Fast-slow injection for recovery from transient response and voltage collapse with avoidance of SSR and SSCI

1. An intelligent impedance injection module (IIM) for use with transmission lines in a power grid, the intelligent IIM comprising:

a plurality of transformer-less impedance injection units (IIUs); and

a controller, to change injection gain of the plurality of IIUs to compensate for power swings, oscillations and other instabilities in a transmission line;

wherein each of the plurality of transformer-less IIUs includes a plurality of switching devices, a capacitor, a sensor for the transmission line, and a power supply that is to extract power from the transmission line to generate a DC voltage across the capacitor.

2. The intelligent impedance injection module of claim 1 , wherein:

the injection gain comprises a first, higher gain or a second, lower gain;

the power swings comprise changes in power transfer due to line switching, or line faults or generator faults that exceed a threshold.

3. The intelligent impedance injection module of claim 1 , wherein the power swings are changes in power transferred over the transmission line that create instability during re-energization of the transmission line after a fault-related incident.

4. The intelligent impedance injection module of claim 1 , wherein:

the IIUs are in a series-parallel configuration in the IIM.

5. The intelligent impedance injection module of claim 1 , wherein:

the intelligent impedance injection module is installed as one of a plurality of IIMs separated from each other on a mobile carrier, each IIM having an insulated standoff.

6. The intelligent impedance injection module of claim 1 , wherein:

the controller has a bypass mode to take the intelligent IIM off-line but retain a capability for fault monitoring and communication.

7. An intelligent impedance injection module (IIM) for use with transmission lines in a power grid, comprising:

a plurality of transformer-less impedance injector units (IIUs); and

a controller, to change injection gain of the plurality of transformer-less IIUs to a first gain for initial response to a power transfer anomaly in a transmission line and change the injector gain of the plurality of transformer-less IIUs to a lower, second gain after a specified time;

wherein each of the plurality of transformer-less IIUs includes a plurality of switching devices, a capacitor, and a power supply that is to extract power from the transmission line to generate a DC voltage across the capacitor.

8. The intelligent impedance injection module of claim 7 , wherein the controller is to change the injector gain to the lower, second gain after the specified time to prevent the power grid from experiencing oscillations induced by rise and fall times of injected impedance.

9. The intelligent impedance injection module of claim 7 , wherein:

the intelligent impedance injection module is installed as one of a plurality of IIMs separated from each other on a mobile carrier, each IIM having an insulated standoff.

10. The intelligent impedance injection module of claim 7 , further comprising:

a sensor, coupled to the transmission line for detection of the anomaly in the transmission line.

11. The intelligent impedance injection module of claim 7 , wherein:

the plurality of transformer-less IIUs are operable with pseudo-ground at a high-voltage power line voltage of the transmission line.

12. The intelligent impedance injection module of claim 7 , wherein:

each switching device comprises a power IGBT (insulated gate bipolar transistor) device.

13. The intelligent impedance injection module of claim 7 , further comprising:

a protection switch arranged to open across impedance injection terminals of the intelligent IIM for impedance injection, and to close across the impedance injection terminals to bypass the impedance injection and protect devices and circuit from damage and failure.

14. A method of impedance injection for a transmission line in a power grid, performed by an intelligent impedance injection module (IIM), the method comprising:

setting injection gain of a plurality of transformer-less impedance injection units (IIUs) of the IIM, coupled to the transmission line, to a first, higher gain state for initial response to a change in power transfer or power flow incident in the transmission line; and

changing the injection gain of the plurality of IIUs to a lower, second gain state after a pre-set time;

wherein the intelligent IIM includes a plurality of switching devices, a capacitor, and a power supply that is to extract power from the transmission line to generate a DC voltage across the capacitor.

15. The method of claim 14 , wherein changing the injection gain to the lower, second gain state after the pre-set time is to prevent buildup of oscillations in the power grid after power flow change has settled.

16. The method of claim 14 , wherein:

setting the injection gain to the first, higher gain state is during a fault, or fault reset or following detection of an increase or decrease in power transfer over the transmission line;

setting the injection gain to the first, higher gain state comprises changing the injection gain from a normal value to the first, higher gain state; and

changing the injection gain to the lower, second gain state comprises resetting the injection gain to the normal value.

17. The method of claim 14 , further comprising:

monitoring the transmission line through a sensor of the IIM, to detect the change in power transfer or power flow incident.

18. The method of claim 14 , further comprising:

receiving high power swing information over a communication link;

determining whether the high power swing information exceeds a fault threshold; and

entering a bypass mode that takes the intelligent IIM off-line, but retaining capability for fault monitoring and communication, responsive to determining the high power swing information exceeds the fault threshold.

19. The method of claim 14 , wherein changing the injection gain to the lower, second gain state comprises changing state of one or more of the plurality of switching devices coupled to the capacitor, in the intelligent IIM.