I have been looking at inverters for grid connected domestic wind turbines. Due to regualtions these must automatically disconnect if the grid goes down.
This got me thinking, how does the inverter know that the grid has gone down if it is producing mains voltage at mains frequency itself? If it was simply sensing the grid voltage/current couldn't it mistakingly sense the voltage and current it itself was putting out .
Does anyone know how this is done?
Emergency Transfer switches monitor the "primary power voltage" When that state changes then they do what they are programed to do. I would think that the inverter would do the same.
There are many methods. One involves what referred to as active islanding detection. In this method, the inverter is always trying to either change the frequency or the voltage. Since it is connected to the "grid", it can do neither (it is small compared to the grid). When the grid goes away, it can change the frequency (or voltage) and knows it is no longer connected to an energized grid.
There are passive methods also. These methods also rely on the fact that the generator is small compared to the grid and assume that the island that is created will not exactly match the demand rating of the generator.
Charles Perry P.E.
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