Circuit Breaker Digital Test Constant Current Source Device And Its Control Algorithms

Circuit breakers are widely used in industry,agriculture and daily life.They are important electrical products in power distribution system.The protection characteristics of low voltage circuit breakers mainly refer to their overload and overcurrent protection characteristics,that is,the relationship between the operating time of circuit breakers and the operating current of overload and overcurrent trippers.For the test of circuit breaker protection characteristics,it is necessary to provide accurate,stable and efficient test current to ensure the reliability of test results.Constant current source device provides continuously adjustable and precise and stable high current to test the protection characteristics of circuit breakers.When testing the protection characteristics of circuit breaker,the output test current of the existing constant current source test device will be affected by the change of load impedance and external interference.In view of this situation,this paper designs an impedance online identification and feedforward compensation combined with PID compound control algorithm.Impedance identification algorithm is used to read load voltage and current signals for two periods.Two sets of first-order differential equations about load impedance are listed.Simultaneous equations are used to obtain load resistance and inductance,which can read load impedance changes in real time and be used for feed-forward compensation control to improve the accuracy of output current.At the same time,the PID control technology is used to overcome the external interference and track the reference current to make the output currentmore stable.Moreover,impedance online identification is used to compensate feed-forward,which reduces the tracking time of PID control technology and achieves the goal of high efficiency.Firstly,the control algorithm of constant current source device is analyzed,and the Simulink simulation environment of constant current source is built.Under different algorithms,the output current is tested and analyzed.In practical design,the circuit structure and composition of constant current source device are explored,including rectifier filter circuit,single-phase bridge inverter circuit,flyback switching power supply circuit,STM32 chip and its peripheral circuit,voltage and current sampling circuit,RS485 communication circuit,etc.Secondly,the current test platform is built,the output side of the constant current source device is connected with transformers,series impedance loads and control circuit contactors.At the same time,the voltage transformer and current transformer are used to detect the load voltage and current.Then,the program is designed and compiled,including impedance identification,feedforward PID control,ADC analog-to-digital conversion,soft start control,advanced timer generation complementary SPWM and interrupt,RS485 communication,DMA,etc.Finally,the feasibility of the control algorithm is verified by simulation,and the simulation results of the output current of feed-forward control,PID control and feed-forward PID control are compared.The simulation and experimental results show that the feedforward PID control scheme has strong adaptive ability,speeds up the system response and improves the anti-interference ability,effectively shortens the verification time and improves the detection efficiency,and ensures the accuracy of test current.