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The basic function of an inverter is to convert DC power produced by the PV panels to 230V / 400V AC power. Inverters are classified as On-Grid, Off-Grid and Hybrid types.
Grid-tie inverter is a power inverter that converts direct current (DC) electricity into alternating current (AC) with an ability to synchronize and interface with another AC source like utility grid or gensets. It converts DC power from solar PV panels or small wind turbines into AC power to work parallel with utility grid / genset. 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 grid tie inverter 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 senses the current AC grid waveform, and outputs 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.
The primary safety feature of an On-Grid inverter is “Anti Islanding Protection”, which simply means when grid power fails for any reason, the inverter should immediately shut down to prevent any power feed-in to the utility grid. On-Grid inverters are available in 1-phase and 3-phase. Micro inverters are also available in this type, which can be connected to 1 or 2 PV panels. The main advantage of micro inverter is to minimize the effects of partial shading.
This is the basic type of inverter, which when connected to a DC power source like a battery bank will produce AC power. Unlike the On-Grid inverter, these types of inverters cannot work parallel with grid or another power source. The normal household inverter is a classic example for this type, which charges the battery bank from grid power whereas in the case of a PV system, the battery bank is charged purely by PV panels. These types of inverters come with various safety features with the battery deep discharge protection the most important.
This is essentially an Off-Grid inverter with PV as the primary source of power and grid / genset as secondary source. When the battery bank is exhausted beyond a preset limit, the grid / genset can automatically switch ON and support the electrical loads or / and charge the depleted battery bank. Very recent developments have given rise to hybrid inverters that can work as Off-Grid & On-Grid based on various grid conditions.
The introduction of hybrid inverters has opened up for many interesting innovations. New inverters let homeowners take advantage of changes in the utility electricity rates throughout the day. Solar panels happen to output the most electrical power at noon – not long before the price of electricity peaks. Your home and electrical vehicle can be programmed to consume power during off-peak hours (or from your solar panels). Consequently, you can temporarily store whatever excess electricity your solar panels in batteries and put it on the utility grid when you are paid the most for every kWh. Smart solar holds a lot of promise. This concept will become increasingly important as we transition towards the smart grid in the coming years.