New Transformer Protection Methods Based On The Multi-Information In Intelligent Substations

The continuity and stability of power supply are significantly influenced by the operating state of transformers,thus,transformer protective devices with good performance and high reliability are very important for the safe and stable operation of power system.Magnetizing inrush has been the core problem of transformer protection,because it can lead to maloperation of current differential protection.Sympathetic inrush is one kind of magnetizing inrush.This kind of inrush is not easily to be detected and the maloperation caused by it is often ignored with harmfulness.Traditional transformer protection only depends on the currents of the transformer.However,the situation can be changed by intelligent substations,in which multi-information sharing is convenient.Therefore,new transformer principles can be studied using the multi-information,such as voltages,currents and switch messages,to improve the performance of transformer protection.Two aspects of research are conducted in this paper based on the convenient multi-information sharing in intelligent substations.On the one hand,the sympathetic inrush is studied on the basis of traditional current differential protection.The physical mechanism of sympathetic inrush is thoroughly investigated and an identification method is proposed.On the other hand,a new transformer protection principle immune to inrush is studied,irrespective of current differential protection.The major contributions of this dissertation are as follows.(1)The physical mechanism of sympathetic inrush is thoroughly studied.First,transformers in parallel and series are modeled,and the flux linkage expressions of the transformers are derived by taking winding resistance into account.The reasons why sympathetic inrush and initial inrush present different characteristics are directly explained by the variation trend of flux linkages as time.Meanwhile,the influences of switching angle,residual flux,system impedance,winding resistance and other factors are investigated by the DC components of flux linkages.The feature that initial inrush and sympathetic inrush are alternately generated in the opposite polarities will not change as these factors vary,which provides the foundation for sympathetic inrush identification.On the other hand,the influences of transformer saturation on the inrush currents are analyzed based on the magnetizing inductance.The interaction between transformers is explained,which is consistent with the basic development rules.(2)A new method for identifying sympathetic inrush is proposed based on the substation-area differential current(SDC),according to the feature of alternate generation of initial inrush and sympathetic inrush.Double wave peaks of the SDC in one cycle are the key to identify sympathetic inrush with the currents and state of CB position used.The alternate generation feature as the core of the proposed method can keep existing during sympathetic inrush,and the method does not depend on the second harmonic content.The simulation results show that the proposed method can identify sympathetic inrush and prevent the current differential protection from maloperation under different conditions.(3)A new method for identifying sympathetic inrush is proposed based on the alternate inductance.The amplitude and phase angle of the fundamental component of magnetizing inductance are used as the basis for the proposed method,according to the alternate inductance due to altenate saturation of the transformers.Voltages,currents and state of circuit breaker are utilized in the method.The proposed mtheod can identify the sympathetic inrush rapidly and substainablely without use of the second harmonic content.The misoperation of the transformer protection can be prevented by the proposed method.(4)A new power differential principle for the transformer is proposed based on the fault component.The differential power in fault component network is used to discriminate internal faults and other situations according to the law of conservation of energy.Since the reliability of the principle relies on the accuracy of the obtained phasor,two phasor estimation algorithms are compared.Using the voltages,currents and switch messages,the principle is immune to the inrush currents and the voltage drops during faults.The proposed protection principle has clear theoretical foundation,and it can be easily applied in practical as well.(5)The test requirements are analyzed for protective devices based on the multi-information in intelligent substations.Key and practical techniques for the tests of these protective devices are developed.Based on the techniques,a real-time test platform is constructed using the Real Time Digital Simulator(RTDS),and the closed-loop test is implemented for a protective device based on the multi-information.The test results show the platform can meet the requirements.