Subharmonic power line carrier based island detection systems and methods
Systems and methods for identifying an island condition in a power distribution system and disconnecting distributed generators in the case of islanding. The systems and methods are used to enable reliable detection of island formation with high false-trip immunity, for any combination of distributed energy resources, and for distributed energy resources using grid support functions.
1. A method for eliminating an island condition in a power grid including at least one distributed energy source selectively electrically coupled to the power grid through a switch electrically coupling a feeder to the power grid, the method comprising:
using a transmitter to inject a subharmonic voltage signal into the power grid through an injection transformer coupled in series with the feeder on a power grid side of the switch;
coupling a receiver on a distributed energy source side of the switch;
detecting the presence or absence of the subharmonic voltage signal using the receiver; and
if the subharmonic voltage signal is absent, disconnecting the at least one distributed energy source from the feeder.
2. The method of claim 1 , wherein the step of using the transmitter includes coupling the injection transformer in series with more than one phase of a multi-phase power feeder.
3. The method of claim 1 , wherein the step of using the transmitter includes coupling the injection transformer in series with a single phase of a multi-phase power feeder.
4. A system for detecting an island condition in a feeder of a power grid, wherein the feeder includes at least one distributed energy source selectively electrically coupled to the power grid through a switch, the system comprising:
a signal generator electrically coupled to the power grid through an injection transformer coupled in series with the feeder, the signal generator including a voltage source inverter, the signal generator emitting a signal through the power grid and feeder at a subharmonic frequency of a frequency of the power grid; and
a receiver electrically coupled to the feeder, the receiver including a digital signal processor providing a logic signal output, the logic signal output indicating whether the signal emitted by the signal generator is detected by the receiver.
5. The system of claim 4 , wherein the subharmonic frequency is 17 Hertz.
6. The system of claim 4 , wherein the signal generator includes an electro-magnetic interference filter and a voltage clamp.
7. The system of claim 4 , wherein the receiver includes a sharp-cutoff low-pass filter.
8. The system of claim 4 , wherein the digital signal processor runs an algorithm.
9. The system of claim 8 , wherein the algorithm is based on a Fourier series calculation.
10. The system of claim 8 , wherein the algorithm is based on a Fourier series calculation with
averaging of results at the subharmonic frequency.
11. The system of claim 4 , wherein the injection transformer is coupled in series with a single phase of a multi-phase power system.
12. The system of claim 4 , wherein the injection transformer is coupled in series with more than one phase of a multi-phase power system.
13. The system of claim 4 , wherein the at least one distributed energy source includes photovoltaic cells.
14. The system of claim 4 , wherein the at least one distributed energy source includes a wind-powered generator.
15. The system of claim 4 , wherein the at least one distributed energy source includes a fossil-fuel powered generator.
16. A method for detecting an island condition in a feeder of an electrical power grid, the feeder including at least one distributed electrical energy source selectively electrically coupled to the power grid through a switch, the method comprising:
coupling a signal generator to a power grid side of the switch using an injection transformer in series with the feeder;
generating a signal at a subharmonic frequency of the power grid using the signal generator, the signal being injected into the power grid and feeder through the injection transformer;
electrically coupling a signal receiver to the power grid and feeder on a distributed energy source side of the switch;
using the signal receiver to detect the presence or absence of the signal; and
if the signal is absent, disconnecting the distributed energy source from the feeder and grid.
17. The method of claim 16 , wherein the step of coupling a signal generator includes connecting the injection transformer in series with a single phase of a multi-phase power system.
18. The method of claim 16 , wherein the step of coupling a signal generator includes connecting the injection transformer in series with more than one phase of a multi-phase power system.