## 2 Implementation of VI-Based Inverters
### Synchrotron  
Synchronverter, also known as SI, is an inverter with a control algorithm which mimics the operation and behavior of an SM [58]. As the term suggests, it is the combination of \"synchronous\" and \"inverter\", which implies self-synchronizing inverter. The underlying operating mechanism of a synchron converter is based on the dynamic swing equations of an SG, which is under the category of SM. The only difference is the mechanical power exchanged with the prime mover or mechanical load in an SM is replaced with the power exchanged with the DC bus in a synchronverter. In general, synchronverter consists of a conventional inverter including the filter inductors, capacitors, and associated controller [59]. It is equivalent to an SM with a small capacitor bank connected in parallel to the stator terminals [60]. Hence, a synchronverter carries over the advantages and disadvantages of an SM, which is based on a complex non-linear system. For example, among the undesirable phenomena, loss of stability due to under-excitation as well as oscillations around the synchronous frequency could occur in a synchronverter. The parameters including inertia, friction coefficient, field inductance and mutual inductances can be manipulated in a synchronverter, unlike in SG. It is notable that the energy loss in the virtual mechanical friction is not lost in reality; it is directed back to the DC bus. This kind of inverters with added virtual inertia, called VSG can contribute to the short-term stabilization of the grid frequency. However, the system dynamics seen from the grid side will be different from those of an SM. The concept of synchronverter is identical to VSM. However, synchronverter exhibits simpler logic control compared to VSM. Instead of using a full seventh-order model, the synchronverter is based on the second-order model swing equation of SM. The second-order model enables the massive reduction of complexity to implement the inertial response of an SM. The simplification of the model removes unnecessary calculations including instantaneous internal virtual fluxes into the control algorithm. The terminology of a synchronverter is to operate the inverter-based DG unit as SG in representing the same dynamic characteristics from the point of view of the grid. It is crucial to allow RES to interface with the power grid without affecting grid frequency and voltage stability. The synchronverter can be modeled with or without the connection of neutral line connected to the grid. In this instance, the synchronverter is modeled without a neutral connection. A conventional DC to AC converter, known as the inverter, is utilized to convert DC power into three-phase AC with certain frequency or vice versa. It typically includes three inverter legs or bridge, which are operated by using PWM in controlling the switching action of IGBT or other switching electronic devices. The LCL filter is used to reduce ripple voltage and current by storing energy in a capacitor and inductor, respectively. A synchronverter is designed to maintain its nominal frequency, maintain its nominal voltage, regulate the generated and consumed real power and regulate the reactive power if it is connected to grid. Synchronverter is modeled by using SG swing equations [61]. Equations (3), (4) and (6) are fundamental equations, used to mathematically describe the dynamic of the SG. Equation (7) defines the vectors of three-phase stator flux (\(\varphi\)), voltage (v), current (i), \(\widetilde{\cos\theta}\), and \(\widetilde{\sin\theta}\). The field excitation current is denoted by \(\mathbf{i_{f}}\) while mutual-inductance between stator and field winding is denoted by \(M_{f}\). \(T_{e}\), \(e\), \(P\), \(Q\), \(i\), and \(\dot{\theta}\) are electromagnetic torque, back electromotive force (EMF) or synchronverter inner generated voltage, active power, reactive power, field current, and Virtual angular speed of the machine, respectively.