The development of converter-based renewable generation (CRG) has fundamentally changed the power system dynamics. As CRG replaced conventional generations, the system oscillation modes changed. Although CRG can provide voltage support, the reduction in the inherent damping and inertia of synchronous generators results in decreased damping of low-frequency electromechanical oscillations.

To address this issue, this paper proposes a framework based on modal analysis for optimizing the deployment and parameter tuning of power system stabilizers (PSS) in CRGs with grid forming (GFM) controls. The proposed method systematically identifies the most effective converter locations and assigns them to specific oscillation modes.

First, modal analysis of the power system including GFMs is carried out. By linearization around the operating point, the system state-space model is obtained and critical oscillation modes can be found. The eigenvalues and the participation factor matrix are evaluated to identify the contribution of converter states to each mode dynamics. In addition, modal controllability and sensitivity are analyzed to assess the effectiveness of each converter in influencing specific modes.

Based on the modal analysis, the converters are ranked according to their ability to damp:

Local oscillation modes: dominated by neighboring generator interactions

Inter-area oscillation modes: involving coherent generating groups across areas/countries

Then PSSs are selectively put into the control loops of the most effective converters. The controller output can be fed into either active power control loop or reactive power/voltage control loop.

Similar to conventional PSS, the GFM PSS consists of a gain, a wash-out filter, and lead-lag blocks, whose parameters are tuned according to modal frequency and phase compensation requirements.

A key feature of the proposed method is the mode-oriented allocation of damping functions, where different converters are explicitly assigned to mitigate different oscillatory modes. This targeted deployment mitigates adverse interactions among multiple controllers.

A multi-machine system is simulated in DIgSILENT PowerFactory with high renewable penetration. The results demonstrate that:

The proposed strategy significantly enhances damping of both local and inter-area oscillations

Oscillation amplitudes are reduced and settling times are improved

Compared to the uniform deployment of PSS, better performance can be achieved using selected PSS locations

Simulation results confirm that GFMs, when properly coordinated using modal analysis, can effectively contribute to oscillation damping and enhance angle stability in large power systems.