**5. Simulation Results**

In order to validate the effectiveness of the power flow strategy and the proposed DCV restoration algorithm, simulations were carried out using the PSIM software. Table 3 lists the system parameters used for simulations. Simulations are carried out in three cases which are the grid-connected case, the islanded case, and the case of grid fault detection delay.


**Table 3.** System parameters of DCMG.

## *5.1. Grid-Connected Case*

In the grid-connected case, the UG is used as the main source to maintain the system power balance under different conditions of wind power, battery status, and load demand. The operation conditions used for the simulation are listed in Table 4.



Figure 11 shows the simulation results for the grid-connected case according to the operation conditions in Table 4. Initially, the system runs stably in operating mode 1 at t = 0.5 s. At this instant, the WPGS works in the MPPT mode to deliver approximately 0.25 kW of wind power to DCMG, the battery is charged with a current of 3 A, and two loads (load 2 and load 3) are connected consuming the total power of 0.8 kW. The UG supplies the power deficit to maintain the system power balance.

**Figure 11.** Simulation results for grid-connected case. (**a**) *a*-phase grid voltage and current; (**b**) Output power of UG connection system; (**c**) Output power of WPGS; (**d**) Load power; (**e**) Battery power; (**f**) Battery current; (**g**) Battery voltage; (**h**) DCV.

At t = 1 s, the load demand is suddenly increased because load 4 is switched in. As shown in Figure 11b, the UG increases the supply power to DC-link via REC mode operation of converter 1 to balance the system power exchange. At t = 1.5 s, the generated power from the WPGS is suddenly changed from 0.25 kW to 2.2 kW, which is higher than the load demand (*PL* = 1.2 kW). According to the PFCS in Figure 2 and Table 1, the system operation is switched into operating mode 6, in which the battery starts DCVM-C mode to absorb the surplus power in DCMG. In addition, the battery is in charge of regulating the DCV in this operating mode, while the UG is released to IDLE mode. At t = 2 s, load 3 is switched out, which decreases the total load demand to 0.9 kW. Even in this case, the battery keeps balancing the power exchange in DCMG by increasing the charging current as seen in Figure 11f. When the generated wind power is further increased to 3.5 kW at t = 2.5 s and this increased power is not acceptable to battery since the charging current cannot be increased due to the limitation of *Imax B*,*cha*, the UG connection system is switched from IDLE to INV mode to maintain the system power balance. In this condition, the battery is charged with the maximum charging current of 6 A, in order to avoid overheating and damage. As the battery maximum voltage is reached at around 3 s, the battery operation is changed to BVCM and the power absorbed by battery is gradually reduced. In contrast, the power injected into the UG is gradually increased to keep the system power balance as shown in Figure 11b. These simulation results clearly demonstrate the coordination of the UG and battery during the grid-connected mode.

## *5.2. Islanded Case*

In the islanded case, the DCV is regulated by the coordination operation of the battery and WPGS. Operation conditions used for this simulation are listed in Table 5.


**Table 5.** Operation conditions for simulation test in islanded case.

Figure 12 shows the simulation results for the islanded case according to the operation conditions in Table 5. Initially, the system is assumed to operate stably in operating mode 3, in which the DCV is regulated by the UG via REC mode of converter 1. The WPGS is in the MPPT mode, providing approximately 1.1 kW of power to DCMG. Also, it is assumed that the battery is in IDLE mode with the maximum *SOC*, and DC loads consist of load 1, load 2, load 3, and load 4, which consume the total power of 1.9 kW. w t = 1 s, the grid fault happens and DCMG operation is changed into the islanded mode. If the grid fault is quickly detected, the system operation is changed to operating mode 4, and the battery starts discharging to control the DCV with DCVM-D mode. At t = 1.5 s, load 5 is switched in and the battery increases the discharging power to supply extra load demand as seen in Figure 12e. When the wind power is reduced to 0.2 kW at t = 2 s, the battery tries to increase the discharging current to compensate for the power deficit. In this condition, however, the battery cannot control the DCV any longer due to the discharging current limit of 8 A. In order to prevent DCMG system from collapsing even in this case, operating mode 5 is activated with the LS algorithm. As shown in Figure 6, as soon as the counter *Cshe* reaches the shedding time delay for load 1, *Tshe*1 defined in Table 3, load 1 which has the lowest priority is disconnected. Operating mode 5 lasts only during a small duration and the behavior of battery current during this interval is also shown in the magnified figure in Figure 12. After disconnecting load 1, the total load demand remains at 2 kW. On the other hand, the possible supply power consisting of the WPGS power and the maximum discharging battery power is

$$P\_W + P\_{B, \text{dis}}^{\text{max}} = 0.2 + 2 \,\, = 2.2 \text{ kW} \tag{2}$$

which is higher than the remaining load demand. Therefore, the system operation can be returned to operating mode 4, in which the battery stably controls the DCV after the LS. As the wind power increases from 0.2 kW to 1.5 kW at t = 2.5 s, the available power on DC-link is calculated from (1) as

$$P\_{DC}^{\text{ranail}} = P\_W + P\_{B, \text{dis}}^{\text{max}} - P\_L = 1.5 + 2 - 2 = 1.5 \text{ kW}. \tag{3}$$

**Figure 12.** Simulation results for islanded case. (**a**) *a*-phase grid voltage and current; (**b**) Output power of UG connection system; (**c**) Output power of WPGS; (**d**) Load power; (**e**) Battery power; (**f**) Battery current; (**g**) Battery *SOC*; (**h**) DCV.

Because *Pavail DC* is larger than the power of load 1 (0.7 kW), the LR can be activated to reconnect load 1 by using the LR algorithm in Figure 7 and the time delay of *Trec* defined in Table 3.

At t = 3 s, as the extracted power from the WPGS is increased to 3.1 kW, which is greater than the power demand of load, the system operation is switched into operating mode 10. Accordingly, the battery operating mode is switched from DCVM-D to DCVM-C to absorb the surplus power. At t = 3.5 s, the WPGS injects the power of 5 kW to DCMG. To maintain the power balance of DCMG system, the battery should continue to absorb the excess power by means of DCVM-C operating mode. However, too large excess power results in the battery overcharging current beyond the maximum value of 6 A. To protect the battery from overheating and damage, the system operation is changed to operating mode 12, in which the battery is charged with the maximum limit of 6 A. Because the battery absorbs only a portion of surplus power in this condition, the WPGS is switched from MPPT to VCM to reduce the power extracted from wind, and consequently, to guarantee the system power balance. As shown in Figure 12, the DCV is effectively regulated and the PFCS satisfactorily works in the presence of variations in wind power and load demand, even during islanded mode.

In order to further demonstrate the e ffectiveness of the PFCS, a comparison in terms of the DCV regulation performance between this paper and the study of [8] is shown in Table 6. In Table 6, the maximum voltage deviation ratio of the DCV is calculated as

$$\text{Maximum voltage deviation ratio} = \frac{\text{Maximum voltage deviation}}{\text{Nominal voltage}} \times 100.\tag{4}$$


**Table 6.** Comparison of the DCV regulation performance.

As shown in Table 6, as compared to the simulation results in Ref [8], the DCV can be regulated at the desirable value with significantly smaller voltage deviation ratio in this paper, in spite of the variations in the generated power from the WPGS and load demand.

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

In this section, simulations are carried out to prove the e ffectiveness of the proposed DCV restoration scheme for the system stability in case of the grid fault detection delay. The DCV restoration can be achieved either by the battery as shown in Figures 13 and 14, or by the WPGS as shown in Figures 15 and 16. In these simulation results, 0.46 s is used for the fault clearance time.

**Figure 13.** Simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 1. (**a**) DCV; (**b**) *a*-phase grid voltage; (**c**) Battery current; (**d**) Output power of WPGS.

Figures 13 and 14 show the simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 1 and operating mode 3, respectively. Before the fault occurs in the UG, DCMG works in operating mode 1 or operating mode 3, in which the system power balance is maintained by the UG via REC mode of converter 1. In both figures, the WPGS is operated in the MPPT mode while the battery is charging in operating mode 1 and is in IDLE mode in operating mode 3. Even though the UG shuts down suddenly at t = 0.4 s, the CC does not recognize it because of the fault detection delay. Consequently, DCMG still operates at operating mode 1 in Figure 13 and operating mode 3 in Figure 14, which results in power imbalance and rapid decrease of the DCV since any power sources do not control the DCV during this period. As soon as the DCV drops to *Vmin*<sup>1</sup> *DC* of 370 V, LECM by LC3 is activated. At this instant, the battery operation starts BCCM, discharging the maximum current to restore the DCV quickly. When the DCV reaches *Vmin*<sup>2</sup> *DC* of 390 V, the operating mode of battery is automatically switched into DCVM-D to gradually regulate the DCV at the nominal value of 400 V. Once the grid fault is detected with delay at t = 0.55 s, the CC changes the system operation to operating mode 4, terminating LECM by LC3. When DCMG operation returns to the normal mode, the battery continuously controls the DCV by means of DCVM-D mode, with a seamless transition between LECM and the normal mode.

**Figure 14.** Simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 3. (**a**) DCV; (**b**) *a*-phase grid voltage; (**c**) Battery current; (**d**) Output power of WPGS.

**Figure 15.** Simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 8. (**a**) DCV; (**b**) *a*-phase grid voltage; (**c**) Battery current; (**d**) Output power of WPGS.

**Figure 16.** Simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 9. (**a**) DCV; (**b**) *a*-phase grid voltage; (**c**) Battery current; (**d**) Output power of WPGS.

Figures 15 and 16 show the simulation results of the proposed DCV restoration in case of grid fault detection delay during operating mode 8 and operating mode 9, respectively. Before DCMG enters the islanded mode due to the fault occurrence, the UG regulates the DCV stably by absorbing surplus power via INV mode of converter 1. In both figures, the WPGS is operated in the MPPT mode while the battery is charged with the maximum charging current in operating mode 8 and is in IDLE mode in operating mode 9. Similarly, the UG has a fault suddenly at t = 0.4 s. In case of the grid fault detection delay, the CC still uses the UG for the DCV regulation. Under this condition, the UG is not able to absorb excess power any longer, a rapid increase of the DCV and power imbalance are introduced in DCMG. When the DCV reaches the maximum level of 420 V, LECM by LC2 is triggered. As a result, the operating mode of the WPGS is instantly changed to VCM to adjust the DCV to the nominal value of 400 V. Once the grid fault is detected with delay, the CC changes the system operation to operating mode 12 in Figure 15 and operating mode 13 in Figure 16, respectively, according to the PFCS shown in Figure 2. This terminates the LECM by LC2, returning to the normal mode in which the WPGS works with VCM with seamless transition between the LECM and the normal mode.

In order to further demonstrate the effectiveness of the proposed DCV restoration algorithm, the simulation results in case of grid fault detection delay during operating mode 3 without the proposed DCV restoration algorithm are shown in Figure 17. As compared with Figure 14, it is shown that the DCV is rapidly dropped due to the delay of grid fault detection without the proposed DCV restoration algorithm in Figure 17, which confirms the effectiveness of the proposed DCV restoration algorithm.

**Figure 17.** Simulation results in case of grid fault detection delay during operating mode 3 without the proposed DCV restoration algorithm. (**a**) DCV; (**b**) *a*-phase grid voltage; (**c**) Battery current; (**d**) Output power of WPGS.
