There may be degradation of modules caused by bypass diode damage, PID effect, shadow, different obliquity, dust and other problem in the application of solar plant. These phenomenons could lead to dropping of string voltage, and voltage mismatching of the strings.
S-MPPT, could control output current and voltage of every string with DC to DC boost converters to work at MPP. It can improve the power loss caused by voltage mismatching.
S-MPPT is installed between the DC combiner box and low voltage string, making it operating at MPP. Also it could avoid interaction between low voltage and high voltage string, improving the power of inverter.