Research On Over Current Protection Technology Of Intelligent Moulded Case Circuit Breaker

With the development of social economic and growing of power consumption, higher requirements of power quality and supply reliability are demanded for power distribution system. Overcurrent is a kind of common fault in power distribution system, which can result in serious accidents and economic losses. Circuit breaker is a kind of indispensable protective device, which can be used in low voltage power supplying system and plays an irreplaceable role in over current protection. Therefore, this dissertation carries out the study of overcurrent protection technology for intelligent molded case circuit breaker.Firstly, the introduction for development of circuit breaker and overcurrent protection is presented, and the operational principle of overcurrent protection for MCCB is illustrated.Secondly, the hardware circuit is designed, which includes current signal transforming circuit, microcontroller and peripheral circuit, auxiliary circuit. The overcurrent protection characteristic is determined by the dynamic response time of current signal transforming circuit, especially the response time under short circuit and false action under short time overcurrent. The current signal transforming circuit meets the requirements through designing, calculating and measuring. The simulation and experimentation of current signal transforming circuit is conducted, and the maximum measurement error is analyzed. The results verifies that the designed current signal transforming circuit can fulfill the design requirements and provide accurate current signal for the microcontroller.Thirdly, the software is designed based on ATMEGA64. Modular design ideas are adopted to write program for each functional module. This paper conducts intensively study of the output characteristic of current transformer, especially the output characteristic of saturation state under large current. MATLAB is used to fitting the output characteristic of current transformer, and the pattern for the output characteristic of current transformer is illustrated. Then saturation compensation algorithm is included in the signal processing program to improve the current detection accuracy and meet the protection requirements.At last, the experimentation for overload long-time delay protection, short circuit short-time delay protection and short circuit instantaneous protection are carried out. The experimental results verifies that the designed intelligent molded case circuit breaker can meet the requirements of the overcurrent protection.