Research On The New Principle And Application Problems For The Fast Main Protection Of Large Power Transformers

Large transformers are important and expensive equipments in the power systems, whose protection scheme should satisfy dependability, reliability, as well as the requirement for fast operation. Based on the existed literatures and the different operating characteristics of large transformers, positive sequence active power differential protection and protection scheme based on the mutation characteristics of sampled value of current (voltage) are newly proposed from the view point of power balance and the comparison between break variable of sampled current(voltage) at different sides, which are analyzed and validated qualitatively and quantitatively in theoretical analysis, and corresponding criterions are given.The dissertation is outlined as following:(1) A nonlinear protection scheme for transformers, i.e. the positive sequence active power differential protection, as well as its implementations are proposed based on detailed analysis to the mechanism and characteristics of the magnetized inrush current of the power transformers. With theoretical analysis and simulation, it is shown that the influence of the magnetized inrush current may be completely avoided with the proposed scheme. Then operation condition for transformer protection with positive sequence active power differential scheme is evaluated, and the possible application is clearly described.(2) According to the different characteristics of the apparent current at both sides with internal and external faults, a new criterion is proposed to compare the direction of the sampled current at both sides of the transformer under the condition of its break variable of absolute value, thus to distinguish external and internal faults. With theoretical analysis and simulation, it is proved that the proposed criterion will operate quickly and reliably to different external faults under different operation conditions.(3) The mutation pattern of the absolute values for the sampled current and voltage at both sides with different internal faults under different operation conditions as well as after external faults clearance is analyzed in detail. Then with the presumption of no external faults, a new criterion is proposed to distinguish the internal faults and the inrush current after external faults clearance. By using theoretical analysis and simulation, it is shown that with this principle, the criterion may operate quickly and reliably against different types of internal faults in transformers.(4) The software flowchart to embed the proposed scheme into the interruption program for microprocessor based on transformer protection software is designed, which solves the problem that one should choose the different criterions of fault treatment for no-load and under-load operation for transformers. A composite comparison flowchart for the positive sequence active power differential protection and the break variable of sampled current(voltage) is presented to analyze the possible problems and corresponding solutions when applied to multi-winding transformers.The proposed scheme is not based on the differential current, therefore it is simple, fast and reliable, which can constitute independent software modules for the fast main protection in microprocessor based on integrated devices for transformer protection.