A grid-tie inverter converts DC from solar panels or wind turbines to AC and can sell excess power to the utility company. It must match the electrical frequency and voltage of the grid. Compensation for excess energy can be through spot metering or feed-in tariffs. The design can use a high-frequency transformer or no transformer.
A grid-tie inverter is a type of power inverter that takes direct current (DC), usually from solar panels or wind turbines, and converts it to alternating current (AC). What makes a grid-tied inverter different from other types of power inverters is that it is grid-tied, allowing excess AC power to be re-routed to the grid and sold to the utility company. It is also called network interactive inverter or asynchronous inverter.
In order for the grid-connected inverter to feed electricity back into the power grid, it must have the same electrical frequency. This frequency is typically 50 or 60 cycles per second, also known as Hertz. The grid-tie inverter must also use an oscillator to ensure that it does not produce a higher voltage than the electrical grid allows; however, the voltage must be slightly higher than that emitted by the network. A modern grid-tie inverter uses integrated circuits to dynamically adjust its output voltage to match that of the electrical grid.
Many countries allow people with electrical systems that use a grid-tied inverter to sell excess power to the utility company, although not all utilities are required to allow this. In most cases, a system called spot metering is used so that the person with excess energy receives direct compensation for the net amount of electricity that goes back into the grid. When power returns from the building to the grid, the power meter jumps back. A feed-in tariff can be used to compensate the power producer for every kilowatt-hour that returns to the power grid, under a contract between the power producer and the utility company. The National Electric Code (NEC) regulates the construction and use of grid tie inverters in the United States.
A modern grid-connected inverter can use a high-frequency transformer to convert the current. This type of transformer goes through several steps to convert DC power to high frequency AC, back to DC, and then back to AC at the desired output voltage. This design requires computer control over the conversion process, but results in less power loss.
Some grid-tied inverters do not use a transformer at all, which substantially reduces its weight and increases its efficiency. However, this design does not work well with an inverter that has negative ground. Grid-connected inverters that do not use a transformer are more common in Europe because the NEC required grid-connected converters to have negative ground until 2005.
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