## 5 Impact of the Control Strategies in the Reciprocal Behavior between MVG and LVGs
### Impact of Different Control Strategies on the Behavior of MVG
#### 5.2.1 Influence of the DTR Connection Point on Q\({}^{\rm ex}\)  
LVGs are connected via DTRs to various points along the MV feeder. As known, the voltage varies along the feeder from bus to bus, and additionally, it changes in each bus depending on the power flows throughout the day. Since the DTRs have fixed taps, the MV feeder voltage changes are forwarded directly to the LVG via the transformer ratio. This means that voltage changes on the MVG directly affect the voltage at the LVGs, and thus also directly affect the operating points of the controllers and the \(Q^{ex}\). Figure 23 shows the daily \(Q^{ex}\) for different control strategies and various LVGs connected to the MV feeder. To understand the influence of the DTR connection point on the \(Q^{ex}\), two connection points are selected: the first at the feeder beginning, and the second at the feeder end, as shown in Figure 23e. Due to the feedback active power supply from low to medium and then to the high-voltage grid, the voltage at the end is higher than at the beginning of the MV feeder. Thus, the voltage at the beginning of the feeder is assumed as 1.01 p.u., while at the end, it is assumed as 1.06 p.u. Power flow simulations are performed on both LVGs as described in Section 4.1 by sequentially setting the two different voltages on the slack bus--i.e., the 20-kV bus of the DTRs. Figure 23a,b correspond to the daily \(Q^{ex}\) for the urban LVG connected at the beginning and end of the feeder, respectively, and when distributed \(cos\varphi(P)\), distributed \(Q(U)\), or concentrated \(L(U)\) local control strategy is exercised. Simulation results for the rural grid are shown in Figure 23c,d. \(Q^{ex}\) consists of the reactive power of load and loss as well as the additional reactive power produced by different voltage control strategies. The reactive power of load and loss depends on the voltage, but this dependence is barely visible in the figures due to the large \(Q^{ex}\) scale. The dependence of the additional reactive power on voltage is determined by the control strategy used. The reactive power produced in case of the \(cos\varphi(P)\) control, \(Q^{cos\varphi(P)}_{inv}\), is independent of the voltage. \(Q^{cos\varphi(P)}_{inv}\) is present at the same period 08:50-15:30 in all of the cases, as shown in Figure 23a-d. The distributed \(cos\varphi(P)\) control is active in accordance with its characteristic, as shown in Figure 19a. Unlike this case, the additional reactive power flow provoked by exercising the distributed \(Q(U)\) control strategy, \(Q^{Q(U)}_{inv}\), strongly depends on voltage at the DTR's connection point. The \(Q(U)\) control may be active even in periods without solar irradiation as long as its local voltage in LVG is outside the dead band of the control characteristic, as shown in Figure 19b. This case is clearly visible in periods 00:00-07:30 and 17:00-24:00 in Figure 23b; 00:00-07:30 in Figure 23c; and 00:00-07:30 and 17:00-24:00 in Figure 23d, where \(Q^{ex}\) is larger than in the \(cos\varphi(P)\) case. In all of the cases, the maximum provoked \(Q^{ex}\) is reached at the critical time, \(t_{crit}\). Around this time, the \(cos\varphi(P)\) control provokes a much larger \(Q^{ex}\) than the \(Q(U)\) control for the urban and rural LVG when the DTRs are connected at the feeder beginning, as shown in Figure 23a,c. When the DTRs are connected at the feeder end, the difference of \(Q^{ex}\) is smaller in the urban LVG case, as shown in Figure 23b, and it becomes almost zero in the rural LVG case, as seen in Figure 23d. As shown in Figure 23a-d, the concentrated \(L(U)\) local control strategy causes the smallest \(Q^{ex}\) in all of the cases. \(Q^{ex}\) depends on voltage at the DTR's connection point, because the \(L(U)\) control is active as long as its local voltage in LVG exceeds the \(U_{set\text{-}point}\).