## 6 Results  
A simulation was performed in which load steps were applied for evaluating the operation of the microgrid under all seven of the operational scenarios described.  
The behavior of the microgrid DC bus voltage for the different control strategies and operating modes is shown in Figure 13. It can be seen how the DC bus voltage tends to the reference value, regardless of the interconnected or isolated condition of the microgrid, except when the microgrid is isolated, the battery is fully charged, and the load demanded power is lower than the DG available power. In this case, the power generated by each DG unit is reduced in approximate proportion to the increase in the DC bus voltage, as expected. As shown in Figure 14, a relatively smooth transition is achieved for each operating mode of the microgrid.  
Initially, the microgrid is connected to the main power system, and three possible situations are considered. (i) DG \(>\) Pload: in this case, since the main microgrid DC/AC converter transfers to the grid the excess of generated power, the DC bus voltage is controlled to remain at the rated value, as shown in Figure 13. Additionally, the PV and wind generation units operate under MPPT mode, producing around 480 W and 1900 W, respectively, as observed in Figure 14. The battery is considered to be fully charged and therefore does not absorb any power. (ii) DG = Pload: at t = 3 s, the load power abruptly increases, becoming approximately equal to the total power produced by the DG units. Due to the net power (Pnet = DG-Pload) reduction, the DC bus voltage starts to decrease, but the DC bus voltage controller asks for the DC/AC converter power (Pgrid) to rise until the DC bus voltage returns to the rated value, which occurs when Pgrid = Pnet = 0.  
Figure 14: Relation of electrical power between the control strategies and modes of operation of the microgrid with PPV, PWT, PESS, Pload, and Pgrid.  
Figure 13: Voltage DC bus microgrid.  
(iii) At t = 6 s, the total load power increases again, making DG \(<\) P\({}_{\rm load}\). In this case, the DC bus voltage controller is forced to absorb active power from the AC mains (P\({}_{\rm grid}\)\(<\) 0) to make V\({}_{\rm DC}\) = 250 V. The microgrid is connected to the main power system and the DG units generated powers are constant since they remain under MPPT operation. (iv) At t = 9 s, the microgrid interconnection circuit breaker is turned off, making P\({}_{\rm grid}\) = 0. At this moment, the battery interface converter assumes the responsibility for the DC bus voltage control and starts to deliver the necessary net power to regulate the DC bus voltage at its nominal value. (v) At t = 15 s, the total load power decreases so that DG = P\({}_{\rm load}\) again. Once the battery is delivering some power to the DC bus voltage, its value tends to increase right after t = 15 s, but the DC bus voltage controller commands a decrease in the power delivered by the battery. (vi) At t = 18 s, an additional decrease in the total load power makes DG \(>\) P\({}_{\rm load}\) again, and the DC bus voltage experiences a transient increase, but the battery converter again regulates this voltage and absorbs the excess power. However, at some instant after t = 20 s (vii), the battery becomes wholly charged (SoC = max), and the battery cannot absorb any more power. Since DG \(>\) P\({}_{\rm load}\), the net power is delivered to the DC bus, the voltage of which naturally starts to increase. During phases (iv), (v), and (vi), the DG units operate under MPPT, but the energy management strategy commands off-MPPT behavior when SoC = max. The power generated by each DG unit decreases in inverse proportion to the DC bus voltage until the total generated power becomes equal to the power demanded by the microgrid loads. As can be seen in Figures 13 and 14, the proposed energy management strategy, together with the proposed DG units' control schemes, operate as theoretically predicted.