ABSTRACT This article investigates the effect of hydrogen aqua equaliser (HAE) with hydrogen repository tank and fuel cell (FC) as an energy storage element for frequency control in an interconnected power system. The system under study consists of a combined structure of hydro – thermal along with wind, diesel and gas units as well as energy storage elements. An advanced fuzzy-based PDF plus PI controller is proposed and a Sunflower optimisation (SFO) technique is used to optimise the controller parameter. The superiority of the prospective SFO algorithm is demonstrated over other techniques like genetic algorithm (GA), differential evaluation (DE) & particle swarm optimisation (PSO). The projected FPDFPI controller is studied over PID and PDFPI controller to show its efficacy. The effect of the HAE-FC units in an interconnected power system is studied. Sensitivity analysis and OP-5700 OPAL-RT Hil module-based Hardware-in-the-Loop (HiL) real-time simulation has also been carried out.