The SMA Sunny Island is a grid-forming battery inverter that can be used for the construction of stand-alone power supply systems. The Sunny Island inverters are capable of forming an AC grid, this sine wave can be used as an interface for PV inverters to synchronize and feed power into the system which is used for supplying loads and recharging batteries.
Off-grid systems can be designed for supplying the complete load, but in order to do so, maximum load and unfavorable weather conditions are to be considered. When designing under these circumstances, the necessary PV system and battery capacity becomes very large which can result in a significant investment.
For the purpose of keeping the system size smaller and therefore reducing the necessary investment, an external AC power source such as a diesel generator set can be used, so that the system can make use of it during periods in which available power is not sufficient for feeding the loads. The generator is also used to supply power to charge the batteries in the case the battery State of Charge (SOC) drops too low.
Here we will be discussing how a generator can be connected to the Sunny Island, the parameters and the settings available for the generator.
Generator and Wiring Requirements:
The generator used must be able to provide a stable grid; typical nominal values are 230V at 50Hz. For a three phase Sunny Island system, a three phase generator will be required. When wiring the generator connections, please ensure that the installation complies with local codes and regulations.
For more details regarding generator compatibility, please refer to the Sunny Island Generator – White Paper, which can be downloaded here.
Connecting the generator to a Single Phase System
The generator must be connected to the AC2 terminal block. In the wiring diagram below, the L, N, and PE are used. The NTT is not used here in the AC2 terminal for an off-grid system.
Connecting the generator to a Single Phase Single Cluster System
In a single phase operation with multiple Sunny Islands i.e. one, two or three phase setup, the generator connections need to be paralleled to the second and third Sunny Island in order to get power from the generator of approximately 11.5kW per Sunny Island. In single—phase single-cluster systems, the Sunny Island inverters must be of device type SI6.0H-11 or SI8.0H-11.
Connecting the generator to a Three Phase System
For the three phase setup, each phase is connected directly to the corresponding Sunny Island, where phase 1 is connected to the master unit, phase 2 is connected to slave 1 and phase 3 is connected to slave 2.
Generator Auto Start Function
The Sunny Island can request the generator to automatically use the built-in relay. To use this function, the generator must support the automatic startup functions. The two signal wires from the generator are connected directly to the built-in relay of the Sunny Island. An example of this wiring is given below.
Setup during commissioning
During the Quick Configuration process done through the Sunny Remote Control (SRC-20), the external source must be set to “Gen”.
The system at this stage should be operational without any further settings and will work based on the default parameters (generator nominal current greater than 16A or approximately 4KVA **). However, some useful settings can be used to fine tune the system.
Settings and Parameters for Installers and Expert Users:
Although the default settings should be enough to run the system, fine tuning the generator parameters will ensure that the system utilizes the generator more efficiently when required. The Sunny Island may need to be in standby mode while this process is being administered and Expert access will be required to access certain parameters. The following settings below can be used to fine tune the generator parameters:
- Relay Settings
The relay 1 and 2 of the Sunny Island can be used for the generator request. Generally, relay 1 is used for the generator control, and the second relay can be used for the other features and controls such as load shedding functions etc.
By default the relay 1 in set to “AutoGn”.
- #234 Gen Control
The settings of the generator can be found in ‘#234 Gen Control.’ The voltage and frequency operating range can be set under these settings. Current drawn from the generator is another major factor, by default the max generator current is set to 16A per Sunny Island. The value can be changed based on the generator rating up to 50A per Sunny Island inverter.
**If the generator’s nominal output current is less than 16A, this parameter must be changed to match the generator’s nominal current to prevent overloading the generator.
Generator warm up time can also be changed. By default it is set at 60s. Please check the generator’s manufacturer recommendations to adjust this timer if required.
One other important parameter is the generator reverse power. The generator manufacturer will provide the maximum reverse current that the generator can handle and the time it is able to withstand the reverse power. By default it is set to 100Watts for 30 sec.
- #235 Gen Start
The setting under this menu determines the conditions to start/stop the generator by the Sunny Island.
The most common settings will be the SOC (State Of Charge) values for generator request. By default the lower SOC is 40% and the higher SOC is 80%. What it means is that the Sunny Island will call the generator by activating the relay once the SOC drops to 40%, and will start charging the batteries from the generator power. The cycle completes by stopping the generator once the SOC reaches 80%.
There is also an option for a time dependent generator request with different SOC values for a 24 hour period in this menu.
Another option is the load dependent generator request. This setting can be used to call the generator when the load consumption exceeds the power rating of the Sunny Island. Setting this feature will ensure that the system will be able to function normally even if there is some unexpected load in the system especially at night time.
Once all the settings and parameters are set, under menu #540, the option can be used to turn on the generator manually by setting the parameter to “Start”. Once the relay activates, the generator will start, and once it is able to form a stable grid, the Sunny Island will then be able to sync to it and start drawing power from the generator.
After the test is completed, the parameter must be set to “Auto” for the automatic generator request feature.
In situations when the PV output power is not sufficient to support the loads or there is a low SOC on the batteries, connecting the generator to the Sunny Island will ensure that the extra power is available to fulfill the consumption/demand. Fine tuning the generator will make the off-grid system more robust and will ensure a smooth transition to the generator grid when extra power is required.
Anwar Mahmood (guest author)
Anwar is a Service Engineer at SMA Australia.