Solar inverter

Solar inverter of a house system

Inside view of a solar inverter

Solar inverter is a device that converts the DC voltage from solar modules into AC voltage and feeds it into the power supply network. The inverter is part of a photovoltaic system . On the input side there is usually a DC / DC converter with a maximum power point tracker , which is controlled by a microcontroller and which feeds the intermediate circuit. On the output side there is a one- to three-phase inverter , which feeds into the low- voltage network or the medium-voltage network and automatically synchronizes with the electricity network.

Device types

Module inverter (English micro-inverter)

Each individual solar module has its own single-phase inverter that can be integrated into the junction box.

This is a DC-DC converter, the purpose of which is to set the voltage so that the connected module is operated at its maximum power point (MPP).

This can be useful for photovoltaic systems that consist of differently oriented or differently shaded subfields, for example in cars or airplanes coated with solar modules.

String Inverter

A mostly single-phase inverter that feeds the energy from a string or a few strings of solar modules into a power grid.

Multi-string inverter

Single or three-phase inverter that is equipped with more than one MPP tracker for several strings (also different ones) of solar modules.

Central inverter

A large electrical system, often in the format of a control cabinet, but also as a station in a container design, which is mostly used from peak outputs of over 100 kW. The modular structure simplifies necessary repairs.

Hybrid inverter

Combination of inverter and internal or external storage batteries. This results in the possibility of an uninterruptible power supply and the optimization of self- consumption in feed-in operation.

Circuit technology and efficiency

Block diagram of a solar inverter

Basically, there are two types of solar inverters:

Devices with a transformer

Here a transformer takes over the galvanic separation between the DC and AC sides. Due to the galvanic isolation, the PV generator can be grounded on one pole - there are no alternating voltage potentials in the system. It is also mandatory in some countries.

Transformerless devices

Here the input side and output side are electrically connected to one another. No transformer is used in this circuit structure, so these devices are usually more efficient . The lack of galvanic isolation, however, requires a different electrical safety concept. Sometimes there are alternating voltages of the solar modules against earth, which can lead to losses and, in the case of thin-film modules, to degradation. Circuit technologies called H5 or Heric topology were developed to further increase efficiency and avoid leakage currents .

There is usually an input converter at the DC input of the solar inverter. This converter is often a step-up converter with a very high efficiency. The output circuit must also have a high degree of efficiency that exists over a wide load range.

To optimize inverters with transformers, the inverter often takes on the function of the input converter, so that the intermediate circuit is omitted. This is called a direct feeder or direct converter . The efficiency improves because only one converter is required. However, such devices have a smaller area with optimal efficiency, so that this advantage is quickly put into perspective, especially in systems with partial shade.

Efficiency

When specifying the efficiency of solar inverters, a distinction is made between three common values:

1. Maximum efficiency: the highest value that the inverter can achieve.
2. Euro efficiency: Weighted average efficiency for radiation conditions in Central Europe . For this purpose, the efficiencies at 5, 10, 20, 30, 50 and 100% inverter utilization are added and given a weighting of how an inverter is typically loaded in Central Europe. ${\displaystyle \eta Euro=0,03\times \eta _{5\%}+0,06\times \eta _{10\%}+0,13\times \eta _{20\%}+0,1\times \eta _{30\%}+0,48\times \eta _{50\%}+0,2\times \eta _{100\%}}$
3. CEC efficiency (California Energy Commission): Weighted average efficiency for radiation conditions in California in the USA. For this purpose, the efficiencies at 10, 20, 30, 50, 75 and 100% inverter utilization are added and given a weighting of how an inverter is typically loaded in California. These values ​​are averaged for the MPP nominal voltage and the upper and lower voltages of the MPP control range. ${\displaystyle \eta CEC={\frac {1}{3}}\times \sum _{k=1}^{3}(0,04\times \eta _{10\%}+0,05\times \eta _{20\%}+0,12\times \eta _{30\%}+0,21\times \eta _{50\%}+0,53\times \eta _{75\%}+0,05\times \eta _{100\%})_{U_{K}}}$

In the solar industry, the term kWp is sometimes used instead of the unit kW to denote the peak power of inverters . However, this does not correspond to the rules of the International System of Units , according to which the unit names are not given any additions. See also: Notation of the unit symbols . In the case of solar modules, however , the kWp value does not indicate the maximum output, but is the dimension under standard test conditions (STC: irradiation = 1000 W / m², cell temperature = 25 ° C, spectrum = AM 1.5). In practice, the power is usually lower, but it can also be higher than the kWp values.

business

In some European countries, a so-called network monitoring device with associated switching devices (ENS) is required on the network side , which switches off the inverter in the event of unintentional islanding. The ENS can be dispensed with in systems with an installed output of more than 30 kW. Frequency and voltage monitoring with all-pole disconnection is sufficient for safe disconnection from the network in the event that it is disconnected 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 and then referred to as the “European efficiency”. However, the efficiency of the inverter alone does not determine the overall efficiency of a photovoltaic system.

Since January 2009, photovoltaic systems in Germany with installed outputs of 100 kW or more must have the option of being reduced in the active power fed in by the grid operator (Section 6.1 EEG ). There is also the possibility that a certain amount of reactive power is made available. In practice, these specifications are implemented dynamically via ripple control receivers that can signal a four-stage active power reduction or  specify an active factor other than 1, for example cos  φ = 0.95 (inductive). By providing inductive reactive power, capacitive overvoltages can be avoided.

From July 2011, smaller systems in the low-voltage network must also offer comparable control functions. Country-specific further regulations lead to delivery bottlenecks and higher generation costs. Counter-concepts such as net metering take a more straightforward approach and shift the problem to the network operator.

For larger systems, which among other things must comply with the medium-voltage directive, further measures for dynamic grid stabilization such as the ability to low-voltage ride through are prescribed. The measures serve to avoid an unwanted and simultaneous shutdown of many systems in the event of short-term local undervoltage, as occurs in the context of short circuits or other errors in three-phase systems .

In Germany, 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 restriction serves the network 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, options for remote maintenance .

Grid frequency

The electrical energy in the power supply network cannot be stored in large quantities for a short time. It is therefore always necessary to establish an energy balance between generation and consumption. To ensure this is used as a control variable in with AC -powered electricity networks the network frequency used. In Europe this is defined as 50.0 Hz. Deviations from the nominal value indicate an energy surplus (increased network frequency) or a lack of energy (reduced network frequency). In order to avoid an oversupply of power in the power supply grid, inverters must therefore continuously monitor the grid frequency and disconnect from the grid if a country-specific limit value is exceeded (in Germany 50.2 Hz). 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 would trigger the opposite effect and thus in turn cause grid instability. For this reason, this limit value was subsequently increased with a random value for installed systems over 10 kW. 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.

Island operation

Switching of a solar power inverter (inverter) in island operation with a battery system

In systems for stand-alone operation , special stand-alone inverters enable the use of conventional consumers for 230 V alternating voltage or, in the case of a three-phase version, the provision of three-phase current . The maximum performance made available is decisive. For this purpose, individual inverters can be connected in parallel, but depending on the size of the network, they require additional control devices for coordination with the other power generators and the energy stores. Small systems are sometimes offered with integrated battery systems, but do not have network synchronization, as this is not specified by other power generators.

literature

• Wolf-Günter Gfrörer: Inverters for solar systems . Franzis, Poing 1998, ISBN 3-7723-4952-8 .

Web links

Commons : Solar Inverter  - Collection of Images

• Photovoltaic energy systems (PDF; 263 kB) Document from the University of Emden / Leer
• Current status of measurement and calculation methods for inverter efficiency levels - energie-experten.org, status April 2014

Individual evidence

1. ↑ Satcon Technology Corporation: PowerGate 100 kW solar PV hybrid inverter. accessed January 23, 2012.
2. ↑ voltwerk electronics GmbH: Voltwerk VS 5 Hybrid, fully integrated energy management system.  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. accessed on January 23, 2012.  
3. ↑ ^{a b} Volker Quaschning: Regenerative Energy Systems: Technology - Calculation - Climate Protection . 10th updated and expanded edition. Carl Hanser Verlag Munich, 2019, ISBN 978-3-446-46113-0 , pp. 261-263 .
4. ↑ Volker Quaschning: Regenerative Energy Systems: Technology - Calculation - Climate Protection . 10th updated and expanded edition. Carl Hanser Verlag Munich, 2019, ISBN 978-3-446-46113-0 , pp. 218 .
5. ↑ SMA, October 2009: Why reactive power is important and correct , accessed February 25, 2012.
6. ↑ Sun, wind and heat, March 2011: [http: Watt_Peak // www.bva-solar.de / sww / pdf / solarstrom / 0776-SWW_0311_124-125_Photovoltaik_Niederspannungsordnung.pdf Photovoltaik - Low Voltage Directive], accessed February 25, 2012.
7. ↑ VDN, March 2004: VDN - leaflet on the VDEW guideline  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 338 kB), accessed on October 2, 2010.  
8. ↑ VDE 50.2Hz problem