# Solutions to Increase PV Hosting Capacity and Provision of Services from Flexible Energy Resources
## 2 Simulation Model and Study Cases
### Drop Control of the DER Units  
Figure 1 in Section 1 presented the type of control functions applied in the studies for WT, PVs, and BESSs. Voltage control of MV and LV distribution network is managed by HV/MV substation OLTC setting as well as by _QU_- and _PUI_-droop control of PV and BESS units. In the simulations _QU_-droop control of the DER units is used as the primary local voltage control method and _PUI_-droop control as the secondary method in order to avoid unnecessary \(P\) curtailment of PV units. Increased reactive power \(Q\) production (Equation (1)) of cables during very high voltages and very low load currents can potentially lead to local overvoltage situations in the distribution network and therefore seasonal OLTC setting values could be used to improve DER hosting capacity of the network. \[Q_{\textit{cable}}=Q_{\textit{production}}-Q_{\textit{consumption}}=\omega CL ^{2}-3\omega L^{2}, \tag{1}\]  
where  
\(\omega\) = angular frequency,  
\(C\) = cable capacitance,  
\(U\) = cable voltage,  
\(L\) = inductance/phase,  
\(I\) = cable current. In order to improve distribution network PV hosting capacity, the OLTC setting during very high load (winter season, total load 6548 kW, 283 kVAr) was chosen to be 20.3 kV (1.015 pu). Reactive power \(Q\) produced by cables is dependent on voltage (Equation (1)) and therefore lower OLTC setting is more suitable during summer time when cable load current \(I\) dependent reactive power consumption is also smaller. In the simulations during very-low load (summer season, total load 970 kW, -660 kVAr) the set value of OLTC was 20.0 kV (1.0 pu) with dead-zone \(\pm\) 0.3 kV/0.015 pu and 7.0 s operation time delay (due to short simulation time \(t\) = 110 s). Total load includes also \(P\) losses and \(Q\) produced/consumed by cables and overhead lines. Respectively, seasonal _QU_- and _PUI_-droops should be utilized for DER units in order to improve the DER hosting capacity of the distribution network. In addition, the seasonal _QU_- and _PUI_-droops should be compatible with the seasonal OLTC setting values as well as coordinated between different type of units. This means that, for example, _PUI_-droop settings of PV and BESS units at the same connection point should be coordinated so that unnecessary active power curtailment of PV unit due to local overvoltage can be avoided by simultaneous charging of the BESS unit. The used _QU_-droops, _PUI_-droops, and _Pf_-droop of PV and BESS units are shown in Figure 2. It can be noted that BESSs 1 and 2 (Figure 1) have also the \(Q_{\text{unb}}\)-control responsibility in order to enable correct operation of passive islanding detection methods [40; 41; 42]. In many grid codes, like in ENTSO-E NC RfG [43], the required reactive power feeding/absorbing capability of the DER units is defined to be range \(\cos(\varphi)=0.95_{\text{ind/cap}}\) and that has been also used in this paper for defining the reactive power limits of the DER unit (PV and BESS) _QU_-droops (Figure 2a,b). The basis for dimensioning the DC/AC converter of the PV unit can be found e.g., from [44]. However, in order to reduce the needed simulation time with multiple connected DER units (PVs, BESSs, and WT), average models with controlled voltage sources are used instead of actual power-electronics switches in PSCAD models utilized in the studies of this paper. In the simulation studies of this paper, PV and BESS units had different chosen _QU_-droops for winter (PV/BESS _QU_-droop 2, Figure 2a,b) and summer time (PV/BESS _QU_-droop 1, Figure 2a,b). Dead-zone of _QU_-droops during summer time was chosen to be smaller in order to compensate more reactive power produced by cables during very low load. In the studies, coordinated _PUI_-droop settings of PV and BESS units were also used (Figure 2c). In addition, PV and BESS units have _Pf_-droop functions (Figures 1 and 2d) in order to participate in frequency control during over-/under-frequency situations.