## 6 Conclusions  
This paper presents a comprehensive review of VI-based inverters, including VSM, VSG and, synchronverter. As RES has become increasingly popular over the years, the incompatibility of converter-based non-dispatchable renewable energy generation causes the deterioration of grid frequency stability. Hence, VI-based inverters are designed to regulate the frequency of future converter-dominated power systems by providing an inertial response through the emulation of SM. The grid support by power converters is mandatory under high penetration of RES. The converter-level instability issues due to the grid support, such as the active power support, reactive power support, and virtual inertia control, should be addressed by the VI-based inverters. On the contrary, the system-level stability improvement using the inertia emulated by energy storage units, such as batteries and flywheels, should complement the VI-based inverter to inject more active and reactive power as required during instability transient. Generally, the suitable VI-based inverter topology can be selected based on the required architecture. The selection criteria are based on the current source or voltage source implementation and desired level of equation order in emulating the exact behavior of SG. To emulate the exact dynamics of SG, synchronverter and VSM are an ideal implementation. In contrast, VSG is focused on the dynamic frequency response without emulating the exact behavior of SG. The VI-based inverter is expected to be evolved into a necessity in a smart grid, modern power system, and RES integration as an ancillary service to improve frequency stability of large power grids. Its development will be accelerated due to the increasing penetration of RES, such as grid-connected solar power and wind power. Apart from grid integrator, similar topology from VI-based inverter can be also applied in the advancement of EV, HVDC, normal inverter, etc. As the integration of RES grows, more governments, academics, and industrial manufacturers are expected to fund the research and development of VI-based inverters. The power system operator will emphasize of the necessity of VI as a standardized requirement in a microgrid or conventional power grid to avoid electricity blackout, frequency instability, and unnecessary frequency relay tripping, even under high penetration of RES. The ancillary service of VI will be a mandatory requirement and standardized under the International Organization for Standardization (ISO) or other relevant code of practice. These regulations are vital to maintain the power quality of the grid. The control strategy of VI-based inverters is expected to be modernized as digital control, under AI-based topology. There are many funded companies including synchronized and democratized smart grid (SYNDEM) and European Union (EU) Commissioning focusing on the research and development (R&D) of VI-based inverter. In the next few years, the next generation of VI-inverters with enhanced controller based on the latest technologies such as model predictive control (MPC) and AI are expected to replace the classical proportional integral (PI) controller. The algorithm is evolving into intelligent, adaptive, and MPC algorithm, instead of a conventional PID controller due to its ageing architecture and slower transient response. The emerging strategies including MPC, adaptive VI, enhanced VI by supercapacitors or ESS, AI algorithm, and others are expected to dominate the next generation VI-based inverters. Their aims are to increase the dampening characteristics of VI, decrease the frequency recovery time (transient time), decrease frequency steady-state oscillation error, and improve overall stability as well as algorithm sophistication. Since the VI-inverter is a relatively new technology, there is a lot of room for improvement and enhancement, to match the state-of-the-art SM which has been technologically matured for decades.