Grid tie inverter

A grid-tie inverter, or a (GTI) is an electrical device that allows solar power users to complement their grid power with solar power. It works by regulating the amount of voltage and current that is received from the direct current solar panels (or other D.C. energy source) and converting this into alternating current. The main difference between a standard electrical inverter and a grid-tie inverter is that the latter also ensures that the power supplied will be in phase with the grid power. This allows individuals with surplus power (wind, solar, etc) to sell the power back to the utility. This is sometimes called "spinning the meter backwards" as that is what literally happens.

On the AC side, these inverters must supply electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage including disconnecting from the grid if the grid voltage is turned off.

On the DC side, the power output of a module varies as a function of the voltage in a way that power generation can be optimized by varying the system voltage to find the 'maximum power point'. Most inverters therefore incorporate 'maximum power point tracking'.

The inverters are designed to connect to one or more strings.

For safety reasons a circuit breaker is provided both on the AC and DC side to enable maintenance. The AC output usually goes through an electricity meter into the public grid.

The meter must be able to run in both directions.

Installations will require, "rate of change of frequency" (ROCOF), vector shift, and a voltage monitor with disconnection of all phases to prevent power being fed back during power outages.

Typical Operation

Inverters work by taking the 12 or 24 volt DC voltage from the source, such as solar panels or micro hydroelectric generators and 'chopping' by turning it on and off at grid supply frequency (e.g. 60 Hz) using a local oscillator and a power transistor. This chopped DC signal is then filtered to make it into a sine wave (removing the upper 3,5,7 harmonics that make up the square wave and then applying it to a transformer to up the voltage to 120 or 240 to supply the needs of load.

A grid tie inverter does the same but has two key differences. Firstly the frequency has to be matched in phase to the grid. This means the local oscillator has to be in sync with the grid. Secondly the voltage of the inverter output needs to be variable to allow it to be slightly higher than the grid voltage to enabling current to flow out to the grid. This is done by sensing current flow and raising the voltage on the output (or duty cycle of the transformer input) until the current flow results in the resulting output power matching the input power from the DC supply.

Effects on Grid Power Quality

In order for grid tie inverters to comply with utility electrical standards, the output power needs to be clean, undistorted and in phase with the AC grid. Typical modern GTI's have a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1 degree of the AC power grid. The inverter has an on board computer which will sense the current AC grid waveform, and output a voltage to correspond with the grid.

Manufacturers

* [http://www.aesolaron.com/ Advanced Energy]
* Beacon Power
* [http://enphaseenergy.com Enphase Energy]
* Fronius IG
* KACO Geraetetechnik GmbH
* OutBack Power Systems
* PV Powered
* [http://www.elettronicasanterno.com SANTERNO]
* [http://www.satcon.com SatCon Technologies]
* [http://www.sma-america.com SMA]
* Solar Energy Australia
* [http://www.sunways.de Sunways AG]
* [http://www.sustainableenergy.com Sustainable Energy Technologies]
* [http://www.xantrex.com Xantrex]

ee also

*Solar inverter
*Inverter (electrical)
*Grid-tied electrical system

External links

* [http://solarfreaks.com/ftopic77_grid-tied-inverters.html SolarFreaks Grid-Tied Inverters]
* [http://www.4lots.com/index.asp?PageAction=VIEWCATS&Category=213 Some examples of commercial grid tie inverters (manufactured by trace engineering)]
* [http://www.xantrex.com/web/id/172/p/1/pt/23/product.asp Xantrex Grid-Tie Example]
* [http://www3.fronius.com/worldwide/usa.solarelectronics/about/index.htm Fronius USA]