EModMineHitOddsBonus = 5,10,15,20

This parameter reduces the risk of hitting a mine. This bonus is not simply added to the normal mine hit odds (like the combat bonuses), it is relative to the normal mine hit odds. For example, the normal mine hit odds are one percent. A level 1 ship has a hit risk of only 0.95% (five percent less than 1%), level 2 has 0.9%, and level 4 has a risk of only 0.8% per light year.

To make a ship immune to minefields, you'd give it the value 100 here.