###### Abstract
Any communication failure could destabilize the whole HVDC grids, which in turn has made the dynamic braking resistor (DBR)-based local control more effective to deal with excess energy in the HVDC [18; 21; 22]. Although DBR (DC chopper) is suitable for wind energy-connected VSC-HVDC system due to the slow response of the wind generator, the PV system can rapidly change the output to control the energy flow in the HVDC link because the photovoltaic system has no inertia and power electronics converter response is fast [23]. Work related to voltage and frequency regulation, reactive power support to AC grid, and active power curtailment during the high frequency of AC grids of PV inverter have been reported in [6; 7; 8]. Usually, active power curtailment is one of the desirable features of a PV inverter for its utility with high PV penetration [24; 25; 26; 27; 28; 29]. PV inverter curtails its output based on frequency rise from the threshold. Similarly, it could inject more energy during the frequency drop if it is operated below the maximum power point [25; 26; 27; 28]. A fuzzy logic controller was employed to control the frequency of the PV-battery-diesel microgrid [27], where battery-free operation has been suggested if the PV system operates below the maximum power point. Droop control-based frequency regulation for PV-connected AC system has been discussed in [30] without taking variable solar radiation and temperature into consideration. Without providing inverter control details, primary frequency response has been addressed for a PV inverter-connected PV simulator system in [24]. An adaptive droop-based frequency and voltage regulation was presented in [30]. A utility size practical exhibition of a PV plant for supporting primary and secondary frequency regulation using active power curtailment has been discussed in [31]. The common feature for all works mentioned in [24; 25; 26; 27; 28; 29; 30; 31] is the manipulation of PV power based on grid frequency command. However, frequency usually remains constant for MMC supported standalone AC grid for renewable energy integration. Hence, other features of the HVDC system need to be explored for active power reduction during the fault. Unlike PV integrated AC grids, fault ride through improvement for PV-integrated VSC-HVDC system employing active power reduction has not been properly explored [23]. A flywheel energy storage is a fast and high power density device and presents high performance, which could be used for excess energy control during the fault. A flywheel energy storage-based fault ride through support has been discussed in [32; 33; 34], where excess energy is stored in the form of kinetic energy depending on the rotating mass and speed. On the other hand, the supercapacitor is also a short-duration energy storage device with high power density and fast response. It has similar potential for fault ride through support of VSC-HVDC link during AC side fault [35; 36]. Besides fault ride through improvement, both systems could be used for power levelling purpose during normal condition. However, both systems are expensive and suffer high energy loss over a longer period. Therefore, it has limited use in the field except for research purposes. In contrast, the cost of a battery-based energy storage system has been going down over the years [37]. Besides, the battery has high energy density and can retain charge over a longer duration. Therefore, many utilities are now utilizing battery-based energy storage system for power levelling, frequency and voltage regulation, and fault ride-through improvement. Usually, transient fault demands a faster response from the energy storage system. The battery is a high energy density but low power density device that means it needs to be oversized to support the transient fault response. Although oversizing increases the cost for energy storage, it increases the energy delivering capacity for a longer duration to mitigate the energy shortage caused by solar radiation, temperature and wind speed change.