A solar PV inverter is an electrical device that converts the variable direct current (DC) output from a solar photovoltaic system into alternating current (AC) of suitable voltage, frequency and phase for use by AC appliances and, where grid connected - for export to the grid. As well as converting the DC to AC, a solar inverter may also provide anti-islanding and maximum power point tracking.
Anti Islanding
A system that is attached to the electricity supply grid can export power to the grid if the local demand for power is exceeded by the power output of the solar system. Regulations require that distributed generators such as solar that are connected to the grid must shut down automatically if the grid itself loses power, a feature called anti-islanding. The reason for this is to protect utility workers, perhaps working to restore grid electricity supplies, from electric shock.
Maximum Power Point Tracking
The output of solar panels varies with the level of solar irradiation and their operating temperature. Under a given set of environmental conditions, the panel current will vary with the voltage, from a maximum current under short circuit conditions (Isc) to a maximum voltage under open circuit conditions (Voc) following a characteristic I-V curve as shown in the diagrams. Solar inverters with maximum power point tracking (MPPT) get the maximum possible energy from the PV array by continuously sampling the output of the solar panels and setting a resistance (load) that produces the maximum power at all times.
The power from a solar panel depends on the operating point
The Maximum Power Point is the combination of current and voltage that gives the highest power output
Types of Inverter
- Stand Alone - normally for applications away from the electricity grid, where solar energy is used to charge batteries and then converted to AC for local use only. Such inverters often include a battery charge controller and would not require anti-islanding functionality.
- Grid tied - where the power from the inverter is used locally in conjunction with the grid, or can be exported to the grid if there is an excess. The AC sinusoid phase must match that of the grid electricity supply. Anti-islanding functionality is also required.
- String inverter - an inverter that converts the DC output from a number of panels that are electrically wired together in series.
- Micro inverter - an inverter that converts the DC output from a single solar panel, or small group of two panels, with multiple micro-inverters combining to create a system.
- Power Optimiser - where the MPPT function is separated from DC-AC conversion, the MPPT occurring at the panel level with individual optimiser units for each solar panel. The DC output of the optimisers is combined into a single inverter to convert to AC.
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