Smart Solar PV Inverters with Advanced Grid Support Functionalities. Rajiv K. Varma

installations up to 100% renewables with substantial share of solar PV systems. Several grid impact studies with 100% Inverter Based Resources (IBRs) and Distributed Energy Resources (DERs) with a major component of solar PV systems have already been performed [2, 3]. While these systems significantly help in reducing overall greenhouse gas emissions, they present unique integration challenges which need to be understood and mitigated to derive full benefits from their applications. The solar PV systems are based on inverters. Power electronics technology provides new “smart” capabilities to the inverters in addition to their primary function of active power generation. These capabilities not only help solar PV systems mitigate different adverse impacts of their integration but also provide several valuable grid support functions.

      This chapter presents the concepts of reactive power and active power control, which form the basis of smart inverter operation. The impact of such controls on system voltage and frequency is explained. The different challenges of integrating solar PV systems on a large scale in transmission and distribution systems are briefly described [4]. The evolution of smart inverter technology is then presented.

1.1 Concepts of Reactive and Active Power Control

      1.1.1 Reactive Power Control

      1.1.1.1 Voltage Control

Injection of reactive power at a bus causes the voltage to rise whereas absorption of reactive power causes the bus voltage to decline. Figure 1.1 illustrates a simple power system having an equivalent voltage E and equivalent network short circuit impedance with reactance X and resistance R. An inductor X_L is connected as load at a bus termed Point of Common Coupling (PCC) to show the effect of reactive power absorption. The PCC voltage and inductor current are denoted by V and I, respectively. The impact of reactive power absorption by the inductor on the PCC voltage is examined through phasor diagrams for three cases of network impedance. The phasor diagrams for cases (a) R = 0 (purely inductive network), (b) X/R = 3 (substantially reactive network), and (c) X/R = 1/3 (substantially resistive network) are depicted in Figure 1.2a–c, respectively. The phasor diagrams are drawn with the phasor V as reference, which has same magnitude in all the three cases. The phasor diagrams can also be drawn with equivalent voltage E as reference phasor having the same magnitude, although the conclusions will be the same in both cases.

Figure 1.1 A simple power system with an inductor connected at PCC.

Figure 1.2 Phasor diagrams for network with inductive load; (a) network with R = 0; (b) network with X/R = 3; (c) network with X/R = 1/3.

      In the absence of inductor X_L, the PCC voltage is E. The lagging inductor current causes a voltage drop IR + jIX across the network impedance, thereby reducing the PCC voltage to V. Stated alternately, the reactive power absorption by the inductor reduces PCC voltage by an amount |E| − |V |.

      For case (a) R = 0, it is evident from Figure 1.2a that the change in voltage is directly proportional to network reactance and the magnitude of inductive current I (which in turn is dependent on the size of the bus inductor X_L). Hence for same inductive current, the larger the network reactance, larger is the change in bus voltage. This also implies that higher reactive power absorption (corresponding to higher I) will cause a larger reduction in voltage in weak systems.

      The impact of system X/R ratio is seen from Figure 1.2b corresponding to X/R = 3, and from Figure 1.2c relating to X/R = 1/3. The same amount of reactive current and reactive power absorption in inductor X_L causes a larger voltage drop in the network with higher X/R ratio.

Figure 1.3 A simple power system with a capacitor connected at PCC.

Consider a capacitor X_C being connected as load at the PCC as depicted in Figure 1.3. The impact of reactive power injection by the capacitor on the PCC voltage is investigated through phasor diagrams for three cases of network impedance. The phasor diagrams for cases (i) R = 0 (purely inductive network), (ii) X/R = 3 (substantially reactive network), and (iii) X/R = 1/3 (substantially resistive network) are displayed in Figure 1.4a–c, respectively. In the absence of capacitor, the PCC voltage is E. The leading capacitor current causes a voltage drop IR + jIX across the impedance of the network, thereby increasing the PCC voltage to V. Stated alternately, the reactive power injection by capacitor increases the PCC voltage by an amount |V | − |E|.

      The change in voltage due to capacitive load is thus directly proportional to network reactance and the magnitude of capacitive current I (which in turn is dependent on the size of the bus capacitor X_C), as seen from Figure 1.4a. Hence for same capacitive current, the larger the network reactance, higher is the change in voltage. This also demonstrates that higher reactive power injection (corresponding to higher I) will cause a larger increase in voltage in weak systems.

      The impact of system X/R ratio is observed from Figure 1.4b corresponding to X/R = 3, and from Figure 1.4c relating to X/R = 1/3. The same amount of reactive current and reactive power injection by capacitor X_C will cause a larger voltage rise in networks with higher X/R ratio.

Figure 1.4 Phasor diagrams for network with capacitive load; (a) network with R = 0; (b) network with X/R = 3; (c) network with X/R = 1/3.

      The above analysis demonstrates that a voltage control strategy based on reactive power exchange at a bus will be more effective in weak systems and in systems with higher X/R ratio, i.e. in largely inductive networks. Conversely, reactive power exchange will be less effective in strong systems and also in substantially resistive networks.

      Low‐voltage distribution systems are typically characterized by low X/R ratios. Hence a purely reactive power based voltage control strategy will