This paper investigates the hydraulic short circuit (HSC) operation of pumped hydro energy storage systems, specifically how it can be modelled and its impact on energy capacity, efficiency and operating range. Hydraulic short circuit operation enables power modulation during charging for fixed-speed pumped hydro systems, thereby allowing them to operate more flexibly. In this work, a pumped hydro storage system with 140.5 MW nominal charging power and 10 h nominal charging time is analysed. The storage is simulated via a dynamic, system-level model composed of detailed physical component models to accurately depict the HSC operating behaviour. HSC enables part-load operation from 39 MW to 107 MW but with significantly lower overall system efficiencies. The HSC operating limits are governed by the turbine's minimum flow and maximum power. The custom-built OpenModelica library employed for this analysis can be downloaded from https://github.com/ibeyers/pumped-storage-analysis.