Solar inverter

Solar inverter means a device that converts the DC voltage from solar modules into alternating current and fed into the power grid. The inverter is thus part of a photovoltaic system. On the input side there is usually a DC-DC converter with maximum power point tracker, the controls a microprocessor and feeds the DC link. On the output side there is a one-to three-phase inverter, which feeds into the low voltage network or large devices using a built- in transformer, the medium-voltage network and automatically synchronized with the power grid.

Device types

Circuit technology and efficiency

Basically, there are two types of solar inverters:

At the DC input of the solar inverter is usually an input transducer. In this converter there often is a boost converter with a very high efficiency. The output circuit must also have a high efficiency, which is over a wide load range.

For optimization of inverters with transformer inverter often takes over the function of the input transducer, so that the intermediate circuit is omitted. This is called a Direkteinspeiser ( cf. cyclo ). The efficiency is improved because only one converter is necessary. However, such devices have a smaller range with optimum efficiency, so that puts particular in systems with partial shading of this advantage quickly.

The efficiency of solar inverters is comparable by the Euro - efficacy which rated particularly partial load cases.

Operation

In some European countries, a so-called device for grid monitoring with assigned switching elements (ENS ) is required on the line side, switches off the inverter in an unintentional islanding. In systems with power ratings above 30 kWp can be dispensed with the ENS. There is sufficient frequency and voltage monitoring with all-pole shutdown for safe disconnection from the network if this is switched off or fails.

It is often advertised with high efficiency of the inverter. In the partial load range, it is slightly smaller and is therefore averaged and then referred to as " European efficiency ". However, the efficiency of the inverter will decide not only on the overall efficiency of a photovoltaic system.

Since January 2009 photovoltaic systems in Germany have come with power from 100 kWp on the ability to be reduced by the network operator in the active power fed ( § 6.1 EEG). Furthermore, there is the possibility that a certain amount of reactive power is provided. In practice, these requirements are realized dynamically via ripple control receiver that can signal a four-stage active power reduction and an active factor other than 1, for example, cos φ = 0.95 pretend ( inductive). Through the supply of reactive power capacitively induced overvoltages can be avoided.

From July 2011, smaller plants must offer comparable control functions in low voltage networks. Local regulations further lead to supply shortages and higher production costs. Opposite concepts such as net metering pursue a more straightforward approach and shift the problem to the network operator.

Single-phase installations may supply into the power grid in Germany only up to a maximum capacity of 5 kWp (4.6 kW continuous output ). This restriction is network stability and avoid unbalanced loads. In addition to the basic function of the energy conversion of a solar inverter has an extensive data collection and partly options for remote maintenance.

In Germany must be programmed later because of the 50.2 -Hz problem installed systems over 10 kW at a randomly chosen slightly higher value in order to guarantee the network stability. Newer systems shall have a power gradient from 50.2 to 51.5 Hz.

Island operation

In systems for stand-alone operation special stand-alone inverters allow the use of conventional consumers for 230V AC. The decisive factor is the maximum power provided. These individual inverters can be connected in parallel, depending on the size of the network but it will need additional control devices for coordination with other power generators and energy storage. Small plants are partially available with integrated battery systems, but have no network synchronization, since their default missing by other power producers.