Grid-Connected Power Systems

Grid-Connected Systems | Grid-Connected Inverters Standards Testing | The Future of Grid-Connected Systems | Links to Suppliers and Further Information | References

Grid-Connected Systems

Renewable energy devices such as PV modules and wind turbines are also being used on a small scale in areas where the electricity grid is available. People who want to make a contribution to the generation of renewable energy usually own these systems, or they are demonstration sites set up by electricity utilities or equipment manufacturers. The output from the renewable energy conversion devices is 'exported' to the grid (after being converted to AC at the correct voltage and synchronised with the grid frequency) during times of excess supply. When there is insufficient renewable energy power is 'imported' from the grid. The home or building owner therefore receives credit for the generated electricity that is offset against the imported power. The rate at which the utility will buy back the renewable energy varies, with some utilities operating a net metering system, where each kWh generated is equivalent to one imported from the grid, whilst other utilities pay a price that is a premium over fossil fuel generated electricity but lower than that charged to domestic customers. In some countries, producers are actually paid far more for the energy they produce than they pay for the electricity from the grid because of government subsidies, therefore making grid-connect renewable systems very attractive.

With a wind-based grid-connected system, the wind turbine will only operate when the utility grid is available as during blackouts the electricity from the wind turbine is not used due to safety issues. Grid connected wind systems can be practical if you live in areas with a good wind resource, have a permit from your local power utility, and have a reasonable power consumption or are remunerated for the power produced at a reasonable amount.

This illustration shows how a grid-connected small wind system works. It shows the wind blowing a three-bladed wind turbine sitting atop a tower, which looks like a pole. The electricity generated by the wind turbine is shown traveling to an inverter. The inverter is a gray-colored, square box with two gauges near the top of the inverter box. From the inverter box, electricity is shown traveling to both a meter (a white, square box) and a house, which is identified as the 'load.' From the meter, the electricity is shown traveling to an electricity transmission, which is drawn as vertical pole with two smaller poles drawn at the top. The pole nearest the top is slighting larger than the one beneath it.

Figure 1 A schematic of a wind turbine grid-connected system.

Grid-connected photovoltaic systems are the most common type of grid-connected system. As electricity produced during the daytime is either used or directed back into the electricity grid, and at night electricity is purchased from the grid, there is no need for an expensive battery bank. This forgoes the added capital expenditure, maintenance and replacement costs of batteries, but this does mean that when neither the grid nor solar power is available there will be no electricity provided to the house.

Figure 2 A schematic of a photovoltaic grid-connected system. (courtesy of Ecolution).

In Australia, most electricity is supplied by utilities or electricity corporations from power stations via power supply networks, called grids. These main grids provide power to the majority of Australians using many large coal and gas-fired power stations, large hydro generation schemes and more recently, some smaller scale wind farms and photovoltaic systems. Remote towns like Port Hedland, Mt Isa and Coober Pedy are not serviced by the main grid and have gas or diesel power stations or combined diesel/wind power stations to provide their power via a mini grid.

Grid Connected Inverters Standards Testing

Standards Australia has released three standards which are pertinent to grid-connected inverter systems. These are:

-AS 4777.1 - 2002 Grid connection of energy systems via inverters Part 1: Installation requirements.
-AS 4777.2 - 2002 Grid connection of energy systems via inverters Part 2: Inverter requirements.
-AS 4777.3 - 2002 Grid connection of energy systems via inverters Part 3: Grid protection requirements.

Inverters must be tested against AS 4777.2 - 2002 (or equivalent) by an appropriate testing laboratory. RISE’s ResLab is accredited to test to AS4777 (Clean Energy Council, 2008). For more information

The Future of Grid-Connected Systems

Installed grid-connected photovoltaic arrays worldwide are increasing at an extremely rapid rate (see Figure 3). These extraordinary levels of growth will most likely continue to be driven by market support mechanisms that focus on grid-connected domestic applications in the urban or suburban environment. With growing interest in all forms of grid-connected renewable energy systems, the future of the grid-connected market seems bright. Figure 3 below shows the worldwide growth of installed capacity in off- and on-grid PV systems.

Figure 3 Cumulative installed PV capacity between 1992 to 2007 in IEA PVPS reporting countries (Source: IEA Photovoltaic Power Systems Programme).

Further Information

RISE Information Portal - Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

RREDC’s PV Watts Performance Calculator for Grid-Connected PV Systems

World Bank – Grid-Connected Renewable Energy

Your Home Technical Manual - Photovoltaic Systems

Clean Energy Council – Off-grid and Grid Connected Renewable Energy Systems

NAPS - Solar Electric Solutions

Sustainable Energy Development Office of Western Australia

References

Clean Energy Council, 2008. “Grid Connected Inverters” (Online) http://www.bcse.org.au/default.asp?id=233 (Accessed 3 December 2008).