I routinely install and test protection systems used on our transmission grid. The protection devices have a myriad of programmable functions. In most cases, a line from one of our stations to another, has 3 different protection functions employed - current differential, impedance and time overcurrent. These systems are duplicated with a primary and secondary device. If the comms go down, the backup impedance will still be there. If the VT signal fails after that the overcurrent will still be there.
When we have to interface with an IPP using IBR, it's a little scary as the IPP generally won't pay for the FO comms, and the settings on the impedance elements are severely restricted. Impedance relays have been protecting T- Lines for about 70 years and rely on certain properties of faults, namely negative sequence current components to determine if the fault is in front or behind the relay. This, combined with communication to the remote end prevents over tripping of lines. IBR's do not produce negative sequence currents, so we have these very unsophisticated custom protection elements that are used in substitution. Fault currents can normally be several thousand amps, which again the relays can respond to. IBR's produce 1.2-1.3 times their FLA. It makes our protection engineers shake their heads and roll their eyes when discussing this. Out T grid has to accept these IBR's on the system and, I guess, has to work with what we have to try to provide coordinated protection.