Research On Problems About Protective Current Transformer Saturation

When fault happens, the primary current of power system may be a lot of times of theload current, and always has DC transient component. In this condition, the magnetic core ofP type protective current transformer (CT) will become saturated, which causes the distortionof secondary current and affects the operation of protection relay and other secondaryequipments. This paper does comprehensive research on the problems about CT magneticcore saturation, the main contents are as follows.1. A new CT digital model based on Preisach theory is presented. By the application ofPreisach theory, this paper avoids the conflict between workload and simulation precisionin the work of CT modeling. The new CT model can simulate magnetizing process ofmagnetic core precisely by only some easily acquired parameters such as limitinghysteresis loop, magnetic core size and CT ratio, and get precise simulation results ofsecondary current, magnetizing current, flux density.2. Analysis is performed about the effective scope of new CT model and some other CTmodels that may always be used in engineering. Based on this, presents some significantfeatures about CT modeling: â‘ Simulating the magnetizing trajectory in the saturationscope precisely is necessary to get right secondary current waveform when the magneticcore is saturated;â‘¡Simulating the magnetizing process of magnetic core precisely isnecessary in the re-close process.3. Analysis about the effects caused by some parameters on CT saturation condition isperformed. These parameters are primary current, secondary load, remanence, material ofmagnetic core, area of cross section, turns of coil and CT scale. Points out for the firsttime that the effects caused by remanence will decrease as the total time of saturationincreases.4. For the first time, presents a new saturation detection method based on the singularcharacters of secondary inductive voltage. The method is called waveform singularitymethod (WS method). By analyzing the physical process of CT saturation, this paperpresents singular characters of secondary voltage and current. And then, presents the CTsaturation detection method by combining those singular characters of secondary voltage.The new method needs little computation, therefore, can be implemented in real time.Moreover, the new method detects saturation by secondary voltage, therefore, eliminatesthe effect caused by power factor of secondary load. At last, the new method doesdetection with essential characters of saturation, therefore, has high precision, stability,practicability and wide effective scope.5. Points out for the first time that the linear artificial neural network (LANN) can be trainedto give out first order prediction of the primary fault current. Based on this, this paperpresents a new secondary current distortion compensation method. The new methodcombines LANN and detection result of WS method, and can compensate the distortedsecondary current in real time. Since the structure of LANN is concise and thecomputation of LANN includes only some additions and multiplications, the method canbe implemented in real time easily.6. Based on real time least mean square algorithm, presents a new secondary currentdistortion compensation method, RLS method. The method compensates the distortedsecondary current by estimating the parameters of primary fault current. Comparing withthe other parameter estimation algorithm, the computation amount of real time least meansquare algorithm in a single step is very little, therefore, the new compensation method isvery fitful of real time implementation. Moreover, the RLS method adjusts estimationresults with new sampling data, therefore, the compensation results has high precision. Atlast, since the estimation result is the optimal solution in some collective sense, themethod has great stability and immune to noise.7. Based on an embed system with DSP+CPLD structure, implements the WS saturationdetection method and RLS secondary current distortion compensation method.Experiment results show that, in the embed system, the WS method and RLS method areall effective, and the program has good real time character.