## 4 Comparison of VI-Based Inverters  
The technical implementations and control strategies of VI-based inverters consisting of VSM, VSG, and synchronverter are compared and evaluated extensively to discuss their key differences. The comparison highlights the features, merits, and demerits of each VI-based inverter. Droop control is excluded from the study because of its inability to provide an inertial response. Table 7 presents a comprehensive comparison of VI-based inverters, which consist of VSM, VSG, and synchronverter. All VI-based inverters are voltage source inverter (VSI) under the category of VSC, which basically generates AC voltage from a DC voltage. Although VSM, VSG, and synchronverter are based on SM-based VI terminology, there are some slight differences among these VI-based inverters. First, the major difference is that synchronverter is a voltage-controlled inverter (via amplitude and frequency) while VSM and VSG are current-controlled inverters. The voltage-controlled inverter provides grid voltage support while the current-controlled inverter provides high-quality grid current. Second, VSM and VSG are unable to work in the standalone (islanded) mode because they are current-controlled inverters, which exhibit inappropriate sharing of reactive power and stability constraint [82]. In other words, VSM and VSG only operate in grid-connected mode. On the contrary, synchronverter can work in both standalone (islanded) and grid-connected mode because it is based on a voltage-controlled inverter. Unlike VSM and VSG, synchronverter inherits appropriate sharing of reactive power and high system stability. Third, the topologies of VSM and VSG are very identical to the conventional droop control with larger inertia. Synchronverter has a more comprehensive and universal controller to integrate inverter with VI. In general, current-controlled inverters including VSM and VSG are ideal to drive low impedance loads and feed excess solar energy into the grid. Conversely, synchronverter which is based on a voltage-controlled inverter is suitable to feed isolated loads and supply power to individual appliances since it is applicable for islanded mode. For the denotation, for VSM, \(P^{*}\) is the virtual mechanical input power, \(P\) is the active power, \(T_{a}\) is the mechanical time constant, and \(P_{d}\) is the damping power. For VSG, \(P_{O}\) is the initial power, \(P_{VSG}\) is the output power, \(d\Delta\omega/dt\) is the RoCoF, \(K\omega\) is the inertia emulation characteristic, and \(K_{D}\) is the damping factor constant. For synchronverter, \(P_{SET}\) is the desired active power setpoint, \(D_{P}\) is the damping factor, and \(\mathrm{f}_{\mathrm{ref}}\) is the reference frequency. There are two types of VI-based inverters, namely grid-forming and grid-following inverters. It is to differentiate the type of VI-based inverter in term of their participation in the regulation of voltage and frequency. Grid-following inverters are also known as a grid-feeding inverter or P-Q control. Table 8 shows the key differences between grid-forming and grid-following inverter. In general, conventional inverter and VSG are categorized as a grid-following inverter, whereas synchronverter and VSM are the grid-forming inverters. As the name suggests, the grid-following inverter is incapable of regulating voltage and frequency because it interfaces to a utility grid with well-regulated voltage and frequency typically. A grid-following inverter is achieved by current source inverter which is based on a current controller with PPL. The grid-following inverter is also known as a grid-feeding inverter or P-Q control. In contrast, a grid-forming inverter is employed by using voltage source inverter. It is equipped with virtual oscillator control and droop control to control the voltage and frequency of the power grid. The grid-forming inverter is a power conversion system for energy storage applications. It assists and participates in the regulation of both voltage and frequency via active and reactive power control.