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Understanding inverters' role in the solar industry

Posted: 01 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:solar power? solar inverter? photovoltaic?

Grid-connected power plants: These systems, also grid-connected, produce a large quantity of photovoltaic electricity in a single point. The size of these plants ranges from several hundred kilowatts to several megawatts. Some of these applications are located in large industrial buildings such as airport terminals or railway stations. This type of large application makes use of already available space and compensates a part of the electricity produced by these energy-intensive consumers.

Off-grid systems for rural electrification: Where no electric mains is available, the system is connected to a battery via a charge controller. An inverter can be used to provide AC power, enabling the use of normal electrical appliances. Typical off-grid applications are used to bring access to electricity to remote areas (e.g., mountain huts and developing countries). Rural electrification means either small solar home system covering basic electricity needs in a single household, or larger solar mini-grids, which provide enough power for several homes.

 solar inverter

Figures 6: Stand-alone (off grid) solar inverter.

Hybrid systems: A solar system can be combined with another source of power!a biomass generator, a wind turbine or diesel generator!to ensure a consistent supply of electricity. A hybrid system can be grid-connected, stand-alone or grid-supported.

Off-grid industrial applications: Uses of solar electricity for remote applications are very frequent in the telecommunications field, especially to link remote rural areas to the rest of the country. Repeater stations for mobile telephones powered by PV or hybrid systems also have a large potential. Other applications include traffic signals, marine navigation aids, security phones, remote lighting, highway signs and waste water-treatment plants. These applications are cost competitive today, as they are able to bring power in areas far away from electric mains, avoiding the high cost of installing cabled networks.

Solar inverters
A typical solar power system consists of an array of solar PV panels and inverters. The PV panels convert the sun's rays directly into electrical energy in the form of DC voltage, and the inverter converts the DC voltage coming out of PV panels into AC voltage which is sent back to the utility grid. Therefore, the inverters have become the central parts of grid-connected systems.

 solar inverter

Figures 7: Grid-connected inverter using linear transformer.

In addition to high efficiencies for DC-AC conversion and maximum power point tracking (MPPT), inverters should produce AC energy at the required quality!with low total harmonic distortion (THD) of current, a high power factor (close to unity) and a low level of electromagnetic interference!to maximize the transfer of energy from the array to the grid. Inverters must also comply with safety requirements for users, equipment and the grid itself.

Several topologies are available for the inverter. One of them is to use a linear transformer driven by an H-bridge. It is the simplest and the most reliable method offering the complete isolation between the power grid and the DC front end. It also prevents the DC current injection into the grid, which is undesirable. But the drawback with this topology lies in the fact that power loss on the linear transformer is significant, hence resulting in low efficiency. The bulkiness and heavy weight of the linear transformer are additional disadvantages in this topology.

Another topology uses the output inductors in place of the heavy linear transformers. This method ultimately offers the highest efficiency among all topologies, and because of the small sizes of the inductor the inverter becomes much lighter and cost effective when compared with their linear transformer counterpart. But the drawback is that it no longer provides any form of electrical isolation between the grid and the PV panels. Some countries, with their strict regulations, do not accept this type of inverter.

 solar inverter

Figures 8: Transformer-less inverter.

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