Auxiliary power outside a substation ground grid

[2banee]

I’ve been tasked with providing auxiliary power to a materials yard directly adjacent to our existing 69 kV to 21 kV substation. The existing ground grid within the substation extends 3 feet beyond the fence. The materials yard will house 240 volt lighting and a future job trailer. I estimate that there will be 100 amps max at build out. I plan on attaching to our substation control building 200 amp main and running secondary to the materials yard. My dilemma is that due to ground faults it’s prohibitive to run a ground/neutral wire from within the substation to the materials yard. I’m looking into the possibility of utilizing an isolation transformer with primary ungrounded 2-120 legs to a secondary grounded 120/240 service. Has anyone dealt with issues like this before and how did you rectify the situation. Also, due to the distances involved, I would have approximately 5% voltage drop to the material yard service. Is it uncommon to have isolation transformers built with 2.5% taps to help boost the utilization voltage? Please advise.