Solar inverter refers to a special type of inverter that converts the DC voltage of the solar cells into alternating voltage and is optimized for operation with solar modules. This inverter is part of a photovoltaic system and has the task of feeding the electrical energy generated into the power grid.
On the input side to the solar cells, there is usually a DC converter with a maximum power point tracker, which is controlled by a microcontroller and which feeds the DC link. On the output side there is a single- to three-phase inverter, which is supplied from the DC link and feeds into the low-voltage grid or the medium-voltage grid. The inverter is designed to automatically synchronize with the power grid. A manual synchronization process, e.g. using a synchronoscope, is not necessary.
A grid monitoring device with associated switching devices (ENS) is required on the grid side, which switches off the inverter in the event of unwanted island formation. For systems with installed capacities of more than 30 kW, the ENS can be dispensed with. In this case, frequency and voltage monitoring with all-pole shutdown is sufficient for safe disconnection from the grid in the event that it is switched off or fails.
It is often advertised that the inverters are highly efficient. In the partial load range, it is somewhat lower and is therefore averaged. However, the efficiency of the inverter alone does not determine the overall efficiency of a photovoltaic system.
Smaller installations in the low-voltage grid will also have to offer comparable control functions. Country-specific more stringent regulations lead to supply bottlenecks and higher production costs. Counter-concepts such as net metering take a more straightforward approach and shift the problem to the grid operator. In the case of larger installations, where the Medium Voltage Directive must be complied with, among other things, further measures for dynamic grid stabilization, such as the capability for low-voltage ride-through, are prescribed. The measures serve to avoid an unintentional and simultaneous shutdown of many systems in the event of short-term local undervoltage, as occurs in the context of short circuits or other faults in three-phase systems.
A single-phase systems are only allowed to feed into the power grid up to a maximum output of 5 kW (4.6 kW continuous output). This limitation serves to ensure grid stability and avoids unbalanced loads. In addition to the basic function of energy conversion, a solar inverter has extensive data acquisition and, in some cases, remote maintenance options.
The electrical energy in the power supply grid cannot be stored in large quantities for a short time. It is therefore always necessary to establish an energy balance between production and consumption. To ensure this, the mains frequency is used as a control variable in power grids operated with alternating voltage. In Europe, this is defined as 50.0 Hz. Deviations from this setpoint indicate an excess of energy (increased grid frequency) or a lack of energy (reduced grid frequency). In order to avoid an oversupply of power in the power grid, inverters must therefore continuously monitor the grid frequency and disconnect from the grid if a country-dependent limit value (50.2 Hz in Germany) is exceeded. Since a significant part of the electrical energy generated in Germany now comes from photovoltaic systems, a hard shutdown of all systems at this limit value would trigger the opposite effect and thus in turn cause grid instability. Therefore, in the case of installed systems over 10 kW, this limit was subsequently increased by a random value. Newer systems must have a power gradient between 50.2 and 51.5 Hz, which, depending on the current grid frequency, reduces or increases the feed-in power and thus actively contributes to grid stabilization.