## 5 Future Research
### Current Challenges and Limitations  
To date, VI-based inverters have several critical issues and limitations. First, the frequency detection and measurement should be accurate, persistent, and consistent for precise control. It is because VI emulation solely depends on frequency variation. It is challenging in term of detailed modeling and complex algorithm development. The cost-effectiveness, algorithm complexity, and computational requirement of VI emulation should be considered as well [93]. Second, the PI systems of VSG, VSM, and synchronverter are analog-based feedback system. The transient response is relatively slow compared with a fully digitalized system. The instability of mathematical modeling and numerical aspect is present. Although the PI system is technologically proven, its scalability, adaptability and lack of self-learning mechanism are the common drawbacks and limitations. The choice to emulate which order of SG is the balance between accuracy and complexity. The higher is the order of SG, the higher is the complexity of the system but with higher accuracy. The most common order of SG is between second and third order. Despite the increasing research on VI-based inverters, there are challenges and technical gaps regarding the integration of RES with the power grid. The existing knowledge of traditional power system operation is unable to replicate completely via VI-based inverters. The present technical gap of VI-based inverters is illustrated in Figure 15.  
Most of the proposed implementations are based on conventional PI controller and verified by small-scaled experimental setup only. The tuning of the PI controller parameters is performed by small-signal analysis approaches. It means that the current research works are only valid under certain assumptions. The tuning process of proportional gain (\(K_{P}\)) and integral gain (\(K_{I}\)) must be robust. It is to ensure that transient response, settling time, overshoots, and oscillations are satisfactory. The robustness of the PI controller is limited because operating environments such as temperature, weather, power surge, sudden load change, and occurrence of faults may affect the performance. The PI controller is not adaptive and does not learn over time. It is required to fine-tune the controllers to ensure the system is stable for every new model. The current limitation of existing VI technique is the requirement of fine-tuning for the frequency droop coefficient, inertia constant, and load frequency controller parameters. The tuning process cannot guarantee the frequency stability of microgrid with low inertia. System reliability should be emphasized to continuously access the robustness of frequency regulation [94]. Generally, VI-based inverter is a relatively new design for a modern power system. Due to its new conceptual design and architecture, there are still many unexplored areas yet to be resolved.  
The implementation of VI has its own areas of controversy and conflict. It includes the problems relating to low inertia and consequential loss of generation equipped with RoCoF protection to identify islanding conditions. Various actions have been taken to make protections less sensitive to mal-operation and low inertia. In the future, VI-based inverters are expected to be equipped with advanced capabilities. It should create system voltage without the supply of clean AC power, support power restoration after the blackout, contribute to fault level that supplies fault current in phase with the system without any delay, and be compatible with the existing synchronous plant. VI-based inverters should act as a sink to counter harmonics, inter-harmonics, and unbalance in system voltage.