Abstract: Abstract:An undervoltage protection false trip accident was analyzed. The voltage transformer wire broken fault occurred firstly due to operator's incorrect manipulation. Then the PT disconnection did not lock the undervoltage protection. So the undervoltage protection tripped incorrectly. The principle and judgement logic of the protection device PT disconnection were analyzed. The influence of outside electromagnetic elements on the voltage sample circuit of the protection device was investigated. The undervoltage protection lockout logic was improved and perfected, and the false trip accident was avoided.

Key words: Keywords:undervoltage relay, electromagnetic interference, PT wirebroken lockout, undervoltage protection, false trip