## 1 Introduction  
As a result of the need to minimize environmental impacts and diversify the electricity mix, the number of grid-connected renewable energy sources (RES), such as wind generators (WGs) and photovoltaic (PV) systems, has been steadily growing. The global cumulative capacity of installed WGs reached 651 GW at the end of 2019, of which 60 GW were installed in 2019 [1]. On the other hand, the global cumulative capacity of installed PV systems reached 627 GW [2]. In Japan, the cumulative capacities of WGs and PV systems reached 3.9 GW and 63 GW, respectively. In particular, the PV market has been growing at a rapid rate. The annual installed PV capacity in 2019 reached 7 GW. With the increase in the number of the grid-connected RES, the number of thermal units composed of synchronous generators (SGs) needs to be reduced to maintain the electricity demand-supply balance. This leads to lower synchronizing power and power system inertia, and thus the transient stability may be negatively affected [3; 4; 5]. In addition, when disturbances occur in the grid, some parts of the power sources without implemented fault ride through (FRT) capability may be disconnected from the grid owing to the voltage drop. The disconnection of the power sources causes a decline of power generation, and thus a further voltage drop may be induced. Ultimately, a large number of power sources may be disconnected from the grid. In this case, due to the decline of power supply caused by the disconnection of many power sources, the conventional SGs may go out-of-step in the decelerating direction [6; 7; 8]. Therefore, this paper proposes a novel control method for enhancing the power system transient stability by using a variable speed wind generator (VSWG). The power converter of a VSWG can individually control the active power and the reactive power. If the active power reference is set to larger value than the mechanical power from the wind turbine, the active power injected from the generator to the grid is compensated for by rotational energy (i.e., kinetic energy). As the response of the energy conversion from the kinetic energy into the electric power is fast, the control strategy using the kinetic energy of the VSWG may be effective for enhancing the stability. On the other hand, the reactive power injected from the VSWG to the grid increases the load voltage and thus the power demand. This leads to enhanced transient stability and reduced release of the kinetic energy of the VSWG. Several control schemes for transient stability enhancement using the active power or both the active and reactive power of a fixed speed WG (FSWG) or VSWG have been suggested [8; 9; 10; 11]. In [8], coordinated control of the active power of doubly fed induction generator (DFIG)-based wind farms and SG tripping was demonstrated. In [9], possible improvements in voltage stability and transient stability resulting from active and reactive power control were investigated. However, a VSWG modeled as a negative load was used for the simulation analysis, and hence the dynamics in the VSWG may have been neglected. In [10], wide area control using both a DFIG and SG to enhance the transient stability was demonstrated. In [11], the active and reactive power of an FSWG were controlled based on a unified inter-phase power controller (UIPC), which was installed between the FSWG and the grid. In this study, the kinetic energy of a VSWG equipped with a full power converter was actively used for transient stability enhancement. Although the rotational speed was adjusted over a large range by using the full power converter, as mentioned above, the speed variation during the control mode could be reduced by the reactive power support. This means that the proposed method can handle more severe disturbances and offer increased reliability as a power system stabilization device. As the proposed control works based on local information, power producers can easily use the proposed control as an ancillary service.