-interface or other interfaces available for programming, it is possible to access thesettings and information of individual element of a multi-unit system. 

The number of units can be seen on the synoptic. 

In the example below there are 2 next3 units, connected to 1 battery. There are in total 4 strings of solar (2x2). There is only the grid and then the 2 AC-Flex connections are configured as Flex-Loads.

Then the access to individual elements can be done with the swipe. Per example below when there are two batteries:

In that case if the settings menu is entered with « Settings », the modifications will apply only to the battery 1. 


7. Extension of an existing installation

It is possible to extend an existing installation by adding one or several similar next3 units in parallel. The software compatibility of the new and old units is mandatory. 

ATTENTION: Equipment belonging to the same system must be operating with the same software version. Download the latest software version from the Studer's website and update the software for all units of the system independently before commissioning. 

For the extension of a battery pack, follow the recommendation of the manufacturer. Very often it is not allowed to put old and new batteries in parallel. 

8. Conclusion

It is possible to have many next3 in parallel. From version up to 2 units are possible, opening possibilities of systems up to 30kVA nominal and 32kVA 30minutes. 

The installer should take care of wiring, especially wire sizing that should cope with large currents. There is only one critical point to respect: the AC-port connection should always be in common. 

Then the paralleling is quite simple from the point of view of the commissioning with the wizard. 

  • Be sure that all elements connected and are seen in the same system on the communication bus 
  • Identify correctly which next3 is on which battery when there are multiple batteries.