## 4 VSG Applications of Power Electronics-Based Units
### Wind Turbine  
In wind farms, the most common types of wind turbines are the permanent magnet synchronous generator (PMSG) and the doubly-fed induction generator (DFIG). The typical schematic for WT generation that is connected to a grid is shown in Figure 7. The power electronic part of a WT often contains back-to-back converters. The conventional WT control strategy is implemented by vector control (VC) based on a phase-locked loop as a synchronizing technique, which decouples the mechanical part and the electrical part [32; 33; 34]. The grid-side converter (GSC) extracts the maximum amount of available power and feeds it to the grid without considering the fluctuation of the system frequency or the rotation of the mechanical parts. Thus, despite the actual spinning parts, the WT provides no inertia contribution or power oscillation damping for the grid. The interaction between the wind farm and the grid only consists of several conditions related to fault ride-through. Through modifying the control strategy and system assembly, a VSG-based WT can provide inertia emulation and power oscillation damping to the grid, as well as work in a standalone scenario and respond to dispatch orders from the upper-level operator [35].  
Figure 6: Schematic of VSG modeling.  
The implementation of a VSG on a WT should consider the operational limit and economic factors. Basically, there are two ways to execute inertia emulation and active power damping. One way exploits the hidden kinetic power stored in the rotating parts of the WT such as the blade and rotor. The kinetic power stored in the rotating parts responds to grid frequency fluctuation by releasing/storing active power and providing damping with inertia for the connected grid. Oscillation damping is achieved through fast converter control of the spare generation capacity of the WT. Correspondingly, the operation point transfers from the maximum power point tracking (MPPT) control curve to the virtual synchronous control curve, as the available operation reserve is between the two. In the VSG mode, the available regulation ability should be monitored for smooth recovery, and the operator should be alerted if unidirectional long-time frequency deviation occurs, as it indicates that the wind farm has lost its response ability [36].  
The other mode uses the attached energy storage apparatus for active power response [37]. In this mode, a fast storage apparatus is added onto the inner DC bus of back-to-back converters and controlled by a fast bidirectional converter. The machine-side converter (MSC) regulates power balance through rotor speed control, while the grid-side converter (GSC) performs VSG control. The storage-side converter (SSC) is performed to shift work modes according to its state of charge (SOC), which can be divided into an upper power limit, normal VSG operation, and low SOC recovery, as shown in Figure 8. The storage apparatus acts ahead of the VSG-GSC as a fast cache to feed/absorb active power for oscillation damping and mitigate the deviation and rate of change in the DC bus, which reduces the impact that constant frequency disturbance has on the GSC. Furthermore, the WT is available for standalone operation, and it is also dispatchable. However, the capacity of energy storage is theoretically assumed to be large enough, but its economic feasibility and maintenance should be further considered.