6. Small-Signal Stability of a Power System with a VSWG Affected by the PLL

Grid connection of a VSWG to a power system is realized by the converter control, which normally adopts the current vector control algorithm. This has been introduced in Chap. 2. Implementation of current vector control needs to track the relative positions between the d − q coordinate of the converter and the common x − y coordinate of the power system in order to determine the d − q coordinate of the converter. As being illustrated by Fig. 2.​8, the direction of the terminal voltage (PCC voltage) of a PMSG in x − y coordinate of the power system is normally taken as that of d axis of d − qcoordinate of the GSC of the PMSG. Figure 2.​10 shows that the direction of the terminal voltage (PCC voltage) of a DFIG in x − y coordinate of the power system is often taken as that of d axis of d − q coordinate of the RSC of the DFIG. Hence, by tracking the phase of the terminal voltage of the VSWG, d − q coordinate of the converter can be determined for implementing the current vector control. This task of phase tracking is normally fulfilled by a phase locked loop (PLL).