## 4 Result and Discussion
### Battery Based Excess Energy Control During Fault at PCC1  
The PV-battery combination supports the FRT and power smoothing during the faults and change of environment condition. In the previous section, PV-based FRT was demonstrated. Active power curtailment is efficient if the battery is fully charged. However, lightly charged battery could be used to take the extra energy during the fault at PCC1 without PV power curtailment. As the battery is a power-limited device, this work has considered an oversized battery for the FRT and power leveling purpose. Hence, this work uses a battery to capture the excess energy in the HVDC link during the fault at PCC1 while keeping the PV power unaffected. As can be seen from Figure 22, PV power remains unchanged while the battery is charged proportionally with the increase of PCC voltage  
Figure 26: PCC voltage, Battery and PV power, SOC, and MMC1-HVDC voltage for the duration of SLG fault.  
during the 3 LG fault at PCC1. When PCC voltage exceeds 1.05 pu, PV energy is delivered to the battery, which in turn keeps the HVDC link voltage below 1.2 pu. During this time, the state of charge (SOC) has increased from 69.85% to 70.3%. Figure 23 shows that the MMC1 has injected the maximum reactive current to PCC1 to support the PCC1 voltage. However, delivered real power is zero during 3 LG fault at PCC1. Similarly, DLG fault at PCC1 has increased the HVDC link voltage, which has increased the PCC voltage, as shown in Figure 24. Consequently, the battery is charged when the PCC voltage exceeds 1.05 pu, while PV power is maintained constant. Battery charging has limited the HVDC link voltage within 1.06 pu. However, the energy delivered into the battery is less compared to 3 LG fault. In contrast, Figure 25 points out that the MMC1 transfers around 50 MW to PCC1 during DLG fault, which is greater than 3 LG fault. Besides, the delivered real current for both cases is 1.1 pu, whereas the reactive current is slightly less for DLG fault than 3 LG fault. During DLG fault, the state of charge (SOC) has increased from 68.31% to 68.38%. Figure 26 shows the battery response during SLG fault at PCC1. As can be seen from Figure 27, MMC1 has transferred almost the same power as before the SLG fault and injected leading reactive current. Note that 3 LG, DLG and SLG faults are 1 s duration faults and are applied at 1 s separately. As a result, HVDC link and PCC voltage have not been increased, and thereby, have not charged the battery as illustrated in Figure 26. Both DLG and SLG fault have introduced double frequency oscillation in the real and reactive current, but no such oscillation is observed during 3 LG fault. In all cases, the battery has effectively absorbed the excess energy from the HVDC link and thus protected the HVDC link without PV power reduction and DBR.  
Figure 27: Id and IQ current and power of MMC1 for the period of SLG fault.  
Figure 28: Solar radiation, Battery and PV power, SOC, and power of MMC1.  
Figure 29: Solar radiation, Battery and PV power, SOC, and power of MMC1.  
Figure 31: Temperature, Battery and PV power, SOC, and power of MMC1.  
Figure 30: Temperature, Battery and PV power, SOC, and power of MMC1.