The grid-friendly PV plant also includes the ability to ride through specific low and high voltages or low- and high-frequency ranges. required ramp rate. As mentioned earlier, some inverters are turned down while others are shut down. note that the control is quite effective even with moderately varying irradiance conditions. Figure 7 also illustrates the plant start command resulting in the controller gradually increasing the plant output by adjusting the inverters' output and turning on the inverters in sequence. An illustration of an additional function related to active power management is shown in Figure 6, which shows the reaction of the plant when it is shut down and started up. the field data in this case illustrates when one block (30 MW), which is under the control of the plant controller, is commanded to shut down. the active power management function reduces plant output while maintaining the Vt REPC_A Vreg Vref Qref Qext Plant-Level V/Q Control Qbranch Pref Pbranch Plant-Level P Control Fref Freg Q Control P Control Pref REEC_B ′ Iqcmd ′ Ipcmd REGC_A Vt Iqcmd Current Limit Logic Ipcmd Iq Generator Model Ip Network Solution Pqflag figure 9. An overall model structure of a solar PV plant. Ibranch Vreg 1 VcompFlag Vreg - (Rc + jXc) × Ibranch 0 Qbranch + Kc + ∑ + 1 - ∑ 1 + sTfltr 1 RefFlag 0 Qmax dbd emax Kp + emin 1 - ∑ 1 + sTfltr + Qmin 1 + sTft Ki S 1 + sTiv Qext Freeze State If Vreg < Vfrz Qref Plant_Pref Pbranch Pmax 1 1 + sTp fdbd1, fdbd2 Freg - - 0 Ddn ∑ + Freg_ref Dup + femax ∑ + + femin Kig Kpg + S 1 1 + sTlag 0 Freg_flag Pref 1 Pmin ∑ + 0 figure 10. An REPC_A model block diagram. 92 ieee power & energy magazine may/june 2014