Why there are big osicilation in my PSS/E Photovoltaic (PV) project dynamic analysis?

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There is very big oscillation when connecting many PVs to the system and do the dynamic analysis. 1. All the PV generation I set control mode to +,-Q limits based on WPF. And I set the remote bus in dyr file as 0. 2. When no fault applied and more than two PVs are connected to the system, the suspect condition is OK, but the simulation output is not flat as output. 3. When I take all PV units and sub network out, the output is flat. The suspect conditions is OK. 4. When I only connect one PV to the system and do dynamic analysis, the output is flat and suspect conditions is OK. I hope someone can help me. Thank you！！