## 2 Implementation of VI-Based Inverters
### Synchrotron
The frequency droop control loop controls the real power, while the voltage droop controls the reactive power. The frequency droop compares the \(\dot{\theta}\) with the \(\dot{\theta}_{r}\) which normally equals the nominal angular frequency of the grid (\(\dot{\theta}_{m}\)). This difference is multiplied with a gain to the active mechanical torque, \(T_{m}\). The mechanical friction coefficient and the frequency drooping coefficient is denoted as \(D_{p}\). This frequency loop regulates \(\dot{\theta}\) of the SG and creates the phase angle (\(\theta\)) for the control signal (\(e\)). The regulation of reactive power flowing out from synchronverter is executed similarly to frequency droop. The voltage droop control is using the voltage droop coefficient (\(D_{q}\)). It regulates the \(M_{\textit{rf}}\) which is proportional to the amplitude of the voltage generated. In general, a synchronverter has power part and electronic part. The power part is the conventional inverter structure with switching devices (metal-oxide-semiconductor field-effect transistor (MOSFET) or insulated-gate bipolar transistor (IGBT)), gate driver, power sources, inductor-capacitor- inductor (LCL) filter, and other electronic components to convert DC to AC. On the contrary, the electronic part of synchronverter is a control unit comprising of sensors, analogue to digital converter (ADC), and measurement devices, interfaced to a microcontroller, DSP, or any computing devices. The function of the electronic part is to control the gate signal of switching devices in the power part via PWM. The modeling of synchronverter is based on the dynamic characteristic of an SG. The mathematical model ranges from second to seventh order. The higher is the order, the more accurate is the modeling, but with higher complexity and longer calculation time. Hence, the third order is most popular due to its balance between accuracy and complexity. Among the proposed control strategies, the synchronverter designed by [PERSON] et al. is the most comprehensive terminology. Their research is the first proposed synchronverter with PLL in 2011 [61]. The original version of synchronverter has been improved since then. The improvements include self-synchronizing synchronverter without PLL [62], digital synchronverter design [63], and bounded frequency and voltage for safety purpose [64]. [PERSON] et al. improved the original synchronverter by eliminating the need for a dedicated synchronization unit (PLL) for initial synchronization. Instead, a self-synchronizing synchronverter has a PI controller in the frequency droop. PLL is replaced by an active power synchronization loop with virtual impedance. By eliminating PLL, it reduces the development cost, complexity in tuning, and computational time [65]. In general, PLL is complex because of the inclusion of a _dq0/abc_ Park's transformation block, quadrature signal generation block, non-linear calculation block and a low pass filter. It is time-consuming, complicated, and complex to fine-tune the PLL parameters to achieve satisfactory performance [66]. The performance of PLL degrades with the presence of steady-state error, harmonic distortions, and voltage sags in a weak grid. Another improvement is bounded frequency and voltage, which ensures synchronverter always operates in a stable region [67]. Virtual inductors, capacitors, resistors, and anti-windup have been added to the original synchronverter to get better performance. Overcurrent protection is also added to the original synchronverter for safety purpose since synchronverter is a voltage-controlled controller and does not have inherent current protection. The synchronverter is scalable and available to integrate multiple RES including wind generator and solar photovoltaic system. It consists of angle, frequency, and power control loops [68]. Typically, the design of a synchronverter is based on second- to third-order system models that represent the dynamic characteristics of an SG sufficiently.