Test results on DC injection phenomenon of grid connected PV system at Rokko test center

Since power output of a PV system is DC and the commercial grid carriers AC, in any linked system, attention is paid to prevent direct mixture of these two currents. Usually alternating and direct currents mix through the inverter, and because of the great variety in types and specifications, phenomena may be extremely complex. To observe the basic phenomena, demonstration tests were conducted in which direct coupling of PV arrays and the grid have been performed. PV arrays' short circuit current is relatively weak, so the effect on transformers are not so substantial as in the case of a storage battery system linkage. After a certain time exciting current appears, and grows, on the AC side of the transformer. Consequently, with increase of the exciting current when DC and AC powers are mixed, higher harmonic currents appear on the AC side. Breakdown phenomenon on the array side is different depending on whether AC injection happens during an active power generating period, or a passive one and also the existence, or lack, of by-pass diodes or blocking diodes is significant. Blocking diodes prevent larger AC currents from flowing through the solar cells, whereas by-pass diodes let those currents get around the blocking diodes.