*4.2. Simulation Results*

Figure 10 shows a power production curve of the common DC system including ESS output with rated voltage level consideration. The ESS charges and discharges depending on the load variation on Figure 8. It can be confirmed that the power supplied from DC, which depends mainly on the amount of PV generation, has been modified according to the complementary power of ESS corresponding to the increase in load of each designed section. Depending on the total output power including extracted power from the ESS, unexpected voltage fluctuations can occur in the DC system, and the main objective of the simulation is to compare the response ability. Since the control was established to reduce the variations of the power supplied, the quantity from the grid can be attenuated as illustrated in Figure 11. Compared with the general conditions in Figure 9, the supplied power from the grid can avoid discrete changes through ESS compensation. However, a minor variation is still exhibited on the power curve, hence more precise compensations are still required.

**Figure 10.** Combined power production of DC network (ESS-strict voltage control).

**Figure 11.** Quantity of power supplied from main grid (ESS-strict voltage control).

The proposed flow expectation method is applied to the same simulating condition. In regards to precise voltage condition, the proposed control attempts to reduce the previous errors and smoothen the power supplied quantity from the grid.

The power production is handled within the rated capacity and the differences between control methods are generated depending on the precision of controllers in terms of voltage. The highlighted charging and discharging sections (2.5–4.5 and 5–7 s) are described in Figure 12. In both cases Figure 12a,b, a strict voltage control method finds it difficult to detect DC voltage variations, and generate mismatches for both sections. The method triggers ESS operation with signal and supports the grid according to the voltage mismatch through comparison of the rated voltage (500 V). Since storage sets perform according to the power capacity, there could be double error generated which could affect the grid power balance. The ESS output curves of the proposed method are displayed together with strict voltage control. The mismatches are reduced in both control modes as described in the figures, and the reliability of the compensation significantly improved in terms of solution. Without the initialization section, the power supplied by ESS almost coincides with the increased variable load value. The power supplied quantity from the main substation is illustrated in Figure 13. Compared with the previous state, as shown in the figure, the fluctuation generated is significantly reduced. The control error via the general method is modified in the case of both charging and discharging processes.

**Figure 12.** Detailed power compensation quantities for ESS with DC voltage variations (**a**) extracted power quantity in the discharging section (2.5 to 4.5 s); (**b**) absorbed power quantity in the charging section (5 to 7 s).

**Figure 13.** Quantity of power supplied from main grid (ESS-proposed control).

The total mismatch quantity for both methods in terms of power support is presented in Table 3. The difference between the demanded control quantity (calculated by strict power references) and the profile is reduced for both control modes, and the reliability of the profiled quantity is significantly improved in regard to energy compensation. In particular, error generated in the discharging state is significantly reduced with the proposed controller.

**Table 3.** Numerical results for imposed order and profiled quantity.

