Inverse-time overcurrent protection based on the theory of decreasing value of thermal resistance in electric power distribution system

The theory of decreasing value of thermal resistance is proposed for the aircraft and civil electrical power distribution systems protections. Trip conditions are studied based on the International Electrotechnical Commission (IEC) inverse-time standard characteristic curve. The thermal resistance and thermal power are calculated. A precise inverse-time protection is designed, and its process of implementation is analyzed with simulation. This paper defines the time delay under 10 times over-current as the time dial and therefore gets the other three inverse-time functions which have uniform and unambiguous quantified parameters, so the parameters dial is set easily. This method overcomes the obstacle that the traditional inverse-time over-current protection can't reflect the real fault state under fast changing fault current. An easy and practical inverse-time characteristic test method is developed, and both static and dynamic characteristic experimental results are presented in the end. Testing results indicate that this method has fast characteristic and high precision for aircraft and civil electrical equipment protection.