Grid Support Current central inverters can provide a host of functions to support the distribution grid or bulk power grid interconnections. Voltage control (VoltVAr) uses reactive current to stabilize the ac voltage at the terminals of the inverter. This characteristic works like a proportional controller and uses positive sequence current only. The control keeps the voltage in a limited range and is designed to increase the stability of the grid. The inverters do not " fight " each other because im - pedances exist between them, comprising transformers and ac cabling. The inverter voltage control characteristic can be combined with a plant controller to provide point of interconnection (POI) voltage controls that respond to both voltage transients and slower voltage variations. With this combination voltage control setup, all plant inverters receive reactive power commands from the plant controller (slow, ~150 ms) to maintain a POI voltage set point. The individual inverters are also set to VoltVAr Weak grid and short circuit characteristics are also important topics as inverterbased generation continues to increase on utility grids. control to provide reaction to voltage transients (fast, <20 ms). Figure 5(a) depicts a voltage dip, and Figure 5(b) illustrates the corresponding inverter reaction of the reactive current. PV inverters can also be configured to provide 24/7 grid voltage support by providing reactive current at night. This function uses a small dc power supply to energize the inverter dc bus from the ac grid connection. Once energized, the IGBTs can be commanded to provide reactive current at night. In addition to voltage control, inverters can be set for reactive current injection during an FRT event. This is a separate feature that tries to increase the positive sequence current while decreasing the negative sequence current. The FRT controls can also be set to simply stop gating the IGBTs during a fault. This results in momentary cessation of all ac output current from the inverter, or limited FRT support. FRT can also be disabled, resulting in inverter tripping during grid voltage or frequency excursions. 1.016 1.015 1.014 1.013 1.012 1.011 Vol_PsNom 127 127.5 128 128.5 Time (s) (a) -0.142 -0.144 -0.146 -0.148 -0.15 -0.152 -0.154 -0.156 -0.158 -0.16 -0.162 127 127.5 128 128.5 Time (s) (b) Figure 5. The inverter's reactive current in response to a voltage dip. (a) AC voltage. (b) Reactive power. p.u.: per unit. IEEE Electrification Magazine / JUNE 2021 47 VArNom Dead Time: <20 ms Settling Time: <1 s 129 129.5 129 129.5 (p.u.) (p.u.)