Transformer 11Kv - High Resistance Valve

Power down the transformer, check all joints on the earth conductor, for corrosion & poor contacts.

...then find all the earth rods and matting, and empty a road tanker full of seawater onto each one. If seawater is unavailable, cow urine makes a tolerable substitute.

What was it before, and what has changed on or near the transformer site since the last and current measurements?

What type of earth rod is used,howmany years ago it was installed,did you measure the resistivity of the soil,were periodic readings taken,try earth enhancing material.