Anti-islanding method and system for distributed power generation systems
View Patent ↗An electrical power supply system connected to an electrical grid, includes at least one source of electrical power, an inverter which receives power from the electrical source and delivers power to a local load, and a connection to the electrical grid. The system also includes a control unit for controlling the inverter, which interrupts the delivery of power by the inverter when at least one electrical parameter of the system exceeds a threshold value. The system also includes anti-islanding logic which detects at least one symptom of an islanding condition of the system and, when the symptom is detected, to cause a variation in the power level delivered by the inverter.
1. A method for controlling a power supply system connected to a power grid, the power supply system including an electrical power source, the method comprising the steps of:
a. delivering power from the power source;
b. detecting a characteristic indicating an islanding condition of the power supply system;
c. in response to detecting the characteristic indicating an islanding condition, causing a temporary variation in the power delivered by the power source;
d. while the power delivered by the power source is temporarily varied, monitoring the power source for a characteristic indicating an islanding condition of the power supply system;
e. if a characteristic indicating an islanding condition of the power supply system is detected while the power delivered by the power source is temporarily varied, interrupting the power source to remove the islanding condition; and
f. if a characteristic indicating an islanding condition of the power supply system is not detected while the power delivered by the power source is temporarily varied, removing the temporary variation in the power delivered by the power source.
2. The method of claim 1 , wherein when the power source includes an inverter and wherein when the characteristic is detected, power from the inverter is temporarily reduced.
3. The method of claim 2 , wherein the power delivered by the power source is varied by modifying a magnitude of current delivered by the inverter.
4. The method of claim 2 , wherein when the characteristic is detected, the power delivered by the inverter is varied by a value between 5% and 20%.
5. The method of claim 1 , wherein the power delivered by the power source is varied for a pre-set time interval.
6. The method of claim 1 , wherein the characteristic indicative of an islanding condition is determined via an impedance measurement.
7. A method for controlling a power supply system connected to a power grid, the power supply system including an electrical power source, an inverter connecting the electrical power source to the power grid, and a load connected to the inverter, the method comprising:
delivering power to the load;
detecting a characteristic indicative of an islanding condition of the system;
when the characteristic is detected, imposing a temporary variation in the power level delivered by the inverter,
detecting a characteristic indicative of an islanding condition of the system while there is a temporary variation in the power level delivered by the inverter; and
interrupting the delivery of power to the load in response to detecting a characteristic indicative of an islanding condition of the system while there is a temporary variation in the power level delivered by the inverter.
8. The method of claim 7 , wherein when the characteristic is detected, the power delivered by the inverter is temporarily reduced.
9. The method of claim 8 , wherein the power delivered by the inverter is temporarily reduced for a pre-set time interval and, if after the pre-set time interval the characteristic is not detected, the power level delivered by the inverter is restored to the level prior to the variation.
10. The method of claim 9 , wherein the power level is varied by modifying the current delivered by the inverter.
11. The method of claim 7 , wherein the characteristic indicative of an islanding condition is determined via an impedance measurement.
12. A method for controlling a power supply system connected to a grid, the power supply system including an electrical power source, an inverter connecting the electrical power source to the grid and a load connected to the inverter, comprising the following phases:
delivering power to the load;
determining a symptomatic condition of islanding of the system;
when a symptomatic condition of an islanding situation is determined, imposing a variation in the power level delivered by the inverter;
if the variation in power level delivered by the inverter causes a variation beyond a pre-set limit of an electrical parameter of the system, causing interruption of the power delivery via the inverter.
13. The method of claim 12 , wherein when the symptomatic condition is detected, the power delivered by the inverter is temporarily reduced.
14. The method of claim 12 , wherein the power level is brought to a varied power level for a pre-set interval of time and wherein, if after the pre-set interval of time the power supply via the inverter has not been de-activated, the power is restored to the level prior to the variation.
15. The method of claim 12 , wherein the symptomatic condition of islanding of the system comprises an electrical parameter and wherein the electrical parameter comprises at least one of an output voltage of the inverter; a power supply voltage of the load an output signal frequency of the inverter; and a power supply frequency of the load.
16. An electrical power supply system connected to a local load and to an electrical grid comprising:
at least one source of electrical power;
an inverter which receives power from the electrical source and delivers power to the local load;
a connection to the electrical grid;
a control unit functionally connected to the inverter, the control unit comprising interrupt logic functional to interrupt the delivery of power;
the control unit further comprising anti-islanding logic functional to detect at least one symptom of an islanding condition of the system and, when the symptom is detected, to cause a variation in the power delivered by the inverter; and
wherein the anti-islanding logic is further functional to cause the interrupt logic to interrupt the delivery of power when at least one symptom of an islanding condition of the system is detected when the power delivered by the inverter is varied.
17. The system of claim 16 , wherein the anti-islanding logic varies the power delivered by the inverter by reducing the power level delivered by the inverter.
18. The system of claim 17 wherein the anti-islanding logic varies the power delivered by the inverter by reducing the current delivered by the inverter.
19. The system of claim 16 , wherein the anti-islanding logic causes a temporary variation in the power delivered by the inverter and after a pre-set interval of time, restores the power delivered to the initial value if power from the inverter has not been interrupted by the interrupt logic.
20. The system of claim 16 , wherein the anti-islanding logic causes the interrupt logic to interrupt the power delivery by the inverter if the variation if the power delivered caused by the anti-islanding device causes at least one electrical parameter of the system to exceed a pre-set threshold value.
21. The system of claim 20 wherein the system electrical parameter includes at least one of an output voltage of the inverter, a power supply voltage of the load, an output signal frequency of the inverter, and a power supply frequency of the load.