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I think you should check following assumptions of your simulations:

1. Did your RoCoFs have the same time frame? Usually, people don't compare RoCoF of the different time frame.
2. Did you simulate the quick frequency response components? That may affect the RoCoF.
3. Check your primary frequency control? Is there any different in the droop of simulations? How many generations taking part in the frequency control in each case? How many active power reserve?

Theoretically, RoCoF at the instant of losing a generator is calculated by the formular:

RoCoF = Delta P/(2H) with Delta P is the active power imbalance and H is the weighted inertia constant of the system.

In which H is calculated by: H = Sigma (Hi*Pi)/(sigma Pi) with Hi and Pi is inertia and power output of generator i.

Given the Hi of solar generation is zero, the increasing in penetration rate of solar will reduce the H of system and increase the RoCoF at the instant.

I think you should check following assumptions of your simulations:

1. Did your RoCoFs have the same time frame? Usually, people don't compare RoCoF of the different time frame.
2. Did you simulate the quick frequency response components? That may affect the RoCoF.
3. Check your primary frequency control? Is there any different in the droop of simulations? How many generations taking part in the frequency control in each case? How many active power reserve?

Theoretically, RoCoF at the instant of losing a generator is calculated by the formular: formula:

RoCoF = f0Delta P/(2H) P/(2Hi*Si) with Delta P is the active power imbalance and H is the weighted inertia constant of the system.

In which H is calculated by: H = Sigma (Hi*Pi)/(sigma Pi) with Hi and Pi Si is inertia and apparent power output rating of generator i.

Given the Hi of solar generation is zero, the SPVs are zero. RoCoF are increase with increasing in penetration rate of solar will reduce the H of system and increase the RoCoF at the instant.SPV penetration rate.