*4.1. L*/*HVRT Controller*

The L/HVRT Controller controls the voltage to keep it within a permissible VRT range in an overor under-voltage situation, which may occur by the ratio of generation to load after recloser opening. If a temporary fault occurs while operating with active and reactive power reference ( *Pre f erence*, *Qre f erence*) in steady-state, the recloser is open and the fault condition, which is unbalanced voltage, is maintained and after the temporary fault is eliminated, it changes to islanding operation state under balanced voltage. At this time, as shown in Figure 4a, the output reference d-axis current value of the controller is controlled through the PI (Proportional-Integral) controller so that the PCC voltage remains within VRT range. In this paper, the rms voltage *VRMS* at the PCC is considered to be controlled to maintain *VRMS-reference*, namely 0.5 p.u., which is the permissible range of VRT operation of IEEE 1547, regardless of the ratio of generation to load. At this time, as shown in Figure 4b, active power output is controlled by the q-axis current value *Iq\_control*, which is the square root of the square of the d-axis current value *Id\_control* subtracted from the square of the rated maximum current *IM* of DG to be controlled within the rated range of the DG output.

**Figure 4.** Block diagram of Low/High Voltage Ride Through (L/HVRT) controller. (**a**) *Id* Current Controller; (**b**) *Iq* Current Controller.
