A grid-tie inverter, or 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 (DC) solar panels (or other DC energy source) and converting it into alternating current. The main difference between an inverter (electrical) and a grid-tie inverter is that the latter also ensures that the power supplied will be in phase with the grid power, so that individuals with surplus power (wind, solar, etc.) can 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, and must 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 such 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 side and the DC side to enable maintenance. The AC output usually goes through across an electricity meter into the public grid.
The meter must be able to run in both directions.
In some countries, for installations over 30kWp a frequency and a voltage monitor with disconnection of all phases is required.
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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 (the upper 3,5,7 harmonics that make up the square wave are removed). Finally, the signal is applied 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. First, the frequency has to be matched in phase to the grid. This means the local oscillator has to be in sync with the grid. Second, the voltage of the inverter output needs to be variable to allow it to be slightly higher than the grid voltage to enable 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 resulting output power matches 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 GTIs have a fixed unity power factor, which means the output voltage and current are perfectly lined up, and the 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
- Advanced Energy
- Beacon Power
- Fronius IG
- KACO Geraetetechnik GmbH
- OutBack Power Systems
- PV Powered
- Satcon Technology
- SMA
- Solar Energy Australia
- Sunways AG
- Trace Engineering (purchased by Xantrex)
- Xantrex
External Links
- SolarFreaks Grid-Tied Inverters
- Some examples of commercial grid tie inverters (manufactured by trace engineering)
- Xantrex Grid-Tie Example
- Fronius USA