*4.1. Simulation Design*

To verify the effectiveness of the control method, a simulation was performed using PSCAD (power system computer-aided design). Figure 5 shows a design of the network with PV sources configuring a common DC section that includes ESS. The basic rated power capacity of a single PV is 400 kVA. The designed PV system was connected to a displayed distribution network through an inverter topology (multistring and central). Preferentially, the individual unit of a PV system is composed of a single string to check calculation load of the multistring scheme (three strings are used for PV). Next, an expanded feasibility test was then carried out with an integrated 1.2 MW PV system with consideration of the maximum computational load as well. The distance of the pi line was reflected using generic model in PSCAD (library with *R*, *Xl*, *Xc* elements), and the residual information is given in Table 2.

**Figure 5.** Simulated distribution network including MW scale PV/ESS.

The configured PV system extracts real power based on environmental variables, as in the previous study. DC voltage fluctuations due to the extracted power may affect the operation of the connected ESS. To continue the feasibility studies based on the verified charging/discharging pattern of ESS, load fluctuations (Figure 6) and ESS control effects (Figure 7) were used. The AC grid voltages at each load connection site are described in Figure 8 to confirm if it causes an impact on the DC network in the simulation. The RMS magnitudes are classified according to the distance from the substation and each section is maintained with stable conditions. The main objective of the simulation is to confirm the reliability of the proposed method. For this reason, individual load fluctuations were designed and ESS operating signal configured to change according to load conditions. An analysis focusing the ESS control according to each load change proceeded.


**Table 2.** Numerical data for the simulation.

**Figure 6.** Imposed load variating condition for case study.

**Figure 7.** Power supply quantity from main grid in designed scenario (with and without ESS).

**Figure 8.** AC side root mean square (RMS) voltage at connection point for loads.

Table 3 details the load parameters applied to the case study. The total simulation time was defined in 8 seconds including initialization. Load changes were made in stages as shown in Figure 6. The ESS was configured to charging/discharging in response to an instantaneous load change (75 kW). The derived average iteration number (adjusting the input values) and expected average solution time, for each topology were represented together. Taking into account the configuration that a single string has 40 modules, the multistring topology configured a system matrix. In the case of the central topology, it was designed with three parallel units that were used for a single string. The number of iterations were not change in each topology, however, the expected solution time increased and the regarded delay increased as well. In this paper, the plan was to apply the 100 ms (10 Hz calculation frequency) interval to the multistring topology and apply not only 200 ms (5 Hz) to the central topology, but 400 ms (2.5 Hz) to consider the additional computational load. If the calculation is not terminated within the imposed constraint, the previous value must be used until the calculation is completed.


**Table 3.** Simulated case study description.

Within the improvement effect shown in Figure 7, the simulation was configured to deduce availability by showing how computational delay affects control. One of the objectives of this study was to confirm if the utilized control can reduce the possible error compared to the strict voltage control as well perform an adequate power supply with expected delay.
