A hierarchical-zoned capacity planning methodology for hybrid power systems connected via cascade multi-terminals transmission: a case study

Given the long distance between geo-dispersed variable renewable energy (VRE) and cascaded hydropower, an innovative and flexible integration of cascaded multi-terminals (CMT) technology has been adapted. Three key issues for hybrid power system, that zoning impact on its optimization, flexibility dispatch between hierarchical converter stations, and solutions under power balance constraints for two-level zones need to be addressed. The strategy leverages the flexibility of pumped storage hydropower (PSH) and cascaded hydropower within each zone, supported by cross-zone hydropower flexibility dispatch, to satisfy the operational demands of CMT and VRE and achieve system optimization. First, a hierarchical-zonal dual-layer model is proposed for Hydro-base integrating dispersed VRE. The model divides the Hydro-base into two zones connected via converter stations, with the two zones linked hydraulically. The method optimizes the operation of the system and satisfies the power balance between the hierarchical converter stations by actively adjusting the hydro-PSH flexibility between upper and lower zones. Second, mathematical formulations for power balance between the hierarchical converter stations and embeds the constraints into the zonal simulation based on voltage stability principles and power balance theory. Nonlinear constraints are linearized to enable solution via MILP, enhancing model solvability and computational efficiency. Case study reveals that: (1) The proposed HZCM model is solvable and feasible, facilitating flexibility adjustment; (2) VRE to hydropower capacity ratio ranges 0.94–1.07:1, optimal hydropower-PSH to VRE ratio is 1:1.60. PSH flexibility enhances VRE capacity; (3) Hydropower flexibility breaks zonal isolation. Zones with higher flexibility resources share 0.5–0.55 of VRE capacity, reducing costs and VRE curtailment.