LOW INERTIA SYNCHRONOUS GENERATOR INSTABILITY

I have an HVDC system. The terminals include an HVDC inverter 1000MW rated, a RES 1000MW rated, another inverter 500MW, a load and a generator using synchronous machine. When i use a machine rating at 1000 MVA, everything works. When i try to reduce the rating to 500MVA maximum, in order to have a case where a country is in its critical case of lower inertia, i get instability when both inverters (1000MW and 500MW) are connected to the system. when i disconnect one of them the system is stable. For the synchronous machine i only changed the Rated power and the transformer voltage to scale down. not the H etc.

Does anyone know how can i fix this?

[Viktor Döhlen]

Is it the frequency that becomes unstable or the voltage? How does your frequency and voltage droop control look like?

[Eleni T]

All the system oscillates, both the voltage and frequency and power etc..

The inverters are grid-following, they have PLL, a PQ controller, and current controller

[Sam Chak]

Hi @Eleni T

To investigate a stability issue like this, it is necessary to examine the mathematical models of the synchronous generator and the controller. All electrical components of the generator can be expressed as mathematical equations. Analyzing the closed‑loop system using Lyapunov methods will generally indicate whether the controller is adequately designed.