Online grid impedance measurement suitable for multiple PV inverters running in parallel

Due to the increased use of photovoltaic (PV) installations, new and stronger grid connection requirements are issued in order to protect the utility network. One of the most severe demand applying to PV installations is the ability to detect islanding conditions. Hence, the continuous grid monitoring is always a matter to be considered in order to fulfill this requirement. The present paper gives a description of one method used for on-line grid impedance estimation by means of signal injection and focus is put on the problems issued by the situation when two or more inverters run in parallel on the same network. The paper reveals a new method to detect a second inverter connected to the grid and the proposed solution is discussed. Practical tests on an existing PV inverter validates the second inverter detection algorithm.