DC current injected into DR (Distributed Resources)
IEEE 1547 Standard for Interconnecting Distributed Resources With Electric Power Systems establishes criteria and requirements for interconnection of DR with the AC power grid. The DR must convert AC or DC power with parameters of the DR to AC power with parameters suitable to the grid. It is possible that the result of the conversion could be AC voltage and current with a DC component or "offset" summed with it. That would be undesirable and IEEE 1547 limits any such DC content such that it does not exceed 0.5% of the full rated output of the DR.

The reason that DC content is undesirable is that it could saturate transformers that are part of the grid equipment. There may be other effects also. I believe the cause would be an imperfection in the switching pattern of the grid-tie inverter or possibly small variations of the switching component characteristics. It would be necessary to dig quite deeply into the design of grid-tie inverters to fully explain the possible causes and determine how significant that is as a design problem.