Study On Intelligent Protective Technique Based On Type A Residual Current

Residual current device (RCD) is an important kind of low voltage electrical apparatus, which is used in low voltage distribution power grid to protect from electrical fire or electrical shock hazard. However, with the dramatic increase of power electronic equipment, there is more and more residual pulsating direct current component in the power grid, and that results in failing protection for the traditional RCDs. This problem has become a common focus of industrial community and academia both at home and abroad.A further study on residual current transformer (RCT) is conducted. Two residual current protection methods (type A residual current protection) to protect residual pulsating direct currents are proposed, and prototypes are designed respectively and tested experimentally.Based on the analysis of operation principle of RCT, the effects of sensitivity and balance characteristic on the performance of RCT are discussed in detail. The analytical expressions were derived to describe the sensitivity and balance characteristic, and the practical design considerations are proposed for RCT. Further more, by numerical simulation, the false residual relationship to the deviation of conductor and spiral angle of core were obtained, respectively. The results provided theoretical basis for the optimal design of RCT. Two residual current protection methods for residual pulsating direct currents are presented: one is based on residual current thresholds, named solution 1, and the other is based on FFT and waveform recognition, named solution 2. And the problems of the traditional RCD which is unable to provided effective protection for residual pulsating direct currents can be solved by using the two methods. Prototypes based on single chip micro-controller or DSP are designed for solution 1 and solution 2.Tests are conducted to the two prototypes in terms of relative standards. The results show that they are verified to t meet he requirements of relative standards with correct action to residual pulsating direct currents and alternative sinusoidal alternating currents. Thus, the feasibility of the design is verified. The differences between the two methods are: solution 1 is easily to implement with low cost, but it has relatively low precision, while solution 2 is relatively complicated with high cost but it has high precision and low unwanted tripping.