Transformer Tap Changing Not Accordingly with Nameplate While Power Plant in Generation Mode

Hi,

Power Transformer of 230/11 KV, 100/133 MVA YNd1.

When the power plant is in generation mode (100MW Generation), Transformer tap changing is not accordingly with Nameplate data. No. 1 tap (Highest Voltage Position) for 253KV & No. 17 tap (Lowest Voltage Position) 2.07KV. No. 9 tap nominal (230KV). Suppose the grid voltage is 230KV but the tap position is 4. I've seen this on ABB, ILJIN & TAI KAI Transformers while I was working.

I've tried to learn regarding this but didn't get any help/documentation. Can anyone please help me regarding this. I'll be really grateful.

Thanks in advance.
MRB Manik