the KPI should show:
1) if the maintenance department is working toward the organization objective?
2) what are the KPI in power transformer maintenance strategy?
For my opinion you are on the right track. Try defining what the organization objectives are, and how maintenance department is/can support those goals. This is an excellent starting point for any discussion around KPIs.
As for specific examples, have a look at this pack here for Electricity Production KPIs, probably it is relevant to what you are looking for.
As for your 2 other question:
“-How can I create KPI that shows improvement in asset condition (POWER TRANSFORMER)
-What are the elements in my project that can be critical in defining if the maintenance department is doing good?”
I’d start looking into the properties that define asset’s conditions, e.g. durability, produced energy, associated costs, for sure there are many technical parameters that engineers can suggest for you.
As for the critical element, I can recommend to start with process chart to see the dependencies that exists between this department and others, if you have a SLA document it would also be helpful.