Abstract: Neutral grounded system brings lower over-voltage and high sensitivity.However, owing to its higher fault current and difficulty on relay setting,transformer neutral points grounding are limited and managed by the dispatching center.This article introduced transformer neutral grounding regulations,and illustrated a typical 220 kV load center substation,which get 3 transformer operating in parallel.By deriving relay actions when system fault occurs,and comparing the consequence of different modes of neutral grounding,it explains the prior grounded transformer neutral.