Inverter anti-islanding method
View Patent ↗A method for detecting and interrupting an electrically islanded condition for a grid connected current source inverter is disclosed in which: A line voltage frequency is monitored at a cycle or sub cycle resolution. Current output from the inverter is periodically phase shifted. If voltage is observed to follow the phase shift of the current output an islanding error signal is generated causing the inverter to stop injecting power onto the line.
1. An inverter connected to a photovoltaic panel comprising:
an anti-islanding monitor, wherein the anti-islanding monitor is configured to:
detect a loss of an AC power grid connected to the inverter and disconnect the connection to the AC power grid when the AC power grid is lost; and
modify AC current periodically and detect a loss of the AC power grid if AC voltage is also modified when the AC current is modified, wherein modifying the AC current includes shifting a phase of the AC current forward in half of the modifications of the AC current and shifting a phase of the AC current backward in half of the modifications of the AC current.
2. The inverter of claim 1 wherein the anti-islanding monitor detects a loss of AC power grid by detecting an over or under frequency condition during a period when the phase of the AC current is shifted.
3. The inverter of claim 1 wherein the AC current is out of phase with the voltage in the AC power grid for one cycle each time the anti-islanding monitor checks for loss of the AC power grid.
4. The inverter of claim 1 wherein the AC current is out of phase with the voltage in the AC power grid for one cycle each time the anti-islanding monitor checks for loss of the AC power grid.
5. The inverter of claim 1 wherein the anti-islanding monitor checks for loss of the AC power grid once every ten cycles.
6. The inverter of claim 1 wherein the phase shift in AC current is less than 40 degrees forward or backward.
7. The inverter of claim 1 wherein the sequence of phase shifts is forward, forward, backward, backward.
8. An inverter connected to a photovoltaic panel comprising:
an anti-islanding monitor that detects a loss of an AC power grid and stops operation of the inverter when the AC power grid is lost, wherein the anti-islanding monitor is configured to:
modify AC current periodically and detect a loss of the AC power grid if AC voltage is also modified when the AC current is modified, wherein modifying the AC current includes shifting a phase of the AC current forward in half of the modifications of the AC current and shifting a phase of the AC current backward in half of the modifications of the AC current.
9. The inverter of claim 8 wherein the anti-islanding monitor detects a loss of the AC power grid if AC voltage is also modified when the AC current is modified.
10. The inverter of claim 8 wherein the AC current is out of phase with the voltage in the AC power grid for one cycle each time the anti-islanding monitor checks for loss of the AC power grid.
11. The inverter of claim 8 wherein the anti-islanding monitor checks for loss of the AC power grid once every ten cycles.
12. A method of operating an inverter connected to a photovoltaic panel comprising:
providing a photovoltaic panel, an output of which is DC power;
providing a set of transistors electrically connected to the photovoltaic panel that controllably convert the DC power into AC power;
connecting to the output of the transistors to an AC power grid;
monitoring for loss of the AC power grid, wherein monitoring includes shifting the phase of the AC current output of the transistors to a sequence of forward, forward, backward, backward, and checking for a modification in AC voltage when the AC current is modified; and
interrupting operation of the inverter when the AC power grid is lost.