*6.1. Grid-Connected Case*

Figure 19 shows the experimental results of DCMG operation for the grid-connected case under the variations of load and wind power. Figure 19a presents DCMG behavior under load power variation. Before load variation, DCMG runs stably in operating mode 1, in which the WPGS is operated in the MPPT mode to inject the maximum power from wind to DCMG and the battery is charged with a constant current of 3 A. The DCV regulation and power balance is achieved by the UG through REC mode of converter 1. At this instant, the load demand is suddenly increased from 0.4 kW to 0.8 kW. To feed extra load demand, the UG increases the supply power to DC-link from 1.2 kW to 1.6 kW as

shown in Figure 19a. The battery still works in the same operating mode and the DCV is maintained at the nominal value of 400 V, in spite of load variation.

**Figure 19.** Experimental results for grid-connected case. (**a**) Load variation at mode 1; (**b**) Transition from mode 6 to mode 8.

Figure 19b illustrates the PFCS performance under the mode transition instant. Initially, DCMG is operated in operating mode 6, in which the DCV is controlled by DCVM-C of battery. Meanwhile, the WPGS is running in the MPPT mode and the UG is in IDLE state. When the battery cannot store excess power in DCMG as a result of the increase of wind power from 1.5 kW to 1.9 kW, surplus energy can be used to inject to the UG. In this situation, to ensure the power balance in DCMG, the system enters operating mode 8 and the UG system should be changed from IDLE to INV mode by regulating the DCV. Instead, the battery releases the DCV regulation to the UG system, operating in BCCM with the charging current of 6 A. Despite the wind power variation and resultant operating mode change, it is shown in this figure that the DCV is well maintained at 400 V, with only small transient.

## *6.2. Islanded Case*

Figure 20 shows the experimental results of DCMG operation for three situations in the islanded case. Figure 20a presents transition results from the grid-connected to islanded mode, Figure 20b shows the results of the LS algorithm, and Figure 20c illustrates the results of LR algorithm, respectively. In Figure 20a, DCMG initially operates stably in operating mode 3. The power balance is maintained by the UG and the battery operation is in IDLE mode. When the UG shuts down abnormally, the system operation is switched to operating mode 4, in which the battery starts DCVM-D mode with discharging current of 1.3 A to compensate the power deficit caused by the UG outage. It is confirmed from Figure 20a that the DCV is stably regulated at 400 V, with only small transient, even under transient conditions in islanded mode.

Figure 20b shows the experimental results for the LS algorithm. At first, the system operates stably in operating mode 4, in which the DCV is controlled by DCVM-D mode of battery and the total load demand is 2.25 kW. As the wind power suddenly drops from 0.75 kW to 0.25 kW, the battery increases its discharging current to compensate this power deficit. Due to the maximum discharging current limit, however, the power balance cannot be achieved by battery discharging. In this case, the LS is inevitable to avoid the system collapse and the DCV reduction. As soon as the LS algorithm is started after 15 ms as seen in Figure 20b, the total load demand is reduced to 1.8 kW by disconnecting load of 0.45 kW. As a result, the battery returns to DCVM-D mode again to regulate the DCV continuously.

Figure 20c shows the experimental results for the LR algorithm. This operation may happen when the wind power in DCMG increases again after the LS. If the wind power increases from 0.25 kW to 0.9 kW after the LS, the CC calculates the available power on DC-link as

$$P\_{D\overline{C}}^{\text{avail}} = P\_W + P\_{B,\text{dis}}^{\text{max}} - P\_L = 0.9 + 2 - 1.8 = 1.1 \text{ kW}. \tag{5}$$

Because *Pavail DC* is larger than the disconnected load power (0.45 kW) by the LS, this load can be reconnected again after *Trec* of 15 ms as shown in Figure 20c. These experimental results coincide well with those of the simulations.

**Figure 20.** Experimental results for islanded case. (**a**) Transition from grid-connected to islanded mode; (**b**) LS; (**c**) LR.

#### *6.3. Case of Grid Fault Detection Delay*

In this section, the experimental results are presented to validate the effectiveness of the proposed DCV restoration scheme. Figure 21 shows the experimental results of the proposed DCV restoration in case of the grid fault detection delay under different operating modes.

In particular, Figure 21a,b show the experimental results of the proposed DCV restoration implemented by using the battery. Before the UG has a fault, the battery is charging with the current of 3 A in operating mode 1 in Figure 21a and is in IDLE in operating mode 3 in Figure 21b, respectively. As soon as the DCV drops to 370 V, LECM is activated and the battery starts BCCM with the discharging current of 8 A. When the DCV is recovered to 390 V, the battery operation is switched into DCVM-D to gradually regulate the DCV at 400 V. As demonstrated, the experimental results in Figure 21a,b are well matched with the simulation results in Figures 13 and 14.

Figure 21c,d show the experimental results of the proposed DCV restoration obtained by using the WPGS. Before the fault occurs in the UG, the battery is charging with the maximum charging current, i.e., 6 A, in operating mode 8 in Figure 21c, and is in IDLE in operating mode 9 in Figure 21d, respectively. Meanwhile, the WPGS works in the MPPT mode to ge<sup>t</sup> the maximum power from wind. As explained, the DCV increases rapidly due to the delay of grid fault detection. When the DCV reaches 420 V, LECM is started and the WPGS operation is switched into VCM mode to regulate the DCV at 400 V. These experimental results are also matched well with the simulation results in Figures 15 and 16. As a result, it is confirmed that the DCV which has an essential role in DCMG operation can be restored effectively by using the proposed scheme.

**Figure 21.** Experimental results of the proposed DCV restoration in case of grid fault detection delay. (**a**) During operating mode 1; (**b**) During operating mode 3; (**c**) During operating mode 8; (**d**) During operating mode 9.
