# Zero-Voltage Ride-Through Capability of Single-Phase Grid-Connected Photovoltaic Systems
## 5 Performance Evaluation  
The performance of the control strategy in ZVRT operation is examined in this section. The SOGI-PLL system is used to detect the fault condition by monitoring the grid voltage frequency and the amplitude [13; 14]. The entire system is simulated in MATLAB/Simulink. A PV array block in MATLAB/Simulink/Renewables/Solar is used to implement an array of photovoltaic (PV) modules. In normal MPPT operation mode, the active power reference \(P^{*}\) is the tracked MPPT output power of the PV panels. In the faulty grid operation mode, the PV panels should reduce its output power when the grid presents a voltage fault. The reduced power level is dependent on the voltage sag depth and the nominal current. The parameters for the system are listed in Table 2. The parameters of the SOGI-PLL are the same as those in Table 1. The results are shown in Figures 12-14.  
Simulation results in the case of ZVRT operations with the normal SOGI-PLL and the modified PLL based on the SOGI-OSG are shown in Figures 12 and 13. It can be seen that at \(t=0.6\) s, a phase-to-ground fault occurs (i.e., the grid voltage sags to 0 V). Once the zero-voltage grid fault is detected, the system is controlled to continuously inject currents in order to support the grid recovery. When the grid voltage recovers to normal conditions at \(t=0.75\) s, both methods guarantee a seamless operation transition to the unity power factor mode. In contrast, the frequency of the injected reactive currents in the ZVRT  
Figure 14: A single-phase PV system under grid voltage fault with modified PLL-based SOGI-OSG: (**a**) grid voltage and current; (**b**) active and reactive power; (**c**) amplitude estimation; (**d**) frequency estimation.  
Figure 12: Performance of the PV system with normal SOGI-PLL during ZVRT: (**a**) grid voltage and current; (**b**) active and reactive power; (**c**) amplitude estimation; (**d**) frequency estimation.  
Figure 13: Performance of the PV system with modified PLL-based SOGI-OSG during ZVRT: (**a**) grid voltage and current; (**b**) active and reactive power; (**c**) amplitude estimation; (**d**) frequency estimation.  
operation with the normal SOGI-PLL has been observed with small steady-state errors. When the grid voltage recovers, the overshoot of the amplitude and frequency estimation in the ZVRT operation with the normal SOGI-PLL is more obvious. However, in the ZVRT operation with the proposed modified PLL structure, both the steady-state error and the overshoot have been reduced. Furthermore, the recovery response time has been improved slightly.  
As it is further shown in Figure 14, a recurring grid voltage fault is evaluated. It can be seen in Figure 14 that at \(t\) = 0.4 s, a phase-to-ground fault occurs when the grid voltage drops to 0.6 p.u., similar to the case in Figure 13a, and the system switches to the LVRT operation mode to adjust the active and reactive power injection to the grid. When the voltage continues to drop to zero at \(t\) = 0.6 s, the system has been switched to the ZVRT operation mode. The active and reactive power are both zero since the grid voltage is zero. However, the reactive current should still be injected for grid support. It is obvious that the power response drops slightly. The overshoots of the estimated amplitude and frequency are small, when the voltage recovers to 0.4 p.u. at \(t\) = 0.75 s, as shown in Figure 14c,d. Furthermore, when the voltage continues to recover to 0.7 p.u. at \(t\) = 0.9 s, the system increases the active power injection to 70% of its rated power. Until the fault is cleared at \(t\) = 1.1 s, the system goes back to the normal operational mode. Compared with the case where the grid voltage directly drops to zero as shown in Figures 12 and 13, the proposed control method guarantees a good dynamic performance under the recurring grid voltage fault.