Implementing High Pass Filter in Simulink

"Oscillation frequency" is a term that describes the frequency where the input-output phase relationship is exactly zero degrees i.e. if this filter were part of a circuit used as an oscillator it would oscillate at this frequency. Sometimes oscillation frequency is quoted at 180 degrees phase shift (inverting filter configurations)

It doesn't mean the circuit on its own will oscillate. Look at the results of an RLC low pass filter: -

Clearly this circuit doesn't oscillate but, all the same, it has an oscillation frequency.

Active damping, for the T LCL filter in your example, can be simplified like this:

1. either get the voltage at the junction (between the inductors), differentiate it, multiply it by a damping constant (sqrt(2) is an optimal [?, not sure] choice), then subtract it from your reference value, or

2. get the current through the capacitor, multiply it by the inverse of the damping constant, then subtract it from your reference value.

The reference values are in whatever frame you're working on. If you did the Clarke, the Park, or whatever transform you concoct up. I made a simple example once, here's a screenshot:

It's for a single phase, only, but you can see this would have been a Clarke transform. The yellow blocks are, from left to right, the PID (PI) controller, the 3-level PWM controller, the bridge. The virtual damping is modelled with both voltage and current, but only one of them is active, set by one of the parameters, so only consider either the lower branch (voltage+diff), or the upper branch (current), not both. The whole ordeal could have been more tiier, particularly the PID values, but, as I said, this is an example, only. I hope it helps.