Thank you for your question regarding distribution and substation grounding systems, it is our pleasure to help.
Most utility power companies treat transmission and distribution lines as separate entities. Transmission lines are those high-voltage power lines that run from power plants to substations, or from substation to substation. Distribution lines are those lines that run from a substation to the consumer (via pole mounted transformers or the like). Most power companies try not to intermix transmission systems with distributions systems when they can avoid it, mostly to protect the distribution systems from the very powerful electrical faults that occur on transmission lines.
That said, this is merely a rule-of-thumb and is difficult in practice to enforce. First of all, what constitutes a distribution substation from a transmission substation? That line is quite fuzzy. Also, it is very common to see 66kV transmission lines with distribution power on the same pole; which means they are inherently grounded to each other via the metallic poles.
The short answer to your question is that all ground systems within a facility must be bonded together. This is true no matter what the source: transmission, distribution, lighting protection, Telco grounds, equipment grounds, isolated grounds, water pipes, building steel, etc.
We hope this has answered your question, if you should have any further concerns, please do not hesitate to contact us again in the future.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions