Anti-islanding device and method for grid connected inverters using random noise injection

A device for detecting islanding of an inverter connected to a grid, the device comprising:

• means for injecting a band limited white noise on an output voltage of the inverter at least once in each voltage cycle;
• means for perturbing a frequency of the output voltage of the inverter by the injected noise upon a loss of the grid;
• and
• means for detecting a frequency error in the output voltage of the inverter that exceeds a predetermined level indicative of the loss of the grid.

A method for detecting islanding of a grid connected inverter, comprising:

• determining a phase angle and a frequency of a grid voltage;
• injecting a band limited white noise to the determined phase angle and frequency at least once a cycle;
• determining a frequency error at every cycle;
• resetting a drift if an absolute value of the determined frequency error exceeds a minimum frequency error threshold;
• accumulating the drift if the absolute value of the determined frequency error does not exceed the minimum frequency error threshold;
• adding the drift to the determined phase angle and frequency;
• and
• generating an inverter current using the drifted frequency and phase angle.

A device for detecting islanding of an inverter connected to a grid, the device comprising:

• a phase locked loop coupled to receive grid voltages of the grid, the phase locked loop determining a grid phase angle, a grid frequency and a frequency difference;
• a self-synchronizing controller coupled to the phase locked loop to receive the grid phase angle and coupled to the inverter to receive inverter currents, the self-synchronizing controller determining a frequency correction;
• a drift accumulator coupled to the phase locked loop to accumulate the frequency difference as a drift;
• a band limited white noise generator coupled to sum a band limited white noise on the drift;
• an angle generator coupled to receive the frequency correction from the self-synchronizing controller and coupled to receive a sum of the grid frequency via the phase locked loop and the drift via the drift accumulator, the angle generator determining an inverter phase angle;
• and
• a current transformer coupled to the angle generator to receive the inverter phase angle and coupled to receive the inverter currents, the current transformer determining actual dq components of the inverter currents;
• and
• a current controller coupled to the current transformer to receive the actual dq components, and to provide switching signals to the inverter.