The high penetration of variable renewable energy raises a flexibility challenge in the power system. This raises the necessity of considering the adequacy of flexibility in power system planning. However

the flexibility of the power system covers a wide range of timescales

from seconds to months. This poses difficulties in planning of multi-timescale flexible resources. This paper proposes a new perspective on the modeling and planning of multi-timescale flexible resources in power systems with high penetration of variable renewable energy. The operational boundaries of flexible resources are transformed into a characteristic domain

where flexibility at different timescales can be added and the balance of flexible supply and demand can be expressed as algebraic equations. Such modeling facilitates rigorous multi-timescale flexibility balance metrics. Furthermore

a planning method for multi-timescale flexibility is proposed based on the model in the characteristic domain. The proposed planning method is tested using data from China's Xinjiang provincial power grid. Results show the proposed method can characterize multi-timescale flexibility with high accuracy

thus making it possible to fully account for flexibility at different timescales.