Reliable balancing of active power generated via wind farms is vital for power system stability and for maintaining system frequency deviations within acceptable limits. This paper presents a backup power balancing technique for the energy-fed voltage source converter high voltage DC transmission systems during different AC side faults based on flywheel energy storage systems. The proposed technique aims to prevent the DC link voltage rise during faults which reduces the voltage and current stresses on the switching devices. An induction machine (IM) based flywheel energy storage system is connected in parallel with the onshore side converter; therefore, the trapped energy in the DC link during AC faults can be stored in the flywheel. During normal conditions, the flywheel storage system is normally used for power leveling. A simulation case study for the proposed system using a 100MW HVDC system is presented, while experimental validation is carried out using a 2.2kW prototype flywheel IM storage system.