Solar inverter

The solar inverter – construction and types

A modern photovoltaic solar cell inverter converts DC voltage into AC voltage which can be fed into a power grid. The general structure of a PV inverter consists of: a standard DC plate on the current input side, with a maximum power point tracker connected to the front, and a control unit that measures the points of maximum efficiency of current flow from the solar cell. The power input side is controlled by a CPU. The power inverter feeds into a low or medium voltage network that is synchronised with the output side.

The two inverter types

PV inverters are divided into two categories. These are solar inverters with a transformer unit, and those without. Models with a transformer are generally only used for converting DC to AC voltage and usually have a single earthed pole; which in some countries is mandatory to avoid floating potentials. This type of inverter takes over the role of the input transformer, resulting in a cost reduction and increase in efficiency; although the optimal power range is reduced. The power input and outputs are interconnected in PV inverters without a transformer. The absence of a transformer means that a higher efficiency can be achieved, although alternative earthing is required due to the removal of the electrical isolation. Transformer-less inverters were designed primarily for systems that require particularly high efficiency rates.

The various solar inverter operational types

There are four different types of solar inverters. Firstly, there is the single-phase string inverter, which combines the power input of one or more solar cells in a power grid. Then there is the module inverter type, where each individual solar cell has its own single-phase inverter module. Usually integrated into the power connectors of the solar cells, they are ideal for systems where several different solar subfields are connected, such as vehicles. The other two varieties are the central inverter and the multi-strand inverter. The former are mainly used in large electrical installations such as switching cabinets, and are often modular to simplify maintenance. Central inverters are used exclusively for high voltage systems. The multi-strand inverter type is a one to three phase solar inverter that transfers the power from several solar modules into a power grid.

Operating mode and operation

Solar inverters are mainly, but not exclusively responsible for the performance of a photovoltaic system. Following 2009 regulations medium-high voltage and since 2011 also low-voltage inverters must provide accurate switching and control options, so that the performance of the inverter can be controlled dynamically, or even throttled. Single-phase systems benefit from these settings, since they can feed electricity into the power grid up to a level of 5kW. If the values are around this threshold they can be easily regulated. This helps to guarantee electricity and grid stability, and avoid dangerous current loads. In addition to these functions PV inverters must also be able to offer remote diagnostic capabilities and digitally store comprehensive operating data in order to make efficient maintenance and repairs possible.

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