## 3 Proposed PV-Battery Based FRT Improvement and Power Levelling Controller  
As can be seen from Figure 5 for any curve, the PV array power is changed with voltage. By moving PV voltage from its optimum point towards the open-circuit voltage, it is possible to curtail the PV power output during the fault. By obtaining the P-V characteristics for all possible cases, the range of effective PV voltages for MPPT is chosen. As can be seen from Figure 5, the range of PV voltage is selected between 1350-2750 V for uniform solar radiation. However, non-uniform solar radiation provides multiple peaks depicted in Figure 6. Usually, the location for large scale PV farm is selected in the open space or in the desert where the efficiency is the highest, and the land is not utilized for forestry, agriculture, and rural and urban development. Besides, the only source of shading for the PV panel is the cloud. Therefore, P-V characteristics are obtained under 50% panel shading at different solar radiations depicted in Figure 6 to consider the worst case scenario. 1000 Wm\({}^{-2}\) solar radiation is considered for the remaining 50% panel. It is inferred from Figure 6 that the peak occurs within the range of MPPT tracking when the solar radiation is above 400 Wm\({}^{-2}\). For all cases below 400 Wm\({}^{-2}\), the peak occurs outside the MPPT range and towards the low voltage side. Without an additional DC-DC converter for each PV module, exploring a wide range of PV voltages using a global tracking algorithm for a central inverter-connected PV system could violate the minimum DC link voltage of the PV inverter for reliable operation and might create stability problem. Besides, active power reduction by changing PV voltage would not be possible if the global peak occurs in the low voltage side. Therefore, the proposed MPPT range-restricted-based active power reduction works well and covers most of the partial shading cases in the open spaces.  
During the normal operating condition, PV inverter operates in MPPT mode. However, any fault at PCC1 rises the HVDC link voltage. Hence, active power from the PV panel is reduced with the increasing HVDC link voltage. Therefore, HVDC link voltage is required to sense and feed into the PV-battery controller. However, this approach is not universal as there might be a considerable distance between the MMC2 converter station and PV inverter, which requires a long wire for sensing the HVDC link voltage. To remove the long wire-based HVDC voltage sensing, this work proportionally increases the PCC voltage with the HVDC link voltage, which in turn is used as the input command for the active power reduction controller of PV-Battery controller.  
Thus, local voltage is monitored instead of remotely located HVDC link voltage. As can be seen from Figure 7a, the positive error is processed through the PI controller and added with the existing MPPT voltage to produce the reference voltage for the DC link voltage controller of VSC-PV.  
The maximum DC link voltage is equal to the open-circuit voltage of the PV panel. Usually, MPPT voltage is approximately 80% of open-circuit voltage of the PV panel. Similarly, battery is charged proportionally if the PCC voltage rises above 1.05 pu during the fault at PCC1 to protect the HVDC link voltage as depicted in Figure 7b. Maximum setpoint for the battery charging power is equal to the MPPT power of the PV panel. During constant PCC voltage, any deviation from the nominal power of PV energy caused by solar radiation and temperature change is filled by the battery as shown in Figure 7c. PSet is the nominal power of PV panel from which actual PV power (PPV_lr_T) deviation during partial shading is compensated by the battery. Positive power command for the battery means discharging; negative power command for the battery means charging.  
Figure 5: PV characteristics during solar radiation and temperature change and active power reduction area.  
Figure 6: PV characteristics under 50% panel shading at different solar radiations.