A solar inverter or PV inverter, is a type of electrical converter which converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical balance of system (BOS)–component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

Classification
Solar inverters may be classified into three broad types:

Stand-alone inverters, used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection.
Grid-tie inverters, which match phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages.
Battery backup inverters, are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection.

Device Types
Modular inverter (micro-inverter)
Every single solar module has its own single-phase inverter, which can be integrated in the junction box.
This is a DC-DC converter whose purpose is to set the voltage so that the connected module is operated in its maximum power point (MPP).
This can be useful in photovoltaic systems, which consist of differently oriented or differently shaded subfields, for example, coated with solar modules cars or aircraft.

String inverters (English String Inverter)
A mostly single-phase inverter that feeds the energy of one strand or few strands of solar modules into a power grid.

Multi-String Inverters
Single or three-phase inverter equipped with more than one MPP tracker for multiple strings (even different) of solar panels.

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

Hybrid Inverter
Combination of inverter and internal or external storage batteries. This results in the possibilities of uninterruptible power supply, as well as the optimization of self- consumption in feed mode.

Circuitry and efficiency
Basically, you can distinguish two types of solar inverters:

Devices with transformer
Here a transformer takes over the galvanic isolation between DC and AC side. Due to the galvanic isolation, the PV generator can be grounded in one pole – there are no AC potentials in the system. It is also mandatory in some countries.

Transformerless devices
Here, input side and output side are electrically connected to each other. In this circuit design, no transformer is used, these devices therefore usually have a higher efficiency. The lack of electrical isolation, however, requires a different electrical safety concept. In part, alternating voltages of the solar modules to ground, which can lead to losses and with thin-film modules for degradation. To further increase the efficiency and avoidance of leakage currents, circuit technologies designation H5 or Heric topology have been developed.
At the DC input of the solar inverter is usually an input converter. This converter is often an up-converter with very high efficiency. The output circuit must also have high efficiency over a wide load range.

To optimize inverters with transformers, the inverter often takes over the function of the input transformer, so that the intermediate circuit is eliminated. This is called a direct feed or direct converter. The efficiency improves, since only one converter is needed. Such devices, however, have a smaller range with optimum efficiency, so that relativized especially in systems with partial shading advantage quickly.

The efficiency of solar inverters is comparable by the euro efficiency, which evaluates particularly partial load cases.

In the solar industry, the term kWp has come to be used to denote peak power instead of kW. However, this does not conform to the rules of the International System of Units according to which the unit designations are not supplemented. See also: Spelling of the unit characters.

Maximum power point tracking
Solar inverters use maximum power point tracking (MPPT) to get the maximum possible power from the PV array. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency known as the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and determine a resistance (load) to obtain maximum power for any given environmental conditions.

The fill factor, more commonly known by its abbreviation FF, is a parameter which, in conjunction with the open circuit voltage (Voc) and short circuit current (Isc) of the panel, determines the maximum power from a solar cell. Fill factor is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc.

There are three main types of MPPT algorithms: perturb-and-observe, incremental conductance and constant voltage. The first two methods are often referred to as hill climbing methods; they rely on the curve of power plotted against voltage rising to the left of the maximum power point, and falling on the right.

Operation
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 unwanted islanding. For systems with installed power over 30 kW, the ENS can be dispensed with. There is sufficient frequency and voltage monitoring with all-pole shutdown for safe isolation from the network, if this is turned off or fails.

It is often advertised with a high efficiency of the inverter. In the partial load range, it is slightly lower and is therefore averaged and then referred to as “European efficiency”. However, the efficiency of the inverter does not alone decide on the overall efficiency of a photovoltaic system.

Since January 2009, photovoltaic systems in Germany with an installed capacity of more than 100 kW must have the option of being reduced by the grid operator in the injected active power (§ 6.1 EEG ). Furthermore, there is the possibility that a certain amount of reactive power is provided. In practice, these specifications are realized dynamically via ripple control receiver, which can signal a four-stage active power reduction or specify a deviating from 1 effective factor of, 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 grid will have to offer comparable control functions. Country-specific further regulations lead to supply bottlenecks and higher production costs. Counter-concepts such as net metering pursue a more straightforward approach and shift the problem to the network operator.

In the case of larger systems, which, inter alia, comply with the Medium Voltage Directive, further measures are required for dynamic network stabilization such as the capability of low-voltage ride-through. The measures are intended to avoid unwanted and simultaneous shutdown of many systems with short-term local undervoltage, as they occur in the context of short-circuits or other errors in the three-phase systems.

Single-phase systems may only feed into the power grid in Germany up to a maximum power of 5 kW (4.6 kW continuous power). This restriction is 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, remote maintenance options.

Mains frequency
The electrical energy in the power grid can not be stored in large quantities in the short term. It is therefore always necessary to establish an energy balance between production and consumption. To ensure this is the mains frequency as a control variable in AC voltage- powered power gridsused. In Europe this is defined as 50.0 Hz. Deviation from the nominal value indicates an energy surplus (increased mains frequency) or an energy shortage (reduced mains frequency). In order to avoid an oversupply of power in the power grid, inverters must therefore constantly monitor the grid frequency and disconnect from the grid when exceeding a country-specific limit (in Germany 50.2 Hz). Since in the meantime a predominant part of the generated electrical energy comes from photovoltaic systems in Germany, a hard shutdown of all systems would trigger a contrary effect with this limit value and in turn cause a network instability. Therefore, for installations above 10 kW, this limit was subsequently increased to a random value. Newer plants must have aPower gradients between 50.2 and 51.5 Hz, which reduces or increases the feed-in power depending on the current grid frequency and thus actively contributes to grid stabilization.

Island operation
In systems for isolated operation, special island inverters enable the use of conventional consumers for 230 V AC. The decisive factor is the maximum power provided. For this purpose, individual inverters can be connected in parallel, but depending on the size of the network but need additional control devices for coordination with the other generators and the energy storage. Small systems are sometimes offered with integrated battery systems, but have no network synchronization because their default by other power generators is missing.

Solar micro-inverters
Solar micro-inverter is an inverter designed to operate with a single PV module. The micro-inverter converts the direct current output from each panel into alternating current. Its design allows parallel connection of multiple, independent units in a modular way.

Micro-inverter advantages include single panel power optimization, independent operation of each panel, plug-and play installation, improved installation and fire safety, minimized costs with system design and stock minimization.

A 2011 study at Appalachian State University reports that individual integrated inverter setup yielded about 20% more power in unshaded conditions and 27% more power in shaded conditions compared to string connected setup using one inverter. Both setups used identical solar panels.

Grid tied solar inverters
Solar grid-tie inverters are designed to quickly disconnect from the grid if the utility grid goes down. This is an NEC requirement that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy it produces from harming any line workers who are sent to fix the power grid.

Grid-tie inverters that are available on the market today use a number of different technologies. The inverters may use the newer high-frequency transformers, conventional low-frequency transformers, or no transformer. Instead of converting direct current directly to 120 or 240 volts AC, high-frequency transformers employ a computerized multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage.

Historically, there have been concerns about having transformerless electrical systems feed into the public utility grid. The concerns stem from the fact that there is a lack of galvanic isolation between the DC and AC circuits, which could allow the passage of dangerous DC faults to the AC side. Since 2005, the NFPA’s NEC allows transformer-less (or non-galvanically) inverters. The VDE 0126-1-1 and IEC 6210 also have been amended to allow and define the safety mechanisms needed for such systems. Primarily, residual or ground current detection is used to detect possible fault conditions. Also isolation tests are performed to ensure DC to AC separation.

Many solar inverters are designed to be connected to a utility grid, and will not operate when they do not detect the presence of the grid. They contain special circuitry to precisely match the voltage, frequency and phase of the grid.

Solar pumping inverters
Advanced solar pumping inverters convert DC voltage from the solar array into AC voltage to drive submersible pumps directly without the need for batteries or other energy storage devices. By utilizing MPPT (maximum power point tracking), solar pumping inverters regulate output frequency to control the speed of the pumps in order to save the pump motor from damage.

Solar pumping inverters usually have multiple ports to allow the input of DC current generated by PV arrays, one port to allow the output of AC voltage, and a further port for input from a water-level sensor.

Market
As of 2014, conversion efficiency for state-of-the-art solar converters reached more than 98 percent. While string inverters are used in residential to medium-sized commercial PV systems, central inverters cover the large commercial and utility-scale market. Market-share for central and string inverters are about 50 percent and 48 percent, respectively, leaving less than 2 percent to micro-inverters.

Inverter/converter market in 2014

Type	Power	Efficiency(a)	Market share(b)	Remarks
String inverter	up to 100 kWp(c)	98%	50%	Cost(b) €0.15 per watt-peak. Easy to replace.
Central inverter	above 100 kWp	98.5%	48%	€0.10 per watt-peak. High reliability. Often sold along with a service contract.
Micro-inverter	module power range	90%–95%	1.5%	€0.40 per watt-peak. Ease of replacement concerns.
DC/DC converter Power optimizer	module power range	98.8%	N/A	€0.40 per watt-peak. Ease of replacement concerns. Inverter is still needed. About 0.75 GWP installed in 2013.
Source: data by IHS 2014, remarks by Fraunhofer ISE 2014, from: Photovoltaics Report, updated as per 8 September 2014, p. 35, PDF Notes: (a)best efficiencies displayed, (b)market-share and cost per watt are estimated, (c)kWp = kilowatt-peak

Source from Wikipedia