You must ground direct-current systems, but where? The answer depends upon whether the source is off-premises or on-premises. Note that the connection in question is to the earth [see the definition in NEC Art. 100], not a metallic path (bonding).
If the source is off-premises, then make the grounding connection at one or more supply stations [NEC Sec. 250.164(A)]. Note that you cannot make a grounding connection at individual services or at any point on the premises wiring.