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

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.

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.