A grid-tie inverter, or a (GTI) is a special type of inverter that is used in a renewable energy power system to convert direct current into alternating current and feed it into the utility grid. The technical name for a grid-tie inverter is "grid-interactive inverter". They may also be called synchronous inverters. Grid-interactive inverters typically cannot be used in standalone applications where utility power is not available.

Residences and businesses that have a grid-tied electrical system are permitted in many countries to sell their energy to the utility grid through a policy known as "net metering", whereby the entity that owns the renewable energy power source receives compensation from the utility for its net outflow of power. So for example, if during a given month a power system feeds 500 kilowatt-hours into the grid and uses 100 kilowatt-hours from the grid, it would receive compensation for 400 kilowatt-hours. In the US, net metering policies vary by jurisdiction.

In the United States, grid-interactive power systems are covered by specific provisions in the National Electric Code, which also mandates certain requirements for grid-interactive inverters.

At the right you'll see a schematic drawing of current-voltage characteristics of a solar cell The area of the yellow rectangle gives the output power. Pmax denotes the maximum power point.

Typical Operation

Inverters work by taking the DC power from the source, such as an array of photovoltaic modules (typically 250 to 600 volts DC) or micro hydroelectric turbines, and inverting it to AC power so it can be fed into the grid. The inverter must also synchronize its frequency with that of the grid (e.g. 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage. 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.

Grid-tie inverters are also 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.

Technology

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 even use no transformer. Instead of converting DC 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. Transformerless inverters, which boast lighter weight and higher efficiencies than their counterparts with