A string inverter using multi-panels in series is a mainstream type of solar inverter, the lowest cost per watt makes this method attractive. But this method has the risk of catching fire caused by DC arc due to the high voltage and harsh environment, and the fire is difficult to extinguish. PV panels always produce high voltage even when the string inverter has stopped working since the sun is always there. Also, when some panels are partially shaded, the output current of the shaded panel decreases, thus, the whole string current decreases since panels are in series, causing the string power to drop significantly as shown in Figure 3-5.

For example, like the ideal working condition on the left of Figure 3-5, 10 PV panels are in series, each one with full irradiation can output 600W power, at 40V and 15A. The whole string power is 10 × 600W = 6000W.

When the string is partially shaded like on the middle of Figure 3-5, one panel cannot have full irradiation, the output current of this panel drops, the string current is 5A now, the whole string power is only 40V × 5A × 10 = 2000W. As a result, total power drops 66.7% just because one panel is shaded. This reduces the profit of the string inverter a lot, so a power optimizer is needed to help the string inverter solve these conditions.

The right side of Figure 3-5 shows a partially shaded string but power optimizers are installed for each panel. Although the shaded panel can only output 40V, 5A, the DC/DC circuit of optimizer can boost up the current to 15A, other panels are not affected. The string power with optimizer is 40V × 5A + 40V × 15A × 9 = 5600W. Additional 3600W power is saved to generate more profit.

Figure 3-5 PV String Working Conditions With and Without Optimizer

A power optimizer can also better protect the PV string. Since the optimizer is connected independently to each panel, the high voltage of the DC link is on the output side of the optimizer instead of the PV side. The optimizer can easily do the rapid shut down (RSD) function, which is mandatory in many countries.

During fault condition, the PLC receiver of the optimizer gets the RSD signal from host. Then the optimizer cuts the PV panel from the string, and allows the current to go through the bypass circuit. By using the LM74610-Q1 turn on, the bypass circuit does not need the MCU to do anything, thus greatly improving the reliability of the circuit. In arc fault, disconnecting PV panels from the string eliminates high voltage in the string inverter, which significantly reduces the rescue risk.