## 5 Future Research
VI-based inverters exhibit larger inertia than droop control with better frequency stability and robustness. VSG control and proposed inertial droop control inherit the advantages of droop control, and, in addition, provide inertia support for the system. The amount of inertia is solely dependent on \(J\), whereas the damping ratio depends on the damping factor (\(D\)) and output reactance (\(X\)). The delay in the governor of VSG reduces the inertia and therefore amplifies oscillation. Hence, the governor delay is unnecessary for VSG control. Contrarily, delay in P droop increases the inertia. VI is gradually becoming an important index and benchmark in microgrids or modern power systems. VI-based inverters have the potential to replace conventional inertia-less droop control [48]. There are multiple research works that have proposed the concept of VI to act as an ancillary service [89; 90]. It is utilized for distorted grid condition to ensure the performance of various power converters. The power grid frequency and voltage amplitude are expected to be regulated by VI. The grid support can be improved by designing active power support, reactive power support and VI-based controller into these supporting components. VI emulation is also applicable to integrate with energy storage units including batteries, flywheels, supercapacitors, and ultracapacitors. The VI is crucial for AC to DC systems to stabilize the conversion of AC to DC. The emulation of any mathematical model-based VI is possible for power electronic system by controlling PWM. The PWM is programmed to emulate any beneficial dynamic and static characteristics of the conventional system. The inducverter and STATCOM are the extended applications of VI. The inducverter is designed to imitate the behavior of the induction generators. It is similar to synchronverter by automatically synchronizing itself without the PLL. STATCOM provides variable reactive in response to voltage variation, similar to reactive power droop control. It serves the same purposes as the VI-based converter, that is supporting the stability of the grid. The Quality of Service (QoS) requirement for smart grid, microgrid, and utility grid is higher, and thus VI is crucial. The open market for power quality control via VI is expected to initiate. The VI-based power converter is designed for the application for electric cars, air conditioners, lighting systems, and electric appliances. The synchronverter is more suitable for an isolated power system because it operates as a  
Figure 14: The applications of conventional SM and VI-based inverters. grid-forming unit. VSG is applicable for interconnected operation as it acts as a grid-following unit with an inertial response. VI emulation has the capacity to develop into standardized electric power ancillary service by the electricity utility company in the future. It includes power quality conditioning, VI emulation, and harmonic resonance damping to upgrade the existing grid-connected RES [91]. This emerging trend is because of the importance of synchronization for a grid-connected converter. VI is a measurable standard in enhancing power converter's capability during interrupted operation in abnormal utility voltage conditions. A robust method with advanced features including expert systems has been identified to inject power into the grid with low total harmonic distortion (THD) of the current [92]. To accelerate the penetration of grid-connected RES, several recommendations should be clearly defined as the following:  
1. The definition of a new grid code and standards: Grid code and standards should enable the integration of large-scale RES into the power grid, by enforcing VI emulation. The frequency regulation should be contributed by RES, and grid-forming inverter should be mandatory. 2. The revision of existing reserve policy: Further technical studies and economic analysis are recommended to revise the current policy to adjust proper frequency reserve margin.