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Off-grid inverters are intended for islanded operation, without connection to the electrical grid, and are suitable for use in photovoltaic systems installed in locations not served by the electrical distribution network or as a backup system in locations that have an electrical grid.
The grid-interactive off-grid inverter has an internal monitoring system and load controller, eliminating the need for an external load controller, as occurs in classic off-grid systems.
As a result, the designer and installer only need to worry about the sizing of the photovoltaic modules and the battery bank, without worrying about the sizing and choice of the charge controller.
All interactive off-grid inverters have an AC (alternating current) input for connection to the electrical grid or connection to a diesel generator. In other words, the inverters are off-grid but have an input for AC power.
If available, the AC power supply can be used as a support to charge the battery bank or supply the loads, performing a bypass. That is, the inverter is bypassed by an internal switch and the AC input can directly power the loads when grid voltage or a generator is present.
The mains input is separated from the output for the loads in different connectors, and the connection between the AC input and the output is carried out internally, depending on the inverter's operating mode. Interaction with the grid through the AC input allows the off-grid inverter to be used as a backup in locations that are already served by the electricity grid.
The grid-interactive off-grid inverter differs from the hybrid solar inverter (which also has a battery connection) as it does not inject excess energy into the electrical grid. In other words, the interactive off-grid inverter does not have the capacity to inject the energy that was stored in the batteries back into the electrical grid.
Some models of interactive off-grid inverters make it possible to perform parallelism, that is, to connect more than one inverter together in the system. With this, we can install single-phase, two-phase and three-phase systems. This is possible when the inverter has the parallelism feature installed, which is an optional accessory on some equipment.
With the parallelism board installed, the inverters communicate with each other and ensure they operate together and in harmony. In the manufacturer's models Growatt, for example, it is possible to connect up to six inverters in parallel.
The inverters have the ability to work with lead-acid batteries and lithium batteries. In the case of equipment from the manufacturer Growatt, which we used to exemplify this article, the BMS (battery management system) communication protocol for lithium batteries is compatible with batteries from the Dyness and Pylontech brands.
Examples of interactive off-grid inverters
Below are some of Growatt's main off-grid inverter models found in Brazil. The nomenclature of inverters refers to their technical characteristics:
- TL: High Frequency/Transformerless;
- T: 60Hz/Transformer;
- LV: Low voltage output is 120v;
- HV: High voltage output is 230V;
- DV: The output with two types of voltage is 120V or 240V;
- M: MPPT controller;
- 48: Battery voltage is 48V;
- Q: Parallel model;
- ES: battery-free functionality.
For example:
- SPF 5000TL HVM-P（5000W, high frequency, 230V output, MPPT, model that allows parallel connection);
- SPF 3000TL LVM-48P（3000W, high frequency, output 120V, MPPT, battery voltage is 48V，parallel model.
Growatt SPF 3000TL LVM-48P
Main features: pure sine wave inverter, battery voltage up to 48 VDC, 3 kW power, paralleling capability, 120 V AC output, internal charge controller with MPPT, operates on lithium batteries, compatible with external generator and has remote monitoring.
With the SPF 3000TL LVM 48P model it is possible to use two inverters in parallel and form a two-phase system (split-phase).
Growatt SPF 3000-5000TL HVM-48P
Main features: pure sine wave inverter, battery voltage up to 48 V, power from 3 kW to 5 kW, ability to parallel with 5 kW model, 230 V AC output, charge controller with internal MPPT, capacity to operate with lithium batteries, compatible with an external generator and has remote monitoring.
This model is ideal for residential applications because of its compact format and complete functionalities. The appearance of the 3000-5000TL HVM-48P model is very similar to the SPF 3000TL LVM-48P model, shown in Figure 4.
Growatt SPF 5000ES
Main features: pure sine wave inverter, battery voltage up to 48 V, power 5 kW, ability to parallelize, 230 V AC output, charge controller with internal MPPT, ability to operate on lithium batteries, compatible with external generator, has remote monitoring, can work with or without a battery, allows you to adjust the charging time and has a battery charge equalization function.
The SPF 5000ES model is one of Growatt's latest off-grid inverter launches. With it, it is possible to create systems of up to 30 kW, single-phase or three-phase.
Growatt SPF 4000-12000T HVM 48
Main features: pure sine wave inverter, battery voltage up to 48 V, power from 4 kW to 12 kW, 230 V AC output, charge controller with internal MPPT, ability to operate on lithium batteries, compatible with external generator , has remote monitoring, has a transformer that allows motors to start.
This family of inverters has a transformer in its construction, which allows the starting of inductive loads (such as motors), as shown in the table below:
Just like inverters grid tie, monitoring the off-grid system is an important feature, as it allows you to monitor the system status, detect faults and generate reports. Manufacturers typically offer dedicated monitoring platforms for their equipment.
In inverters from the manufacturer Growatt, for example, it is possible to carry out remote monitoring via computer (Shine Server) or through an application (Shine Phone). To do this, you need to connect the data logger Shine WIFI-F and configure it to send and receive data via the customer's router.
With monitoring, it is possible to check solar generation, battery use, the consumption curve and the amount of energy that needed to be used from the electricity grid, if available.
Conclusion
The interactive off-grid inverter differs from the traditional off-grid inverter as it has an AC input that allows the batteries to be charged and the loads to be directly supplied, functioning as a backup system. The inverter can also be used in completely off-grid systems, which use only photovoltaic modules and batteries.
It can also, alternatively, have its AC input powered by an external diesel generator (a useful feature when the battery charge is insufficient). A great advantage of the interactive off-grid inverters shown in this article is the presence of a charge controller with internal MPPT, which eliminates the need for external controllers (as occurs in traditional off-grid systems).
Manufacturers such as Growatt offer off-grid inverter models with different technical characteristics, ensuring great flexibility in projects and solutions for customers. In summary, these are some characteristics that can be observed when choosing an off-grid inverter:
- Presence of AC input (grid-interactive inverter);
- Possibility of parallelism for the formation of higher power systems;
- Possibility of operation in single-phase and three-phase arrangements;
- Availability of monitoring system;
- Peak power capacity for starting motors and inductive loads.
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