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The inverter is the heart of every PV plant; it converts direct current of the PV modules into grid-compliant alternating current and feeds this into the public grid. At the same time, it controls and monitors the entire plant. This way, it ensures on the one hand that the PV modules always operate at their radiation- and temperature-dependent maximum power. On the other, it continually monitors the power grid and is responsible for the adherence to various safety criteria.
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.
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.
Tasks of the PV inverter
The tasks of a PV inverter are as varied as they are demanding:
1. Low-loss conversion
One of the most important characteristics of an inverter is its conversion efficiency. This value indicates what proportion of the energy “inserted” as direct current comes back out in the form of alternating current. Modern devices can operated with an efficiency of around 98 percent.
2. Power optimization
The power characteristics curve of a PV module is strongly dependent on the radiation intensity and the temperature of the module – in other words, on values that continually change over the course of the day. For this reason, the inverter must find and continually observe the optimal operating point on the power characteristics curve, in order to “bring out” maximum power from the PV modules in every situation. The optimal operating point is called the "maximum power point" (MPP), and the search for, and tracking of, this MPP is correspondingly called "MPP tracking." MPP tracking is extremely important for the energy output of a PV plant.
3. Monitoring and securing
On the one hand, the inverter monitors the energy yield of the PV plant and signals any problems. On the other, it also monitors the power grid that it is connected to. Thus, in the event of a problem in the power grid, it must immediately disconnect the plant from the grid for reasons of safety or to help support the grid – depending on the requirements of the local grid operator.
In addition, in most cases the inverter has a device that can safely interrupt the current from the PV modules. Because PV modules are always live when light is shining on them, they cannot be switched off. If the inverter cable is disconnected during operation, this can lead to dangerous light arcs forming, which do not go out on account of the direct current. If the cutout device is integrated directly in the inverter, installation and wiring efforts are reduced considerably.
Communication interfaces on the inverter allow control and monitoring of all parameters, operational data, and yields. Data can be retrieved and parameters can be set for the inverter via a network connection, industrial fieldbus such as RS485, or wireless via SMA Bluetooth®. In most cases, data is retrieved through a data logger, which collects and prepares the data from several inverters and, if desired, transmits them to a free online data portal (e.g. Sunny Portal from SMA).
5. Temperature management
The temperature in the inverter housing also influences conversion efficiency. If it rises too much, the inverter has to reduce its power. Under some circumstances the available module power cannot be fully used.
On the one hand, the installation location affects the temperature – a constantly cool environment is ideal. On the other hand, it directly depends on the inverter operation: even an efficiency of 98 percent means a power loss of two percent –in form of heat. If the plant power is 10 kW, the maximum thermal capacity is still 200 W. Therefore, an efficient and reliable cooling system for the enclosure is very important – such as SMA’s “OptiCool” cooling concept. The optimum thermal layout of the components allows them to dissipate their heat directly to the environment, while the whole encasing acts as a heat sink at the same time. This allows the inverters to work at maximum rated capacity even at ambient temperatures of up to 50° C.
A weather-proof enclosure, ideally built in line with protective rating IP65, allows the inverter to be installed in any desired place outdoors. The advantage: the nearer to the modules the inverter can be installed, the lower the expenditure for the comparatively expensive DC wiring.
Professional planning and plant design takes the conditions at the set-up location into account in terms of module selection and wiring: roof pitch, any shade and, of course, alignment. In Germany, maximum yield is achieved when the modules are aligned to the south at an angle of around 35 degrees.
Next, the selection of a suitable inverter in terms of performance and technology is absolutely essential. The rated capacity of the PV array may be up to ten percent above the rated capacity of the inverter. If an inverter is greatly undersized, this can have a negative effect on plant yield, since the inverter can no longer process part of the module power supplied during periods of high radiation. It is also important that the maximum DC voltage never exceeds the permissible inverter input voltage – otherwise damage to the inverter may be the result.
Basically, almost every PV plant is unique and has to be designed customized for the specific location and requirements involved. For installers to make planning a plant easier, manufacturers, like SMA, provide professional planning tools. The free software Sunny Design allows solar specialists to design a tailor-made grid-tied PV plant for their customers. The program accesses a database containing all the current PV plants and high-resolution weather data, verifies the technical components, works out cable lengths and cross-sections and delivers data for an economic evaluation of the plant. Preparing the way for an optimally designed PV plant.