The financial value of security service provided by dual-branch controller of a large-scale photovoltaic system (LPS) has been investigated in this paper. To perform this task, it is necessary to investigate the impact of LPS on small-signal stability of power network. To study the impact and performance of LPS on low-frequency oscillatory modes, a systematic scheme is developed to achieve an accurate model of multi-machine power network including of the LPS technology. A fully-described LPS model with all network-relevant control functions will be obtained. The proposed model has been then employed in the study of financial compensation strategy of security service provided by the LPS controller. The proposed compensation strategy in this paper is based on correlation of swing damping rate (SDR) criterion with generation cost (GC) criterion as dynamic and pecuniary indices, respectively. A weighted sum of performance and robustness indices is considered for SDR. The financial profit created by dual-branch controller of LPS as ancillary service (AS) is evaluated by a two-purpose optimization based on SDR and GC indices. The proposed strategy is tested on a modified 6-machine 3-area power system. Comprehensive discussion is provided to describe the performance of the presented method.