Research Of Over-Load Protection Mathematical Model And Algorithm

The over-load protection technology is an indispensable constituent of the low voltage apparatus intellectualization and is widely used in the products, such as low-voltage circuit-breaker, overload relay and so on. Moreover, this technology is very important to ensure the electric motor and the distribution line safe operation. The study of inherent laws of over-load influence for the electric motor and the distribution line, building of over-load protection model for the different objects, and new digital algorithm of over-load protection is meaningful to improve the efficiency of the over-load protection validity.This paper establishes a mathematical model for the dynamic over-load protection, based on the heat balance differential equation, to determine how temperature increases with the change of the protection object's current. Different from the common inverse time characteristic, this model conforms to the load actual operating law and the protection request, and takes the analysis of measurement error into consideration. And, the equivalent current algorithm for the electric motor heat accumulation is proposed based on the symmetrical component method.Wavelet analysis, as a new kind of time-frequency representation technique, has made great progress recently and been applied widely in different engineering practice. To solve the problem of the full-wave Fourier algorithm which is disable to filter decaying DC component and has worse frequency characteristic, it is presented a algorithm depending on the combination of subtraction filter and full-wave Morlet complex wavelet amplitude algorithm to get out the amplitude value of fundamental component of the current in the distribution line by studying each parameter's influence on the performance of Morlet complex wavelet amplitude algorithm. It can filter efficiently decaying DC component, has clear frequency characteristic, and make calculation with high precision.The hardware and software modulation systems of the intelligent over-load protector are designed with the core of microprocessor PIC18F458. Moreover the remote bi-directional communication, between superior computer and over-load protector, is achieved by the RS-485 serial communication interface and software.