A large number of PV inverters is available on the market – but the devices are classified on the basis of three important characteristics: power, DC-related design, and circuit topology.
1. Power
The available power output starts at two kilowatts and extends into the megawatt range. Typical outputs are 5 kW for private home rooftop plants, 10 – 20 kW for commercial plants (e.g., factory or barn roofs) and 500 – 800 kW for use in PV power stations.
2. Module wiring.
The DC-related design concerns the wiring of the PV modules to the inverter. In this connection, distinctions are made between string, multistring and central inverters, whereby the term "string" refers to a string of modules connected in series. Multistring inverters have two or more string inputs, each with its own MPP tracker (Maximum Power Point, see below). These make a particularly sensible choice when the PV array consists of differently oriented subareas or is partially shaded. Central inverters only have one MPP tracker despite a relatively higher power output. They are especially well-suited for large-scale plants with a homogeneous generator.
3. Circuit topology
With regard to circuit topology, distinctions are made between one- and three-phase inverters, and between devices with and without transformers. One-phase inverters are usually used in small plants, in large PV plants either a network consisting of several one-phase inverters or three-phase inverters have to be used on account of the unbalanced load of 4.6 kVA. However, transformers serve the purpose of galvanic isolation (required in some countries) and make it possible to ground the PV module (necessary for some types of modules). Whenever possible, however, inverters without transformers are used. They are a little smaller and lighter than transformer devices and operate with a higher efficiency.
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