Grid-tie inverter

Inverter for grid-connected PV
Example of a large three-phase inverter for commercial and utility scale grid-tied PV systems

A grid-tie inverter is a power inverter that converts direct current (DC) electricity into alternating current (AC) with an ability to synchronize to interface with a utility line. Its applications are converting DC sources such as solar panels or small wind turbines into AC for tying with the grid.[1]

Residences and businesses that have a grid-tied electrical system are permitted in many countries to sell their energy to the utility grid. Electricity delivered to the grid can be compensated in several ways. "Net metering" is where 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. Another policy is a feed-in tariff, where the producer is paid for every kilowatt hour delivered to the grid by a special tariff based on a contract with distribution company or other power authority.

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.

Typical operation

Inverters take DC power and invert it to AC power so it can be fed into the electric utility company grid. The grid tie inverter must synchronize its frequency with that of the grid (e.g. 50 or 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage. A high-quality modern GTI has 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. However, supplying reactive power to the grid might be necessary to keep the voltage in the local grid inside allowed limitations. Otherwise, in a grid segment with considerable power from renewable sources voltage levels might rise too much at times of high production, i.e. around noon.

Grid-tie inverters are also designed to quickly disconnect from the grid if the utility grid goes down. This is an NEC requirement[2] that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy it transfers from harming any line workers who are sent to fix the power grid.

Properly configured, a grid tie inverter enables a home owner to use an alternative power generation system like solar or wind power without extensive rewiring and without batteries. If the alternative power being produced is insufficient, the deficit will be sourced from the electricity 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 without 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.[3] Transformerless inverters, lighter and more efficient than their counterparts with transformers, are popular in Europe. However, transformerless inverters have been slow to enter the US market because historically there have been concerns about having transformerless electrical systems feeding into the public utility grid. This is due to the lack of galvanic isolation between the DC and AC circuits could allow the passage of dangerous DC faults to be transmitted to the AC side.[4] However, since 2005, the NFPA's NEC allows transformerless (or non-galvanically) inverters by removing the requirement that all solar electric systems be negative grounded and specifying new safety requirements. 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.

The concept of sinusoidal PWM (pulse width modulation) was introduced in an attempt to reduce the harmonic contents at the output voltage [12][13]. Level shifted modulation technique gives better result compared with the phase shifting technique. Level shifting is done to reduce the harmonics at the output voltage [14][15].Most grid-tie inverters on the market include a maximum power point tracker on the input side that enables the inverter to extract a maximum amount of power from its intended power source. Since MPPT algorithms differ for solar panels and wind turbines, specially made inverters for each of these power sources are available.

Characteristics

Inverter manufacturers publish datasheets for the inverters in their product line. While the terminology and content will vary by manufacturer, datasheets generally include the information listed below.

See also

References and further reading

  1. ↑ http://www.osti.gov/bridge/servlets/purl/463622-TtEMSp/webviewable/463622.pdf OSTI
  2. ↑ NEC Handbook 2005, Section 705, "Interconnected Electric Power Production Sources," Article 705.40 "Loss of Primary Source"
  3. ↑ Solar Energy International (2006). Photovoltaics: Design and Installation Manual, Gabriola Island, BC: New Society Publishers, p. 80.
  4. ↑ "Summary Report on the DOE High-tech Inverter Workshop". Sponsored by the US Department of Energy, prepared by McNeil Technologies. eere.energy.gov. Retrieved 2011-06-10.
  5. ↑ gosolarcalifornia.org, "List of Eligible Inverters", accessed July 30, 2009,

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