Reaseach On Detecting Technology Of Residual Currents Under Complicated Waveform Conditions

Residual current devices (RCDs) are widely used in low voltage power system to preventpersonal electric shock, electrical fires, personal injury and electrical equipment damages causedby ground fault. With the development of smart grid and increasing number of electricalequipment, and the widely employment of power electronics products, the types of electricalequipment are diversified. When power electronic equipments fail, the residual current generatedis not just sinusoidal, but also includes pulsating DC, or even smooth DC. At the same time, thefrequency of residual current can be as high as1000Hz in some cases, or even higher. Therefore,the traditional technology is no longer applicable. Electromagnetic current transformers, Hallcurrent sensors and magnetic modulation current transformer are widely used in currentdetecting. However, non-sinusoidal residual current is a kind of weak signal, and the traditionalcurrent detecting methods are no longer applicable. Based on analysis of current detectingprinciples and techniques, a kind of voltage source magnetic modulated AC/DC residual currentdetecting method is introduced. The detection and protection for complicated residual currentcan be achieved, without defects of traditional magnetic modulation current detection method, bydetecting the excitating current, which is coupled to the residual current.A mathematical model of AC/DC residual current detection is built based on coremagnetization curve. Residual currents are only related to the low frequency components ofexciting current when the frequency of exciting current is much higher than that of the highestharmonic of exciting current. Therefore, it provides a basis for measurement of residual currentsunder complicated wave conditions. In order to analyze the dynamic process of exciting current,two kinds of models of core magnetization curve, which are called piecewise-linear model andanti-tangent nonlinear model, are used. The mathematical model of dynamic process of exciting current based on voltage type magnetic modulation principle was deduced and it provides amathematical method for design of residual current transformer by using piecewise-linear model.The dynamic process of exciting current based on voltage type magnetic modulation principlewas simulated using Simulink with anti-tangent nonlinear model. In addition, it provides a moreaccurate description of variation of exciting current.The characteristics of frequency spectrum of residual currents under complicated conditionsare analyzed. Moreover, the demodulation methods to extract signals are present, includingsoftware demodulation method based on all phase FFT (apFFT) and hardware demodulationmethod based on low-pass filter and residual current detection under complicated waveformconditions are implemented. The problem of frequency leakage can be solved effectively by A/Dconversion and digital signal process by using apFFT for the exciting current. In order to reducethe requirement for A/D conversion and digital sampling, a hardware low-pass filter is used toextract the residual current from exciting current.Two types of RCDs are designed based on apFFT software demodulation method andhardware low-pass filter method. Residual current transformer, signal detecting circuit, low-passfilter design, power supply circuit and trip circuit are including in hardware design. Differentcontrol software is designed separately according different types of demodulation methodsincluding apFFT software demodulation. And different signal processing methods are used toimplement residual current instantaneous protection as well as reducing malfunction. Thus, thereliability of RCD is improved.Finally, prototypes based on apFFT and low pass filter are tested separately, includingdetecting range test, linearity test and frequency ranges test. In addition, it is experimentallyverified test that the prototypes can meet the requirements of relevant standards. Meanwhile, testresults are consistent with the theoretical analyses, which verify the correctness of the simplifiedmathematical model for further.